10/23/2012 12:14:34

Reconsidering the Global Thermostat

Caltech researcher and colleagues show outcome of geoengineering can be tunable

PASADENA, Calif.—From making clouds whiter and injecting aerosols into the stratosphere, to building enormous sunshades in space, people have floated many ideas about how the planet's climate could be manipulated to counteract the effects of global warming—a concept known as geoengineering. Many of the ideas involve deflecting incoming sunlight before it has a chance to further warm the earth. Because this could affect areas of the planet inequitably, geoengineering has often raised an ethical question: Whose hand would control the global thermostat?

Now a team of researchers from the California Institute of Technology (Caltech), Harvard University, and the Carnegie Institution says there doesn't have to be just a single global control. Using computer modeling, they have shown that varying the amount of sunlight deflected away from the earth by season and by region can significantly improve the parity of the situation. The results appear in an advance online publication of the journal Nature Climate Change.

Previous geoengineering studies have typically assumed uniform deflection of sunlight everywhere on the planet. But the pattern of temperature and precipitation effects that would result from such efforts would never compensate perfectly for the complex pattern of changes that have resulted from global warming. Some areas would end up better off than others, and the climate effects are complex. For example, as the planet warms, the poles are heating up more than the tropics. However, in models where sunlight is deflected uniformly, when enough sunlight is redirected to compensate for this polar warming, the tropics end up colder than they were before man-made activities pumped excess carbon dioxide into the atmosphere.

In the new study, the researchers worked with a climate model of relatively coarse resolution. Rather than selecting one geoengineering strategy, they mimicked the desired effect of many projects by simply "turning down the sun"—decreasing the amount of sunlight reaching the planet. Instead of turning down the sun uniformly, they tailored when and where they reduced incoming sunlight, looking at 15 different combinations. In one, for example, they turned down the sun between January and March while also turning it down more at the poles than at the tropics.

"That essentially gives us 15 knobs that we can tune in order to try to minimize effects at the worst-off regions on the planet," says Doug MacMartin, a senior research associate at Caltech and lead author of the new paper. "In our model, we were able to reduce the residual climate changes (after geoengineering) in the worst-off regions by about 30 percent relative to what could be achieved using a uniform reduction in sunlight."

The group also found that by varying where and when sunlight was reduced, they needed to turn down the sun just 70 percent as much as they would in uniform reflectance to get a similar result. "Based on this work, it's at least plausible that there are ways that you could implement a geoengineering solution that would have less severe consequences, such as a reduced impact on ozone," MacMartin says.

The researchers also used the tuning approach to focus on recovering Arctic sea ice. In their model, it took five times less solar reduction than in the uniform reflectance models to recover the Arctic sea ice to the extent typical of pre-Industrial years.

"These results indicate that varying geoengineering efforts by region and over different periods of time could potentially improve the effectiveness of solar geoengineering and reduce climate impacts in at-risk areas," says Ken Caldeira of the Carnegie Institution. "For example, these approaches may be able to reverse long-term changes in the Arctic sea ice."

The group acknowledges that geoengineering ideas are untested and could come with serious consequences, such as making the skies whiter and depleting the ozone layer, not to mention the unintended consequences that tend to arise when dealing with such a complicated system as the planet. They also say that the best solution would be to reduce greenhouse gas emissions. "I'm approaching it as an engineering problem," MacMartin says. "I'm interested in whether we can come up with a better way of doing the geoengineering that minimizes the negative consequences."  

In addition to MacMartin and Caldeira, David Keith of Harvard University and Ben Kravitz, formerly of the Carnegie Institution but now at the DOE's Pacific Northwest National Lab, are also coauthors on the paper, "Management of trade-offs in geoengineering through optimal choice of non-uniform radiative forcing."

Written by Kimm Fesenmaier