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Climate Change Information Sheet 5
How will the climate change ?
- If nothing is done to reduce emissions, current climate models predict a global warming of about 2oC between 1990 and 2100. This projection takes into account the effects of aerosols and the delaying effect of the oceans. This oceanic inertia means that the earth's surface and lower atmosphere would continue to warm by a further 1-2oC even if greenhouse gas concentrations stopped rising in 2100.
- The range of uncertainty in this projection is 1oC to 3.5oC. Even a 1oC rise would be larger than any century-time-scale trend for the past 10,000 years. Uncertainties about future emissions, climate feedbacks, and the size of the ocean delay all contribute to this uncertainty range.
- The earth's average sea level is predicted to rise by about 50 cm by 2100. The uncertainty range is large - 15 to 95 cm - and changing ocean currents could cause local and regional sea levels to rise much more or much less than the global average. The main cause of this rise is the thermal expansion of the upper layers of the ocean as they warm, with some contribution from melting glaciers. Slightly faster melting of the Greenland and Antarctica ice sheets is likely to be balanced by increased snowfall in both regions. As the warming penetrates deeper into the oceans and ice continues to melt, sea level will continue rising well after surface temperatures have levelled off.
- Regional and seasonal warming predictions are much more uncertain. Although most areas are expected to warm, some will warm much more than others. The largest warming is predicted for cold northern regions in winter. The reason is that snow and ice reflect sunlight, so less snow means more heat is absorbed from the sun, which enhances any warming: a strong positive feedback effect. By the year 2100, parts of northern Canada and Siberia are predicted to warm by up to 10oC in winter, but less than 2oC in summer.
- Inland regions are projected to warm faster than oceans and coastal zones. The reason is simply the ocean delay, which prevents the sea surface from warming as fast as the land. The size of this delay depends on how deep any warming penetrates into the oceans. Over most of the oceans, the uppermost few hundred metres do not mix with the water beneath them. These upper layers will warm within just a few years, while the deep ocean stays cold. Water mixes down into the ocean depths in only a few very cold regions, such as the Atlantic south of Greenland and the Southern Ocean near Antarctica. In these regions, warming will be delayed because much more water needs to be warmed up to get the same temperature change at the surface.
- Aerosols may counteract some of the effects of greenhouse warming in the vicinity of major industrialised regions. Clouds of superfine sulphate particles from burning coal and oil should counteract greenhouse warming over much of the Eastern USA, Eastern Europe, and parts of China. But since some action is likely to reduce sulphur emissions because of acid rain, the size of this effect is unpredictable.
- Total precipitation is predicted to increase, but at the local level trends are much less certain. Wintertime precipitation in the far north is likely to rise, but what happens in mid-latitudes and in the tropics depends very much on the details of the particular climate model and the emissions scenario. Including the effects of aerosols, for example, significantly weakens the Asian summer monsoon in the two models which have so far run this experiment.
- More rain and snow will mean wetter soil conditions in high-latitude winters, but higher temperatures may mean drier soils in summer. Local changes in soil moisture are clearly important for agriculture, but models still find it difficult to simulate them. Even the sign of the global change in summertime soil moisture - whether there will be an increase or a decrease - is uncertain.
- The frequency and intensity of extreme weather events such as storms and hurricanes may change. However, models still cannot predict how. The models used to simulate climate change cannot themselves simulate these extreme weather events, so the evidence is indirect. There is some concern that patterns of extreme weather may change because the models predict changes in ocean surface temperatures and other factors that are known to affect storm and hurricane development. However, it will be many years before scientists can predict whether individual regions will become more or less stormy.
- Rapid and unexpected climate transitions cannot be ruled out. The most dramatic such change, the collapse of the West Antarctic ice sheet, which would lead to a catastrophic rise in sea level, is now considered unlikely in the next 100 years. There is evidence that changes in ocean circulation which have a significant impact on regional climate (such as a weakening of the Gulf Stream that warms Europe) can take place in only a few decades, but it is unknown whether or not greenhouse warming could trigger any such change. External factors, such as a series of volcanic eruptions or a change in the power output of the sun, could also have a major impact, but the consensus is that climate change over the 21st century as a whole is likely to be dominated by the effects of greenhouse gas emissions.
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