The Impacts of Global Warming
What is the impact of such changes, particularly on human communities? Let me briefly mention two impacts. First of all, the sea level will rise. Our best estimate is that it will rise by about half a metre by the end of the century with a range of uncertainty between 10 and 90 cm. The reasons are mainly thermal expansion of the oceans and the melting of glaciers; not, as is often thought, melting of the icecaps. For the oceans, at first, just the top layer is heated, say the first hundred metres, then the next hundred metres and so on. Since there are kilometres depth of ocean to heat, increase of sea level is going to continue for many centuries - even if we succeeded sometime this century in reducing carbon dioxide emissions sufficiently to prevent further climate change. So far as the ice caps are concerned, the best estimates are that Antarctica might grow slightly, the increased snow fall probably being greater than the ablation round the margins. For Greenland, with a temperature rise of more than 3ºC the icecap would probably start a melt down that would take many thousands of years, with an eventual total sea level rise of about 7 metres.
Such a rate of sea level rise over the next 100 years would not be too much of a problem for the UK, although the Thames barrier would need modification quite soon and substantial resource would have to go into improved sea defences, especially along the low lying east coast regions. Other developed countries with the necessary resources such as The Netherlands would be able to cope, although it would cost. But the situation would be very different in a poor developing country like Bangladesh, where there are around 10 million people living within 1 metre of present sea level, and where the provision of adequate sea defences along the complicated coastline of the river deltas is not an option. Even more people would be affected in southern China. Many islands in the Indian and Pacific oceans are likely to become uninhabitable and, since a high proportion of the world's population lives close to the coast, many people in other coastal regions would be presented with severe problems (fig 11).
A second major impact is on precipitation and water supplies. It almost goes without saying that water is an important resource. We're using much more water than we used to, some ten times as much a hundred years ago (fig 12), and its use is still increasing, largely because of the demands of irrigation. And there is enormous competition for water. It was recently reported that the Rio Grande river in north America no longer reaches the sea; it is all used before it gets there - as has been the case with the Colorado river for some time. Butrous Butrous Ghali, the last Secretary General of the UN, said he thought the next war would be about water not about oil, and he may well be right.
So what will happen in a globally warm world? As might be expected, the models all predict increases in average precipitation relating to the increases in temperature. But they are not very good at predicting detailed changes at the regional level. However, a robust result from all models is that heavy rainfall will tend to get heavier, and light rainfall will often tend to get lighter (fig 13). It is quite easy to understand why that might be. In a warmer world more water is evaporated from the surface, more water vapour enters the atmosphere and more water vapour condenses to form clouds. Since the latent heat of condensation of water vapour provides the main energy source for the atmosphere's local circulations, the hydrological cycle will become more intense. It is not surprising therefore that heavy precipitation will tend to become heavier. But why does lighter rainfall tend to become lighter? That is because, in a more intense hydrological cycle, uprising currents in the atmosphere will tend to go higher, where it's colder and dryer. So the air in the downdraughts will tend to be dryer. So in some parts of the world, there will be a tendency to increased floods and in others, increased droughts. There is evidence that this increased tendency to heavy rainfall events is occurring already (fig 14), with an increase of a few percent in many parts of the US over the past century.
How does this impact on human activities? Taken together, floods and droughts are the biggest disasters the world knows. They cause more loss of life, more damage, more misery, more economic loss, than any other disasters we know. An increase in the frequency and intensity of floods and droughts will be of substantial concern to countries at mid latitudes, like the UK, the USA or China, that possess at least some of the necessary infrastructure to respond to such problems. But in sub-tropical regions where there are developing countries much more fragile in their agriculture, their living and their ability to cope, the impacts will be much greater. We know something of their vulnerability from the two floods in Mozambique in the year 2000 and from recent droughts in Ethiopia and Eritrea. Although, because of the large natural variability of climate, we cannot say that any given flood or drought is a result of global warming, we can say with some certainty that we expect them to occur in the future with more intensity and frequency. Because of this, there is a great deal of interest in the El Niño phenomenon, a large climatic variation that occurs about every five years associated with unusual warming of the Pacific ocean close to south America. Associated with El Niños are climate extremes such as floods and droughts, particularly in the Australian, African and American continents. The 1980s and 1990s have seen some unusual El Nino events and a number of researchers are investigating the evidence for the possibility that El Niños may become more intense as global warming occurs.
Due to the two impacts I have mentioned, substantial numbers of people are likely to be displaced. Norman Myers working at Oxford1 has estimated the number of refugees that might occur because of sea level rise and because of disruption of agriculture due to precipitation changes. It's a fairly conservative study, but he estimates that there could be perhaps 150 million refugees by the year 2050. That is a very large number, for example it is half the population of the USA. It is not, of course, an easy estimate to make and his assumptions may be open to question, but it provides some indicator of the extent of the possible impact of global warming.
How do these and other impacts translate into economic terms? Estimates are not, of course, easy to make, but most studies suggest that the damage, associated with a doubling of atmospheric carbon dioxide concentration, will represent in developed countries a loss of about 1% of GNP, and in developing countries possibly 5%, or even 10% of their GNP. But then such studies do not quantify the disruption or misery or the refugee problem that are hard to represent in money terms.
There is a further possible impact I should mention that is connected with the ocean circulation. A major source for the deep ocean circulation (fig 15), a ‘conveyer belt' as it's sometimes called, is located in the North Atlantic between Greenland and Scandinavia. Water there that has travelled northwards from the region of the Gulf of Mexico is cold and also comparatively salty because of the evaporation that has occurred on its journey. It is therefore more dense than the water around and tends to sink to the bottom, providing much of the source water for this slow, deep, snakish current.
Now, in the warmer, wetter world we are expecting quite a lot more rainfall, particularly at higher latitudes, which will introduce more fresh water into that part of the ocean. The water will become less salty and will not sink so readily. All models with coupled atmosphere-ocean circulations show that circulation weakening as the carbon dioxide in the atmosphere increases. Some models show it cutting off in a few hundred years. We know that this can occur because palaeo records show that it has occurred at various times in the past.
What impact would that have? It could have a significant impact on northwest Europe. No model suggests that this region would be colder, but it would be likely to be less warm than it would otherwise be. In some places that might be seen as an advantage although, because of the increased land-sea temperature contrast, stronger winds might also be a consequence. I mention it, not only because of its scientific interest, but because it demonstrates the possibility of inadvertently altering major systems of the atmosphere and of the ocean that could cause climate changes in ways that we do not well understand as yet.
I've mentioned some of the deleterious impacts of climate change. Are there likely to be beneficial impacts as well? More very hot days will be difficult to cope with, especially in some cities but less cold days will tend to bring benefits. At northern latitudes in Siberia or northern Canada, a longer growing season is likely to provide for more and possibly different crops. Increased carbon dioxide also acts as a fertiliser for some plants and again, if other conditions are also right, could lead to higher agricultural production. This higher production is likely to occur in developed countries at mid-latitudes. By contrast, many developing countries may not be able to produce as much for their increasing populations. So there are factors which are positive; but careful consideration of the impacts demonstrates that negative ones outweigh the positive ones. Most of the reasons for this arise from the very rapid rate of change that is expected and the difficulties for both humans and ecosystems to adapt. In some cases, ecosystems cannot move or regenerate fast enough to survive, so the connection between global warming and biodiversity is also a serious issue.