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reduce the carbon dioxide in the atmosphere, and elevated levels
of that gas have consequences beyond raising the temperature.
One is that the ocean absorbs more carbon dioxide and becomes
more acidic as a result. That harms shellfish and some forms of
plankton, a key source of food for fish and whales. The fishing
industry could be devastated. What's more, carbon dioxide lev-
els will continue to rise if we don't address them directly, so any
sunlight-reducing technology would have to be continually ratch-
eted up to compensate for their warming e ects.
And if the geoengineering had to stop---say, for environmental or
economic reasons---the higher levels of greenhouse gases would cause
an abrupt warm-up. "Even if the geoengineering worked perfectly,"
says Raymond Pierrehumbert, a professor of geophysical sciences
at the University of Chicago, "you're still in the situation where the
whole planet is just one global war or depression away from being
hit with maybe a hundred years' worth of global warming in under
a decade, which is certainly catastrophic. Geoengineering, if it were
carried out, would put the earth in an extremely precarious state."
Figuring out the consequences of various geoengineering plans and
developing strategies to make them safer and more e ective will
take years, or even decades, of research. "For every dollar we spend
figuring out how to actually do geoengineering," says Schrag, "we
need to be spending 10 dollars learning what the impacts will be."
To begin with, scientists aren't even sure that sulfates deliv-
ered over the course of decades, rather than in one short volcanic
blast, will work to cool the planet down. One key question is how
microscopic particles interact in the stratosphere. It's possible
that sulfate particles added repeatedly to the same area over time
would clump together. If that happened, the particles could start
to interact with longer-wave radiation than just the wavelengths
of electromagnetic energy in visible light. This would trap some
of the heat that naturally escapes into space, causing a net heating
e ect rather than a cooling e ect. Or the larger particles could fall
out of the sky before they had a chance to deflect the sun's heat. To
study such phenomena, David Keith, the director of the Energy
and Environmental Systems Group at the University of Calgary,
envisions experiments in which a plane would spray a gas at low
vapor pressure over an area of 100 square kilometers. The gas would
condense into particles in the stratosphere, and the plane would fly
back through the particle cloud to take measurements. Systemati-
cally altering the size of the particles, the quantity of particles in
a given area, the timing of their release, and other variables could
reveal key details about their microscale interactions.
Yet even if the behavior of sulfate particles can be understood
and managed, it's far from clear how injecting them into the strato-
sphere would a ect vast, complex climate systems. So far, most
models have been crude; only recently, for example, did they start
taking into account the movement of ice and ocean currents. Sul-
fates would cool the planet during the day, but they'd make no
di erence when the sun isn't shining. As a result, nights would
probably be warmer relative to days, but scientists have done lit-
tle to model this e ect and study how it could a ect ecosystems.
"Similarly, you could a ect the seasons," Schrag says: the sulfates
would lower temperatures less during the winter (when there's
less daylight) and more during the summer. And scientists have
done little to understand how stratospheric circulation patterns
would change with the addition of sulfates, or precisely how any
of these things could a ect where and when we might experience
droughts, floods, and other disasters.
If scientists could learn more about the e ects of sulfates in the
stratosphere, it could raise the intriguing possibility of "smart"
geoengineering, Schrag says. Volcanic eruptions are crude tools,
releasing a lot of sulfur in the course of a few days, and all from
one location. But geoengineers could choose exactly where to send
sulfates into the stratosphere, as well as when and how fast.
"So far we're thinking about a very simplistic thing," Schrag says.
"We're talking about injecting stu in the stratosphere in a uniform
way." The e ects that have been predicted so far, however, aren't
evenly distributed. Changes in evaporation, for example, could
be devastating if they caused droughts on land, but if less rain
falls over the ocean, it's not such a big deal. By taking advantage
of stratospheric circulation patterns and seasonal variations in
weather, it might be possible to limit the most damaging conse-
quences. "You can pulse injections," he says. "You could build smart
systems that might cancel out some of those negative e ects."
Rather than intentionally polluting the stratosphere, a di er-
ent and potentially less risky approach to geoengineering is to pull
carbon dioxide out of the air. But the necessary technology would
be challenging to develop and put in place on large scale.
In his 10th-floor lab in the Manhattan neighborhood of Morn-
ingside Heights, Klaus Lackner, a professor of geophysics in the
Department of Earth and Environmental Engineering at Colum-
bia University, is experimenting with a material that chemically
binds to carbon dioxide in the air and then, when doused in water,
releases the gas in a concentrated form that can easily be captured.
The work is at an early stage. Lackner's carbon-capture devices
look like misshapen test-tube brushes; they have to be hand dipped
in water, and it's hard to quickly seal them into the improvised
chamber used to measure the carbon dioxide they release. But he
envisions automated systems---millions of them, each the size of
a small cabin---scattered over the countryside near geologic reser-
voirs that could store the gases they capture. A system based on
Scientists explain different geoengineering methods:
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