Evidence that the World has Warmed in the Late Nineteenth Century

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  • Evidence that the Earth has Warmed since the Late Nineteenth Century
    • Increase in Global Temperatures
      • Consistent rise of global land and ocean temperatures in the C20
    • Shrinking Valley Glaciers and Ice Sheets
      • Everywhere in the past century, valley glaciers have been retreating and some have completely disappeared
        • If trend continues, valley glaciers in the Alps may shrink by 80-96% by the end of the century
          • Present day melting of polar ice sheets adds approximately 1mm to sea level every year
            • Losses are due to:
              • Warming of the atmosphere, which melts the ice surface
              • Warming, which produces meltwater that penetrates the ice and increased the velocity of glacial flow
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    • Rising Sea Level
      • Began to rise mid-nineteenthcentury
      • Since 1900 the average rise has been 1.0-2.5 mm/year
        • Recent satellite imagery suggests a faster rate - 3mm/year
      • Two processes account for the global sea level rise:
        • The thermal expansion of oceans as the worlds climate warms
        • The melting of land-based ice sheets and glaciers
    • Increasing Atmospheric Water Vapour
      • The most important greenhouse gas (GHG)
      • It traps huge amounts of energy radiated from the Earth's surface and creates a natural greenhouse affect
      • Amount of water vapour in the atmosphere is directly related to temperature and rates of evaporation
      • In a warmer world there will be more atmospheric moisture
      • For every 1 degree increase in temperature caused by enhancingCO2 levels, rising levels of water vapour will double the warming
      • Large concentrations of water vapour will amplify the effects of warming on positive feedback; hence more vapour intensifies the greenhouse effect
        • Increases evaporation, which in turn leads to more atmospheric vapour, more evaporation, more warming...
    • Decreasing snow cover and sea ice
      • 2% decline per decade since 1966 in the northern hemisphere
      • Snow has a high albedo, reflecting 70-80% of incoming solar radiation compared with just 10-20% for soil and vegitation surfaces
        • Demising snow cover therefore increases the absorption of solar radiation
      • With the suns energy being used to warm the ground air temperatures rise
        • Creates positive feedback, which explains the rapid warming currently taking place in the Arctic
      • Arctic sea ice has declined dramatically - 8% in summer and 3-4% in winter
        • Also decreasedthickness
      • Less sea ice results in more solar energy absorption by open sea surface, raising air temperatures and causing even more rapid melting

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