Tim Lenton, BBC Green Room 4 Mar 09;
Climate change is a massive problem that needs big and bold solutions, says Professor Tim Lenton. In this week's Green Room, he outlines the reasons why "geo-engineering" projects, such as reflecting sunlight back into space, could help win the battle against dangerous climate change.
The climate is undoubtedly changing, and it is changing faster than many scientists thought it would, especially in the Arctic.
Regardless of the ineffectual Kyoto Protocol, carbon dioxide emissions from human activities increased by 3% per year during 2000-2006.
Even if we can globally get our act together and reduce CO2 emissions by 80% by 2050, we are still heading for at least a 2C (3.6F) warmer world.
This may be too much for elements of the climate system, including the Arctic sea ice and Greenland ice sheet, which could pass a tipping point on the way there.
The resulting climate change may well be "dangerous"; and if so, mitigation alone cannot avoid it.
But reducing CO2 emissions is not the only way. There are two "geo-engineering" approaches that could complement it: reflecting more sunlight back to space, or actively removing CO2 from the atmosphere.
Both aim to cool the planet; but one tackles the symptoms (higher temperatures) while the other, like mitigation, addresses the underlying cause (elevated CO2 concentrations).
But do these approaches really offer a silver bullet to solve the climate problem?
From mirrors in space to reflective roofs on our homes, reducing the amount of sunlight the Earth absorbs could counteract (to varying degrees) the extra heat radiation trapped by the increasing greenhouse effect.
For a given option, the strength of the cooling effect is determined by the change in reflectivity applied, the area it is applied over, and the altitude of application.
Air-cooled
Among the most potent options is injecting tiny sulphur particles into the stratosphere to scatter more sunlight.
In principle, this could counteract the projected warming resulting from the future concentrations of atmospheric CO2 up to at least twice the pre-industrial level.
Such options might be developed within decades, and if they are deployed, their cooling effect would be very fast acting.
However, the cooling effect will also be short-lived, so if activities start on a global scale (which is necessary for them to be effective), there will need to be a commitment to maintain them for many centuries.
Stopping the activity would result in dangerously rapid climate warming, far worse than the steadier warming they were designed to counteract.
In effect, such technologies imply an unprecedented duration and level of international co-operation to maintain them.
Who today is willing to commit future generations to collectively controlling the planetary thermostat?
There are also undesirable side effects of this kind of medicine.
We know from past volcanic eruptions that reducing incoming sunlight weakens the water cycle, promoting drought in regions including India and the Sahel in northern Africa.
Growing attraction
So, what about the other geo-engineering approach?
From planting trees or fertilising the ocean to chemical "scrubbing", there are several ways of creating carbon "sinks" to remove CO2 from the air.
In general, these options act more slowly and progressively than those that reflect sunlight.
CO2 removal activity has to be ramped up and maintained for several decades (and in some cases centuries) for it to have a significant effect on atmospheric CO2 and climate.
For it to be truly effective, the carbon must be transferred to a long-lived reservoir such as charcoal in soil, or geological storage for liquid CO2.
The proposals vary greatly in their potential effectiveness, which of course also depends on the area or scale of application.
Perhaps the most potent option, on the century timescale at least, is to grow plants to get CO2 out of the air and then convert their biomass to both charcoal and (bio)fuels.
The charcoal would be added to soil as "biochar" and the fuels used for combustion, but ideally with capture of the CO2 and transfer to geological reserves.
The attraction of such an approach is that it produces energy and heat as well as agricultural benefits, and might (according to some estimates) generate revenue.
Most, if not all, other geo-engineering schemes will cost money and thus rely on willingness to pay.
The climate side-effects of creating carbon sinks are generally less than for sunlight reflection options, but the socio-economic side effects may be considerable.
Follow the money
So where do we go next? Many fear that even discussing geo-engineering options undermines mitigation efforts.
But mitigation alone may not be able to avoid dangerous climate change, so we must consider what other activities could complement it.
There is no simple silver bullet among the geo-engineering options, but some could make an invaluable contribution.
Potent and rapidly deployable sunlight reflection options could be held in reserve as an emergency response should we get some early warning of approaching tipping points.
The creation of significant CO2 sinks is just as valid as reducing the sources of the greenhouse gas, because the atmosphere cannot tell the difference.
Together, they give the best chance to stabilise atmospheric CO2 and ultimately reduce it.
But economic assessment of the various geo-engineering options is badly needed, because cost will likely determine which, if any, are deployed.
Ultimately climate change is a problem of risk management. There is already a substantial risk of dangerous impacts, even if we do start meaningful global mitigation of CO2 emissions.
Given this context we must weigh up the balance of risks of using, or not using, geo-engineering.
Professor Tim Lenton is an Earth system scientist based in the School of Environmental Sciences, University of East Anglia, Norwich, UK
The Green Room is a series of opinion articles on environmental topics running weekly on the BBC News website
