By Alex Kirby
BBC News website environment correspondent
One of the most highly charged topics preoccupying the governments of the world is to be thrashed out at a UK conference starting on Tuesday.
Clouds are one of the big unknowns
But Avoiding Dangerous Climate Change, a three-day meeting at the Met Office in Exeter, is mainly about the science.
The participants, more than 200 in all, will try to agree how to define what is a danger level, and what it should be.
This, they hope, will lead to a better understanding of methods the world can employ to avoid catastrophic warming.
The conference, sponsored by the Department for Environment, Food and Rural Affairs (Defra), was announced last September by the UK Prime Minister, Tony Blair.
It will try to answer three questions:
The secretary of the steering committee which has organised the conference is Dr Geoff Jenkins, a veteran of 30 years' work at the Met Office.
- for different levels of climate change what are the key impacts, for different regions and sectors, and for the world as a whole?
- what would such levels imply in terms of greenhouse gas stabilisation concentrations and emission pathways required to achieve such levels?
- what technological options are there for achieving stabilisation of greenhouse gases at different concentrations in the atmosphere, taking into account costs and uncertainties?
He told the BBC News website: "The UN climate convention calls on countries to act to prevent 'dangerous anthropogenic (human-caused) interference with the climate system' from the build-up of greenhouse gases.
Forests may produce vast amounts of carbon
"So the conference will be aiming to identify what's dangerous and what that implies for greenhouse emissions, though without specifying any actual numbers.
"It'll look at the impacts for different levels of warming, but it's very unlikely to say, for example, that a rise of 2C is the limit so we shouldn't let atmospheric carbon concentrations rise beyond 450 parts per million (ppm)."
A number of the papers to be presented deal with areas where science is far from certain about what will happen but remains apprehensive - high-impact low-probability events, as they are known.
Examples include the possible melting of the Greenland ice sheet, disruption to ocean circulation, and the fate of methane hydrates - lumps of frozen methane on the seabed which could conceivably thaw and accelerate the warming process.
The European Union has said global average temperature should not rise more than 2C above its present level in order to avoid damaging climate change.
Greenland's ice could start an unstoppable thaw
One paper, Emission Implications Of Long-term Climate Targets, says carbon dioxide concentrations will have to be stabilised at 450 ppm or lower to achieve a 50% certainty of reaching the EU target.
They are already at almost 380 ppm, up from about 280 ppm before the Industrial Revolution, and have recently been rising at two ppm annually.
Another paper, Tropical Forests And Atmospheric Carbon Dioxide, says the forests may become "a mega-source of carbon", leading to atmospheric concentrations reaching 980 ppm by 2100, or even higher.
Dr Jenkins said: "The big problem is the uncertainties. But the science is hardening up quite a lot, and it's come on by leaps and bounds since the Intergovernmental Panel on Climate Change first met in 1988.
"There's been enormous progress in observations, in our understanding of the processes and our modelling of them - they've all moved on brilliantly.
"The more you understand, though, the more you realise how much you don't understand. In some areas our ignorance is woeful."
Dr Jenkins said the evidence pointed to the likelihood of a temperature rise of about 3C by 2100.
"I'm more convinced now than I was in 1988 that we're seeing climate change that's due to human activities", he said. "We have more confidence that we're in the right ballpark."