This week the Royal Society published a special edition of its journal, Philosophical Transactions, dedicated to "geo-engineering" interventions to combat global warming. Its initiative deserves to be welcomed, not rejected out of hand. The time may come when we need to geo-engineer in order to maintain our planet in a livable state.
Doug Parr, Greenpeace UK's chief scientist, made the case against: we should muster serious political will, and equally serious finance, to reduce greenhouse gas emissions. Using existing and proven clean technologies from wind turbines to concentrated solar power, we need to bring about a worldwide renewable energy revolution. If we do the above, he implied, we will not need any "outlandish" or "outright dangerous" geo-engineering solutions.
He is right in everything he is calling for. In fact, he could have gone further. We need major investments in energy efficiency and conservation, as well as in renewables. We need to bring an end to deforestation and rebuild ravaged forest ecosystems. We also need an agricultural revolution in which farmers draw down excess carbon from the atmosphere into soils, enhancing their ability to retain moisture and nutrients as well as mitigating global warming.
But even if we do all the above, can we be sure of preventing climate catastrophe? No. The Earth's climate system is characterised by feedback loops which can amplify even a small initial perturbation. And it seems that following an initial post-industrial warming of 0.8C, one major positive feedback process is already well under way, in the Arctic.
Last year saw a record melting of Arctic sea ice. This year, that record has been broken: for the first time in history, the northern ice cap can be circumnavigated. And with melting ice, more sunshine is absorbed rather than reflected back into space. The result is more warming, and more melting. In turn this increases the degassing of methane from Arctic bogs, lakes and thawing permafrost - and methane is a powerful greenhouse gas in its own right, 70 times stronger than CO2 over 20 years.
If we rapidly cut our emissions of greenhouse gases, it might bring an end to the "Arctic amplifier". Or it might not. It is entirely possible that the melting of the sea ice and the emissions of Arctic methane have already reached a point of no return that will lead to a warming world no matter what we do. It would be imprudent not to insure ourselves against this possibility.
This means setting up a global research programme into geo-engineering options. The most valuable options are those that will have immediate effect by directly altering the Earth's thermal balance. Two proposals stand out. First, the introduction of sulphate aerosol to the stratosphere to reflect sunlight. There are fears that this could damage the ozone layer, but then we know that volcanoes routinely discharge millions of tonnes of sulphate into the stratosphere, cooling the Earth without inflicting long-term harm.
James Lovelock, who developed the Gaia hypothesis, warns of global acidification as the stratospheric sulphate mist slowly falls to the ground. However this problem is easily solved. We already emit 50m tonnes of sulphur a year, mainly as sulphate from power stations and shipping. This is 10 times more than would be used for geo-engineering purposes, and that could be more than offset by additional cuts in emissions.
Better still is the proposal by physicist John Latham and colleagues to raise the reflectivity of marine clouds. This would involve a fleet of wind-powered yachts criss-crossing the world's oceans, controlled by a global network of satellites, blowing out a mist of ultra-fine salty droplets to act as cloud condensation nuclei. Latham's solution promises to be inexpensive, highly effective, environmentally benign, and reversible in a matter of days as the droplets are washed from the sky in rain.
One major question is that of who should be responsible for any interventions. The best-placed body is the UN climate convention (UNFCCC), whose major achievement to date is the Kyoto protocol. Its main objective, to "prevent dangerous anthropogenic interference with the climate system", by stabilising greenhouse gas concentrations, should be extended to accommodate the possibility of geo-engineering.
This would also provide clarity as to the purpose of geo-engineering. Doug Parr decries it as an "expression of political despair" that would only produce business-as-usual emissions. Lovelock sees it as a temporary palliative before inevitable disaster strikes. We must make sure they are both wrong. Geo-engineering should be developed strictly as a firefighting capability to maintain long-term climatic stability, not as a substitute for all the other actions we should be taking.
If the topic enters into discussions at the UNFCCC meeting in Poznan this December, member states must ensure that the two approaches are firmly linked so that no geo-engineering is permitted unless accompanied by deep and rapid cuts in emissions.
Oliver Tickell, the author of Kyoto2.