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Global Climate Change
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| Observed global climate change |
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| Temperature |
The global mean temperature rose by 0.74 ò¾ during the hundred year period between 1906 and 2005. The rising rate was 0.13ò¾ per decade in the 50 years from 1956 to 2005, nearly twice the rate in the past 100 years. Nine (2010, 2005, 2003, 2002, 2009, 2006, 2007, 2004, 2001) of the last ten years (from 2001 to 2010) rank among the 10 warmest years on record.
Observed changes in global average surface temperature. Changes are relative to corresponding averages for the period 1961-1990. Smoothed curve represents decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals. (Source: Intergovernmental Panel on Climate Change 2007)
Using complex climate models, the Intergovernmental Panel on Climate Change in their Third Assessment Report forecast that global average surface temperature will rise by 1.4ò¾ to 5.8ò¾ by the end of 2100. The latest Fourth Assessment Report has slightly revised the forecast to the likely range of 1.1-6.4ò¾ with best estimate of 1.8-4.0ò¾.
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| Precipitation |
Global warming (land and ocean) will affect the atmospheric moisture, precipitation and atmospheric circulation. Increases in temperature lead to increases in the moisture-holding capacity of the atmosphere and enhance the hydrological cycle, altering the characteristics of precipitation amount, frequency, intensity, duration, type, etc.
Analysis of long term data shows that, unlike the global temperature rise, the regional variation in precipitation trends is large. Some regions had a rising trend while some had a downward trend.
The diagram shows the precipitation trends (1900-2005) at various regions. Precipitation curves with white background are having rising trends and those with yellow background falling trends. (Source: IPCC, 2007)
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| Mean sea level |
Under the effect of global warming, sea level will rise mainly due to two processes. Firstly, warmer temperature would cause thermal expansion in the upper layer of the ocean. Secondly, melting of glaciers, ice caps and the Greenland and Antarctica ice sheets would add water to the ocean.
According to the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) released in 2007, global average sea level rose at an average rate of 1.8 mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 mm per year. It is predicted that global average sea level will rise 0.18 to 0.59 m by the end of 21st century.
Observed changes in global average sea level rise from tide gauge (blue) and satellite (red) data. Changes are relative to corresponding averages for the period 1961-1990. Smoothed curve represents decadal averaged values while circles show yearly values. The shaded areas are the uncertainty intervals. (Source: Intergovernmental Panel on Climate Change 2007)
Sea level changes showed geographical variations because of uneven changes in ocean temperature, salinity, ocean currents and winds. Regional sea level is affected by climate variability on shorter time scales, leading to regional decadal variations which can be greater than the global change.
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| Extreme weather events |
Extreme weather events (e.g. heavy rainfall, heat wave, cold spell, drought, etc.) are infrequent weather phenomena which are at the extreme of the historical distribution.
According to the Fourth Assessment Report of IPCC, over the last 50 years, there were widespread changes in extreme temperatures. In many places, hot days, hot nights and heat wave have become more frequent, while cold days, cold nights and frost have become rarer. Moreover, the frequency of heavy rain events has also increased over most land areas. Since the 1970s, more intense and longer droughts have been observed over wider areas, particularly in the tropics and subtropics.
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