(2) Global Warming & Global Society



What is the IPCC?

The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 jointly by the United Nations Environmental Panel and World Meteorological Organization because of worries about the possibility of global warming. The purpose of the IPCC is the continued assessment of the state of knowledge on the various aspects of climate change, including scientific, environmental, and socio-economic impacts and response strategies. The IPCC is recognised as the most authoritative scientific and technical voice on climate change, and its assessments have had a profound influence on the negotiators of the United Nations Framework Convention on Climate Change (UNFCCC) and its Kyoto Protocol.

The meetings in The Hague in November 2000 and in Bonn in July 2001 were the second and third attempts to ratify (i.e. to make legal) the Protocols laid out in Kyoto in 1998. Unfortunately, President Bush pulled the USA out of the negotiations in March 2001. However, 186 other countries made history in July 2001 by agreeing the most far-reaching and comprehensive environmental treaty the world has ever seen. But the Kyoto Protocol has yet to be ratified.

The IPCC is organised into three working groups plus a task force to calculate a number of greenhouse gases produced by each country. Each of these four bodies has two co-chairmen (one from a developed and one from a developing country) and a technical support unit. Working Group I assesses the scientific aspects of the climate system and climate change; Working Group II addresses the vulnerability of human and natural systems to climate change, the negative and positive consequences of climate change, and options for adapting to them; and Working Group III assesses options for limiting greenhouse gas emissions and otherwise mitigating climate change, as well as economic issues. Hence the IPCC also provides governments with scientific, technical, and socio-economic information relevant to evaluating the risks and to developing a response to global climate change. The latest reports from these three working groups were published in 2001 and approximately 400 experts from some 120 countries were directly involved in drafting, revising, and finalising the IPCC reports and another 2,500 experts participated in the review process. The IPCC authors are always nominated by governments and by international organisations including Non-Governmental Organisations. These reports are essential reading for anyone interested in global warming.

The IPCC also compiles research on the main greenhouse gases: where they come from, and the current consensus concerning their warming potential (see below). The warming potential is calculated in comparison with carbon dioxide, which is allocated a warming potential of one. This way the different greenhouse gases can be compared with each other relatively instead of in absolute terms.

The Global Warming potential is calculated over a 20- and 100-year period. This is because different greenhouse gases have different residence times in the atmosphere because of how long they take to break down in the atmosphere or be absorbed in the ocean or terrestrial biosphere. Most other greenhouse gases are more effective at warming the atmosphere than carbon dioxide but are still in very low concentrations in the atmosphere. As you can know there are other greenhouse gases which are a much more dangerous mass for mass than carbon dioxide but these exist in very low concentrations in the atmosphere, and therefore most of the debate concerning global warming still centres on the role and control of atmospheric carbon dioxide.

What is climate change?

Many scientists believe that the human-induced or anthropogenically enhanced greenhouse effect will cause climate change in the near future. Even some of the global warming sceptics argue that though global warming may be a minor influence, natural climate change does occur on human timescales and we should be prepared to adapt to it. But what is climate change and how does it occur?

Climate change can manifest itself in a number of ways, for example, changes in regional and global temperatures, changing rainfall patterns, expansion and contraction of ice sheets, and sea-level variations. These regional and global climate changes are responses to external and/or internal forcing mechanisms. An example of an internal forcing mechanism is the variations in the carbon dioxide content of the atmosphere modulating the greenhouse effect, while a good example of an external forcing mechanism is the long-term variations in the Earth’s orbits around the sun, which alter the regional distribution of solar radiation to the Earth. This is thought to cause the waxing and waning of the ice ages. So in terms of looking for the evidence for global warming and predicting the future, we need to take account of all the natural external and internal forcing mechanisms. For example, until recently the cooling that occurred globally during the 1970s was unexplained until the ‘external’ and cyclic variations every 11 years in the sun’s energy output, the so-called sunspot cycle, was taken into consideration.

We can also try to abstract the way the global climate system responds to an internal or external forcing agent by examining different scenarios. In these scenarios, I am assuming that there is only one forcing mechanism which is trying to change the global climate. What is important is how the global climate system will react. For example, is the relationship like a person trying to push a car up a hill which, strangely enough, gets a very little response? Or is it more like a person pushing a car downhill, which, once the car starts to move, it is very difficult to stop. There are four possible relationships and this is the central question in the global warming debate, which is most applicable to the future.

