The response of clouds to warming is one of the biggest "known unknowns"
Pier Luigi Vidale smiles fondly as he gazes at the image unfolding on his screen.
It is a rare and beautiful view of Planet Earth.
Curlicues of cloud formations swirl around the Antarctic at the bottom of the screen as if captured by time-lapse photography.
The image resembles a view of the Earth from space, stretched full frame.
But a small yellow ball scudding along the bottom of the screen hints at another story.
The ball is the Sun, heating the surface as it passes and provoking a daily puff of cloud from the Amazon rainforest in this computer-generated climate model.
The animation comes from research led by Dr Vidale at Reading University's Walker Institute.
It is designed to provide long-term data to help scientists distinguish between heating trends and natural climatic fluctuations.
This week, about 150 of the world's top climate modellers have converged on Reading for a four day meeting to plan a revolution in climate prediction.
And they have plenty of work to do. So far modellers have failed to narrow the total bands of uncertainties since the first report of the Intergovernmental Panel on Climate Change (IPCC) in 1990.
Vicky Pope from the Met Office explains how climate models have changed over the years
And Julia Slingo from Reading University admitted it would not get much better until they had supercomputers 1,000 times more powerful than at present.
"We've reached the end of the road of being able to improve models significantly so we can provide the sort of information that policymakers and business require," she told BBC News.
"In terms of computing power, it's proving totally inadequate. With climate models we know how to make them much better to provide much more information at the local level... we know how to do that, but we don't have the computing power to deliver it."
If we ask models the questions they are capable of answering, they answer them reliably
Professor Jim Kinter
Professor Slingo said several hundred million pounds of investment were needed.
"In terms of re-building something like the Thames Barrier, that would cost billions; it's a small fraction of that.
"And it would allow us to tell the policymakers that they need to build the barrier in the next 30 years, or maybe that they don't need to."
Knowing the unknowns
One trouble is that as some climate uncertainties are resolved, new uncertainties are uncovered.
Some modellers are now warning that feedback mechanisms in the natural environment which either accelerate or mitigate warming may be even more difficult to predict than previously assumed.
Research suggests the feedbacks may be very different on different timescales and in response to different drivers of climate change.
Just last week, preliminary research at the Leibniz Institute of Marine Sciences in Kiel, Germany, suggested that natural variations in sea temperatures will cancel out the decade's 0.3C global average rise predicted by the IPCC, before emissions start to warm the Earth again after 2015.
IPCC authors said this was not incompatible with their models; but the German research provoked some sceptics to ask whether models could be believed at all.
"If we ask models the questions they are capable of answering, they answer them reliably," counters Professor Jim Kinter from the Center for Ocean-Land-Atmosphere Studies near Washington DC, who is attending the Reading meeting.
"If we ask the questions they're not capable of answering, we get unreliable answers."
Models work by simulating the multitude of influences on the climate, including the Sun, the Earth's orbital wobbles, volcanoes, ocean currents, rainforests and man-made pollution.
The modellers draw on science to create equations which, once inside the computer, project a picture of the future climate.
All the models used in the IPCC's vast report last year forecast warming of at least 2C if CO2 doubles (up from a 1.5C minimum rise in the organisation's 2001 report).
This warming is predicted to increase some crop yields in northern countries, but also bring more floods and droughts - particularly in poor nations.
Double or even triple pre-industrial CO2 levels are inevitable by the end of the century unless emissions are very radically cut.
Many scientists believe that exceeding a 2C rise would substantially increase the risk of causing irreversible feedbacks.
Significantly, most IPCC models suggest a rise of 3C if CO2 doubles.
This is forecast to cause increased droughts for more than a billion people, bring about widespread death of coral reefs, and put up to a third of all species at risk of extinction.
The most extreme IPCC model projects over 7C, and other models indicate that rises of double figures are possible.
The models paint a big picture. They struggle to capture regional detail and precipitation; none can successfully forecast the eminently predictable daily rainfall in the Amazon, for instance.
Professor Slingo emphasises the importance of having good information on rainfall in a world hungry for more food.
Cycles of life
Differences between high and low projections in climate models used by the IPCC stem mainly from uncertainties over feedback mechanisms - for example, how the carbon cycle and clouds will react to future warming.
Roger Harrabin explains how cloud feedback works
If clouds trap more heat or the planet releases more carbon, then climate change may accelerate dangerously.
If clouds reflect heat or the planet takes up carbon, we could get away with higher CO2 emissions.
Carbon cycle feedback explained
Professor Andrew Watson at the University of East Anglia researches carbon uptake in the oceans.
He fears dangerous climate change; but he told BBC News that basic science on the carbon cycle is too poorly understood to make a meaningful contribution to models.
"We have to try to model an immensely complex system all the way from the tropical rainforest, the oceans, the northern hemisphere forests, the soil - and we have no fundamental equations to do that with," he says.
"When we are modelling the physics of the oceans and the atmosphere, we do have some fundamental equations.
"We don't have those for the living parts of the system."
Clouds are the biggest "known unknown". Those with low tops bounce the Sun's energy back into space to keep us cooler, whereas high-level clouds tend to trap heat radiating upwards from the Earth.
The crucial question is which sort of clouds will be favoured in the warmer world most scientists expect.
All but one of the IPCC models predict that clouds will accelerate future warming.
Professor Roy Spencer, a meteorologist from the University of Alabama at Huntsville and a noted critic of the IPCC, accepts that this forecast may be right - but believes that the opposite will prove true.
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