International climate experts now believe that measures to reduce industrial pollution will have an unintended side-effect - they will also help to speed up global warming.

The experts, members of the Intergovernmental Panel on Climate Change (IPCC), say the world's temperature will rise over the next century much faster than they had been expecting.

They had thought it would be about two degrees higher by 2100 than it is now.

But a new draft estimate says the increase is likely to be nearly half as much again - 2.8 degrees.

The IPCC, an internationally respected group of climatologists, is expected to issue a final report next April. But the draft has been published by the Kyodo news service in Japan.

A no-win choice

The reason for this startling upwards revision is bizarre - cutting pollution will actually turn up the heat.

The key to understanding the IPCC forecast is a peculiar property of sulphuric acid particles.

These are emitted in the form of sulphuric oxide from the combustion of fossil fuels, especially from industry.

Sulphuric oxide shows there are no easy choices

When the particles are dispersed in the atmosphere they act as a sort of insulator, and help to limit the amount of solar energy reaching the earth.

This in turn helps to slow down the pace of global temperature increase.

The IPCC's previous estimates of global warming a century from now had assumed a growth in future emissions of sulphuric particles.

But it now says that pollution control technology will achieve a steep drop in sulphuric oxide emissions from factories between 2020 and 2050.

Scenarios agree

The draft report follows usual IPCC practice in spelling out several probable scenarios, rather than saying that one is certain to happen.

But all the scenarios agree that a drastic drop in the quantity of sulphuric particles is likely.

They have also been designed to take account of factors such as projected population increase and economic growth.

Acid rain has laid waste to some of the world's greatest buildings

The report will face policy-makers with a cruel dilemma, bevause there are compelling reasons for trying to reduce sulphuric oxide pollution as quickly as possible.

The particles damage air quality, which threatens human health.

And they are responsible for acid rain, which destroys stonework and has turned some Scandinavian lakes into lifeless wastes.

Showers falling in Japan have been found to be as acid as vinegar, reputedly capable of discolouring clothing.