Long marginalized as a dubious idea, altering the climate through 'geoengineering' has staged something of a comeback. Oliver Morton reports.
In the first week of June 1991,
Michael MacCracken, a climate physicist from Lawrence Livermore National
At the same time half a world away,
something like 20 million tonnes of sulphur dioxide dissolved in searingly hot
magma a few kilometres underneath the Philippines was preparing to show him and
his audience how it's done.
The day after the conference ended, the first of that magma emerged from the crater of
The Pinatubo particles cooled the
Earth more or less exactly in line with the figures that MacCracken had offered
at the meeting for the effects of 'artificial volcanoes' any technology for
injecting sulphur high into the atmosphere. Had there not been a simultaneous
El Niρo, 1992 would have been 0.7 degrees cooler, worldwide, than 1991. And
this demonstration of cooling power took place at a crucial time. The first
report of the Intergovernmental Panel on Climate Change (IPCC) warning of
greenhouse warming came out the year before Pinatubo; the UN Framework
Convention on Climate Change was opened to signatures while its aerosols were
still enlivening the skies. In a world awakening to the prospect of global
warming, you might have expected such an object lesson in global cooling to
sharpen the debate over artificial volcanoes of the sort that MacCracken had
First cut is the deepest
But things went the other way. Once global warming started to be seen as real and important, climate scientists shied away from such speculation, preferring to hammer home the message that greenhouse-gas emissions had to be cut quickly and deeply. 'Geoengineering' the climate through artificial modifications was seen as a dangerous distraction from the business of slashing emissions. In the decade and a half that followed Pinatubo, talk of geoengineering went into eclipse. From 1995 to 2005, more research went into technological responses to asteroids that might one day endanger the Earth than into direct responses against the sunlight already heating the planet.
Much of the climate community still views the idea with deep suspicion or outright hostility. Geoengineering, many say, is a way to feed society's addiction to fossil fuels. "It's like a junkie figuring out new ways of stealing from his children," says Meinrat Andreae, an atmospheric scientist at the Max Planck Institute for Chemistry in
This new interest in geoengineering was set off by an article by Andreae's friend and colleague Paul Crutzen, published in the journal Climatic Change in August 2006. The article contained relatively little that wasn't already in the literature when Pinatubo blew its top, but it had a major impact because of who was saying it. "In this case, the messenger is the message," says Stephen Schneider, a climate scientist at
Pollution to save the world
If the identity of the author was striking, so too was the matter-of-fact way that he chose to frame the issue. Mankind, Crutzen pointed out, already puts more than 100 million tonnes of sulphur dioxide into the atmosphere every year the equivalent of at least five Pinatubos. Unfortunately, the aerosols that this sulphur produces sit in the lower atmosphere, the part we breathe, and they do us no good; they are estimated to contribute to 500,000 premature deaths every year. But clearing away this pollution has the unintended consequence of increasing the rate of global warming, because even in the lower atmosphere the sulphates stop sunlight from reaching the surface. Crutzen looked at the idea of introducing one or two million tonnes of sulphur into the stratosphere every year, where it could produce a long-lived aerosol, as a way to keep the protective effects while getting rid of the short-lived aerosols in the lower atmosphere.
At both the beginning and end of his article, Crutzen stressed that he would rather see global warming controlled by a reduction in emissions. But he admitted that, so far, he saw little cause for optimism. He also pointed out that sulphate aerosols can act to cool the climate immediately; reducing emissions, on the other hand, takes decades or generations. If something really bad starts to happen, aerosols could provide a prompt cooling response in a way that emissions control simply could not.
On hearing of Crutzen's paper, Tom Wigley, a veteran climate scientist at the
A little geoengineering might make an equivalent objective a lot more achievable, Wigley argued. Imagine an aerosol effort that starts fairly soon and is quickly ramped up to a Pinatubo's worth of sulphates being injected into the upper atmosphere every two years, before being phased out completely after 80 years. The resulting cooling effect would allow carbon dioxide emissions to keep climbing for a few more decades without the world warming any more than if they levelled immediately. In Wigley's model the peak level of atmospheric carbon dioxide could climb to well over 500 parts per million without the Earth's temperature getting any higher than it would with stabilization at the much-harder-to-obtain 450 parts per million. Emissions would still have to be cut very steeply from the middle of the century on. But for Wigley, those extra decades of room to manoeuvre are all important.
