ENVIRONMENT GLOBAL WARMING

El Nino/La Nina Explainer (Video)

EL NIÑO, LA NIÑA & ENSO

What are El Niño and La Niña?

Global vegetation

The vegetation zones of the world are controlled by the annual average and seasonality of both temperature and precipitation. Temperature follows a latitudinal gradient with warmest conditions in the tropics and coldest at the poles. As we have seen there are three main rainfall belts, the convection rainfall belt in the tropics and the convergent rainfall belt in the mid-latitudes of the Northern and Southern Hemispheres. The two main desert regions of the world lie between these rainfall belts.

Vegetation follows these climate zones. So rainforest is found in the tropics where there is a lot of rainfall all year round. Savannah is found in the tropics when rainfall is high seasonally, but there are also long dry seasons lasting over 4 months. The world’s largest deserts are found in the mid-latitudes. Here the seasonality of rainfall is critical, as while many deserts have the same rainfall as, say, London, this rain falls over a very small period of time, with the rest of the year being extremely arid. When the rainfall occurs only in the winter months followed by a very dry summer period, the unique Mediterranean flora is found, such as in California, South Africa, and of course around the Mediterranean. In high mid-latitudes are the temperate or boreal forests. In areas with low annual rainfall, steppe vegetation is found. In high latitudes where the temperature is the limiting factor, tundra is found. Other factors can influence where different vegetation can exist, for example we have seen that major ocean currents can allow temperate-weather vegetation to exist much further north than would usually be expected. Later we will see that mountain ranges and plateaus have a huge influence on where rainfall occurs and thus where deserts form.

Finally, it should be remembered that vegetation has its own influence on climate. First, vegetation changes the albedo of any area, so tropical rainforests absorb much more solar radiation than does tundra. Second, vegetation is very good at recycling water and maintaining a moist atmosphere. For example, 50 per cent of all the rainfall in the Amazon Basin comes from water recycled by the trees, evaporating and creating new clouds.

Weather versus climate

Many people get weather and climate confused. This confusion is exacerbated when scientists are asked to predict climate 50 years from now when everyone knows they cannot predict the weather a few weeks ahead. So climate is generally defined as ‘the average weather’. The original definition of climate was ‘the average weather over 30 years’, this has been changed because we now know that our climate is changing and significant changes have been seen every decade for the last 50 years. The chaotic nature of the weather can make it unpredictable beyond a few days, while understanding the climate and modelling climate change is much easier as you are dealing with long-term averages. A good comparison is that though it is impossible to predict at what age any particular person will die, we can say with a high degree of confidence that the average life expectancy of a person in a developed country is about 80. The other confusion is that people always remember extreme weather events and not the average weather. So for example everyone remembers the heat waves in the UK in 2003 and the USA in 2012, or the floods in Pakistan and Australia in 2010. So our perception of weather is skewed by these events rather than by an appreciation of the average weather or climate.

Chaos theory

The National Weather Service in the USA spends over $1 billion per year ensuring the country has the most accurate weather prediction possible. In other countries similar resources are poured into weather agencies, as predicting the weather is big business and getting a storm prediction right can save billions of dollars and many lives. Today three-four day forecasts are as accurate as the two-day forecasts were 20 years ago. Predictions of rain in three days’ time are as accurate as one-day forecasts were in the mid-1980s. The accuracy of flash flood forecasts has improved from 60 per cent correct to 86 per cent, moreover potential victims of these floods get nearly an hour’s warning instead of the 8 minutes they would have had in 1986. The lead times of advance warnings of tornadoes, in other words, the time that residents have to react, has increased from 5 minutes in 1986 to over 12 minutes. Severe local thunderstorms and similar cloudbursts are typically seen 18 minutes beforehand rather than 12 minutes over two decades ago. Seventy per cent of all hurricane paths can be predicted at least 24 hours in advance and the landfall of a hurricane can be predicted to within 100 miles (160 km).

These are great achievements but it does not explain why with all our technology and our understanding of the climate system we cannot predict the weather 10 days, a month, or a year in advance. Moreover, think of all those times that the weather report on television has said it will be sunny today and then it rains. So why is it so difficult to predict the weather? In the 1950s and 1960s it was thought that our weather prediction was limited by our lack of data and that if we could measure things more accurately and clearly understand the fundamental processes we would be able to achieve a much higher level of prediction. But in 1961 Edward Lorenz a meteorologist at Massachusetts Institute of Technology made a cup of coffee that radically changed the way we think about natural systems. In 1960 Lorenz had produced one of the first computer models of weather. One day in the winter of 1961, Lorenz’s computer model produced some very interesting patterns, which he wanted to look at in greater detail. So he took a short-cut and started mid-way through the run. Of course this was one of the earliest computers so he had to retype all the starting numbers. Instead of typing them into six decimal places (e.g., 0.506127) he only typed the first three to save time and space, and then went and made the famous coffee. When Lorenz came back he found that the weather patterns had diverged from the initial run so much that there was no recognizable similarity between them. It seems the model was very sensitive to the very small changes, that one part in a thousand instead of being inconsequential had had a huge effect on the outcome. This original work has lead to the development of chaos theory. Chaos theory shows us that very small variations in atmosphere temperature, pressure, and humidity can have a major and unpredictable or chaotic effect on large-scale weather patterns.

Nevertheless, chaos theory does not mean there is a complete lack of order within a system. Far from it, chaos theory tells us that we can predict within certain boundaries what the weather will be like: we all know, for example, that most tornadoes occur in May in the USA and that winters are wet in England. But when it comes to more detailed prediction everything breaks down due to what has become known as the ‘butterfly effect’. The idea is that small changes represented by the flapping of the wings of a butterfly can have a large effect on the weather, for example altering the strength and direction of a hurricane. As errors and uncertainties multiply and cascade upward through the chain of turbulent features from dust devils and squalls up to continental size eddies that only satellites can see. In effect, we will never know which of the small weather changes will combine to have these large effects. While Lorenz used 12 equations in his weather model, modern weather computers use 500,000. But even the best forecasts, which come from the European Centre for Medium Weather Forecasts based at Reading in England, suggest that weather predictions for more than four days are at best speculative and beyond a week worthless, all because of chaos. So chaos theory says that we can understand weather and we can predict general changes but it is very difficult to predict individual events such as rain storms and heat waves. The study of climate, however, has one great advantage over meteorology because it only examines averages and thus chaos theory does not affect it. Moreover when it comes to modelling future climate change we can now understand that an increase in the Earth’s average temperature will make some weather phenomena more frequent and intense for example heat waves and heavy rainfall events, while others will become less frequent and intense, for example extreme cold events and snow fall.

Decadal and quasi-periodic climate systems

The climate system contains many cycles and oscillations that complicate our ability to predict the weather. These include decadal cycles such as the North Atlantic oscillation (NAO), the Atlantic multi-decadal oscillation (AMO), Arctic oscillation (AO), and the Pacific decadal oscillation (PDO). So the first of these is the NAO, which was first described in the 1920s by Sir Gilbert Walker (14 June 1868-4 November 1958), a British physicist and statistician. The NAO is a climate phenomenon in the North Atlantic Ocean and is represented by the atmospheric pressure difference at sea level between Iceland and the Azores. The difference in Icelandic low-pressure and the Azores highpressure systems seems to control the strength and direction of westerly winds and storm tracks across the North Atlantic Ocean. This in turn controls where and when in Europe it rains. Unlike the El Niño–Southern Oscillation, the NAO is largely controlled by changes in the atmosphere. The NAO is closely related to the AO and though both seem to change on a decadal scale there seems to be no periodicity. The NAO should not, however, be confused with the AMO. The AMO is the decadal-scale variability in the sea-surface temperatures of the North Atlantic Ocean. Over the last 130 years, 1885-1900, 1927-1947, 1951-1961, 1998–present day, the North Atlantic Ocean temperatures have been warmer than average and the time in between colder. The AMO does affect air temperatures and rainfall over much of the Northern Hemisphere, in particular North America and Europe, for example the North Eastern Brazilian and African Sahel rainfall and North American and European summer climates. It is also associated with changes in the frequency of North American droughts and it may influence the frequency of severe Atlantic hurricanes. There are also irregular or quasi-periodic cycles such as the Indian Ocean Dipole and El Niño-Southern Oscillation (ENSO). Of these ENSO is by far the best known.

