Sunblock For The Earth
October 12, 2012

Solar Management Needs Depend On Uncertain Factor Climate Sensitivity

April Flowers for - Your Universe Online

When you want to protect your skin from the harmful rays of the Sun, you employ some sort of sun block. It might be a lotion or a cream; it might even be a shirt or an umbrella to filter the effects. But what would you use to protect the entire planet from the Sun?

Apparently that is the question that an increasing number of scientists are trying to answer; ways to temporarily reduce the amount of sunlight reaching the Earth to stave off some of the worst effects of climate change. No one sees this as a permanent fix for a variety of reasons including the fact that it would only temporarily reduce temperatures, it would do nothing for the health of the oceans, and it would affect different regions unevenly. Most previous studies have examined this strategy by itself, without looking at simultaneous attempts to reduce greenhouse gas emissions.

A new study from the Department of Energy's Pacific Northwest National Laboratory has created a computer analysis of future climate change that considers emissions reductions along with a "sunblock" technology. The findings, published in the journal Climatic Change, show that such drastic steps to cool the earth would only be necessary if the planet heats up easily with added greenhouse gases. Such analysis might prove useful in helping policy makers with future climate change plans.

The research team explored sunlight reduction methods, or solar radiation management, in a computer model that followed emissions' effect on climate. Their findings show that there is a fundamental connection between the need for emissions reductions and the possible need for some sort of solar dimming.

"It's a what-if scenario analysis," said Steven Smith with the Joint Global Change Research Institute, a joint venture between PNNL and the University of Maryland. "The conditions under which policymakers might want to manage the amount of sun reaching earth depends on how sensitive the climate is to atmospheric greenhouse gases, and we just don't know that yet."

Solar radiation management is a type of geoengineering method that might include shading the earth from the sun's heat by either brightening clouds, mimicking the atmospheric cooling from volcanic eruptions or putting mirrors in space. The research team combined the solar radiation management techniques in virtual worlds with different techniques for reducing greenhouse gas emissions, which limits the amount of heat in the earth system due to greenhouse gas accumulation.

"Solar radiation management doesn't eliminate the need to reduce emissions. We do not want to dim sunlight over the long term – that doesn't address the root cause of the problem and might also have negative regional effects. This study shows that the same conditions that would call for solar radiation management also require substantial emission reductions in order to meet the climate goals set by the world community," said Smith.

How much sun blocking, or solar dimming, would be needed depends on an uncertain factor called climate sensitivity. Think about your skin and the sun again. The earth might be sensitive to carbon dioxide, like the redheaded girl at the beach who needs to slather on an SPF 50. Then again, the earth might not be that sensitive, like the person who only needs an SPF 10 to be fine in the sun.

Climate sensitivity is measured by how many degrees the atmosphere warms up if the concentration of carbon dioxide doubles.

If the climate has a medium sensitivity of about 3 degrees Celsius per doubling of carbon dioxide, said Smith, "it's less likely we'd need solar radiation management at all. We'd have time to stabilize the climate if we get going on reducing emissions. But if it's highly sensitive, say 4.5 degrees Celsius per doubling, we're going to need to use solar radiation management if we want to limit temperature changes."

The earth's temperature has already risen about 0.62 degrees Celsius since the beginning of the 20th century as the carbon dioxide in the atmosphere has grown from 290 parts per million to 379 parts per million, according to NOAA's August report.

Earth's atmosphere hasn´t reached equilibrium yet, and even if humans stopped putting more carbon dioxide into the air today, it would still take a while for the climate change to stop. It is still unknown at what temperature Earth will reach equilibrium, mostly because scientists don't know how sensitive the planet is to greenhouse gases.

When coupled with emission reductions, the amount of solar radiation management needed could be far less than the amount generally considered by researchers so far, according to the authors of the new study.

"Much of the current research has examined solar radiation management that is used as the sole means of offsetting a doubling of carbon dioxide concentrations. What we showed is that when coupled with emissions reductions, only a fraction of that amount of 'solar dimming' will be needed. This means that potential adverse impacts would be that much lower," said Smith. "This is all still in the research phase. We do not know enough about the impacts of potential solar radiation management technologies to use them at this time."

The most immediate use for this study is to help policy makers evaluate solar reduction technologies side-by-side. The team devised a metric to quantify how much solar radiation management will be needed to keep global warming under a certain temperature change threshold. This metric, called degree-years, can be used to evaluate the need for potential sunlight dimming technologies.