Realms of the unknown
If a burst of sulphates might allow the world to postpone the effects of emissions control for a few decades, would a consistent effort allow the world to do without control altogether? Wigley points to at least one reason why not. Carbon dioxide does more than just warm it also acidifies the ocean. Even if the warming effects of ever-increasing carbon dioxide could be cancelled out, the effects on corals, shellfish and eventually the entire marine food web would still be disastrous. And even the most vigorous proponents of geoengineering do not suggest that it can defer any need to reduce emissions indefinitely. "If you are digging a hole and want out of it, certainly slowing your digging rate is good," says Gregory Benford, an astrophysicist at the
Even a strictly term-limited scheme
has potential pitfalls. Wigley's model deals only with average global
temperatures, and there is much more to the climate than that. For decades,
climate scientists dubious about geoengineering schemes have pointed out that
the pattern of warming expected from carbon dioxide, and the pattern of cooling
expected from aerosols, would differ in both space and time. Aerosols cool
things only when the Sun is shining, and they cool things most where the Sun
shines brightest. They thus cool only in the day and more in summer and the
tropics. Greenhouse gases warm things night and day, and their effect is
greater at the poles. The two factors could thus cancel each other out in terms
of global average, while fundamentally changing the way that the climate works
region by region.
In 2000, Ken Caldeira then of the
The result surprised Caldeira, who had undertaken the research in part to show a colleague, Lowell Wood, that geoengineering was more complex than Wood imagined. Wood is a forceful spokesman for extreme ideas, most notoriously the proposed X-ray laser that was to have formed the cornerstone of Ronald Reagan's Star Wars programme. In the 1990s, he had become enamoured of radiation management, as had his mentor, Edward Teller,
Caldeira had wanted to show that the
world was more complex than simple physics suggested. His results, though,
edged things the other way, making geoengineering look more plausible, rather
than less. Perhaps as a result, they were hardly followed up at all. Only six
years later, under the influence of the Crutzen paper, are other researchers
with GCMs starting to look at radiation management. Last month, for instance,
Wigley's colleague Phil Rasch unveiled some preliminary results in a seminar at
Caldeira, too, while stressing that he is not an advocate of moving ahead with geoengineering, has recently revisited the topic using a different GCM to the one he used in 2000. He finds similar results, with somewhat larger shifts in precipitation than in temperature. His new work also suggests that natural sinks for carbon might expand in a geoengineered world. With more carbon dioxide, plants are more productive and thus suck up more carbon dioxide. In a greenhouse world, this tendency is counterbalanced by the effect of temperature increases on the respiration of soil microbes warmer microbes produce more carbon dioxide. But in a greenhoused-and-cooled world, the plant effect remains while the respiration effect is capped, and so significantly more carbon dioxide gets used up.
Climate modellers at NASA's Goddard Institute for Space Studies in
The very thing that motivates people like Crutzen to study geoengineering the risk of large surprises that require immediate action leads others to see the whole idea as fundamentally unworkable. Although models agree that the world will warm and climatic patterns will change as carbon dioxide rises, they don't agree on the amount of warming or the patterns of change. Indeed, that uncertainty is one of the reasons that climate change is such a difficult issue. "How can you engineer a system whose behaviour you don't understand?" asks Ronald Prinn, a climate scientist at the Massachusetts Institute of Technology in
One answer to this question is "as carefully and reversibly as you can". Caldeira and MacCracken have now joined Wood and Benford to investigate a radiation-management proposal aimed at the
Caldeira and his colleagues reason that cooling the
But even modest, local geoengineering could have disproportionate effects far away. Alan Robock and his colleagues at
The fact that that is what seems to have happened in the past does not necessarily mean that it would happen in a geoengineered future. But it is easily argued that betting the monsoon on the ability of models to accurately capture such subtleties would require a foolhardy level of trust, a remarkable lack of concern for hundreds of millions of livelihoods or a startling desperation in the face of the alternative.
