Predicted Climate Change in the Twenty First Century

Results of climate model simulations performed for a whole spectrum of possible greenhouse gases and aerosols emission scenarios indicate that globally averaged surface air temperature will rise till the end of the twenty-first century by additional 1-6 °C compared to the present one (Figure 4). This temperature rise, however, will not be uniform and the warming trend over the continents and high latitudes is expected to be much stronger than the averaged one (Figure 5a). In the Southern Hemisphere, where the ocean area is much larger than in the Northern Hemisphere, warming will occur at a lower rate.

Another important aspect of global warming is change in the hydrological cycle. All climate models predict an increase in the globally averaged precipitation due to global warming; however, simulated regional patterns of precipitation changes are much less robust and show a low correlation between different models. This is related to a strong spatial variability of precipitation and the large number of factors affecting precipitation changes. Still, some common features can be derived from model simulations (Figure 5b). In particular, most of the climate models predict the largest increase in precipitation in the equatorial region and middle latitudes, while in the subtropics they predict the precipitation to remain unchanged or even to decrease. Due to the increased contrasts between the land and ocean temperatures, climate models predict considerable intensification of the Asian and African monsoons.

Results of model simulations and analysis of paleocli-matological data suggest that the climate system represents a strongly nonlinear object and its response to gradual changes in external and internal forcing may not necessarily be smooth and reversible. There are several components of the Earth system which are suspected for such strongly nonlinear behavior. One of them is the Atlantic thermohaline circulation which is known to be sensitive to variations in the freshwater flux and did experience major reorganizations during the glacial age. Some model simulations indicate that global warming and associated changes in the hydrological cycles and melting of the Greenland ice sheet can cause a complete shutdown of the Atlantic thermohaline circulations and, as a result, severe and abrupt changes in the regional climate and sea level. The shutdown of the Atlantic thermohaline circulation would also have a pronounced effect on marine ecosystems.

Another component of the climate system which can respond strongly to the future climate change is the West Antarctic ice sheets. There is a possibility that global warming can destabilize the West Antarctic ice shelf, which in turn may trigger abrupt destabilization of the grounded ice sheet. The latter will led to an additional sea-level rise of up to 5 m within several centuries. Due to very complex and still not well understood dynamics of the West Antarctic ice sheet, it is impossible so far to quantify a probability of such collapse, but recent disintegrations of smaller ice shelves and associated accelerations of adjacent ice streams add to the concern about potential instability of the West Antarctic ice sheet.

Although it is still impossible to predict all important consequences of anthropogenic climate change, there is a

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Climate Change Twenty First Century

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Figure 5 Mean annual temperature (a) and precipitation (b) changes around the year 2080 compared to the year 1990, simulated with the HadCM3 coupled climate model for SRES A1FI scenario. Temperature changes are in °C. Precipitation changes are in mm per day. The data are from the IPCC data distribution center (http://www.ipcc-data.org).

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