El Niño-Southern Oscillation

One of the most important and mysterious elements in global climate is the periodic switching of direction and intensity of ocean currents and winds in the Pacific. Originally known as El Niño (‘Christ child’ in Spanish) as it usually appears at Christmas, and now more often referred to as ENSO (El Niño–Southern Oscillation), this phenomenon typically occurs every 3 to 7 years. It may last from several months to more than a year. ENSO is an oscillation between three climates: the ‘normal’ conditions, La Niña, and ‘El Niño’. ENSO has been linked to changes in the monsoon, storm patterns, and occurrence of droughts throughout the world. For example the prolonged ENSO event, in 1997 to 1998, caused severe climate changes all over the Earth including droughts in East Africa, northern India, north-east Brazil, Australia, Indonesia, and Southern USA; and heavy rains in California, parts of South America, the Pacific, Sri Lanka, and east central Africa. The state of the ENSO has also been linked into the position and occurrence of hurricanes in the Atlantic Ocean. For example, it is thought that the poor prediction of where Hurricane Mitch made landfall was because the ENSO conditions were not considered and the strong Trade Winds helped drag the storm south across central USA instead of west as predicted.

An El Niño event is when the warm surface water in the western Pacific moves eastward across to the centre of the Pacific Ocean. Hence the strong convection cell or warm column of rising air is much closer to South America. Consequently the Trade Winds are much weaker and the ocean currents crossing the Pacific Ocean are weakened. This reduces the amount of the cold, nutrient-rich upwelling off the coast of South America and without those nutrients the amount of life in the ocean is reduced and fish catches are dramatically reduced. This massive shift in ocean currents and the position of the rising warm air changes the direction of the jet streams that upset the weather in North America, Africa, and the rest of the world. However if you ask what causes El Niño, then the answer is of the chicken and egg variety. Does the westward ocean current across the Pacific reduce in strength, allowing the warm pool to spread eastward and moving with it the wind system? Or does the wind system relax in strength, reducing the ocean currents, and allowing the warm pool to move eastwards? Many scientists believe that long period waves in the Pacific Ocean that move between South America and Australia over time help shift the ocean currents which produce either an El Niño or a La Niña period.

La Niña is a more extreme version of the ‘normal’ conditions. Under normal conditions the Pacific warm pool is in the western Pacific and there are strong westerly winds and ocean currents keeping it there. This results in upwelling off the coast of South America, providing lots of nutrients and thus creating excellent conditions for fishing. During a La Niña period the temperature difference between the western and eastern Pacific becomes extreme and the westerly winds and ocean currents are enhanced. La Niña impacts on the world’s weather are less predictable than those of El Niño. This is because during an El Niño period the Pacific jet stream and storm tracks become strong and straighter and it is therefore easier to predict its effects. La Niña on the other hand weakens the jet stream and storm tracks, making them more loopy and irregular, meaning that the behaviour of the atmosphere and particularly of storms becomes more difficult to predict. In general where El Niño is warm, La Niña is cool, where El Niño is wet, La Niña is dry. La Niñas have occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988, 1995, 1999, 2008, and 2011, with the 2010-2011 La Niña being one of the strongest ever observed.

Predicting ENSO

Predicting an El Niño events is difficult but a lot of work has gone on for the last three decades to better understand the climate system. For example, there is now a large network of both ocean and satellite monitoring systems over the Pacific Ocean, primarily aimed at recording sea-surface temperature, which is the major indicator of the state of the ENSO. By using this climatic data in both computer circulation models and statistical models, predictions are made of the likelihood of an El Niño or La Niña event. We are really still in the infancy stage of developing our understanding and predictive capabilities of the ENSO phenomenon. There is also considerable debate over whether ENSO has been affected by global warming. The El Niño conditions generally occur every 3 to 7 years; however, in the last 20 years, they have behaved very strangely, returning for 3 years out of 4: 1991-1992, 1993-1994, and 1994-1995, then not returning until 1997-1998, and then not returning for 9 years, finally arriving in 2006-2007 and 20014-2015. Reconstruction of past climate using coral reefs in the western Pacific shows sea-surface temperature variations dating back 150 years, well beyond our historical records. The sea-surface temperature shows the shifts in ocean current, which accompany shifts in the ENSO and reveal that there have been two major changes in the frequency and intensity of El Niño events. First was a shift at the beginning of the 20th century from a 10-15-year cycle to a 3-5-year cycle. The second was a sharp threshold in 1976 when a marked shift to more intense and even more frequent El Niño events occurred. Moreover during the last few decades the number of El Niño events has increased, and the number of La Niña events has decreased. Even taking into account the effect of decadal cycles on ENSO the size of the ENSO variability in the observed data seems to have increased by 60 per cent in the last 50 years. However, as we have seen, to predict an El Niño event 6 months from now is hard enough, without trying to assess whether or not ENSO is going to become more extreme over the next 100 years. Most computer models of ENSO in the future are inconclusive; some have found an increase and others have found no change. This is, therefore, one part of the climate system that we do not know how global warming will affect. Not only does ENSO have a direct impact on global climate but it also affects the numbers, intensity, and pathways of hurricanes and cyclones, and the strength and timing of the Asian monsoon. Hence, when modelling the potential impacts of global warming, one of the largest unknowns is the variation of ENSO and its knock-on effects on the rest of the global climate system.

Modelling climate

The whole of human society operates on knowing the future weather. For example, a farmer in India knows when the monsoon rains will come next year and so they know when to plant the crops. While a farmer in Indonesia knows there are two monsoon rains each year so each year they can have two harvests. This is based on their knowledge of the past, as the monsoons have always come at about the same time each year in living memory. But weather prediction goes deeper than this as it influences every part of our lives. Our houses, roads, railways, airports, offices, cars, trains, and so on are all designed for our local climate. Predicting future climate is, therefore, essential as we know that global warming is changing the rules. This means that the past weather of an area cannot be relied upon to tell you what the weather in the future will hold. So we have to develop new ways of predicting and modelling the future, so that we can plan our lives and so that human society can continue to fully function.

There is a whole hierarchy of climate models, from relatively simple box models to the extremely complex three-dimensional general circulation models (GCMs). Each has a role in examining and furthering our understanding of the global climate system. However, it is the complex three-dimensional general circulation models which are used to predict future global climate. These comprehensive climate models are based on physical laws represented by mathematical equations that are solved using a three-dimensional grid over the globe. To obtain the most realistic simulations, all the major parts of the climate system must be represented in sub-models, including the atmosphere, ocean, land surface (topography), cryosphere, and biosphere, as well as the processes that go on within them and between them. Most global climate models have at least some representation of each of these components. Models that couple together both the ocean and atmosphere components are called atmosphere-ocean general circulation models (AOGCMs).

Over the last 25 years there has been a huge improvement in climate models. This has been due to our increased knowledge of the climate system but also because of the nearly exponential growth in computer power. There has been a massive improvement in spatial resolution of the models from the very first Intergovernmental Panel on Climate Change (IPCC) in 1990 to the latest in 2007. The current generation of AOGCMs has a resolution of one point every 110 km by 110 km, and this is set to get even finer when the next IPCC Science Report is published in late 2013. The very latest models or as some groups are now referring to them ‘climate simulators’ include much better representations of atmospheric chemistry, clouds, aerosol processes, and the carbon cycle including land vegetation feedbacks. But the biggest unknown or error in the models, is not the physics, it is the estimation of future global greenhouse emissions over the next 90 years. This includes many variables, such as the global economy, global and regional population growth, development of technology, energy use and intensity, political agreements, and personal lifestyles.

