Climate Change during Twentieth Century

Historical data based on meteorological observations over the last hundred years clearly show a warming trend on the global and hemispheric scales as well as over most regions and for all seasons. In accordance with observational data, the globally averaged surface temperature has increased by c. 0.6 °C during the twentieth century (Figure 2). In some regions, especially over the continents in the middle and high latitudes of the Northern Hemisphere, the temperature has risen much faster than the globally averaged.

Although observed temperature rise represents the most direct manifestation of climate change during the last century, other climate characteristics related to the hydrological cycle, cryosphere, and extreme weather events also have experienced detectable trends. These changes are not only important as additional indicators of global warming, but also because they affect natural

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Figure 2 Global Northern and Southern Hemisphere surface air temperature anomalies over the last 140 years. Color bars represent annual data, while solid line shows 10-year running mean. Temperature anomalies are calculated relative to the averaged period 1950-1979. The data are from the HadCRUT2 database (http://www.cru.uea.ac.uk/cru).

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1920 1940 Year

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Figure 2 Global Northern and Southern Hemisphere surface air temperature anomalies over the last 140 years. Color bars represent annual data, while solid line shows 10-year running mean. Temperature anomalies are calculated relative to the averaged period 1950-1979. The data are from the HadCRUT2 database (http://www.cru.uea.ac.uk/cru).

and anthropogenic components of the Earth system. In particular, the observations show that the total precipitation over the land area has increased by about 2 % during the twentieth century, and at high latitudes in the Northern Hemisphere this increase was as large as 5-10%. However, the increase of precipitation was not uniform during the past several decades. For example, Northern Africa suffered long and devastating droughts.

Global warming leads to a gradual 'shrinking' of the cryosphere, that is, a reduction of snow and sea-ice cover. The data show a decrease in snow and sea-ice areas in the Northern Hemisphere during the twentieth century. Not only is sea-ice area decreasing, but the ice layer is becoming considerably thinner. These trends are statistically significant and consistent with the results of model simulations. Mountain glaciers are probably the most sensitive to climate changes. It is known that glaciers worldwide have been retreating since the beginning of the twentieth century. Because the mass balance of glaciers is affected both by temperature and precipitation, in some areas, where increase of precipitation dominated, some glaciers advanced in recent decades, but the overall number of retreating glaciers is much higher than the advancing glaciers.

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