The Middle East must leave 40 percent 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 Wednesday.

Globally, a third of oil, half of gas and over 80 percent 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 $670 billion (565 billion euros) last year searching for and developing new fossil fuel resources," said McGlade's colleague and co-author Paul Ekins.

"They will need to rethink such substantial budgets if policies are implemented to support the 2 C limit, especially as new discoveries cannot lead to increased aggregate production."

For their project, McGlade and Ekins made a best estimate of quantities and locations of oil, gas and coal resources, and then assessed how much could be safely used, per region, up to 2050.

They used two scenarios -- one with "widespread deployment" of carbon capture and storage (CCS) facilities, a technology in its infancy, and one with no CCS.

Even if CCS is widely implemented, over 430 billion barrels of oil must be left unburned, the pair said.

Under this scenario, the Middle East has over half the world's "unburnable" oil -- more than 260 billion barrels or 38 percent of its stock -- equal to about eight years of global production calculated at 2013 levels of 87 million barrels per day.

Central and South America must leave 58 billion barrels alone, Canada 39 billion and the former Soviet states 27 billion.

About 95 trillion cubic metres (3,354 trillion cubic feet) of gas should remain unburned, the study found -- again the bulk (46 trillion cubic metres) is held in the Middle East and 31 trillion by former Soviet states.

- Winners and losers -

Countries with large stocks of coal, including developing nations that rely heavily on the resource to fuel their fast growth, also face challenges.

China and India would have to leave nearly 70 percent of their reserves under a scenario with CCS, and Africa almost 90 percent, according to the findings.

Among developed nations, Europe would have to leave 78 percent of its coal and the United States 92 percent.

"These results demonstrate that a stark transformation in our understanding of fossil-fuel availability is necessary," said the study.

"Although there have previously been fears over the scarcity of fossil fuels, in a climate-constrained world this is no longer a relevant concern: large portions of the reserve base... should not be produced if the temperature rise is to remain below 2 C."

Experts Michael Jakob and Jerome Hilaire, in an analysis published with the study, said the uneven distribution of "unburnable" reserves highlighted challenges to finding a carbon-cutting pact.

"Only a global climate agreement that compensates losers and is perceived as equitable by all participants can impose strict limits on the use of fossil fuels in the long term," they wrote.