Over 20 completely independent AOGCMs have been run using selected future carbon dioxide emission scenarios for the IPCC 2007 report, producing global average temperature changes that may occur by 2100. This is a significant change from the IPCC 2001 report, in which only 7 of these models were used. Using the widest range of potential emission scenarios the climate models suggest that global mean surface temperature could rise by between 1.1°C and 6.4°C by 2100. Using the best estimates for the 6 most likely emission scenarios, then this range is 1.8°C to 4°C by 2100. Model experiments show that even if all radiation forcing agents were held at a year-2000 constant, there would still be an increase of 0.1°C per decade over the next 20 years. This is mainly due to the slow response of the ocean. Interestingly, the choice of emission scenario has little effect on the temperature rise to 2030, making this a very robust estimate. All models suggest twice the rate of temperature increase in the next two decades compared with that of the 20th century. What is significant is that the choices we make now in terms of global emissions will have a significant effect on global warming after 2030. The next IPCC report to published in late 2013, use  greatly improved emission scenarios, will have a very similar potential change of warming by the end of the century. What is amazing and very reassuring is that over the last 25 years the climate models have consistently given us the same answer, meaning we do understand the climate system and we can understand the consequences of our past and future actions.

Extreme climates

Humans can live, survive, and even flourish in the extreme climates ranging from that of the Arctic to that of the Sahara. We have populated every continent except Antarctica. We can deal with the average climate of each region through our adaptations of technology and lifestyle. The problems arise when the predictable boundaries of local climate are exceeded, for example by heat waves, storms, droughts, and/or floods. This means that what we define as extreme weather, such as a heat wave, in one region may be considered fairly normal weather in another. Each society has a coping range, a range of weather with which it can deal: what is seen as a heat wave in England would be normal summer conditions in Kenya. However, one of the most unpredictable and dangerous elements in our climate systems are storms. In this chapter we examine how and why storms are formed and their impact. Hurricanes, tornadoes, winter storms, and the monsoons will all be discussed.

Hurricanes

A hurricane is a severe cyclonic tropical storm that starts in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, west coast of Mexico, or the northeast Pacific Ocean. They are called typhoons in the western Pacific and simply tropical cyclones in the Indian Ocean and Australasia. They are, however, all the exactly the same type of storm and here we will call all of them hurricanes. Hurricanes occur in the tropics between 30°N and 30°S, but not near the Equator as there is not enough atmospheric variation to generate them. For a storm to be classified as a hurricane, the sustained wind speed must exceed 120 km/hr. Of course in a fully developed hurricane, wind speeds can exceed 200 km/hr.

A hurricane is a tropical storm run amok, a rotating mass of thunderstorms that has become highly organized into circular cells, which are ventilated by bands of roaring winds. Hurricanes develop over the oceans and tend to lose their force once they move over land—this is because unlike temperate storms hurricanes are driven by the latent heat from the condensation of water. The sun is most intense close to the Equator where it heats the land, which in turn heats the air. This hot air rises and consequently sucks air from both the north and south producing the Trade Winds. As the seasons change so does the position of the clash of the Trade Winds, which is called the Inter-tropical Convergence Zone (ITCZ). To generate a hurricane the sea temperature must be above 26°C for at least 60 m below the surface and the air humidity must be at about 75–80 per cent. This combination provides the right amount of heat and water vapour to sustain the storm once it has started. For example these conditions occur during the summer in the North Hemisphere when the tropical North Atlantic Ocean heats up enough and its water starts to evaporate. Initially the warm ocean heats the air above it and causes that to rise. This produces a low-pressure area which sucks in air from the surrounding area. This rising air contains a lot of water vapour due to pronounced evaporation from the hot surface of the ocean. As the air rises it cools and can no longer hold as much water vapour; as a result some of it condenses to form water droplets and then clouds. This transformation from water vapour to water droplets releases energy called ‘latent heat’. This in turn causes further warming of the air and causes it to rise even higher. This feedback can make the air within a hurricane rise to over 10,000 m above the ocean. This becomes the eye of the storm and the spiraling rising air it produces creates a huge column of cumulo-nimbus clouds. You can see a mini version of this with steam coming out of a kettle. As the hot air rises from the kettle it hits the colder air and it forms steam, a mini-cloud. If you have ever put your hand near the steam you can feel it is very hot and this is because of all the energy being released as the water vapour changes from a gas back to a liquid. When the air inside the hurricane reaches its highest level it flows outwards from the eye producing a broad canopy of cirrus cloud. The air cools and falls back to sea level where it is sucked back into the centre of the storm. Because of the Coriolis force, the air that is sucked into the bottom of the hurricane spins into the storm in a clockwise direction, while the air escaping at the top spins out in a counter-clockwise direction. This pattern is the opposite in the Southern Hemisphere. Hurricanes form at least 345 miles or 5° of latitude away from the Equator, where the Coriolis Effect is strong enough to give the required twist to the storm. The size of hurricanes can vary from 100 km to over 1,500 km. A hurricane can form gradually over a few days or in the space of 6 to 12 hours and typically the hurricane stage will last 2–3 days and take about 4-5 days to die out. Scientists estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day, equivalent to about 1 PW (1015 Watt). This rate of energy release is equivalent to 70 times the human world energy consumption and 200 times the worldwide electrical generating capacity, or to exploding a 10-megaton nuclear bomb every 20 minutes. Hurricanes are measured in the Saffir-Simpson scale and go from a tropical storm through category 1 to the worst at category 5.

However, the formation of hurricanes is much rarer than might be expected given the opportunities for them to occur. Only 10 per cent of centres of falling pressure over the tropical oceans give rise to fully fledged hurricanes. In a year of high incidence, perhaps a maximum of 50 tropical storms will develop to hurricane levels. Predicting the level of a disaster is difficult as the number of hurricanes does not matter-it is whether they make landfall. For example, 1992 was a very quiet year for hurricanes in the North Atlantic Ocean. However, in August, one of the few hurricanes that year, Hurricane Andrew, hit the USA just south of Miami and caused damage estimated at $26 billion.

Hurricane Andrew also demonstrates that predicting where a storm will hit is equally important-if the hurricane had hit just 20 miles further north it would have hit the densely populated area of Miami City and the cost of the damage would have doubled.

In terms of where hurricanes hit in developed countries, the major effect is usually economic loss, while in developing countries the main effect is loss of life. For example, Hurricane Katrina, which hit New Orleans in 2005, caused 1,836 deaths while Hurricane Mitch, which hit Central America in 1998, killed at least 25,000 people and made 2 million others homeless. In both cases the greatest damage was caused by the huge amount of rainfall. Honduras, Nicaragua, El Salvador, and Guatemala were battered by 180-mile (290 km) per hour winds, and more than 23 inches (60 cm) of rain every day. Honduras, a small country of only 6 million inhabitants, was the worst hit. The Hamuya River, normally a calm stretch of water about 200 feet (60m) wide rose by 30 feet (9 m) and became a raging torrent, ripping out trees as tall as a city block from the ground. Eighty-five per cent of the country ended up under water. Over 100 bridges, 80 per cent of the roads, and 75 per cent of its agriculture were destroyed, including most of the banana plantations.

In New Orleans the worst damage by Hurricane Katrina was caused by both the intense rainfall and the storm surge. Together they caused 53 different levees to break, submerging 80 per cent of the city.

The storm surge also devastated the coasts of Mississippi and Alabama. Hurricane Katrina was not the worst storm that has hit the USA; a storm that hit Miami in 1926 was 50 per cent larger but did little damage because Miami Beach was then still undeveloped. In the USA coastal population has doubled in the last 10 to 15 years making the country much more vulnerable to storm related losses. There is also a large financial difference if a hurricane hits a developed or developing country. For example, the immediate economic impact of Hurricane Katrina was over $80 billion, but its subsequent effect on the US economy was to boost it slightly, by 1 per cent, that year due to the billions of dollars spent by the Bush administration to aid reconstruction of the region. Compare this with Hurricane Mitch in 1998, which set back the economy of Central America by about a decade. Hurricanes also occur elsewhere in the world. An average of 31 tropical storms roam the western North Pacific every year, with typhoons smashing into Southeast Asia from June to December; most at risk are Indonesia, Hong Kong, China, and Japan, otherwise known as ‘Typhoon Alley’. Why does Typhoon Alley get so many typhoons? And why can they occur almost any time of year. The answers lie in the oceans. The key is the ‘warm pool’ of ocean water that sits in the western tropical Pacific.

All year long the Trade Winds and the ocean current push the surface water warmed by the tropical sun to the far western side of the North Pacific. Hurricane seasons come and go in other parts of the world but the water of the ‘warm pool’ is always warm enough to start a hurricane-though they are most common between June and December.

