The greenhouse ellect keeps the earth warm enough for life, and lack of it makes mountains cold (Chapter I). But now the warming from the greenhouse ellect is intensifying, as humans push more and more of the so-called greenhouse gases into the atmosphere (see Box Section 1.1). Leading amongst these gases is C02. released by fuel-burning and deforestation. Its concentration is already 40% higher than it was 200 years ago, and it looks set to double by 2050 (Chapter 7). Methane is another important greenhouse gas, produced largely by Hooded rice fields and by cattle. Its concentration has more than doubled since the year 1800, as human population and agriculture have expanded. Together with several other more minor greenhouse gases resulting from human activity, these additions should already have produced a warming of something like 1°C since 1800.
If the increase in greenhouse gas concentrations continues as expected, by around 2100 there could be somewhere between a 3 and 5 C increase in global temperature. 'This forecast is based on sophisticated climate models, known as general circulation models (or GCMs), that divide the land surface, oceans and atmosphere into their basic climatic components (more about these models in Chapter 5).
The amount by which greenhouse gases have increased over the past 200 years should already have been enough to produce a noticeable warming of the global climate. The GCMs suggest that the warming already will have been around T C. Temperature records from around the world, and a variety of other indicators of climate, seem to confirm that there has been a warming during that timespan, enough to have had at least some noticeable cffccts on vegetation. The warming seems particularly rapid and steady during the last 25 years, suggesting that the effect of greenhouse gases is now starting to dominate against the background of natural climatic variability.
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