If the IPCC's draft version leaked on to the Internet last month is anything to go by, this United Nations document will show that the scientific consensus is strengthening: pollution is very likely to alter climate over much of the earth's surface in the next century. The scientists are saying that we are probably seeing this already, in the run of exceptionally warm years in the 1980s and 1990s. This year will also turn out to be an unusually hot one, not just in Britain but in terms of average temperatures around the globe.
When the final version of the IPCC science report is released the qualifications will be there. One hundred per cent certainty cannot be provided for systems as complex as the earth's atmosphere, oceans, ice-caps and life, which all interact in shaping the planet's response to humanity's massive intervention.
But even the qualified conclusions will be sufficient to spark an explosion of alarming and vivid headlines. It's a funny business, this global warming. Most of the time the media and society ignore the issue. Every now and then we have big stories warning us that sea levels will rise and tropical plagues spread as temperatures rise. Droughts and floods will become more common. Then, not quite as frequently, we get the big debunking pieces which tell us that another group of scientists has disproved global warming and it is all a big scare.
So what are we to conclude from all this? And what measures would it be sensible for modern industrial societies to take in the face of continuing uncertainty about a threat that was first recognised almost 100 years ago, when Sven Arrhenuis, a Swedish chemist, made the first prediction about man-made global warming?
In 1896 Arrhenuis calculated that if the burning of fossil fuels doubled the concentration of carbon dioxide in the atmosphere, then temperatures would rise by around 5 degrees Centigrade. In the 99 years since then we have frantically burnt coal, oil and gas, and we can be sure that if we go on at present rates the atmospheric concentration of carbon dioxide will, midway through the next century, reach double pre-industrial levels. We also now know that we are adding several other kinds of "greenhouse gas" to the atmosphere - methane, CFCs - which are even better than carbon dioxide at trapping heat in the lower atmosphere.
But although the hundreds of climate scientists who contribute to the IPCC have made much progress in the past decade in understanding what we are doing to the planetary thermostat, they are still at least five years away from giving a precise estimate of how rapidly average global temperatures will rise.
They are fairly certain that Arrhenuis's alarming 5 degrees for a doubling of carbon dioxide is a little too high. Their low- to middle-range estimates imply rates of warming and resulting sea level rise that are within the bounds of what advanced industrial societies (but not crowded, poor countries) can easily cope with for the next 50 years - and whoever thinks further ahead than that?
What every government wants to know most of all is how regional climates will change. Which countries will suffer the most damaging changes; which ones might even benefit from benign climate shifts?
The scientists are probably at least 10 years from making good regional predictions. They need computing power much greater than that of the number- crunching supercomputers they now employ for their simulations of the world's changing atmosphere and oceans.
This is high science, involving dozens of research groups which collaborate and debate. It is not the kind of work that throws out an abrupt consensus, when suddenly everyone can agree that pollution has already caused this much climate change and will go on to do that much more in the next x years. It churns out probabilities, not pat answers.
Those with vested interests watch this unfold, then put their own spin on things. The USA's gigantic fossil fuel industry, along with oil exporters like Kuwait and Saudi Arabia, play up the uncertainties. Their lobbyists stoop to suggesting that the scientists exaggerate in order to get their research grants.
Some environmentalists, meanwhile, suggest that every extreme weather incident is a manifestation of man-made climate change - forgetting that at any one time extreme and unusual weather is being experienced somewhere in the world. The nuclear power industry also likes global warming because it generates electricity with far less of the "greenhouse gas" pollution of its fossil fuel rivals.
The media simply wants good, strong stories. So climate change is either very serious, or a scare, or off the agenda.
While the scientists plod ahead in trying to ascertain the threat, the sensible response is not to do nothing, relying on adapting to climate change once it happens. It is to take measures that reduce our reliance on fossil fuels without harming the economy.
These so called "no regrets" measures are a moral as well as a rational response to the threat, which is why many politicians advocate them. The problem, as ever, is that they tend to pay lip service to them.
Take one example close to home. At a time when the real cost of household energy is falling (gas and electricity bills are coming down) there is an excellent environmental case for placing higher taxes on fossil fuels. That would encourage people to use less of them and curb pollution.
The revenue raised should first be used to make sure those worst affected (the poor and the elderly) can keep warm in winter, by installing better insulation and more efficient heating systems. The second call on this revenue should be to reduce taxes that keep people out of work, such as income tax and employers' National Insurance contributions.
But what do we get? A botched, enormously controversial introduction of VAT on electricity and gas in which no politician seriously advocated the environmental case. The Government introduced it simply because it needed to raise the money, and Labour is hinting at getting rid of it simply to raise votes. And the bigger picture of a planet in real danger is ignored.Reuse content