Tornadoes

Tornadoes are nature’s most violent storms. Nothing that the atmosphere can dish out is more destructive: they can sweep up anything that moves; and they can lift buildings from their foundations, making a swirling cloud of violently flying debris. They are very dangerous, not only because of the sheer power of their wind, and the missiles of debris they carry, but because of their shear unpredictability. Tornado strength and destructive capability is measured on the Fujita Scale. A tornado is a violent rotating column of air, which at a distance appears as an ice cream cone-shaped cloud formation. Other storms similar to tornadoes in nature are whirlwinds, dust-devils (weaker cousins of tornadoes occurring in dry lands), and waterspouts (a tornado occurring over water). Tornadoes are most numerous and devastating in central, eastern, and northeastern USA, where an average of 5 per day are reported every May. They are also common in Australia (15 per year), Great Britain, Italy, Japan, Bangladesh, east India, and central Asia. While the greatest number of fatalities occurs in the United States, the deadliest tornadoes by far have occurred in the small area of Bangladesh and east India. In this 8,000 mile2 (21,000 km2) area, 24 of the 42 tornadoes known to have killed more than a 100 people have occurred. This is likely due to the high population density and poor economic status of the area as well as a lack of early warning systems.

We can see tornadoes as miniature hurricanes. Although tornadoes can form over tropical oceans they are more common over land. The formation of tornadoes is encouraged when there is warm, moist air near the ground and cold dry air above. This occurs frequently in late spring and early summer over the Great Plains of the USA. Intense heating of the ground by the sun makes warm, moist air rise. As it does so it cools and forms large cumulo-nimbus clouds. The strength of the updraft determines how much of the surrounding air is sucked into the bottom of what becomes a tornado. Two things help the tornado to rotate violently; the first is the Coriolis force and the second is the high level jet stream passing over the top of the storm, adding an extra twist to the tornado. Because of the conditions under which tornadoes are formed they can easily occur beneath thunderstorms and hurricanes.

In the USA nearly 90 per cent of tornadoes travel from the southwest to the northeast, although some follow quick changing zigzag paths. Weak tornadoes or decaying tornadoes have a thin ropelike appearance. The most violent tornadoes have a broad dark funnel shape that extends from the dark wall cloud of a large thunderstorm. There have even been reports of some tornadoes practically standing still, hovering over a single field, and of others that crawl along at 5 miles per hour. On the other hand, some have been clocked at over 70 miles per hour. However, on average, tornadoes travel at 35 miles per hour. It has been noted that most tornadoes occur between 3pm and 9pm, but they have been known to strike at any time of day or night. They usually only last about 15 minutes, staying only a matter of seconds in any single place-but then some tornadoes just do not fit any of these rules, for example on 18 March 1925 a single tornado travelled 219 miles in 3.5 hours through Missouri, Illinois, and Indiana killing 695 people.

Tornado Alley

Tornado Alley is the nickname for the area in which most tornadoes occur in the USA, and it expands through spring and summer as the heat from the sun grows warmer and the flow of warm moist air from the Gulf of Mexico spreads further north. An area that includes central Texas, Oklahoma, and Kansas is at the hard core of Tornado Alley, but before the season is over it can have expanded to the north to Nebraska and Iowa. It shrinks and swells over time but there is only one Tornado Alley. Nowhere else in the world sees weather conditions in a combination so perfect to make tornadoes.

The key reasons for this special area are:

(1) Beginning in spring and continuing through summer, low-level winds from the south and southeast bring a plentiful supply of warm tropical moisture up from the Gulf of Mexico into the Great Plains;

(2) From down off the eastern slopes of the Rocky Mountains or from out of the deserts of northern Mexico come other flows of very dry air that travel about 3,000 feet above the ground; and

(3) At 10,000 feet high the prevailing westerly winds, sometimes accompanied by a powerful jet stream, race overhead, carrying cool air from the Pacific Ocean and providing a large temperature difference, which will drive the tornadoes and the twists to get started.

In 2011 there were 1,897 tornadoes reported in Tornado Alley in the USA beating the record of 1,817 tornadoes recorded in 2004. The year 2011 was also an exceptionally destructive and deadly year in terms of tornadoes, killing at least 577 people worldwide. Of those, an estimated 553 were in the United States, which compared to 564 US deaths in the prior 10 years combined. That year saw the second greatest number of deaths due to tornadoes in a single year in US history. However, this is still a long way off from the most deadly tornado on record, which occurred on 26 April 1989 in Bangladesh and killed over 1,300 people, injured 12,000 people, and destroyed everything but a few trees from Daultipur to Salturia.

Winter storms

For people living in the mid-latitudes weather seems to be a permanent topic of conversation. This is because it is always changing. In Britain there is a saying, ‘if you do not like the weather wait an hour and it will change’. This is because the climate of the mid-latitudes is dominated by the titanic clash between the cold polar air moving southward and the warm sub-tropical air moving northwards. This clash of air masses takes place at the Polar Front.

The Polar Front moves north and south with the seasons. In summer when the sub-tropical air is warmer it moves further towards the pole. During winter when conditions are much colder the polar air mass is dominant and the Polar Front moves towards the Equator. Where these two air masses meet rain is formed. This is because warm air can hold more water vapour and when it clashes with the cold air this vapour condenses into clouds, which in turn produce rain. But it is the upper atmosphere which really controls the shape and thus the weather of the Polar Front. The upper atmosphere is characterized by fast ‘jet streams’ that race around the planet. These powerful jet streams push the Polar Front around the Earth, but as it does so the Front wrinkles and becomes a mass of so-called planetary waves moving gradually round our planet. These waves have a great effect on our weather, causing us all to complain about the weather being so changeable and of course wet. One of these waves can pass over a town in about 24 hours. The weather will be experienced as starting out to be relatively cold but with clear skies. As the warm front passes overhead the conditions get warmer and it starts to rain-usually light rain or drizzle. As the centre of the warm air mass reaches the town the weather turns cloudy and muggy and the rain stops. Then the second front, the cold front, passes overhead; temperatures drop and there is a short period of very heavy rainfall.

Then it is back to cold, clear weather until the next wave reaches the town. As we have seen there are many storms that are associated with distinct areas of atmospheric circulation described in the section on Hadley Cells. Ice, wind, hail, and snow storms are associated with either the Polar Front or high mountain regions and are worse in the winter time. In the Northern Hemisphere these types of storms are common over North America, Europe, Asia, and Japan. For snow to reach the ground the temperature of the air between the base of the cloud and the ground must be below 4°C, otherwise the snowflakes melt as they travel through the air. For hailstones to form the top of the storm must be very cold. High up in the atmosphere water droplets can become super cooled to less than 0°C, which collide in the atmosphere to form ice balls or hailstones. If you cut open a hailstone you can see the layers of ice that have built-up like an onion. The stones can vary from between 2 mm and 20 cm. Their size depends on how strong the updraft of air is, as this determines how long they stay in the atmosphere before dropping out. The worst storm conditions are called blizzards. These combine strong winds, driving snow, ice, and hail, with air temperatures as low as −12°C and visibility less than 150 meters.

Caught in the cold

When your body loses the battle against the cold, it is often someone else who will notice it. This is why you should always be on the look-out for the symptoms of cold weather exposure in your companions. When the cold has started to affect you badly, you are not always the best judge of the seriousness of the problem. You still think that you are okay you just need another minute’s rest. These are the signs to look out for:

• You cannot stop shivering

• You are fumbling your hands

• Your speech is slow and slurred and may even be incoherent

• You stumble and lurch as you walk

• You are drowsy and exhausted and feel the need to lie down even though you are outside

• Maybe you have rested, but cannot then get up

A person acting like this needs to get into dry clothes and a warm bed. This is because the core temperature of that person has started to drop, which is extremely dangerous for the body; if it is not stopped it will result in death. They need a warm hot water bottle, heating pad, or warm towels on their body. They need warm drinks. They do NOT need an alcoholic or caffeinated drink, as these speed up the person’s heart rate, causing them to lose yet more heat; they also dehydrate the body, which hinders its recovery. Also do NOT massage or rub the person, as this again takes away heat from the body core where it is most required. The person should also always be seen by a doctor.

