Long before mankind, and even longer before the industrial age led to large-scale burning of fossil fuels, the climate warmed and cooled in cycles. Among the natural factors affecting it, then and now, are volcanic eruptions, fluctuations in solar radiation and quirks in the earth's rotation. Dissenters argue that any impact from man's actions will have been dwarfed by such age-old natural forces. The trouble is that man's actions affect climate in many complex ways. CO2 is produced when fossil fuels are used, or when forests are burnt.
Agriculture releases other powerful GHGs, such as methane and nitrous oxide.
The release of particulates, however, promotes cloud formation and so has a cooling effect. A study by a team led by James Hansen of America's space agency, NASA, has looked in detail at the net effects of these factors. Under normal circumstances, the earth releases heat at the same rate at which it absorbs energy from the sun. Indeed, the more that researchers look at the problem, the more convinced they are that GHGs are the main culprit for the earth's recent warming. The evidence they cite is largely based on mathematical models of climate change.