For the past six years, more than 2,000 scientists from around the world have been writing the most definitive and up-to-date assessment of climate change. It is the fourth report of the Intergovernmental Panel on Climate Change (IPCC) since it was set up by the United Nations and the World Meteorological Organisation in 1988.
The first volume of this assessment - called Climate Change 2007: The Physical Science Basis - will be published today at a meeting in Paris. It will be followed later in the year by two more volumes concerned with what we can expect this century in terms of climate change effects, and what we can do to minimise the impact that those effects will have on our way of life.
It is clear from the draft version of the IPCC's fourth assessment report on the science of climate change there is little doubt that global warming is a reality, and increases in the man-made emissions of carbon dioxide over the past 250 years are largely responsible.
There are still many uncertainties surrounding the issue of climate change, but these uncertainties should not be allowed to cloud the facts. The first is that levels of carbon dioxide in the atmosphere and increases in average global temperatures are higher than at any time over the past 650,000 years - and we know one can exacerbate the other.
It is also virtually certain that the burning of fossil fuels such as coal is responsible for the observed rise in carbon dioxide, the most important greenhouse gas in terms of global warming.
The IPCC has therefore concluded that the global warming we have observed in recent decades is the likely result of human activity.
The IPCC will point out that 11 of the 12 warmest years since 1850 have occurred since 1995. It expects global temperatures to rise by a further 3C by 2100.
Today's report is also expected to warn that sea levels will continue to rise this century, and the century after that, whatever we do to curb carbon dioxide levels. This is because of the inherent inertia of the climate system - the oceans contain about 1,000 times more heat than the atmosphere and so they respond more slowly to changes in global temperature.
If we want to avoid catastrophic increases in sea levels we must attempt to limit the melting of the giant ice sheets off Greenland and west Antarctica.
If one or both of the ice sheets disintegrate, sea levels would rise disastrously to inundate most of the major cities of the world as well as low-lying and densely populated countries such as Bangladesh.
Many scientists believe that we have about 10 years left to enact policies that will curb dangerous climate change.
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