Geoengineering. It’s not the sexiest sounding topic, but a small group of scientists say it just might be able to save the world.

The basic idea behind geonengineering (or climate engineering) is that humans can artificially moderate the Earth's climate allowing us to control temperature, thereby avoiding the negative impacts of climate change. There are a number of methods suggested to achieve this scientific wizardry, including placing huge reflectors in space or using aerosols to reduce the amount of carbon in the air.

It's a hugely controversial theory. One of the main counter-arguments is that promoting a manmade solution to climate change will lead to inertia around other efforts to reduce human impact. But the popularity of geoengineering is on the rise among some scientists and even received a nod from the IPCC in its recent climate change report.

In a fast-flowing and sometimes heated head-to-head climate professors David Keith and Mike Hulme set out the for and against. Keith, a geoengineering advocate, doesn't believe that this science is a solve-all but says "it could significantly reduce climate impacts to vulnerable people and ecosystems over the next half century." While Hulme sets out his stall in no uncertain terms: "Solar climate engineering is a flawed idea seeking an illusory solution to the wrong problem."

Enjoy the debate and do add your comments at the end.

David Keith: Gordon McKay professor of applied physics (SEAS) and professor of public policy at Harvard Kennedy School

Deliberately adding one pollutant to temporarily counter another is a brutally ugly technical fix, yet that is the essence of the suggestion that sulphur be injected into the stratosphere to limit the damage caused by the carbon we've pumped into the air.

I take solar geoengineering seriously because evidence from atmospheric physics, climate models, and observations strongly suggest that it could significantly reduce climate impacts to vulnerable people and ecosystems over the next half century.

The strongest arguments against solar geoengineering seem to be the fear that it is a partial fix that will encourage us to slacken our efforts to cut carbon emissions. This is moral confusion. It is our responsibility to limit the impact that our cheap energy has on our grandchildren independently of the choices we make about temporary solar geoengineering.

Were we faced with a one-time choice between making a total commitment to a geoengineering program to offset all warming and abandoning geoengineering forever, I would choose abandonment. But this is not the choice we face. Our choice is between the status quo—with almost no organized research on the subject—and commitment to a serious research program that will develop the capability to geoengineer, improve understanding of the technology's risks and benefits, and open up the research community to dilute the geo-clique. Given this choice, I choose research; and if that research supports geoengineering's early promise, I would then choose gradual deployment.

Mike Hulme: professor of climate and culture in the School of Social Science & Public Policy at King's College London

David, your ambition to significantly reduce future climate impacts is one of course we can share along with many others. But I am mystified by your faith that solar climate engineering is an effective way of achieving this. More direct and assured methods would be to invest in climate adaptation measures—a short-term gain—and to invest in new clean energy technologies—a long-term gain.

My main argument against solar engineering is not the moral hazard argument you refer to. It is twofold. First, all evidence to date—from computer simulations and from the analogies of explosive volcanic eruptions—is that deliberately injecting sulphur into the stratosphere will further destabilize regional climates. It may reduce globally-averaged warming, but that is not what causes climate damage. It is regional weather that does that—droughts in the US, floods in Pakistan, typhoons in Philippines. Solar climate engineering in short is a zero-sum game: some will win, some will lose.

Which leads me to my second argument. The technology is ungovernable. Even the gradual deployment you propose will have repercussions for all nations, all peoples, and all species. All of these affected agents therefore need representation in any decisions made and over any regulatory bodies established. But given the lamentable state in which the conventional UN climate negotiations linger on, I find it hard to envisage any scenario in which the world's nations will agree to a thermostat in the sky.

Solar climate engineering is a flawed idea seeking an illusory solution to the wrong problem.

DK - You are correct that climate impacts are ultimately felt at the local scale as changes in soil moisture, precipitation, or similar quantities. No one feels the global average temperature. Precisely because of this concern my group has studied regional responses to geoengineering.