(a) Linear and synchronous response. In this case, the forcing produces a direct response in the climate system whose magnitude is in proportion to the forcing. In terms of global warming, an extra million tonnes of carbon dioxide would cause a certain predictable temperature increase. This can be equated to pushing a car along a flat road: most of the energy put into pushing is used to move the car forward.

(b) Muted or limited response. In this case, the forcing may be strong, but the climate system is in some way buffered and therefore gives a very little response. Many global warming sceptics and politicians argue that the climate system is very insensitive to changes in atmospheric carbon dioxide so very little will happen in the future. This is the ‘pushing the car up the hill’ analogy: you can spend as much energy as you like trying to push the car but it will not move very far.

(c) Delayed or non-linear response. In this case, the climate system may have a slow response to the forcing thanks to being buffered in some way. After an initial period, the climate system responds to the forcing but in a non-linear way. This is a real possibility when it comes to global warming and why it is argued that as yet only a small amount of warming has been observed over the last 100 years. This scenario can be equated to the car on the top of a hill: it takes some effort and thus time to push the car to the edge of the hill; this is the buffering effect. Once the car has reached the edge it takes very little to push the car over, and then it accelerates down the hill with or without your help. Once it reaches the bottom, the car then continues for some time, which is the overshoot, and then slows down of its own accord and settles into a new state.

(d) Threshold response. In this case, initially, there is no or very little response in the climate system to the forcing; however, all the response takes place in a very short period of time in one large step or threshold. In many cases, the response may be much larger than one would expect from the size of the forcing and this can be referred to as a response overshoot. This is the scenario that most worries us, as thresholds are very difficult to model and thus predict.

However, thresholds have been found to be very common in the study of past climates, with rapid regional climate changes of over 5°C occurring within a few decades. This scenario equates to the bus hanging off the cliff at the end of the original film The Italian Job; as long as there are only very small changes, nothing happens at all. However, a critical point (in this case weight) is reached and the bus (and the gold) plunge off the cliff into the ravine below.

Though these are purely theoretical models of how the global climate system can respond, they are important to keep in mind when reviewing the possible scenarios for future climate change.

Moreover, they are important when we consider why different people see different global warming futures despite all having access to the same information. It depends on which of the above scenarios they believe will happen. An added complication when assessing climate change is the possibility that climate thresholds contain bifurcations. This means the forcing required to go one way through the threshold is different from the reverse. This implies that once a climate threshold has occurred, it is a lot more difficult to reverse it. The bifurcation of the climate system has been inferred from ocean models which mimic the impact of fresh water in the North Atlantic on the global deep-water circulation, and we will discuss this can of worms in great detail later.

Linking global warming to climate change

We have seen that there is clear evidence that greenhouse gas concentrations in the atmosphere have been rising since the industrial revolution in the 18th century. The current scientific consensus is that changes in greenhouse gas concentrations in the atmosphere do cause global temperature change. However, the biggest problem with the global warming hypothesis is understanding how sensitive the global climate is to increased levels of atmospheric carbon dioxide. Even if we establish this, predicting climate change is complex because it encompasses many different factors, which respond differently when the atmosphere warms up, including regional temperature changes, melting glaciers and ice sheets, relative sea-level change, precipitation changes, storm intensity and tracks, El Niño, and even ocean circulation. This linkage between global warming and climate change is further complicated by the fact that each part of the global climate system has different response times. For example, the atmosphere can respond to external or internal changes within a day, but the deep ocean may take decades to respond, while vegetation can alter its structure within a few weeks (e.g. change a number of leaves) but its composition (e.g. swapping plant types) can take up to a century to change. Then, add to this the possibility of natural forcing which may be cyclic; for example, there is good evidence that sunspot cycles can affect climate on both a decadal and a century timescale.

There is also evidence that since the beginning of our present interglacial period, the last 10,000 years, there have been climatic coolings every 1,500 ±500 years, of which the Little Ice Age was the last. The Little Ice Age began in the 17th and ended in the 18th century and was characterized by a fall of 0.5–1°C in Greenland temperatures, significant shift in the currents around Iceland, and a sea-surface temperature fall of 4°C off the coast of West Africa, 2°C off the Bermuda Rise, and of course ice fairs on the River Thames in London, all of which were due to natural climate change. So we need to disentangle natural climate variability from global warming. We need to understand how the different parts of the climate system interact, remembering that they all have different response times. We need to understand what sort of climatic change will be caused and whether it will be gradual or catastrophic. We also need to understand how different regions of the world will be affected; for example, it is suggested that additional greenhouse gases will warm up the poles more than the tropics. All these themes concerning an understanding of the climate system and the difficulty of future climate prediction are returned later.

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