Geoengineering to Block Sunlight Cooling Effects Could Prove Difficult to Measure

Even if the world started souring the skies to block sunlight any cooling effects would be hard to detect

Ash cloud of Pinatubo during 1991 eruption.
Image courtesy of Wikimedia Commons/D. Harlow

Imagine talking to a friend in a crowded bar, while music blares and the surrounding din nearly drowns out your conversation.

That experience of trying to track and follow the conversational thread is what statisticians would call a signal to noise problem.

The signal is your friend's voice. The noise is everything keeping you from hearing it.

This problem, of course, is not limited to a happy-hour conversation. In the field of climate science, researchers are finding that the practice of geoengineering is likely to have some significant signal to noise problems, as well.

"There's the saying: If you can't measure it, you can't manage it. If we are trying to manage the climate system to control it and engineer it, we have to be able to measure what we are doing to it," said Dian Seidel, a climate scientist at the National Oceanic and Atmospheric Administration's Air Resources Laboratory.

Geoengineering is the idea of making changes to the Earth in order to counterbalance the effects of climate change.

A popular example of this is called solar radiation management (SRM). This is the practice of making the Earth more reflective, so that more of the sun's radiation bounces away instead of warming the planet.

In an ideal SRM geoengineering scenario, even as humans warm the Earth by releasing increasing amounts of heat-trapping gases, that warmth would be counterbalanced, since more heat-causing radiation would also be reflected.

Actual geoengineering, however, is a long way from reality. One step on the way toward moving the idea forward involves the ability for geoengineers to know if their efforts are working, and to what degree.

NOAA's Seidel is the lead author of a perspective paper published yesterday in Nature Climate Change that examines just how difficult it would be to detect these effects.

Because of the signal to noise problem, said Seidel, the impacts of solar radiation management can be hard to see.

"I think the main message is that it would be very challenging to detect either a short-term experiment or a long-term deployment of solar radiation management unless it was a pretty large signal," she said.

Clues hiding amid natural variability
Scientists use a NASA satellite called CERES to measure how much radiation enters and exits the Earth.

In theory, CERES would be able to measure the changes caused by geoengineering to, say, make clouds more reflective.

The problem, Seidel explained, is that the Earth's reflectivity (also known as albedo), and thus the radiation coming off of it, can vary a lot.

The globe gets more reflective when snow is on the ground, as it is now across much of the East Coast, and less when that snow melts away.

Reflectivity changes with the seasons. It changes as clouds, which are reflective, form and dissipate.

When trying to measure geoengineering effects, researchers can control for some of that variability, which is cyclical -- sort of like turning the music volume down in the bar.

But just as the noise from other bar patrons can still drown out a conversation even if the music is muted, researchers are unable to control for other things that affect reflectivity, such as widespread snowfall, or the effect an El Niño year might have on cloud formation.

Because of this, knowing whether the radiation changes CERES measures come from geoengineering or some other effect proves difficult -- unless, that is, the engineering effort is so big it drowns out the noise of natural variability.

This poses a particular challenge for the early stages of geoengineering. It would make sense to first geoengineer a small part of the globe and see if it worked, and if there were any unanticipated effects.

That's even harder to detect, according to Seidel's calculations. "It turns out that the smaller the region, the higher the noise," she said.

Seidel said there are other ways to measure if geoengineering is having an effect. Researchers could engineer clouds and use instruments to detect changes in them.

However, even if they could measure those changes, geoengineers still would not be able to know how much of an effect their efforts were having on the Earth's reflectivity, she pointed out.

Although a number of groups have investigated the potential for geoengineering, researchers continue to find problems with the concept, and former Vice President Al Gore has called the idea "insane."

Other recent research on geoengineering using solar radiation management has also found that if the practice did begin but was then stopped, it could lead to rapid climate change with potentially hazardous consequences (ClimateWire, Nov. 27, 2013).

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