One source of such problems is the
fact that the stratosphere is not just a sheet of glass to be tinted at will.
It is a circulating system in which physics and chemistry interact; it is tied
to the troposphere below in complex ways that greenhouse warming is already
changing; and aerosols warm it or cool it in different ways depending on the
size of the particles involved. True, compared with most other components in
Earth's system it is relatively simple. (For a start, nothing lives there.) But
it still has its subtleties.
A tempting way around this problem is to put the sunblock even higher in orbit, where among other things it can be turned off at will. Discussions of orbital sunshades have been around almost as long as those of artificial volcanoes. The most technically sophisticated was published by Roger Angel of the
Up and away
Angel was looking for a way to put up a sunshade that, unlike earlier proposals, did not require humans in orbit or the resources to be found on the Moon or nearby asteroids. His solution was to use a fleet of almost-transparent 'fliers', the size of dustbin lids, that would be launched from Earth in prepacked stacks by means of a vast electromagnetic cannon. Once in orbit, the gossamer-thin fliers would peel off these stacks and arrange themselves in orbits that keep them between the Earth and the Sun at almost all times. The shadow of this cloud of spacecraft 1.85 million kilometres away, Angel calculated, would be a little larger than the Earth, and would cut down sunlight by about 1.8%. The details of Angel's proposal are meticulously worked out, and their cost is suitably astronomical about $5 trillion, or a decade's worth of
Setting the standard
Nevertheless, Ralph Cicerone, a climate scientist and president of the US National Academy of Sciences, singles the paper out for praise for the painstakingly careful way it was done. "He went back to it again and again," Cicerone says. "In its standard of elegance and completeness it was exemplary." For him and many others, such academic excellence is the main point of publishing research on geoengineering. For these researchers, the aim is not to find feasible solutions but to do good science that provides a standard against which to judge the less good, or flatly foolish, schemes that might otherwise accrete around the idea. Cicerone points to quack schemes for ozone replacement in the 1980s as the sort of thing that needs to be forestalled: back then, he says, "poor ideas got as far as they did because of [the community's] silence."
Cicerone says he would welcome a body of work on geoengineering that is substantial enough to deserve a chapter of its own in the next IPCC assessment report, due in about six years. At the same time, he favours a moratorium on any moves towards deploying such a system, and agrees with the consensus of the climate community that much greater efforts towards mitigation of emissions remain the highest priority. After all, no one thinks that, in the short term, a world cooled by engineering would be preferable to one cooled by a reduction in carbon dioxide levels. And no one thinks that, as yet, we know enough to embark on any sort of large-scale engineering. Models of geoengineering's benefits need to be a lot more accurate than models of the harm that will be done in its absence. As Caldeira puts it, if you can be no more precise about the chances of harm under the status quo than to give them as 50%, that's still something to worry about. But if a proposed intervention has a 50-50 chance of doing good or harm, that's something to avoid.
A few voices argue that it is too late for this thinking that we are already engineering nature by exerting a vast influence over the nitrogen cycle, the carbon cycle, the radiative balance of the atmosphere and everything else. In this sense we live in an engineered world, and the question is simply how to engineer it better. But in the scientific community this argument has achieved little traction. The key point, articulated by climate scientist David Keith from the
That is why economist and philosopher Herbert Simon famously grouped it with the social and some of the human sciences under the rubric of 'the sciences of the artificial', a category created as a deliberate counterpart to the intention- and imperative-free natural sciences.
Although in the past two decades climate
scientists have been confronted with the social, technological and economic
implications of their work, they are not scientists of the artificial. Hans
Feichter, a climate modeller at the Max Planck Institute for Meteorology in
In the past year, climate scientists have shown new willingness to study the pathways by which the Earth might be deliberately changed, although many will do so in large part simply to show, with authority, that all such paths are dead-end streets. But they are not willing to abandon the realm of natural science, and commit themselves to an artificial Earth.
The original version of this story said that Ralph Cicerone been awarded the Nobel Prize. It was Sherwood Rowland who shared that Nobel with Paul Crutzen and Mario Molina.
Oliver Morton is Nature's chief news and features editor.
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