A third of the world's oil reserves, half of gas reserves and 80 percent of current coal reserves should not be used in the coming decades if global warming is to stay below an agreed 2 degrees Celsius target, scientists said Wednesday.

In a study published in the journal Nature, researchers said the vast majority of coal reserves in China, Russia and the United States should stay in the ground, as well as more than 260,000 million barrels of oil reserves in the Middle East, equivalent to all of Saudi Arabia's oil reserves.

The Middle East should also leave more than 60 percent of its gas reserves in the ground, the study found.