The Middle East must leave 40 per cent of its oil reserves in the ground, and China, the US and Russia most of their coal if global warming is to be curbed, researchers said today.

Globally, a third of oil, half of gas and over 80 per cent of coal reserves must be left untouched until 2050, according to a novel breakdown of "unburnable" fossil-fuel stocks published in the journal Nature.

This is the sole way to meet the UN target of limiting warming to 2.0 degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, said co-author Christophe McGlade of University College London.

"Policymakers must realise that their instincts to completely use the fossil fuels within their countries are wholly incompatible with their commitments to the 2 C goal," he warned.

The UN's Intergovernmental Panel on Climate Change estimates that for a good chance of meeting the target, further emissions of Earth-warming greenhouse gases must be limited to about 1,000 billion tonnes (gigatonnes).

But global fossil fuel reserves, on current estimates, would emit three times this total if used, said the study.

UN members are negotiating a global carbon-cutting climate pact to be signed in Paris in December.

At current rates, emissions are steering the world towards potentially catastrophic warming of 4 C or more, scientists say.

"Companies spent over USD 670 billion last year searching for and developing new fossil fuel resources," said McGlade's colleague and co-author Paul Ekins.