A Climate Minute - The Greenhouse Effect

What Is the Greenhouse Effect?

Atmosphere and oceans

We will examine the effects of both the atmosphere and the oceans on climate and how they store and redistribute solar heat around the globe. We will explain why the ocean dominates in the movement of heat away from the Equator while the atmosphere dominates in the mid- to high latitudes. The chapter will finish by summarizing the major climate zones of the world and explaining why there are globally three main rain belts and two main desert belts.

The atmosphere

The atmosphere is the home of our weather. It begins at the surface of the Earth and becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The arbitrary Kármán line at 62 miles (100 km), named after Theodore von Kármán (1881-1963), a Hungarian-American engineer and physicist, is usually used to mark the boundary between atmosphere and outer space. The layer of atmosphere in which weather takes place is thinner at about 10 miles thick. The oceans also play an important part in controlling our weather and climate. The oceans are on average about 2.5 miles deep, so the total thickness of the layer controlling our climate is 12.5 miles thick.

The atmosphere is a mechanical mixture of gases, not a chemical compound. What is significant is that these gases are mixed in remarkably constant proportions up to about 50 miles (80 km) above the surface of the Earth. Four gases, nitrogen, oxygen, argon, and carbon dioxide account for 99.98 per cent of air by volume. Of special interest are the greenhouse gases that despite their relative scarcity have a great effect on the thermal properties of the atmosphere, which include carbon dioxide, methane, and water vapor.

Content of the atmosphere

Nitrogen is a colorless, odorless, tasteless, and mostly inert gas and makes up ~78 per cent by volume of the Earth’s atmosphere. Argon is also a colorless, odorless, tasteless, and completely inert gas and makes up ~0.9 per cent by volume of the Earth’s atmosphere. In contrast oxygen is a very reactive gas and makes up ~21 per cent of the Earth’s atmosphere by volume. Oxygen sustains all life on Earth and is constantly recycled between the atmosphere and the biological processes of plants and animals. It combines with hydrogen to produce water, which in its gaseous state, water vapor, is one of the most important components of the atmosphere as far as weather is concerned.

Oxygen also forms another gas called ozone or trioxygen, which is made up of three oxygen atoms instead of the usual two. This is an extremely important gas in the atmosphere as it forms a thin layer in the stratosphere (between 6 and 31 miles) that filters out harmful ultraviolet radiation that can cause cancer. However, even in this ‘layer’ the ozone concentrations are only two to eight parts per million in volumes, so most of the oxygen remains of the normal dioxygen type. Much of this important gas was being destroyed by our use of CFCs, and ozone holes have been found over the Arctic and Antarctic, until governments worldwide agreed (for example, in the Vienna Convention for the Protection of the Ozone Layer in 1985 and then in the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987) to stop the use of all CFCs and related compounds.

Carbon dioxide makes up 0.04 per cent of the Earth’s atmosphere and is a major greenhouse gas, important for keeping the Earth relatively warm. Until recently, the level of carbon dioxide has been balanced through its consumption by plants for photosynthesis and its production by plants and animals in respiration. However, human industry over the last 100 years has caused a lot  more carbon dioxide to be pumped out into the atmosphere, upsetting this natural balance.

Aerosols are suspended particles of sea salt, dust (particularly from desert regions), organic matter, and smoke. The height at which these aerosols are introduced will determine whether they cause regional warming or regional cooling. This is because high up in the atmosphere they help reflect sunlight thus cooling the local area, while at low altitudes they absorb some of the warmth coming off the Earth thus warming the local air. Industrial processes have increased the level of aerosols in the atmosphere, which has lead to smog in urban areas, acid rain, and localized cooling causing ‘global dimming’. But the most important effect of aerosols is to help clouds form. Without these minute particles water vapor cannot condense and form clouds; and without cloud precipitation there is no weather.

Water vapor is the forgotten but most important greenhouse gas, which makes up about 1 per cent by volume of the atmosphere, but is highly variable in time and space as it is tied to the complex global hydrological cycle. The most important role that water vapor plays in the atmosphere is the formation of clouds and the production of precipitation (rain or snow). Warm air can hold more water vapor than cold air. So whenever a parcel of air is cooled down, for example as air rises or meets a cold air mass, it cannot hold as much water vapor, so the water condenses on to aerosols and produces clouds. An important point which we discuss later is that as water changes from a gas to a liquid it releases some energy, and it is this energy which can fuel storms as large as hurricanes.

Clouds come in all sorts of shapes and sizes and are an excellent way of telling what sort of weather is coming up!

Greenhouse effect

The temperature of the Earth is determined by the balance between energy from the sun and its loss back into space. Of Earth’s incoming solar short-wave radiation (mainly ultraviolet radiation and visible ‘light’), nearly all of it passes through the atmosphere without interference. The only exception is ozone, which luckily for us absorbs energy in the high-energy UV band (which is very damaging to our cells), restricting how much reaches the surface of the Earth. About one-third of the solar energy is reflected straight back into space. The remaining energy is absorbed by both the land and ocean, which warms them up. They then radiate this acquired warmth as long-wave infrared or ‘heat’ radiation. Atmospheric gases such as water vapor, carbon dioxide, methane, and nitrous oxide are known as greenhouse gases as they can absorb some of this long-wave radiation, thus warming the atmosphere. This effect has been measured in the atmosphere and can be reproduced time and time again in the laboratory. We need this greenhouse effect because without it, the Earth would be at least 35°C colder, making the average temperature in the tropics about −5°C. Since the Industrial Revolution we have been burning fossil fuels (oil, coal, natural gas) deposited hundreds of millions years ago, releasing the carbon back into the atmosphere as carbon dioxide and methane, increasing the ‘greenhouse effect’, and elevating the temperature of the Earth. In effect we are releasing ancient stored sunlight back in to the climate system thus warming up the planet.

Hadley, Ferrel, and Polar Cells

As we have seen the shape of the Earth sets up a temperature imbalance between the Equator and the poles. Both the atmosphere and the oceans act as transporters for this heat away from the Equator. But as always with climate things get a little more complicated. At the Equator the intense heat from the sun warms up the air near the surface and causes it to raise high into the atmosphere. Warm air rises because the gas molecules in warm air can move further apart making the air less dense, and correspondingly cold air sinks. This loss of air upwards creates a space and low atmospheric pressure, which is filled by air being sucked in. This produces the Trade Winds in both the North and South Hemispheres. The northeast and southeast Trade Winds meet at the Inter-Tropical Convergence Zone (ITCZ). This causes a problem as the climate system is desperately trying to export heat away from the region around the Equator and these in-blowing winds do nothing to help this removal of heat. So in the tropics it is the surface currents of the ocean that transport most of the heat.

These currents include the Gulf Stream, which takes heat from the tropical Atlantic and transports it northward keeping Europe’s weather mild all year round. Other currents include the Kuroshiro current in the western North Pacific, the Brazilian current in the western South Atlantic, and finally the East Australian current in the western South Pacific However, the hot air which has risen high into the atmosphere in the tropics slowly cools, due to both its rise and its movement towards the poles, and at about 30° north and south it sinks, forming the sub-tropical high pressure zone. As this sinking air reaches the surface it spreads out, moving both north and south. This sinking air has lost most of its moisture and therefore dries out the land it sinks onto, producing some of the major deserts around the world. The southward air links into the first atmospheric cell called the Hadley Cell and becomes part of the Trade Wind system. While the northward-bound air forms the Westerlies and it is from here northwards that the atmosphere takes over from the oceans as the major transporter of heat. The movement of warm sub-tropical air northward is only stopped when it meets the cold Polar air mass at the Polar Front. The intense cold at the poles causes air to become super chilled and sink, causing out-blowing winds. When this cold Polar air meets the warm, moist Westerlies at the Polar Front the clash causes the Westerlies to lose a lot of their moisture in the form of rain. It also forces the warm sub-tropical air to rise, as the cold Polar air is much heavier. This rising air completes the other two cells, the Ferrel or mid-latitude cell, and the Polar Cell-because as the air rises it spreads out to both the north and south. To the south this high-rise air meets with tropical air coming northward and sinks forming the middle Ferrel Cell. The northward component of this rising air drifts over the poles where it is chilled and sinks forming those Polar out-blowing winds which complete the third Polar Cell. The names of two of the three cells come from George Hadley, an English lawyer and amateur meteorologist, who in the early 18th century explained the mechanism which sustained the Trade Winds. In the mid-19^th century William Ferrel, an American meteorologist, developed Hadley’s theories by explaining the mid-latitude atmospheric circulation cell in detail. An important component of these cells is the high altitude, fast flowing, narrow air currents called jet streams. The main jet streams are located near the tropopause, which represents the transition between the troposphere and the stratosphere. The major jet streams are westerly winds that flow west to east. Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around 7-12 km above sea level, and the higher and somewhat weaker sub-tropical jets at around 10-16 km. The Northern and the Southern Hemispheres each have both a Polar jet and a sub-tropical jet. The Northern Hemisphere Polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the Southern Hemisphere Polar jet mostly circles Antarctica all year round. Jet streams are caused by a combination of the Earth’s rotation and energy in the atmosphere; hence they form near boundaries of air masses with significant differences in temperature.

