The Gatekeepers of Sunlight

NASA ISS024-E-15122.jpg Managing the amount of energy from the sun that enters our planet’s atmosphere may well be the only saving grace we have left from global warming. But that doesn’t mean that what researchers call solar radiation management is any less controversial in scientific and public spheres.

The Sun provides the energy that drives Earth’s climate, but not all of the energy that reaches the top of the atmosphere finds its way to the surface. That’s because aerosols—and clouds seeded by them—reflect about a quarter of the Sun’s energy back to space. (NASA)

I first heard David Keith speak about geoengineering during ‘The Science of Science Communication’ #Sackler conference held this May in Washington, D.C. (I wasn’t actually in D.C., but watched the live webcast! Very handy…)

David Keith, Gordon McKay Professor of Applied Physics at Harvard University and geoengineering expert, has been listed as one of TIME magazine’s Heroes of the Environment 2009 (article).

Geoengineering involves large-scale engineering and manipulation of Earth’s environment (Royal Society). Solar radiation management, a unique type of geoengineering, is all about adjusting the amount of sunlight that the Earth absorbs. Turning the dial down on sunlight could potentially offset climatic changes that are occurring as greenhouse gases like carbon dioxide increase in our atmosphere.

Critics of solar radiation management often object to geoengineering on the principle of distraction away from true mitigation efforts. In other words, critics worry that in taking efforts to turn down the dial on sunlight, we will forget to fix the source of the problem: overproduction of greenhouse gases. But as David Keith said during a recent ScienceLive event, “Mitigation means reducing the amount of carbon we put into the atmosphere. In the long run, putting carbon to the atmosphere increases climate risk. There’s no getting away from that even with SRM [solar radiation management].”

According to Keith, geoengineering CAN NOT save the world, but it may be an option to take seriously, as global temperatures will continue to rise long after we take significant measures to cut carbon dioxide emissions – if indeed we succeed at that before irreversible planetary changes occur.

One option proposed to ‘turn down the dial’ on sunlight involves ejecting sulfate aerosols into the atmosphere. This occurs naturally, for example, when volcanoes erupt and spew ash and sulfate aerosol-generating gases into the lower atmosphere. Traditionally thought of as air pollution, the ejection of sulfate aerosols may actually cause the atmosphere to absorb less energy from the sun. Think of artificial clouds, like the ones in this picture I look of the Raleigh-Durham airport here in the U.S. (very pretty airport indeed). If we can create more artificial clouds to block out sunlight, we may be able to slow down Earth’s currently staggering rate of global warming.

IMG_8741.JPG Could artificial clouds help protect our planet from the impacts of intense global warming?

According to NASA: “Aerosols play an important role in Earth’s climate. Most aerosols are brighter than land or ocean, and cool the Earth by reflecting sunlight back to space.”
So what would it take to see solar radiation management deployed as a way of saving Earth’s environment from the damaging effects of extremely warm global temperatures? I interviewed David Keith for this week’s From The Lab Bench blog post on geoengineering.

1) Can you describe the lay of the land (briefly) of current climate change mitigation research, and where (solar) geoengineering currently stands as an effective mitigative option? What are some of the promises AND potential pitfalls of geoengineering.

For me, mitigation means reducing emissions. Solar geoengineering, or SRM, is an entirely different option as distinct from mitigation as is adaptation. This usage of the term ‘geoengineering’ is standard but not universal in climate policy.

Mitigation research should focus on finding low-cost large-scale ways to cut emissions of carbon dioxide from the energy sector. This means research into new low-carbon energy sources and the policy measures to drive their diffusion in a cost-effective manner. Globally, there is a lot of effort in this area. Bloomberg estimates that worldwide investment in “clean energy” has risen steadily to $260billion in 2011 – that’s 0.4% of world Gross domestic product (GDP) ( But this investment does not seem to be effectively focused on low-cost high impact measures, and emissions continue to increase rapidly.

Solar geoengineering offers the potential to substantially reduce climate impacts over the next half century. At present, relatively little research has been done, so one cannot confidently say how effective SRM might be. But the reasons to take it seriously are simple: basic physics along with every climate model in which it has been tested suggest that SRM could substantially reduce the rates of climate change – not just the rate of temperature change but also the rates of change of other climate variables such as precipitation. It is therefore likely that it could substantially reduce climate change impacts.

SRM is fast, cheap and imperfect. It entails significant risks. At present I think there is a very strong case for a substantial global research program given (a) the scale of the potential benefits, and (b) the fact that early evidence suggests that both the costs and risks are comparably small.

2) What is your take on public opinion of serious climate change mitigative action and geoengineering in particular? What do you think are the key aspects to improving public approval of mitigative policies and geoengineering options?

Few people in the general public have strong opinions about these technologies. People’s opinions are not set in stone but rather depend on the way the issue evolves in the public arena. Early evidence suggests that a significant fraction of population in North America and the UK is in support of research on SRM, although this is by no means conclusive and it clearly depends on the way the question is asked (for an academic article on public perception see

3) Tell me more about solar geoengineering, and some of the most up-to-date research on solar engineering. What do you think are some of the long-term consequences (both good and bad) of pursuing this option for climate change mitigation?

The central positive consequence of pursuing this option is that it could materially reduce climate risks for people and the natural world.

In my view, the central risks are political, not technical. Put simply, our species lacks effective tools to manage the technology that gives us such large leverage over our Earth’s climate. The ability to manage climate could increase as well as decrease international tensions and might lead to an overemphasis on SRM without the appropriate efforts to reduce emissions.

4) What are the 3-5 most important things that the public should know about solar geoengineering, in order to inform their decisions on potential future geoengnineering policies?

1. We cannot improve our understanding of the way these technologies work, of their risks and potential benefits, without a substantial research program. That program could be tiny (less than 1%) compared to the cost clean energy research, but a program that starts out at the least at a few tens of millions of dollars a year globally makes sense. If early results are promising, the program will need to ramp up from there.

2. No kind of SRM can eliminate long-term climate risks unless we cut emissions of carbon dioxide towards zero.

3. In concert with a technical research program, there is a need to develop methods of managing technology like SRM, with public as well as political and scientific expert input.

Thank you David Keith.

What questions do you have about geoengineering?


1) Earth Observatory, NASA, on Aerosols:

2) Image: Image Science and Analysis Laboratory, NASA-Johnson Space Center. “The Gateway to Astronaut Photography of Earth.” <>06/24/2012 04:18:59.

David Keith (1992). A serious look at geoengineering Transactions, American Geophysical DOI: 10.1029/91EO00231