By Gerard Wynn - Analysis
LONDON (Reuters) - New research, to be published in the journal Climatic Change in November, suggests humankind may have to remove carbon dioxide (CO2) from the atmosphere on a vast scale if emissions keep rising after 2020.
The series of articles provide scenarios which will form the basis of the next report by the U.N.'s Intergovernmental Panel on Climate Change (IPCC) in 2013 and 2014.
At present emissions levels, in less than 20 years the sky would effectively be full, meaning every extra tonne of carbon dioxide (CO2) emitted would have to be removed to stay within safer climate limits, one lead author says.
That so-called "negative emissions" approach, where excess carbon dioxide is removed from the atmosphere, is a less radical step than direct manipulation of the climate, called geo-engineering, which includes blocking sunlight using artificial clouds or mirrors in space.
Both approaches are getting more serious consideration, reflecting concern at rising emissions and a target held by world governments to keep average warming below 2 degrees Celsius compared with pre-industrial levels.
Some scientists say that the 2 degrees limit is too arbitrary and not proven to link to dangerous weather events.
It was calculated partly as a threshold beyond which Greenland ice sheets may melt irreversibly, adding seven meters to world sea levels over centuries.
"If we want to stay below 2 degrees and possibly achieve 1.5 in the 22nd century then we're not going to get around these negative emissions," said one lead author, Malte Meinshausen, of Germany's Potsdam Institute for Climate Impact Research.
"This is a crucial change in perception, that there is a point and it is very close at which time if we put CO2 into the atmosphere future generations will have to take it out again."
Sharp emissions cuts now could avoid or would delay that moment to later this century.
In its next report, the IPCC will for the first time estimate what the world must do to have a likely chance of keeping long-term warming below 2 degrees Celsius. Temperatures have already risen about 0.8 degrees Celsius since pre-industrial times.
Meinshausen's study calculates that the world would have to halt rises in global greenhouse gas emissions within five years.
By 2070, humans should have a net output of minus 3.5 billion tonnes CO2 annually to reduce temperature rises further below 2 degrees in the long-term and so slow sea level rise.
Researchers say that allowing emissions to continue to rise after 2020 would involve passing 2 degrees as early as mid-century.
After that, the only way back would be CO2 removal from the atmosphere on a massive scale -- a net output of minus 18 billion tonnes of CO2 annually during the next century and for about 100 years, they calculated in the new series of studies.
That compares with actual emissions of 33 billion tonnes CO2 last year from burning fossil fuels. Emissions have risen 2-3 percent per year over the last several decades.
The view that extreme steps are needed is therefore becoming more accepted.
"If we really are going to avoid more than 2 degrees of warming, we're either looking at geo-engineering in the sense of sun shields in space, or negative emissions type of geo-engineering in the second half of this century," said Oxford University climate scientist Myles Allen.
"That's increasingly where the thinking is."
Technologies which drive negative CO2 emissions include burning plant matter called biomass and trapping the resulting carbon emissions and burying these underground.
That achieves net negative emissions because the plants themselves absorbed CO2 from the air. But the idea only exists in the lab and pilot projects.
Other techniques could include crushing limestone, which absorbs CO2, but appears improbable because of the vast quantities of rock to be quarried. Engineers have also suggested using artificial photosynthesis to mimic plants.
Instead of mopping up CO2, an alternative geo-engineering approach is to screen out sunlight, for example, by spraying sulphur into the upper atmosphere. This causes water droplets to form and create hazy clouds and is to be trialed by British engineers next month.
The problem is a threat of unforeseen consequences.
"It's not the same as just rewinding things back to where we were in terms of greenhouse gases. You're doing another change which will potentially bring the temperature back but could lead to less rainfall," said Reading University's Peter Stott.
Some climate scientists are alarmed by how far predictions have been borne out or exceeded since the last IPCC report.
Other experts say it isn't clear how far specific changes are the result of emissions or simply natural effects.
"There's no final decision," said the Potsdam Institute's Vladimir Petoukhov.
For example, last week it emerged that Arctic sea ice this summer melted to a record low extent, or a close second. Natural weather effects partly explained the previous record in 2007, scientists say, and may help explain this year's, said Petoukhov.
In other climate changes, a study last week found rapidly rising temperatures in the northeast Atlantic Ocean driving major shifts in fish stocks.
And scientists say they can now detect a human fingerprint on trends in global rainfall.
"What's clear is that the changes do seem to be happening and consistent with the projections," said Reading's Stott.
"That's indicating that the climate is already changing, not just the global temperatures but the rainfall patterns. Then we're getting to things that actually affect people."
(Reporting by Gerard Wynn; editing by Jason Neely)