In 2007 the Intergovernmental Panel on Climate Change will report four times. With the first set of conclusions in hand Richard Black weighs the evidence and predictions for the fate of the planet

A world that is maybe two, maybe four degrees Celsius warmer by the end of the century. Sea levels that are higher, by perhaps ten,
perhaps 40cm. More major storms away from the tropics; more droughts, more deserts, and more floods.

The 2nd of February, 2007 will perhaps be remembered as the day where the question mark was removed behind the debate on whether climate change has anything to do with human activity Achim Steiner, Executive Director of the United Nations Environment Programme

These are the hot-off-the-press headline conclusions of the Intergovernmental Panel on Climate Change (IPCC), the global scientific body that, in February, published its first major report in five years. But even as I write, I am laid siege by déjà vu; are these not the very words I wrote in 2001, when the IPCC's previous report appeared? Was I not writing them a decade ago, in fact, in a period when climate science was clad in nappies compared with its current sophisticated dress?

Well, yes I was, with minor variations. Yet the new report is the result of research by several thousand scientists across the world during the past five years or so. So what have they been doing all this time?

They have been sending robotic floats out on oceans to measure temperature and salinity. They have been looking for evidence of past climatic change in kilometre-long cores of Antarctic ice. They have been studying the sun's behaviour, asking whether changes in its energy output could be driving major changes on Earth. They've been honing computer models of the planet's climate in abstract form, making them replicate - ever more accurately - what happens, which in turn gives ever more confidence about their projections.

And as their work is gathered together in the fourth IPCC report, we see the fruits of the billions of dollars that all of this costs every year. The boundaries of our possible future are narrowing. Whereas the last report put the average global temperature rise at between 1.4? and 5.8?c over the coming century, the projected probable bounds are now 1.8? and 4.0?. In 2001, sea level rise was put at anywhere between nine and 88cm by 2100; that range has now come down to between 28 and 43cm.

From the IPCC 4th Assessment Report • Observations since 1961 show the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise

This is useful; it starts to give policy-makers and ordinary Joe Publics the parameters needed to plan for a climatically different future. If you are to build sea defences, for example, you need to know whether 88cm is a possibility or not. In parallel, observational science and computer modelling are getting better at making regional forecasts, vital to planners. Scientists now understand parts of the Earth well enough to know that a rise of two to three degrees can end the Amazon as we know it; a more modest rise could be enough to start an irreversible thaw of the mighty Greenland ice cap.

Even so, many will fault the IPCC and science in general for not coming up with hard, firm numbers. It will seem ridiculous that so many clever men and women can work for so long with so much public money, and still not know exactly how hot it is going to be in which country and at what time.
But even if scientists understood all the workings of the Earth's climate systems and could model them perfectly, which is certainly not the case, their forecasts would still lack one crucial ingredient: the behaviour of human societies. We may choose at some point in the century to abandon fossil fuels. We may cover the Sahara with solar panels, or scatter the ocean with giant wind turbines; who knows, we might even get nuclear fusion to work. Or maybe choices will be forced upon us; perhaps oil supplies will run down sharply, as some believe; or perhaps conflict will perturb the global economy, the trade in fuels, and even the inexorable rise of the global population.


At these factors, the IPCC can only take an educated punt, and put up with a still imperfect understanding of the workings of the atmosphere and oceans. This is why its projections still fall unsatisfyingly short of full precision.

Not all scientists agree with the IPCC. Some think that climate models are inherently unreliable, others that physical processes involving cosmic rays from the sun and clouds are a much bigger problem than greenhouse gases. They are relatively few in number; and there should be no doubt that the IPCC does represent the consensus.

But a consensus can hide a hundred compromises; and interpretation is everything. In the middle of last year, the BBC commissioned a panel of seven eminent scientists to debate climate change. Over the century, they concluded unanimously that it will be severe; but on whether the word
'catastrophic' was appropriate, they were split. Such matters are judgements; if a temperature rise of 3?c, for example, spells very different things for Norway and for Niger, how should the word 'catastrophic' be used?

Scientists have, with relatively small variation, been writing the same thing with the same implications for more than a decade. In that time, the political rhetoric has increased hugely in volume; the action, hardly at all. I am sure that one reason among many for this is that the IPCC predictions have been vague, allowing the comforting thought that perhaps doing nothing is an option. What is changing is that the boundaries are narrowing; and with them, the window for reducing greenhouse gas emissions - before the Amazon basin dries out and the Greenland ice cap begins irreversibly to melt.

Always fascinated by science and the natural world, although not destined to become a scientist, Richard Black spent years covering scientific and environmental issues for BBC radio, he is now Environment Correspondent for BBC News online, where his brief includes climate change and conservation

This is our planet. Since life began it has gone through extraordinary changes. But now it is being transformed - not by natural events, but by the actions of one species: mankind Sir David Attenborough

In April, Sir David Attenborough presents a two-part series for BBC World TV's Climate Watch season.
BBC World Service and BBC News online will carry extensive coverage and analysis of the publication of the next stages of the IPCC reports throughout 2007
Visit the BBC Climate change portal