Though he had serious reservations about geoengineering, he decided
to explore the idea. In 2009, he and a
colleague published a paper suggesting that seeding cirrus clouds with tiny
particles of bismuth tri-iodide, an inorganic compound that may break down
into the necessary sub-micrometer size,
might substantially o;set climate change.
More recently, Mitchell estimated that it
would take around 160 tons of the material annually to seed clouds in the areas he
has in mind, at a cost of about $6 million.
Not everyone agrees the proposal
would work. A 2013 paper in Science, led
by MIT atmospheric scientist Dan Cziczo,
concluded that the formation of ice crystals around dust, known as heterogeneous
ice nucleation, is already the dominant
mechanism creating cirrus clouds. That
might mean adding more dust would, on
balance, create thicker clouds that trap
more heat. The larger problem with the
idea, Cziczo argues, is that clouds are the
least understood part of the climate system. We do not have nearly enough knowledge about cloud microphysics, or accurate
enough measurements, to precisely
manipulate climate in this way, he says.
But Mitchell’s most recent research,
relying on observations of ice crystal concentrations from NASA’s Calipso satellite, has further convinced him that cloud
seeding could work, as long as it’s done in
regions where cirrus clouds form primarily
without dust particles. On the monitor in
his o;ce, Mitchell pulls up a page of maps
from a paper he presented at the National
Center for Atmospheric Research in late
February. Navy- and light-blue dots, representing Cziczo’s heterogeneous clouds,
dominate the mid-latitudes, covering
much of South America and Africa. But
the higher latitudes are covered in red, yellow, orange, and green dots that indicate
the sorts of clouds Mitchell has in mind.
The satellite images suggest that in
very cold and humid conditions, toward
the poles and particularly during winter,
tiny ice crystals can form on their own,
spontaneously, without dust. That suggests that cloud seeding could work, if
it’s targeted to those areas during those
months. Mitchell even thinks he’s come
up with a way to get nature to carry out a
field experiment to test his theory. During
spring and winter, strong winds regularly
stir up major dust storms in the deserts of
Mongolia and the western edge of China.
The fine particles blow across the Pacific
and run into an atmospheric wave that
rolls over the Rocky Mountains.
If Mitchell is correct, the dust should
promote thinner cirrus clouds in an area
where the thicker type otherwise tends
to dominate. There was no way to properly observe this phenomenon—until
late last year, when the National Oceanic and Atmospheric Administration
launched a satellite equipped with some
of the most powerful imaging technology ever launched into space, as well as
sensors that can measure the temperatures of clouds. The satellite should be
able to capture exactly what happens as
the dust rides over the Rockies, detecting the subtle shifts under way in cloud
Mitchell submitted a research proposal to NOAA last year, asking the
agency to use the satellite to make such
observations. He knows it’s a long shot,
particularly in light of the Trump administration’s e;orts to slash funding for climate science. But if NOAA agrees, the
test could lend weight to his theory—or,
of course, contradict it.
Another outdoor geoengineering
experiment should occur even sooner.
By this time next year, Harvard professors David Keith and Frank Keutsch hope
to launch a high-altitude balloon from a
site in Tucson, Arizona. This will mark the
beginning of a research project to explore
the feasibility and risks of an approach
known as solar radiation management.
The basic idea is that spraying materials
into the stratosphere could help reflect
more heat back into space, mimicking a
natural cooling phenomenon that occurs
after volcanoes blast tens of millions of
tons of sulfur dioxide into the sky (see
“A Cheap and Easy Plan to Stop Global
Warming,” January/February 2013).
Scientists generally believe the technique would ease temperatures, but a lingering question is: what else will it do?
Notably, volcanic eruptions have also significantly altered rainfall patterns in certain areas, and sulfur dioxide is known to
deplete the protective ozone layer.
Keith has done extensive climate modeling to explore whether other materials,
including alumina, diamond dust, and
calcium carbonate, might have a neutral
or even positive impact on ozone. During a conversation in his office at Harvard, he stressed that the experiments
wouldn’t constitute a test of geoengineering itself. But they would allow his group
to subject its models to real-world data,
revealing more about the relevant stratospheric physics and chemistry. “Theory
alone doesn’t tell you what will happen in
the atmosphere,” Keith says. “You can fool
yourself if you don’t go out and make direct
kind of drastic
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