Engineering Earth 'is feasible'
A UK Royal Society study has concluded that many engineering proposals to reduce the impact of climate change are "technically possible".
Such approaches could be effective, the authors said in their report.
But they also stressed that the potential of geo-engineering should not divert governments away from their efforts to reduce carbon emissions.
Such engineering projects could either remove carbon dioxide or reflect the Sun's rays away from the planet.
Suggestions range from having giant mirrors in space, to erecting giant CO2 scrubbers that would "clean up" the air.
Ambitious as these schemes seem, the report concluded that many of them potentially had merit, and research into them should be pursued.
The authors stated, however, that some of the technology was barely formed and there were "major uncertainties regarding its effectiveness, costs and environmental impacts".
One of the technologies considered "too risky" was pouring iron filings into the ocean to grow algae which, the authors said, could cause "substantial damage" to marine life and freshwater, estuary and coastal ecosystems.
Buying time
The study stressed that engineering approaches would only have a limited impact, and that efforts should continue to be focused on reducing CO2 emissions.
"(Governments) should make increased efforts toward mitigating and adapting to climate change and in particular agreeing to global emissions reductions of at least 50% on 1990 levels by 2050 and more thereafter," the authors wrote.
But, they continued, there should be "further research and development" into geo-engineering options "to investigate whether low-risk methods can be made available if it becomes necessary to reduce the rate of warming this century".
Injecting sea salt into the clouds could cool the planet
Of the two basic geo-engineering approaches, the report concluded that those involving the removal of carbon dioxide were preferable, as they effectively return the climate system closer to its pre-industrial state.
But the authors found that many of these options were currently too expensive to implement widely.
This included "carbon capture and storage" methods, which require CO2 be captured directly from power plants and stored under the Earth's surface.
Current proposed methods also work very slowly, taking many decades to remove enough carbon dioxide to significantly reduce the rate of temperature rise.
Of the carbon removal techniques assessed, three were considered to have most potential:
1. CO2 capture from ambient air: This would be the preferred method, as it effectively reverses the cause of climate change.
2. Enhanced weathering: This aims to enhance natural reactions of CO2 from the air with rocks and minerals. It was identified as a prospective longer-term option.
3. Land use and afforestation: The report found that land-use management could and should play a small but significant role in reducing the growth of atmospheric CO2 concentrations.
CO2 capture and storage is still "too expensive", the report says
So-called solar radiation management methods do not take carbon dioxide out of the atmosphere, and, according to some climate models, may be ineffective in altering shifts in rainfall patterns and storms, the report said.
Geo-engineering and its consequences are the price we may have to pay for failure to act on climate change.
Professor John Shepherd, University of Southampton
But the authors said that the door should not be shut to the approach, which could be a faster way to reduce the rate of increase in global temperatures.
Some suggestions include: a giant mirror on the Moon; a space parasol made of superfine aluminium mesh; and a swarm of 10 trillion small mirrors launched into space one million at a time every minute for the next 30 years.
The study also said that many of these approaches had huge logistical demands, and it could take several decades for them to be implemented.
But if temperatures rose to such a level where more rapid action needed to be taken, three techniques were considered to have most potential:
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