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Floods in Pakistan have left millions homeless and at least one-fifth of the country inundated. In Russia, droughts have sparked wildfires that sent crippling smog over main cities, claiming scores of lives while destroying crops and costing billions of dollars in damage.

The extreme weather of 2010 is likely to be remembered in these regions for many years to come. There as well as in the rest of the world, the broader question is whether, as climate scientists predict, this type of weather is set to become more common – and how certain we can be about that.

This has been an unusual year – the warmest January-June period on record around the world, and the driest on record in some regions. But however extreme, the events of one year cannot be taken as proof of climate change. Natural variability brings periodic extreme floods, droughts and heatwaves around the world, and it takes years of data to distinguish this from any underlying trend.

The most scientists are willing to say is that the weather in Pakistan and Russia is consistent with predictions of what will happen in a warming climate, driven by greenhouse gas increases.

For it has also been a year in which the science of global warming has been questioned as never before. On Monday, climate research will come under the microscope again. A panel of the world’s most august scientific bodies will pass judgment on climate science, and specifically on the Intergovernmental Panel on Climate Change, the grouping of the world’s leading experts whose advice forms the basis of international policy.

Unjolly hockey sticks

The “hockey stick” graph reproduced in the 2001 report by the Intergovernmental Panel on Climate Change has long been derided by sceptics. It shows temperature data for the last 1,000 years. As widespread instrumental records have only been available for 150 years, data for other centuries were reconstructed from proxies such as tree rings and ice cores, which carry traces of previous climatic conditions. Tree ring data were a source of controversy in the University of East Anglia’s “climategate” e-mails. In a subsequent investigation, Britain’s leading statistician judged the graph flawed, though he added that better methods would not have drastically altered the pattern.

The investigation, and this year’s extreme weather, have thrown a spotlight on some of the murkiest corners of climate science: the areas where scientists are simply not sure what to expect. These uncertainties include the exact nature of the changes from a warming world, when and where these will strike, and how severely.

Clearing up these unknowns is vital. If the scientists are right, worldwide greenhouse gas emissions must begin to fall within the next 10 years or so in order to have a good chance of avoiding the worst effects of global warming. That would require drastic changes to the world economy, a revolution in consumer behaviour, stiff regulation and – in the short term at least – higher costs to business.

With all this at stake, politicians and business leaders are demanding answers – but sometimes, answers that scientists cannot give. Scientists are now addressing some of the areas of greatest concern with a new urgency and candour, following the “climategate” debacle that surrounded disputed data six months ago. Sir John Beddington, the UK government’s chief scientific adviser, tackled the issue of uncertainties head-on when launching a new climate website last month.

“The main uncertainties are how bad things will get,” he said. “There are enormous differences between different geographies around the world. The Arctic, for example, is enormously problematic. The most optimistic prediction is for an 8 degree [Celsius] rise, the most pessimistic a 16 degree rise.”

The difference in climate terms is huge – today’s temperatures are only on average about 6°C higher than in the last ice age. Either of Sir John’s predictions would lead to a North Pole free of ice, but the warmer the temperatures the faster the melting of the vast Greenland ice sheet and therefore the sharper the rise in sea levels.

Sir John pointed to another big uncertainty for politicians – how quickly the world is warming. We could be in for 4°C of warming by 2060, Sir John noted – or it might not be until 2100. For scientists, this variance is well within the expected margins – but for policymakers and business it makes a world of difference. It will determine how far and how urgently emissions need to be cut and the world economy reformed.

David Easterling of the US National Oceanic and Atmospheric Administration is, like most scientists, confident that the basic research and conclusions are correct. “The direction of travel is clear,” he says. “We are certain the world has warmed, and virtually certain that a lot of this came from human activities.”

NOAA, working with most of the world’s other big climate research units, recently conducted the first big review of recent climate science since the IPCC’s 2007 report and found “unmistakable” signs of warming. Peter Stott, of the UK’s Met Office, goes further, saying that the signs of warming are so clear that they “have human fingerprints” all over them.

Climate sceptics disagree with that analysis. But it is when scientists stray into more complex predictions that the real difficulties arise – and some of these can be crucial. What politicians really want from scientists is firm answers they can translate into targets and policy goals. So at last year’s Copenhagen summit, developed and developing country governments agreed to try to prevent temperatures rising more than 2°C above pre-industrial levels.

That figure – adopted by governments as the limit of safety, beyond which the effects of climate change may become catastrophic and irreversible – ultimately derives from the IPCC report. But in the report, the 2-degree figure merely appears as one point amid a range of possible temperature increases, ranging from about 1.5°C to 4°C.

According to governments, sticking to the 2°C target will require a halving of global emissions by 2050. That gives countries a handy reckoner by which to judge their climate policies. But scientists are not so sure. According to Vicky Pope of the Met Office, quoting recent research, halving global emissions by 2050 would give only a 50 per cent chance of avoiding the higher temperatures. These differences display the difficulty of shoehorning scientific ranges of probabilities into the confines of political expression.

Other important areas of uncertainty are underestimated, critics assert. Contrast the reaction to this summer’s Russian drought and Pakistan’s floods with the response to the unusually deep and prolonged cold snap suffered by much of Europe and the US earlier in the year. While sceptics seized on the latter to ridicule global warming, many climate scientists dismissed it as a blip.

Those differing reactions angered many sceptics. “Any record cold snap or harsh winter is, correctly, attributed to the dynamics of natural variability,” says Benny Peiser, director of the Global Warming Policy Foundation, a climate sceptic think-tank. “Yet extreme weather events that occur during the summer are habitually linked to man-made climate change. Numerous climate scientists are on record for claiming that such weather events ‘match IPCC projections’ of global warming.”

This “inconsistent” way of treating extreme weather events is, says Mr Peiser, a prime reason why “climate science as a whole is, regrettably, haemorrhaging trust and respect”.

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