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Many scientists believe that the human-induced or anthropogenic-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

Table 1: Main greenhouse gases and their comparative ability to warm the atmosphere

Greenhouse gas

Chemical formula

Pre-industrial concentrations



Human source

Global warming potential 20 years

Global warming potential 100 years

Carbon dioxide


278 ppmv

358 ppmv (30% increase)

Fossil-fuel combustion Land-use changes Cement production





700 ppbv

1721 ppbv (240% increase)

Fossil fuels Rice paddies Waste dumps Livestock



Nitrous oxide


275 ppbv

311 ppbv (15% increase)


Industrial processes

Fossil-fuel combustion





Does not exist naturally and is human generated

0.503 ppbv

Liquid coolants/ foams





Does not exist naturally and is human generated

0.105 ppbv

Liquid coolants



Perfluoro methane


Does not exist naturally and is human generated

0.070 ppbv

Production of aluminium



Sulphur hexa-fluoride


Does not exist naturally and is human generated

0.032 ppbv

Dielectric fluid



ppmv = part per million by volume ppbv = parts per billion by volume

time time


5. Possible climate system responses to a linear-forcing 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 (see Figure 5). 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 a person trying to push a car up a hill which, strangely enough, gets g very little response? Or is it more like a person pushing a car a downhill, which, once the car starts to move, it is very difficult to a stop. There are four possible relationships and this is the central i question in the global warming debate, which is most applicable to ? the future.

(a) Linear and synchronous response (Figure 5a). 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 (Figure 5b). In this case the forcing may be strong, but the climate system is in some way buffered and therefore gives 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 (Figure 5c). 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 (Figure 5d). In this case, initially, there is no or jj very little response in the climate system to the forcing; however, all j| the response takes place in a very short period of time in one large 3 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 in Chapter 3 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 (see Figure 5e). 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 in Chapter 7.

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