Next week's landmark UN climate science report is likely to set a figure on the amount of fossil fuels that can be burnt if global warming is to be limited to safe levels.
It could warn that less than a quarter of the planet's oil, coal and gas reserves can be consumed between now and 2050, and set a worldwide "carbon emission budget."
Previous studies have indicated that the world has already used a third of this budget it needs to stay within to avoid warming of more than 2°C. Scientists say rises about this level are likely to be dangerous.
"For the first time ever scientists will say to governments there is a budget of greenhouse gases that you can emit over the next century, beyond which you cannot go if you want to have a reasonable change of avoiding 2°C," said Bob Ward, Policy and Communications chief at the Grantham Institute of the London School of Economics.