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
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
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.
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.