Poster's note : trop NaCl injection for heatwaves is a potential gateway
drug for SRM.

http://www.sciencemag.org/news/2018/03/dusting-salt-could-cool-planet


Scientists have proposed injecting salt into the upper troposphere, above
the clouds, as a form of geoengineering.

ISTOCK.COM/ALEKSANDARGEORGIEV
A dusting of salt could cool the planet

By Paul Voosen <http://www.sciencemag.org/author/paul-voosen>Mar. 21, 2018
, 12:10 PM

THE WOODLANDS, TEXAS—​A last-ditch remedy for a climate disaster might be
waiting in your kitchen. If efforts to control greenhouse gases fail,
finely powdered salt spread through the upper troposphere could hold off
the sun's rays and cool the planet, researchers reported here today at the
Lunar and Planetary Science Conference. The approach could be more benign
than other schemes for putting a temporary hold on climate change.

For several decades, scientists have suggested ways to “geoengineer” the
climate
<http://www.sciencemag.org/news/2017/01/us-should-pursue-controversial-geoengineering-research-federal-scientists-say-first>.
Several proposals call for injecting microscopic particles, called
aerosols, into the stratosphere, the quiet region of the atmosphere above
the troposphere about 18 kilometers up from the equator. There they reflect
sunlight back into space, mimicking the influence of large volcanic
eruptions that have temporarily cooled the planet in the past.

Such proposals often involve sulfates, particles that form in the
stratosphere from sulfur dioxide ejected by volcanoes, or other molecules
with high reflectivity, such as diamond dust or alumina (aluminum oxide).
But all these approaches have drawbacks, says Robert Nelson, a senior
scientist at the Planetary Science Institute who is based in Pasadena,
California. Sulfur dioxide, for example, could eat away at the ozone layer
or cause acid rain. Alumina could be even worse, Nelson says. Although it
is extremely reflective, it could embed in the lungs if inhaled and cause
chronic disease similar to silicosis. “I was raised in Pittsburgh,
[Pennsylvania,] and I remember as a child seeing black lung victims
struggling to get down the street.” Still, given the limited amount of
alumina that could be required, it’s far from certain such a health risk
would be a genuine concern.

So Nelson continued to look for other reflective compounds that might be
less hazardous to human health. In 2015, he was studying evaporated salts
on the surface of other solar system bodies such as the dwarf planet Ceres
<http://www.sciencemag.org/news/2015/12/dwarf-planet-ceres-may-harbor-clouds-water-ice>.
He soon realized that simple table salt is more reflective than alumina,
while also harmless to humans. Just as important, Nelson believes that
salt, when ground into small enough particles of the right shape and
dispersed randomly, would not block outgoing infrared heat released by
Earth, adding to its cooling effect
<https://www.hou.usra.edu/meetings/lpsc2018/pdf/1834.pdf>.

Nelson is not the first to consider salt, says Matthew Watson, a
volcanologist at the University of Bristol in the United Kingdom. Watson
led a geoengineering experiment, called the Stratospheric Particle
Injection for Climate Engineering project, that was canceled in 2012. His
group briefly considered salt for stratospheric injection, he says, but
problems popped up.

First, there's a lot of chlorine in salt, and chlorine can contribute to
destroying ozone. That alone could be enough to kill salt as a candidate,
Watson says. Few would likely welcome injecting a particle that could
reopen the ozone hole. "[This] could be a big problem," agrees David Keith,
an energy and climate scientist focused on geoengineering at Harvard
University. Salt is also highly attracted to water, and water is scant
enough in the stratosphere that injecting even limited amounts of salt
could potentially alter, for example, the formation of the realm’s wispy
clouds, to unknown effects.

Nelson hopes these concerns could be addressed by injecting salt in the
high troposphere, above the clouds but below the stratosphere. He
also plans to look more closely at salt's properties; if he can resolve
some of these questions, he'd like to see a test of the particles above a
region forecasted to experience life-threatening extreme temperatures. This
would test the science while potentially benefiting society in the short
term, he says. Such a research effort could only come after thorough
engagement with the public, Nelson adds.

But like nearly all scientists interested in geoengineering, Nelson
stresses that the strategy is no substitute for action to curb carbon
emissions. No type of solar radiation management, for example, would
prevent rising carbon dioxide from acidifying the oceans. This research
should only be done so the world can potentially buy itself some time,
Nelson says. “This would be a palliative, not a [long-term] solution.”

**Correction, 21 March, 1:50 p.m.: This story was updated to clarify that
Nelson seeks to inject in the upper troposphere, not the stratosphere, as
previously stated.*

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