In the first quantitative look at the effectiveness of solar geoengineering we found—to our surprise—that it can reduce changes in both temperature and precipitation on a region-by-region basis. This work has now been replicated by much larger study using a whole set of climate models led by Alan Robock, one of the more skeptical scientist working on the topic, and they got the same result. While there are claims in the popular press that it will "destabilize regional climates"—presumably meaning that it will increase local variability—I know of no scientific paper that backs this up.

I have no faith in geoengineering. I have some faith in empirical science and reasoned argument. It's true that we don't have mechanisms for legitimate governance of this technology. Indeed in the worse case this technology could lead to large-scale conflict. This exactly why I and others have started efforts to engage policy makers from around the world to begin working on the problem.

MH - David, The point here is how much faith we can place in climate models to discern these types of regional changes. As the recent report from the UN's Intergovernmental Panel on Climate Change has shown, at sub-continental scales state-of-the-art climate models do not robustly simulate the effects of greenhouse gas accumulation on climate.

What you are claiming then is that we can rely upon these same models to be able to ascertain accurately the additional effects of sulphur loading of the stratosphere. Frankly, I would not bet a dollar on such results, let alone the fate of millions.

You may say that this is exactly why we need more research—bigger and better climate models. I've been around the climate research scene long enough to remember 30 years of such claims. Are we to wait another 30 years? What we can be sure about is that once additional pollutants are injected into the skies, the real climate will not behave like the model climate at scales that matter for people.

As for getting political scientists to research new governance mechanisms for the global thermostat - you again place more faith in human rationality than I. We have had more than 20 years of a real-world experiment into global climate governance: it's called the UN Framework Convention on Climate Change. It's hardly been a roaring success! You must be a supreme optimist to then expect a novel system of global governance can be invented and sustained over the time periods necessary for solar climate engineering to be effective.

DK: You made a very strong claim that geoengineering is zero-sum. If true, I would oppose any further work on the technology. I responded that results from all climate models strongly suggest that this is not the case. Your response was to dismiss climate models. Assume for the moment that climate models tell us nothing about regional climate response, on what then do you base your claim that solar geoengineering is zero sum - that is, that it just shuffles winners and losers?

When climate skeptics rubbish models, I defend science by agreeing if all we had was complex models I too would be a doubter; but, I then argue, that we base our conclusions on a breadth of evidence from basic physics and a vast range of observations to simple—auditable—models as well as the full-blow three dimensional climate models. Models of atmospheric circulation and aerosols developed for earth make good predictions of the climates of other planets. This is a triumph of science.

The same science that shows us that carbon dioxide will change the climate shows that scattering a bit more sunlight will reduce that climate change. How you do you accept one and reject the other?

On the other points: I am not excited by an endless round of climate model improvements nor do it think that political scientist will solve this. We need less theory and more empiricism.

MH: David, I agree that we need less theory and more empiricism. This is one of the reasons why I am skeptical that climate models are able to reveal confidently what will happen to regional climates—especially precipitation—once sulphur is pumped into the stratosphere.

I don't dismiss climate models, but I discriminate between what they are good for and what they are less good for. Having spent nearly half of my professional life studying their ability to simulate regional and local rainfall—by comparing simulations against observations, empiricism if you will—I have little faith in their skill at the regional and local scales.

But let's assume for a moment that climate models were reliable at these scales. Another argument against intentional solar climate engineering is that it will introduce another reason for antagonism between nations. There are those who claim that their models are good enough to precisely attribute specific local meteorological extremes—and ensuing human damages—to greenhouse gas emissions. There will be nations who will want to claim that any damaging weather extreme following sulphur injection was aerosol-caused rather than natural- or greenhouse gas-caused. The potential for liability and counter-liability claims between nations is endless.

I am against solar climate engineering not because some violation of nature's integrity - the argument used by some. I am against it because my reading of scientific evidence and of collective human governance capabilities suggests to me that the risks of implementation greatly outweigh any benefits. There are surer ways of reducing the dangers of climate change.

This debate originally appeared on the Guardian Sustainable Business blog.

Can Geoengineering Save the Planet?

Jan 15, 2014