The Aerosol Silhouette in Geoengineering: Both Problem and Opportunity

Holding a hand near the projection screen casts a well-defined shadow. Moving the hand back toward the projector makes the shadow indistinct: the edges are too fuzzy. Were thumb and forefinger creating a silhouette with a hole, it would become difficult to discern the hole.

There is a similar difference between dimming the sun from millions of miles away and doing it in the stratosphere close to the target.

Uniformity of aerosol application is difficult to accomplish—and maintain—via sun-scattering aerosols in the upper atmosphere. This means that the cooling will tend to be as patchy. What happens with patchy cooling from this aerosol silhouette?

Temperature differences create density differences—and thus winds are generated between the air column of the cooling silhouette and uncooled neighboring regions. At many altitudes, more dense air will flow sideways out of the silhouette column. A patchy silhouette could rearrange the generally eastward track of storm systems, though simulations are needed to estimate how sensitive the storm tracks are to this.

Weather disruption might be avoided by doing the uneven scattering at some distance toward the sun, though that does nothing for the shadowed nighttime side of the globe with its warmer nights. And even a uniform application, pole to pole, will create an ever-moving stirring along the terminator that is not the natural one.

There is, however, a circumstance where patchy cooling from stratospheric aerosols might be of use—say, for cooling a region with out-of-control forest fires. There is now a need to break up the blocking highs that stall eastbound weather systems to cause three-day downpours, prolonged heat waves, and facilitate the prolonged arctic outbreaks into the subtropics.

In the same manner that a long barrier island can be cut in half by waves, one can imagine an aerosol application scenario whose changing cooling silhouette temporarily creates a channel for the stalled storm systems to pass through the ridge of high pressure.

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William H. Calvin is a professor emeritus at the University of Washington’s medical school in Seattle and the author of Global Fever: How to Treat Climate Change  (University of Chicago Press, 2008). The latest version of the CO2 cleanup was a finalist in MIT's 2013 geoengineering climate contest.

November 2014    WCalvin@UW.edu

The Emergency Cleanup of Excess CO2 via a Second Manhattan Project

Briefly: Pull up sunken nutrients to create CO₂-capturing plankton blooms, then push new organic carbon into the ocean depths before it reverts to CO₂. This push-pull pump avoids many of the problems of up-only pumps for fertilizing the ocean surface.

Suppose our CO₂-based overheating became an emergency via abruptly rearranging the winds—say, a supersized El Nino that doesn’t quit? More emissions reduction would be too little, too late—nor would it fix ocean acidification.

   An emergency drawdown of atmospheric CO₂ would address all three issues—but it would need to be big, quick, and sure-fire.

   How big? Aim at removing all 350+ GtC emitted since 1750.

   How quickly? We must back out of the danger zone before being weakened by resource wars and economic collapse. During a 20 yr project period, another 250 GtC are likely be emitted from business-as-usual, so make that goal 600 GtC. That's 30 GtC/yr. Once the drawdown is complete, half of the sequestration capacity might still be needed to continuously counter out-of-control emissions from developing countries; the rest goes on standby for future emergencies.

   Sure to work the first time? With no second chance, our initiative needs to be sure-fire, since we must avoid the human population crash that a global economic collapse would trigger.

   Most candidates suitable for long-run improvements will be too small, too slow, or too uncertain for an emergency. Even fertilizing the ocean surface enough to settle out 30 GtC/yr of the usual debris into the depths would require an unachievable 3x increase in ocean productivity worldwide.

The proposed push-pull pump plantations need less than 1% of the ocean surface. Pump up nutrients from the depths to enhance plankton production (what winter winds do)—but with an essential addition.

Simultaneously, emulate the natural downwellings of eddies and whirlpools. Pump down the carbon-enriched surface waters within a week, before the new organic carbon reverts to CO₂. This also sinks the 240x larger amounts of organic carbon from feces and decomposition, which are dissolved in surface waters. This sinks far more organic carbon than is needed to offset any upwelled CO2.

A plankton plantation that uses windmill power. 

Wave-powered pumps should be more economical.

   Just as farmers grow a nitrogen-fixing crop of legumes and then plow it under, we would be growing a carbon-fixing crop of plankton and then pumping it under.

   This simplified sketch shows the ballpark in which we are forced to play. Charge the experts gathered for the Second Manhattan Project with deploying this or something equally big, quick, and sure-fire within four years using wartime priorities. If nothing major intervenes in the following ten years, the climate threat might be cut in half.

This latest version of the CO2 cleanup was a finalist in MIT's 2013 geoengineering climate contest.

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William H. Calvin is a professor emeritus at the University of Washington’s medical school in Seattle and the author of Global Fever: How to Treat Climate Change  (University of Chicago Press, 2008). 

September 2014    WCalvin@UW.edu      faculty.washington.edu/wcalvin