The world needs a Plan B on climate change because politicians are failing to reduce carbon emissions, according to a UN report.
It warns governments if they overshoot their short-term carbon targets they will have to cut CO2 even faster in the second half of the century to keep climate change manageable.
If they fail again, they will have to suck CO2 out of the atmosphere.
This could be achieved by burning wood and capturing the CO2 emissions.
The gas could then be stored in rocks underground.
But a leaked draft of the UN report also says that the technology for carbon dioxide removal is untested at such a scale.
The authors warn that carbon removal systems may encounter resistance from the public - and if the policy goes wrong, it could damage forests and ecosystems.
The final draft report to the Intergovernmental Panel on Climate Change (IPCC) adopts a new tone of realism in the face of repeated failures by governments to meet their rhetoric on climate change with action.
It warns that governments are set to crash through the global CO2 safety threshold by 2030. Humans have tripled CO2 emissions since 1970, it says - and emissions have been accelerating rather than slowing.
The experts advise governments that it will be cheaper overall to cut the greenhouse gas before 2030 if they want to hold emissions at 430-480ppm CO2 - a level that's calculated to bring a 66% chance of staying within a desired 2C threshold of warming by the end of the century.
These are not recommendations - the IPCC isn't allowed to make them - but they are an acknowledgement that many countries appear to lack the will or the ability to cut emissions.
A Greenpeace spokesman said: "This new report captures the choices we face. It's not too late; we can still avoid the worst impacts of global warming but only if the clean energy technologies that can slash carbon pollution are given the green light.
"The more we wait, the more it will cost. The sooner we act, the cheaper it will be."
But Bob Ward, from the LSE's Grantham Institute, said it was crucial to reach safe levels by 2100.
"We are in a much worse situation politically than we were seven years ago," he said.
"The current lack of action means that we may have to consider overshoot scenarios, which would be better than abandoning our temperature target threshold of 2 degrees. Some people think there's a degree of political dishonesty in allowing governments to claim they will keep to their targets in the short term."
The report says emissions are running at the high end of projections. Concentrations of CO2 in the atmosphere are likely to break the 450ppm threshold by 2030. It adds that current pledges by governments made at climate summits in Copenhagen and Cancun currently exceed this cautionary limit.
The share of clean energy sources needs to triple or even quadruple by 2050, relative to 2010, the final draft explains. Delaying emissions reductions beyond 2030 will increase the challenge of bringing down CO2 to a safe level by the end of the century.
The report will be discussed by government representatives and experts through the week.
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