Though the general wind patterns of Earth follow this simple three-celled, two jet stream per hemisphere model, in reality they are much more complicated. First because the Earth is spinning and this adds the influence of the Coriolis Effect. This means that air masses trying to flow northward or southward are deflected by the spinning of the Earth. For example this causes large meanders in the jet streams, which are called planetary waves. These can have a huge effect on our weather, for instance in spring and summer 2012 the planetary waves within the Polar jet became fixed and brought a major heat wave to the USA and the wettest April, May, and June on record for England. Second, the continents heat up much quicker than the oceans, which can cause the surface air over the land to rise, which can alter the general circulation of surface wind. This can cause local land–sea breezes and, on a much larger scale, cause the monsoon systems. The seasons, then, can have a huge effect on atmospheric circulation, because during the summer in each hemisphere the land heats up much more than the ocean, hence the ITCZ is pulled southwards towards Australasia, and across South America and Southeast Africa during Southern Hemisphere summer and northwards across India, Southeast Asia and North Africa during Northern Hemisphere summer. The Hadley Cells however do explain why there are three main rainfall belts across the Earth, the convection rainfall belt which moves north and south of the Equator and the two convergent rainfall belts one in the Northern and one in the Southern Hemisphere where warm, moist sub-tropical air meets cold dry Polar air. They also explain why there are two main desert belts in the world, which are usually found between the rainfall belts with super dry air sinking between the Hadley and Ferrel Cells. In the Northern Hemisphere good examples are the Sahara desert in North Africa and the Gobi desert in China, while in the Southern Hemisphere, Central Australia and the Kalahari desert in South Africa are good examples.

The Hadley Cells can also be used to define the three main storm zones. First are ‘winter storms’ at the Polar Front. Second are the sub-tropical highs and the Trade Wind belt, which are the spawning ground for hurricanes. Third is the ITCZ, where the rapidly rising air cools and produces tropical thunderstorms with heavy rainfall, producing monsoons as it moves over the land.

Surface ocean circulation

As we have seen the surface ocean is important in transporting heat around the globe. The circulation of the oceans starts with the wind, because it is the action of the wind on the surface ocean that makes it move. As the wind blows on the surface water, the friction allows energy to be transferred from the winds to the surface water, leading to major currents. The wind energy is transferred to greater depths in the water column turbulence, which allows wind driven currents to be very deep. There are three main types of current flow: (a) Ekman motion or transport; (b) Inertia currents; and (c) Geostrophic currents.

Ekman motion or transport

Vagn Walfrid Ekman (3 May 1874-9 March 1954) was a Swedish oceanographer who calculated that with a constant wind over an ocean that was infinitely deep and infinitely wide with the same density, the Coriolis effect would be the only other force acting on the water column. The further away from the surface and the diminishing influence of the wind, the greater the effect of Coriolis, which results in a spiral of water movement. The result is that the net movement of the surface of the ocean is at 90 degrees to wind direction. This phenomenon was first noted by Fridtjof Nansen, during his arctic expeditions in the 1890s, when he recorded that ice transport appeared to occur at an angle to the wind direction. The direction of transport is of course dependent on the hemisphere. In the Northern Hemisphere this transport is at a 90° angle to the right of the direction of the wind, and in the Southern Hemisphere it occurs at a 90° angle to the left of the direction of the wind.

Inertia currents

Surface water masses are huge. For example, the Gulf Stream measures about 100 Sverdrup (Sv). One Sverdrup is 106m3/s or a million tones of water per second. The entire global input of freshwater from rivers to the ocean is equal to about 1 Sv. Hence these water masses have a huge momentum, and thus the currents continue to flow long after the wind has ceased pushing. When the wind stops blowing only friction and the Coriolis Effect continues to act on the water mass. If the water mass does not change latitude then the current will flow along the line of latitude. If it changes latitude then the Coriolis Effect acts and thus the path of the current will become even more steeply curved.

Geostrophic currents

Contrary to Ekman’s assumptions, oceans are not infinitely wide and infinitely deep. The oceans have boundaries-the continents-and the water driven by the wind tends to ‘pile up’ on one side of the ocean against the continent. This causes a sea-surface slope, and affects the hydrostatic pressure with water flowing from areas of high to those of low pressure. This force is known as the horizontal pressure gradient force, and is also influenced by the Coriolis Effect, producing what are known as geostrophic currents. One way of studying geostrophic currents is to look at the dynamic topography of the sea-surface-in other words, areas of the sea that are higher than the rest.

The combination of wind-blown Ekman currents, inertia currents, and geostrophic currents produces most of the major circulation features of the world’s oceans. One of the major features is the gyres in each of the ocean basins. These large systems of rotating ocean currents are found in the North and South Atlantic Oceans, North and South Pacific Oceans, and the Indian Ocean. There is, however, another influence on surface ocean circulation and that is the pulling created by the sinking of surface water when deep-water currents are formed.

Deep-ocean circulation

The circulation of the deep ocean is one of the major controls on global climate due to its ability to exchange heat between the two hemispheres. In fact, the deep ocean is the only candidate for driving and sustaining internal long-term climate change (of hundreds to thousands of years) because of its volume, heat capacity, and inertia. Today the tropical sun heats the surface water in the Gulf of Mexico. This heat also causes there to be a lot of evaporation sending moisture into the atmosphere starting the hydrological cycle. All this evaporation leaves the surface water enriched in salt. So this hot salty surface water is pushed by the winds out of the Caribbean along the coast of Florida and into the North Atlantic Ocean. This is the start of the famous Gulf Stream. The Gulf Stream is about 500 times the size of the Amazon River at its widest point and flows along the coast of the USA and then across the North Atlantic Ocean, past the coast of Ireland, past Iceland, and up into the Nordic Seas. As the Gulf Stream flows northward it becomes the North Atlantic Drift and it cools down. The combination of a high salt content and low temperature increases the surface water density or heaviness.

Let us now examine the difference between freshwater and seawater. As freshwater is cooled down, something amazing happens-it becomes denser down to a temperature of 4°C, after which it becomes lighter, and then freezes at 0°C. This means that when ponds freeze they do so from the top as the heaviest water sits on the bottom and is at 4°C, perfect for protecting any life within the pond or lake. As you progressively add salt to water, its freezing point drops, which is why we put salt on roads to stop them freezing, but also the temperature of greatest density drops.  At 26 grams of salt per kilogram of water the temperature of greatest density and the freezing point coincide. This means seawater, which has 35 grams of salt per kilogram, will continue to get heavier and heavier until it freezes. When water freezes then another amazing thing happens-ice is formed, a solid that is lighter than its liquid form.

When the surface water reaches the relatively fresh oceans north of Iceland, the surface water has cooled sufficiently to become dense enough to sink into the deep ocean. The ‘pull’ exerted by the sinking of this dense water mass helps maintain the strength of the warm Gulf Stream, ensuring a current of warm tropical water flowing into the northeast Atlantic, sending mild air masses across to the European continent. It has been calculated that the Gulf Stream delivers the same amount of energy as a million nuclear power stations. If you are in any doubt about how good the Gulf Stream is for the European climate, compare the winters at the same latitude on either side of the Atlantic Ocean, for example London with Labrador, or Lisbon with New York. Or, better still, compare Western Europe and the west coast of North America, which have a similar geographical relationship between the ocean and continent-for example, Alaska and Scotland, which are at about the same latitude.

