CANCUN, Mexico: Scientists have estimated that planned cuts in global emissions will fall well short of the level necessary to cap temperature rises at 2 degrees. The shortfall, about 5 gigatonnes a year of CO2 equivalent, is equal to the emissions of all the world's cars, trucks and buses.

Even then, that outcome is dependent on all countries meeting pledges from last year's Copenhagen summit. Deeper cuts still would be required to hold temperature rises on the earth's surface to 1.5 degrees.

The statistic, issued by the UN Environment Program, underscores the monumental task facing the world and negotiators racing to reconcile the disparate needs of 192 nations meeting in Cancun, where world-renowned coral reefs have been endangered by the effects of pollution and warmer water.

The report on the deficiency of emissions cuts is the work of 30 researchers drawn from 25 centres in a variety of countries, including Australia.

Its formal release yesterday coincided with a plea from the UN Secretary-General, Ban Ki-moon, for nations to work harder to narrow their differences and reinvigorate momentum in the battle against climate change.