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The temperatures of the air and the oceans have increased by 0.75 °C compared to the period 1860 - 1900. This observed increase is not due to the growth of urbanisation, even if this latter has had important local effects. For thirty years or so, the temperature of the land masses has increased twice as quickly as the temperature of the seas (0.25 °C and 0.13 °C by decade respectively). The temperature of the seas increases less rapidly than that of the land, because the thermal conductivity of water is lower than that of air and the rapid evaporation of the oceans means that they lose heat more quickly. The Northern hemisphere has more continental land masses than the Southern hemisphere and therefore warms more.
Temperatures in the lower troposphere (that part of the atmosphere situated between the Earth’s surface and an altitude of around 8 to 15 km ; the precise thickness varying with the season and the latitude) have increased by about 0.2 °C by decade since 1979 (measurements made by satellites).
According to Nasa, 2005 was the warmest year ever observed since reliable thermometers became generally available (end of the 1800’s). The World Meteorological Organisation believes that 2005 was the second warmest year after 1998. In any case, the average of the last few years beats all preceding records.
Historically speaking, it is thought that the temperature was relatively stable during the two millennia before 1850. There was a short warm period in the Middle Ages, in the Xth and XIth centuries, which allowed the Vikings to colonise Greenland (at that time a green land!). More recently, towards 1700, there was a short cold period, sometimes called « the Little Ice Age », the cause of a lot of difficulty for French peasants at the end of the reign of Louis XIV. The warming of the last 150 years seems to stand out clearly from the pattern of natural variations in global temperature. (see picture 1) |