The newly formed deep water in the Nordic Seas sinks to a depth of between 2,000 meters and 3,500 meters in the ocean and flows southward down the Atlantic Ocean, as the North Atlantic Deep Water (NADW). In the South Atlantic Ocean, it meets a second type of deep water, which is formed in the Southern Ocean and is called the Antarctic Bottom Water (AABW). This is formed in a different way to NADW. Antarctica is surrounded by sea ice and deep water forms in coast polnyas (large holes in the sea ice). Out-blowing Antarctic winds push sea ice away from the continental edge to produce these holes. The winds are so cold that they super-cool the exposed surface waters. This leads to more sea-ice formation and salt rejection because when ice is formed it rejects any salt that the freezing water contains, which produces the coldest and saltiest water in the world. AABW flows around the Antarctic and penetrates the North Atlantic, flowing under the warmer and thus somewhat lighter NADW. The AABW also flows into both the Indian and Pacific Oceans. The NADW and AABW make up the key elements of the great global ocean conveyor belt (Figure 15), which allows heat to be exchanged between the two hemispheres on the timescale of hundreds and thousands of years.

The balance between the NADW and AABW is extremely important in maintaining our present climate, as not only does it keep the Gulf Stream flowing past Europe, but it maintains the right amount of heat exchange between the Northern and Southern Hemispheres. Scientists are worried that the circulation of deep water could be weakened or ‘switched off’ if there is sufficient input of fresh water to make the surface water too light to sink. This evidence has come from both computer models and the study of past climates. Scientists have coined the phrase ‘dedensification’ to mean the removal of density by adding fresh water and/or warming up the water, both of which prevent seawater from being dense enough to sink. There is concern that climate change could cause parts of Greenland to melt. This could lead to more fresh water being added to the Nordic seas, thereby weakening the NADW and the Gulf Stream. This would bring much colder European winters with generally more severe weather. However, since the influence of the warm Gulf Stream is mainly in the winter, this change would not affect summer temperatures. So, if the Gulf Stream fails, global warming would still cause European summers to heat up. Europe would end up with extreme seasonal weather very similar to that of Alaska.

Vertical structure of the atmosphere

The atmosphere can be divided conveniently into a number of well demarcated horizons, mainly based on temperature.

Troposphere

The lowest layer of the atmosphere is the zone where atmospheric turbulence and weather are most marked. It contains 75 per cent of the total molecular mass of the atmosphere and virtually all the water vapor. Throughout this layer there is a general decrease in temperature at a mean rate of 6.5°C/km, and the whole zone is capped by a temperature inversion layer. This layer, called the ‘tropopause’, acts as a lid on the troposphere and on weather.

Stratosphere

The second major atmospheric layer extends upwards from the tropopause to about 50 km. Although the stratosphere contains much of the ozone, the maximum temperature caused by the absorption of ultraviolet radiation occurs at the ‘stratopause’ where temperatures may exceed 0°C. This large temperature increase is due to the relative low density of the air at this height.

Mesosphere

Above the stratopause average temperatures decrease to a minimum of -90°C. Above 80 km temperatures begin rising again because of absorption of radiation by both ozone and oxygen molecules. This temperature inversion is called the ‘mesopause’. Pressure is extremely low in the mesosphere decreasing from 1 mb at 50 km to 0.01 mb at 90 km (surface pressure is about 1,000 mb).

Thermosphere

Above the mesopause, atmospheric densities are very low. Temperatures rise throughout this zone due to the absorption of solar radiation by molecular and atomic oxygen.

Blond hair and ocean circulation

The Gulf Stream may have also given us blond, fair skinned people. The warming effect of the Gulf Stream on Western Europe is so great that it means that early agriculturalists could grow crops incredibly far north in countries such as Norway and Sweden. These early settlers were living as far north as the Arctic Circle, which is on the same latitude as the middle of the Greenland ice sheet or the northern Alaska tundra. But there is one major drawback to living so far north and that is the lack of sunlight. Humans need Vitamin D, without it children develop rickets, which causes softening of the bones, leading to fractures and severe deformity. Vitamin D is made in the skin when it is exposed to ultraviolet light from the sun. This of course was no problem for our ancestors who evolved in Africa-quite the reverse, and dark skin was essential protection from the strong sunlight. However, as our ancestors moved further and further north there was less and less sunlight and less production of Vitamin D. In each generation only those with the lightest skin and hair color could avoid getting rickets since the lighter your skin and hair, the more sunlight you can absorb, and thus the more Vitamin D you can make. So there was a very strong selection pressure in these areas in favor of fair skinned, blond haired people. On the other hand, Vitamin D is also found in food, such as fatty fish species and mushrooms, which may be why the same selection pressure did not apply to the Arctic Inuit. However, it is interesting to think that if it were not for the Gulf Stream and the stubbornness of the early Scandinavian settlers, relying only on crops and eating little or no fish, we would not have real blonds.

How does the climate system work?

UNEP - CLIMATE CHANGE

The comfort zone ranges from about 20°C to 26°C and from 20 to 75 per cent relative humidity. However, we live almost everywhere in the world, meaning that conditions are frequently outside this comfort zone, and we have learnt to adapt our clothing and dwellings to maintain our comfort. So while you may think the clothes you have hanging in your wardrobe simply reflect your fashion taste or lack of, in reality they reflect the climate in which you live and how it changes throughout the year. So you have a thick padded coat for a Canadian winter and a short-sleeved shirt for a business meeting in Rio. Our wardrobes also give hints about where we like to take our holidays.

If you are a budding Polar explorer then there will very warm Arctic clothes hanging up-if you love sunning yourself on the beach, then there will be shorts or a bikini instead.

Our houses are also built with a clear understanding of local climate. In England almost all houses have central heating as the outside temperature is usually below 20°C, but few have air conditioning as temperatures rarely exceed 26°C. On the other hand, in Australia most houses have air conditioning but rarely central heating.

Climate also affects the structure of our cities and how transport systems around the world operate. In Houston, Texas, there is a network of 7 miles of underground tunnels connecting all the major downtown buildings; this is fully climate controlled and links 95 heavily populated city blocks. People use the tunnel when it is raining or hot outside, because for at least 5 months of the year the average temperature in Houston is above 30°C. Similarly there are underground malls in Canada to avoid the problems of heavy snow and extreme cold.

Climate controls where and when we get our food, because agriculture is controlled by rainfall, frost, and snow, and by how long the growing season is, which includes both the amount of sunlight and the length of the warm season. So in a simplified way, rice is grown where it is warm and very wet, while wheat can grow in much more temperate climes. The climate can also affect the quality of our food, for example it is well known that the very best vintages of French wine are produced when there are a few short sharp frosts during the winter, which harden the vines, producing excellent grapes. Farmers can also ‘help’ the local climate, for example by growing tomatoes in a greenhouse or by irrigating the land to provide a more constant supply of water.

Climate also influences where there will be extreme weather events such as heat waves, droughts, floods, and storms. However in many cases our perception of extreme events is determined by local conditions, so for example in 2003 northern Europe was hit with a ‘heat wave’ and 100°F (37.8°C) was recorded for the first time ever in England. However in countries of the tropics a heat wave would not be recorded until temperatures were above 45°C. Climate also has a large effect on our health, as many diseases are temperature and humidity controlled. For example incidences of influenza, commonly called the flu, reach a peak in winter. Since the Northern and Southern Hemispheres have winter at different times of the year, there are actually two different flu seasons globally each year.

The influenza virus migrates between the two hemispheres after each winter, giving us time to produce new vaccinations based on the new strain of flu that has appeared in the previous six months in the other hemisphere. There have been many arguments about why flu is climate controlled and the theory is that during cold dry conditions the virus can survive on surfaces longer and so be more easily transmitted between people. Another suggestion is that vitamin D might provide some resistance or immunity to the virus. Hence in winter and during the tropical rainy season, when people stay indoors, away from the sun, their vitamin D levels fall and incidences of influenza increase.

