There have clearly been many variations in the Earth’s climate over and above those produced by the cycles and wobbles of its orbit. Some changes are due to variations in the radiation reaching Earth from the Sun. over its life time so far, the Sun has been getting slowly brighter, but Earth has compensated for this by lowering carbon dioxide levels and reducing the greenhouse effect. The sun also has periods of low sunspot activity, known as Maunder minima, during which its radiation falls by a few percent. One such period caused the “little ice age” between 1530 and 1850, when fairs were held on the frozen river Thames.
Long term trends in climate are very difficult to measure against the back ground of seasonal and annual variation. But there is evidence to suggest the average world temperatures have risen by about 0.5 degree C during the last century. This follows closely the increasing levels of carbon dioxide caused by the large scale clearing of the world’s forests and the burning of fossil fuels. Climate models in computers mostly agree that, if the present trend continues, the overall climate could warm by about 2.5 degree C by the year 2050.
That may not seem much, but this is an average figure and local variations could be much more extreme. Some models suggest that the climate will become more polarized, with droughts in the tropics and more rainfall and storms in temperate zones.