Brighten the Water: Proceed with Caution

Update, April 1, 2011: Alan Robock's response to Seitz's study, which have both now been published in the April issue of Climatic Change, reflects his own opinions and not that of the journal's editors. The original version of this post incorrectly described Robock's response as “official.”


Harvard physicist Russell Seitz suggests that brightening the ocean can help reflect solar radiation. Credit: Mr.Thomas/flickr

People take comfort in having a plan B; back-up plans can help people prepare for the possibility things might not work at as expected, or hoped. Which is why the subject of geoengineering keeps popping up in discussions of future climate change.

If humans don’t dramatically lower greenhouse gas emissions and stabilize the atmospheric concentration of carbon dioxide (CO2), then manipulating the planet to keep temperatures cool could be a good backup plan. (Note: even the staunchest proponents of geoengineering say that is still the way to think of it, as a plan B only to be put into effect if emissions aren’t sufficiently minimized).

One method of geoengineering is to increase the amount of sunlight that is reflected away from the planet, a strategy known as solar radiation management (SRM). Scientists have proposed a few different ways to achieve this, including putting reflective mirrors or aerosols into the atmosphere, increasing the reflectivity of clouds, or covering large areas of the ground with white or reflective material. If more of the sun’s rays are bounced back out of the atmosphere, then the planet should absorb less heat.

Recently, another potential way to reflect sunlight has been discussed. Last spring, Harvard physicist Russell Seitz suggested that brightening the ocean could have advantages over using aerosols in the atmosphere, especially because two-thirds of the planet is covered by water. Because deep blue ocean water absorbs as much as 93 percent of incoming solar radiation, even slight brightening can change how much light is reflected away. By increasing the number of microscopic bubbles at the ocean’s surface, Seitz says, the water’s reflectivity will increase. More light will be scattered away and the planet could be kept cooler.

Seitz’s findings, published in Climatic Change in December 2010, says that this approach to SRM has the advantage of being more localized than spraying aerosols into the atmosphere, so it is less risky to humans. He proposes that teeny tiny bubbles can be easily injected into the water, at low cost and without requiring much energy. Sounds great, you might think.

Not so fast, says Alan Robock.

As an atmospheric scientist at Rutgers University, Robock has been studying the potential effects of different types of SRM on the planet. A few days ago, he posted an official response to Seitz’s “brighten the water” pitch, also in Climatic Change. In it, Robock points out that although increasing reflectivity of ocean water takes away some of the risks that come along with pumping sulfate aerosols into the air, it also introduces a host of new ones — risks that can’t be fully evaluated or easily managed.

For example, Robock points out that if parts of the ocean are engineered to be more reflective and consequently absorb less heat, the movement of heat in the ocean may change, and there is the potential that regional climate and weather would be affected.

Just think about how much global climate has been influenced this year by the cool ocean waters of La Niña in the Pacific: increased rainfall in Asia, Australia and parts of South America; heavier snowfall in parts of the U.S.; and more severe drought in the American Southwest.

Affecting the way oceans absorb and retain heat may impact crucial ocean currents, causing unusual weather in some places. Credit: NOAA

Furthermore, cooler ocean temperatures and less sunlight is bound to have some influence on marine life, although it is difficult to predict exactly what would happen. Colder water also holds more CO2, so Robock mentions that there might be more risk of ocean acidification using Seitz’s technique.

Robock writes:

“Bubble generation in the ocean should be added to the list of solar radiation management options being considered as part of geoengineering, but it needs to be rigorously evaluated, [along with marine cloud brightening and stratospheric aerosol generation], in terms of its costs, benefits, and risks.”

Ultimately, Seitz’s and Robock’s comments seem to boil down to a similar discussion that circle around almost all types of geoengineering. Brightening the water is sure to have some risks — but without large-scale testing, there is no way to assess how large the risks are. Which means it is also unclear whether using these microscopic bubbles will be more helpful than harmful in the future. So much for a plan B.