A team of Swiss and British scientists have compared major research on climate change, showing where consensus on the issue ends and disagreements begin.
Almost all scientists agreed that carbon dioxide (CO2) emissions by humans were mainly responsible for global warming, but opinions differed on future levels of warming.
Professor Reto Knutti of the Institute for Atmospheric and Climate Science at the Federal Institute of Technology in Zurich and Gabriele Hegerl of Edinburgh University's School of Geosciences studied climate models and research findings from the past century.
"Our summary shows that although the studies published during the past ten years have provided many fascinating new insights into our climate system, they have brought little additional certainty as to the expected maximum long-term warming," they said in the study, published by the online journal Nature Geoscience.
"However, the summary might provide decision-makers with a basis for further measures."
By compiling past research, the team found most scientists estimated that equilibrium global temperatures could increase by 2-4.5 degrees Celsius during a doubling of atmospheric CO2 concentrations – a level that could be reached before the end of the century.
But there was less agreement on a maximum amount, with some research pointing to a possibility of the earth warming by up to ten degrees.
Knutti told swissinfo that although the likelihood of this is relatively small, the consequences would be catastrophic.
"We have a relatively small probability of a disastrous outcome which is an interesting situation because it raises the question of how to deal with the low-probability but high-impact risk.
"In our daily lives we tend to ignore these things. But we should be worried about the possibility of a high-impact event," he said.
"After all, we all pay for insurance in the unlikely event that bad things happen."
To estimate long-term change, the team looked at "climate sensitivity", the effect of CO2 on the climate system.
Knutti says the first estimates of this figure go back more than 100 years. In early studies of the properties of the CO2 molecule scientists found it had an effect on solar radiation and calculated a five-degree rise over time.
Knutti and Hegerl found most research had been carried out in the past 30 years and refer to 100 previous studies in their paper.
"Different people have found different ways of trying to calculate how sensitive the climate is to changes in CO2. We summarise all these.
"The picture of what we know is reasonably balanced and accurate. But the fact is, what we know is not so good," warned Knutti.
The study follows the publication in 2007 of the Fourth Report by the Intergovernmental Panel on Climate Change, which mainly reviewed recent research into climate change.
Based on the findings, Knutti and Hegerl also identified major short-term trends the planet faces under climate change.
Current temperature increases by two-tenths of a degree every decade are set to continue for the next 20 years.
There will be more rainfall in high latitudes in winter and drying of the sub-tropics. Extreme weather changes will increase, bringing more heat waves, droughts, floods and heavy rainfall events. Sea levels will rise the same as in the past by 3mm a year; the arctic sea ice will continue to decline and snow cover will decrease.
"The short-term [effects] are relatively well known from what we have observed in the past. The long-term is a bit uncertain. I assume we will still be uncertain in ten years," the authors said.
The team says its findings provide an objective basis for discussion and determining policy.
"We try to say what this means now... how should we act, how should we make decisions in terms of policy. I think that is even the more interesting part of the paper," Knutti said.
He argues that the world needs counteraction soon and should not wait years for science to come to a unanimous conclusion on the impact of climate change.
"CO2 has a long lifetime – once these things are in the atmosphere they stay there for a long time. So if we were to make a decision to limit CO2 to certain levels, we would have to act very soon," he said.
"We have to reduce emissions within the next few years and achieve a reduction of maybe 80 per cent by the end of the century. Otherwise we will not prevent a warming of two degrees. Uncertainty should not be an excuse for not making a decision."
swissinfo, Jessica Dacey
Temperatures in Switzerland have risen by an average of 0.57 degrees Celsius each decade since 1970. The increase is twice as much as the average for the northern hemisphere.
Carbon dioxide is one of the major gases responsible for the greenhouse effect and global warming. In Switzerland it represents around 80% of harmful emissions. Other gases include methane, nitrous oxide and hydrocarbons.
Despite ambitious emission targets, greenhouse gas emissions have actually risen by 0.4% in Switzerland since 1990.
Calculating climate sensitivity
Scientists say that by doubling the amount of CO2 in the atmosphere – as society is on track to do by the end of the century – and keeping all other climatic variables such as cloud changes fixed, the earth will warm by one degree Celsius as CO2 continues to absorb radiation.
As a result of that one-degree increase, snow will decrease, cloud patterns will vary, humidity will increase and the temperature will go up again in a cyclical process. The extent of these variables is uncertain.
Climate researchers assume that the temperature increase caused by CO2 emissions will be around three degrees. Knutti and Hegerl found the range of uncertainty was 2-4.5 degrees, based on past research. There was a large uncertainty with regard to the maximum value: the models and calculations showed that the possibility of the earth warming up by 8-10 degrees could not be excluded.