Keeping global temperatures from rising more than a few degrees above an agreed upon threshold is unlikely to occur at the rate the world is currently producing greenhouse gases, scientists said Wednesday.

The European Union and countries in other regions have set a goal of keeping global temperatures from rising more than two degrees Celsius above pre-industrial levels, but two papers to be published this week in the journal Nature say that goal may be difficult for policy-makers to achieve.

In one paper, University of Oxford researcher Myles Allen and colleagues say that a rise in global temperatures of about two degrees Celsius by 2050 would most likely come about if the total cumulative carbon introduced into the atmosphere totalled about one trillion tonnes, or the equivalent of 3.7 trillion tonnes of carbon dioxide, about half of which has already been emitted.

They suggest any targets on limiting global warming must take into account the cumulative effect of carbon emissions on the planet.

The second group of researchers, using different methods, found that the chances of the temperature rise staying below two degrees Celsius are less than 50-50 if total greenhouse gas emissions from 2000 to 2050 is the equivalent of about 1,400 million tonnes of CO2. Since emissions over the last seven years totalled about 250 million tonnes of CO2, the researchers suggest there's a greater than 50 per cent chance the world will warm more than two degrees Celsius if emissions stay "business as usual."

The researchers note that keeping mean global temperatures below two degrees Celsius is no guarantee that some of the anticipated effects of global warming, such as droughts in deserts and flooding in low-lying regions, will be avoided. But the difficulty in keeping to the limit serves as a guide to the scale of the problem.

The bottom line is that avoiding dangerous change to climate will be difficult, said NASA space scientist Gavin Schmidt and University of Chicago researcher David Archer in a related commentary in Nature.

"Unless emissions begin to decline very soon, severe disruption to the climate system will entail expensive adaptation measures and may eventually require cleaning up the mess by actively removing CO2 from the atmosphere," they wrote.

"Like an oil spill or groundwater contamination, it will probably be cheaper in the long run to avoid making the mess in the first place."