British climatologists say massive reductions in emissions of carbon dioxide (CO2) will have a fairly modest effect in slowing climate change.

In a report, scientists at the Hadley Centre for Climate Prediction and Research, part of the United Kingdom Meteorological Office, assess the global impacts of stabilising atmospheric CO2 at different levels.

The report investigates what would happen with emissions reduced so that CO2 concentrations stabilised at 550 parts per million or 750 ppm.

Delayed impacts

Carbon dioxide is the main global warming gas produced by human activities: concentrations now are about 360 ppm, the highest for 160,000 years. But this will inexorably increase as emissions continue to build up in the atmosphere.

Weaning the world off fossil fuels will take time

The Hadley Centre team say their work shows that "many of the changes and consequent impacts resulting from unmitigated emissions of CO2 are delayed by 50-100 years if emissions scenarios leading to the stabilisation of CO2 are considered".

Global temperatures, for instance, would rise more slowly, sea level rise would be more gradual, and tropical forests would not be so severely affected.

The snag is that achieving the sort of stabilisation the report suggests is far beyond the bounds of what appears to be politically possible.

Massive cuts needed

The international agreement on tackling climate change, the Kyoto Protocol, commits signatories to a global reduction of greenhouse gas emissions of about 5% over the next decade.

To stabilise CO2 concentrations at their present level - where many scientists say the early signs of climate change are already discernible - would require immediate emission cuts more than ten times as large.

They would need to come down by 50-70%, with further reductions later.

Global temperatures are set to go on rising

A report published last month by the United Nations Environment Programme looked at the prospects for stabilising CO2 emissions close to 550 ppm, the lower level researched by the Hadley Centre team.

"If stabilisation at below 550 ppm were to be aimed for, the annual mean per capita CO2 emission for the whole world would need to be approximately five tonnes during the next century, and below three tonnes by 2100.

"Current levels are about four tonnes/capita as a world average, with a maximum emission of nearly 20 tonnes/capita in North America and a minimum of less than one tonne/capita in many parts of Africa."

The UNEP report comments: "Meeting all the targets agreed at Kyoto will have an insignificant effect on the stabilisation levels of CO2."