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Human Influence

The rapid increase in atmospheric concentrations of the three main man-made greenhouse gases is clear from the data sets for these gases over the last 1000 years.

Since around the time of the industrial revolution in Western countries levels of carbon dioxide, methane and nitrous oxide of all risen dramatically. Fossil fuel combustion, increasingly intensive agriculture and an expanding global human population have been the primary causes for this rapid increase.

Methane concentrations have seen the biggest relative increase in the last 200 years, concentrations more than doubling. The rate of methane increase appeared to be lessening over the last decade, but rose again in 2007. It is concentrations of carbon dioxide, methane and nitrous oxide which are the human-made greenhouse gases which we are likely to see increase most in the next 100 years.

Sulphate aerosols, though not greenhouse gases, are nonetheless very important to global climate. Sulphate in our atmosphere has a net cooling effect (see below) and so goes some way to reduce the warming effect of the greenhouse gases.

The same increases in fossil fuel burning which have led to elevated greenhouse gas concentrations in the last two hundred years have also led to an increase in sulphate emissions. However sulphate particles have a much shorter ligetime in our atmosphere than greenhouse gases.

Cleaner fuel technologies are today leading to a reduction in sulphate emissions and their incidental cooling effect on our climate. If greenhouse gas emissions continue to increase then their overall warming effect may therefore become even more intense.

Figures courtesy of the Intergovernmental Panel on Climate Change (IPCC). Click figures to enlarge.

Our impact on the global climate since the industrial revolution has been complex. Though emissions of greenhouse gases, like carbon dioxide and methane have had a net warming effect, emissions of sulphate have had a net cooling effect.

The overall effect is one of net global warming, but the complex interaction of these positive and negative influences on global warming make predicting future warming difficult.

The problem is exacerbated by our poor level of understanding of exactly how some factors, like land-use albedo (the reflectance of the land), operate and interact.

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