(2) Climate System

A Climate Minute - The Greenhouse Effect

What Is the Greenhouse Effect?

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-19th 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.

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