Hot and cold Earth

The climate of our planet is caused by the Equator of the Earth receiving more of the sun’s energy than the poles. If you imagine the Earth is a giant ball, the closest point to the sun is the middle or the Equator. The Equator is where the sun is most often directly overhead and it is here that the Earth receives the most energy. As you move further north or south away from the Equator, the surface of the Earth curves away from the sun, increasing the angle of the surface of the Earth relative to the sun. This means the sun’s energy is spread over a larger area, and thus causes less warming. If we lived on a flat disc we would get much more energy from the sun-about 1,370 Watts per square meter (W/m2)-instead the planet surface averages about 343 W/m2  due to its curved nature. The Earth also receives a very small fraction of the energy pumped out of the sun. If you consider how small the Earth is compared with the sun, for every Watt we receive from the sun, it emits 2 billion Watts. This is why in many science fiction novels the authors imagine a strip or even a sphere around a star to collect all that energy that is simply being lost into space.

Solar energy distributed over a sphere

About one-third of the solar energy we receive is reflected straight back into space. This is because of ‘albedo’, which means how reflective is a surface. So, for example, white clouds and snow have a very high albedo and reflect almost all of the sunlight that falls on them, while darker surfaces such as the oceans, grassland, and rainforest absorb a lot more energy. Not only do the poles receive less energy than the Equator, but they also lose more energy back into space: the white snow and ice in the Arctic and Antarctic have a high albedo and bounce a lot of the sun energy back into space. On the other hand, the darker much less reflective vegetation at lower latitudes absorbs a lot more energy. These two processes working together mean that the tropics are hot and the poles are very cold. Nature hates this sort of energy imbalance, so energy, in the form of heat, is transported by both the atmosphere and oceans from the Equator to both poles, and this affects the climate.

Earth in space

Our climate is controlled by two fundamental facts about the relationship between the Earth and the sun. The first is the tilt of the Earth’s axis of rotation, which causes the seasons. The second factor is the daily rotation of the Earth that provides us with night and day and drives the circulation of both the atmosphere and the oceans.

The Earth’s axis of rotation is tilted at an angle of 23.5° and results in a seasonal difference in the amount of energy received by each hemisphere throughout the year. The seasonal changes are by far the largest effect on climate. It is amazing to think that if the Earth were not tilted and stood straight up on its axis then we would not have spring, summer, autumn, and winter. We would not have the massive change in vegetation in the temperate latitudes and we would not have the monsoon and hurricane seasons in the tropics. The reason for the seasons is the change in the angle of the sunlight hitting the Earth through the year. If we take 21 December as an example, the Earth’s axis is leaning away from the sun, so the sunlight hitting the Northern Hemisphere is at a greater angle and spreading its energy over a wider area. Moreover the lean is so great that in the Arctic the sunlight cannot even reach the surface and this produces 24 hours of darkness and winter in the Northern Hemisphere.

However, everything is opposite in the Southern Hemisphere, since it is then leaning towards the sun and hence the sunlight is more directly overhead. This means that Antarctica is bathed in 24 hours of sunlight and people in Australia have Christmas dinner on the beach, while topping up their tan. As the Earth moves round the sun, taking about 365.25 days (hence the leap year every fourth year), the angle of the axis stays in the same place. Hence when it comes to June the Earth’s axis is leaning towards the sun, so the Northern Hemisphere has lots of direct sunlight and thus summer, while the Southern Hemisphere is shielded from the sunlight and is plunged into winter.

Solstice and equinox caused by the tilt of the Earth

If we follow the sun through a year we can see how this tilt affects the Earth through the seasons. If we start at 21 June the sun is overhead at midday at the Tropic of Cancer (23.4°N), the northern summer solstice. The angle of the sun moves southward until 21 September when it is overhead at midday over the Equator, the equinox or autumn equinox in the Northern Hemisphere. The sun appears to continue southward and on 21 December it is overhead at midday at the Tropic of Capricorn (23.4°S) the southern summer solstice. The sun then appears to move northward until it is directly overhead at midday at the Equator on the 21 March the equinox or spring equinox in the Northern Hemisphere and so the cycle continues.

The seasons signal by far the most dramatic change in our climate; if we take for example New York, winter temperatures can be as low as -20°C while summer temperatures can be over 35°C-a 55°C temperature difference. Moreover as we will find out the seasons are one of the major reasons for storms.

Moving heat around the Earth

The second big factor affecting the climate of the Earth is its daily rotation. First this plunges the Earth in and out of darkness causing massive changes in diurnal temperature. For example the Sahara desert during summer can have daytime temperatures of over 38°C (100°F) and then nighttime lows of 5°C (40°F); while Hong Kong has a diurnal temperature range of little more than 4°C (7°F).

Depending on the season, different areas also get varying amounts of daylight. The days can vary from 24 hours’ daylight to 24 hours’ darkness at the poles to around 12 hours’ sunlight every day at the Equator. This change in the daylight compounds the seasonal contrasts, because not only during summer do you get more direct ‘overhead’ sunlight but also you get it for much longer.

But the spinning of the Earth also makes the transport of heat away from the Equator more complicated. This is because the spinning of the Earth makes everything else including the atmosphere and oceans turn. The simple rule is that rotation of the Earth causes the air and ocean currents to be pushed to the right of the direction they are travelling in the Northern Hemisphere and to the left of the direction they are travelling in the Southern Hemisphere. This deflection is called the Coriolis Effect and its strength increases the further you go towards the poles.

An everyday example of this, which is always quoted, is the way water flows down a plughole or a toilet. In the Northern Hemisphere water is said to flow clockwise down the plughole while in the Southern Hemisphere it is anti-clockwise. However, I hate to tell you that the direction the water drains out of your bath or toilet is not related to the Coriolis Effect or to the rotation of the Earth.

Moreover no consistent difference in rotation direction between toilets in the Northern and Southern Hemispheres has been observed. This is because the Coriolis Effect has such a small influence compared with any residual movement of the water and the effect of the shape of the container. This also means the wonderful cottage industry of communities living on the Equator showing tourists the Coriolis Effect is simply done by a sleight of hand. For example in Kenya there are big signs up telling you when you are crossing the Equator; if you care to stop at the road side locals will happily pour water from a bucket into a large funnel and seeming to demonstrate clearly that it goes a different way round when you are standing on one side of the sign than when you are standing on the other. However, this change is all in the wrist and how the water is poured in; affecting which way it goes round. Still, even though it is completely fake, I love these demonstrations as it means loads of locals and tourists get to hear about the Coriolis Effect!

Back to climate, so why do the ocean currents and winds have this deflection? Imagine firing a missile from the Equator directly north. Because the missile was fired from the Earth which is spinning eastward, the missile is also moving east. As the Earth spins the Equator has to move fast through space to keep up with the rest, as it is the widest part of the Earth. As you go further north or south away from the Equator the surface of the Earth curves in, so it does not have to move as fast to keep up with the Equator. So in one day the Equator must move round 40,074 km (the diameter of the Earth) a speed of 1,670 km/hour, while the Tropic of Cancer (23.4°N) has to move 36,750 km, with a speed of 1,530 km/hour, and the Arctic Circle (66.6°N) has to move 17,662 km so has a speed of 736 km/hour. At the North Pole there is no relative movement at all so the speed there is 0 km/hour. A practical demonstration of this is if you hold hands with a friend and stand in the same place while spinning them around, they will travel much faster than you do. Therefore the missile, fired from the Equator, has the faster eastward speed of the Equator; as it moves northward towards the Tropic of Cancer; the surface of the Earth is not moving as fast eastward as the missile. This gives the appearance that the missile is moving northeast as it is moving faster eastward than the area it is moving into. Of course the closer you get to the poles the greater this difference in speeds so the greater the deflection to the east.

The climate system is very straightforward. It is controlled by the different amount of solar energy received at the Equator and the poles. Climate is simply the redistribution of energy to undo this imbalance. It is the atmosphere and the oceans which undertake this redistribution, as we will see later. Complications are added because the Earth’s axis of rotation is at an angle with respect to the sun, which leads to there being a strong season cycle. On top of this the Earth rotates every hour, plunging the Earth in and out of darkness. It also means the redistribution of energy away from the Equator takes place on a spinning ball. This creates the Coriolis Effect and helps to explain why nearly all weather systems seem to spin.