It’s always about this time of year—when the first air-sucking, clothes-wilting, soul-smothering heat wave hits a big swath of the country—that people who rarely think about climate change start to worry. Never mind that a single sweltering summer can never be traced directly to global warming. Hot weather causes even some of the noisiest climate-change deniers to start to get serious.
And it’s at that point too that the big ideas—the quick, imaginative fixes that could reverse the climate change crisis and spare us all that grown-up bother of getting off fossil fuels and switching to clean, renewable energy—start to get trotted out. Couldn’t we just plant a trillion trees? What about seeding the oceans with iron to encourage carbon-eating plankton blooms? How about putting giant mirrors into orbit to reflect some sunlight back into space?
This week, another of the perennials got a good, close look when a study by the Carnegie Institution and the Indian Institute of Science explored the idea of seeding clouds to make them whiter and more reflective—essentially the mirror idea but without the actual mirrors. The good news: It works! The bad news: You’d better like monsoons.
The study did not involve any actual cloud seeding. Instead, the investigators used a computer model and programmed it with something close to a worse-case (if not worst-case) scenario: a near-future Earth where atmospheric carbon concentrations are twice what they are today. That’s a point we’ll reach by 2070 if emissions continue to rise at present rates, and by 2100 if they’re merely stabilized, according to some projections. The investigators then adjusted the models to reduce the size of water droplets in all of the clouds over all of the Earth’s oceans. Small water droplets are what you get in white clouds that are not preparing to release rain. Gray rain clouds have large droplets. Said the Carnegie Institute’s Ken Caldeira, a co-author of the study:
Rain clouds…tend to be grey and absorb sunlight, whereas clouds with smaller droplets tend to be white and fluffy and reflect more sunlight to space. In practice this could be done by shooting a fine spray of seawater high into the air, where the tiny salt particles would create condensation nuclei to form small cloud droplets.
The researchers chose to alter only oceanic clouds for two reasons: Since most of the clouds would have dumped their rain on the water and not on the land, the fact that they’d be stopped from raining at all wouldn’t meaningfully affect world precipitation levels. What’s more, preventing sunlight from reaching the ocean as opposed to the land gives you the greatest bang for your temperature-moderating buck, since the oceans’ dark color gives them a low albedo—or reflectivity—which means they absorb heat and light especially well.
In the study, the treated clouds did reduce the overall warming effects of CO2, while reducing global precipitation by only 1.3%. But that was the worldwide average. Rainfall over the 30% of the Earth’s surface that is covered by land increased a very significant 7.5%, and most of that was concentrated in the tropics. Why? Caldeira explains:
Monsoons occur when air masses over land are warmer than air masses over the ocean, and this draws in cool, moist air from over the ocean which then drops rain over the land. Our basic result calls into question previous assumptions about the impact of this geoengineering scheme. It merits further investigation.
That, of course, is the scientist speaking, and a good scientist will always argue for further investigation since that way lies knowledge—even if it’s knowledge you wind up wishing you didn’t have. But good policymakers—let’s say, oh, Senators in Washington with a climate and energy bill idling in their chamber, for example—are required to make tougher, real world decisions that don’t involve magic bullets to make problems go away. What’s more, they can’t computer-model their yes votes to see how they play out at the polls. It’s time they just stood up and cast them.