Climate Change > Expected effects

Expected effects

Effects on temperature, percipitation, sea level etc.

Results from IPCC Working Group I

Most forecasts of future emissions, and all the scenarios the IPCC uses in its report, assume that levels of greenhouse gases in the atmosphere will increase compared to the present. None of the IPCC’s scenarios make any assumptions regarding international agreements to limit emissions.

In the lowest emission scenario, B1, it is assumed that technical and social development permits the reduced use of fossil fuels. In this case the carbon dioxide level is expected to continue rising, but to stabilize at a level that is roughly twice the pre-industrial level.

The highest emission scenario, A1FI, assumes continued use of fossil fuels and hence a continued rapid rise in the carbon dioxide level. By 2100 the carbon dioxide concentration is forecast to be around three times higher than the pre-industrial level.

Climate simulations were carried out for the period 1990–2095. In the case of the lower emission scenario (B1) these show a rise in the mean global temperature of 1.8°C from the present level, with an uncertainty range of 1.1 to 2.9°C. The higher emission scenario (A1FI) gives a temperature rise of 4.0°C with an uncertainty range of 2.4 to 6.4°C.

One new factor introduced in some models is feedback between the natural carbon cycle and physical climate changes. Rising temperatures may lead to reduced land and ocean uptake of carbon dioxide. This is one of the main reasons why several scenarios give a higher limit for the uncertainty range in comparison with the 2001 IPCC report.

The IPCC also described what could happen if concentrations of greenhouse gases in the atmosphere are stabilized at year 2000 levels. The temperature rise by 2100 would then be around 0.6°C (0.3–0.9°C) above the current level, i.e. 1.3°C above the pre-industrial level.

The sea level will continue to rise. The lowest emission scenario (B1) predicts a rise of between 0.18 and 0.38 metres, while the highest scenario indicates between 0.26 and 0.59 metres by 2100, compared to present levels. These calculations do not take into account the risk of accelerated glacial melting in Greenland and the Antarctic that could result from continued warming. Such processes could give an additional increase in sea level of 0.1–0.2 metres.

Warming will not be distributed uniformly around the world. Over the Arctic and land masses in the northern hemisphere, warming will be significantly greater. In fact over the Arctic it will be roughly twice as high as the global mean. Over the seas of the southern hemisphere and in the north Atlantic, warming will be less than the global mean. The latter is linked to weakening of the Gulf Stream, which will mean that less heat is transferred to the sea areas around Iceland and southern Greenland. It is however highly unlikely that warming will lead to a sudden collapse of the Gulf Stream.

It is very likely that precipitation will increase at high latitudes, such as Europe and America, and likely that it will decrease over most subtropical land regions. The pattern of these changes is similar to what has been observed during the 20th century.

It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent. Sea ice is projected to shrink in both the Arctic and Antarctic under all scenarios. In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century.

It is likely that the frequency of severe tropical cyclones will increase as the climate gets warmer. There is a tendency for the westerly wind belt to shift northwards, with the consequence that low pressure corridors and precipitation patterns are also shifted north.

Continued warming and rising of the sea level is likely after 2100 even if the concentrations of greenhouse gases in the atmosphere are stabilized. Among other things, this means that the melting of the Greenland ice sheet will continue. Global warming of between 1.9 and 4.6°C compared with the pre-industrial level – which is not at all improbable – could lead to the entire Greenland ice sheet melting within a few thousand years. This would cause the sea level to rise seven metres and submerge many cities around the world.

The Antarctic ice sheet would be likely to grow in a warmer climate since much of the ice sheet is still colder than zero degrees. It is however possible that large masses of ice will break loose as the climate warms. This will lead to sudden ice losses and rises in sea level.

Future emissions of carbon dioxide over the next century will remain in the atmosphere for more than a thousand years and contribute to the continued warming and rising of the sea level.

Increasing atmospheric carbon dioxide concentrations also lead to increasing acidification of the ocean. Projections give reductions in average global surface ocean pH of between 0.14 and 0.35 units over the 21st century, adding to the present decrease of 0.1 units since pre-industrial times.

Effects on natural world and human society

Results from IPCC Working Group II

The IPCC report, published in April, summarizes published scientific research up to 2005. It describes how climate change is affecting people and the environment. It predicts future impacts, by region and sector, and estimates how far mitigation and adaptation measures can help reduce them.

The report concludes that climate change is already having major impacts. Future climate change will make dry areas drier and increase the risk of flooding in others. More than a billion people may face fresh-water shortages by 2050. In some countries, crop yields could halve by 2020. Health problems will increase.

There are also likely to be widespread biodiversity losses: 20 to 30 per cent of plant and animal species will face an increased risk of extinction if the average global temperature increases by more than 1.5 to 2.5 degrees Celsius. The IPCC forecasts a three-degree rise this century under business as usual.

The four areas of the world thought to be the most vulnerable to climate change are:

• the Arctic, where temperatures are rising fast and ice is melting;

• sub-Saharan Africa, where dry areas are forecast to get dryer;

• small islands, because of their inherent lack of capacity to adapt, and

• Asian mega-deltas, where billions of people will be at increased risk of flooding.

For further details, see article and factfile in Acid News 2/07.

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Last modified: 21 July 2007.