Solar Geoengineering
October 25, 2021
Kitchen chemistry, such as the use of
baking soda and
baking powder, is important, but many of us have employed some kitchen
physics without realizing it. When
baking bread, it's a common practice to cover the
loaf with
aluminum foil near the end of the baking process to prevent excess
browning. The principle involved is that of
reflection, with the foil reflecting the
infrared radiation of the
electric heating elements, which are
hotter than the overall
oven temperature, away from the bread. This same heat reflection principle has been proposed as a method of
solar geoengineering to reflect
sunlight back to
space to
mitigate climate change.
A solar geoengineering themed cartoon. I wrote about how sunlight-reflecting paint can mitigate excess heating of buildings and vehicles in two previous articles,
White Roofs, March 19, 2012, and Solar Reflecting Paint, November 19, 2018 (Nympha Palermo by Gemma Grimaldi. (Click for larger image.)
Painting building rooftops white, and doing similar reflective measures for
parking lots and
roadways, are simple
ground level cooling measures that use the reflection principle. These measures are also effective against the
urban heat island effect that makes large
cities up to ten degrees
Celsius hotter than
rural areas. Other
global measures seem more
science fiction than practical.
It's been proposed that a
space sunshade,
space mirror, or
space lens could be deployed to mitigate
global warming, although there would be a high
cost involved in
launching materials for these into space. The
physical principles for a sunshade or sun reflector are obvious. Less obvious is the operating principle of a space lens.
Fresnel lenses can be made as
flat sheets with less material than conventional lenses. They have lower
imaging quality than conventional lenses, but they are ideal for such applications as focusing lenses for
lighthouses. For space-based reduction of
solar radiation, a lens would be created to diverge light to radiate around the
Earth. Since just a 1% reduction in
insolation is required, a 1000
kilometer lens at the
L1 point should cost just a few tens of billions of
dollars to construct and maintain. Of course, much prior
research is needed to develop materials that would be
robust against the extreme conditions in space that include radiation and the
solar wind.
Fresnel lens construction. Just as in a conventional lens (left), the refractive material of a Fresnel lens (right) retards the speed of light passing through it. This results in a bending of the light rays as explained by Snell's law. A flat Fresnel lens is possible if the refractive index can be varied along the plane of the sheet. Another option is a diffraction grating in which diffraction of light is used, rather than refraction, to steer a beam. (Wikimedia Commons image (modified) by Pko.)
Some ground level measures for solar geoengineering include the idea that additional reflective
Arctic ice can be created by
pumping cooler
water from the
ocean depths to its
surface. Likewise, water could be pumped out of the ocean onto the
Antarctic ice sheet. As an example of solar
bioengineering,
crops can be
genetically modified into
strains with high
albedo. A March, 2021, report by the
US National Academy of Sciences, Engineering, and Medicine, examined ground level solutions for solar geoengineering.[1] These are as follow:
Stratospheric Aerosol Injection - Increasing the number of aerosols, small reflective particles, in the stratosphere to reflect incoming sunlight.
Marine Cloud Brightening - Adding particles to the lower atmosphere to increase the reflectivity of low-lying clouds over oceanic regions.
Cirrus Cloud Thinning - Modification of high-altitude ice clouds to increase atmospheric transparency to outgoing thermal radiation.
Three methods of atmospheric solar geoengineering. (Data from a National Academies of Sciences, Engineering, and Medicine 2021 report.[1] Background image via Wikimedia Commons. Click for larger image.)
Scientists from
PARC, the Palo Alto Research Center, (Palo Alto, California) and the
University of Washington (Seattle, Washington) have reported on the
Marine Cloud Brightening Project in a recent
article in
IEEE Spectrum.[2] The Marine Cloud Brightening Project is an open, international
collaboration to explore the potential for intentionally brightening
marine low clouds to reflect sunlight by aerosol particle creation.[3]
The goals of this project are as follow.[3]
Develop spray technology capable of generating controlled sizes of sub-micrometerr seawater particles in sufficient numbers to brighten low marine clouds.
Conduct field experiments with the spray technology to provide data to accurately model the efficacy of this cloud brightening approach.
Improve existing models and develop better models of aerosol-cloud interactions and their impact on climate change.
Use observations of aerosol-cloud interactions that are already brightening low marine clouds. ship emissions are one source of aerosols that affect marine clouds.
Explore advanced analysis techniques, such as machine learning, to understand the efficacy and affects of marine cloud brightening.
A checkerboard pattern of clouds off the coast of Spain on January 16, 2018, created by ships traveling over the North Atlantic Ocean.
This image was taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Aqua satellite.
(NASA image by Jeff Schmaltz, the MODIS Land Rapid Response Team, NASA GSFC. Click for larger image.)
It's been known for
decades that aerosols emitted from ships affect low-lying
stratocumulus clouds above the oceans.[2] As in the example of the above image,
satellites have seen cloud streaks above
shipping lanes, and these reflect more sunlight than the ocean water beneath them.[2] Similar
cloud condensation nuclei are formed naturally by such processes as sea salt boosted into the air through
wave action.[2]
The focus of the Marine Cloud Brightening Project is to determine whether injecting more sea salt into the atmosphere by spraying seawater from ships can cause
planetary cooling.[2] Such seawater sprays would instantly dry in the air to form salt particles that would be transported to the cloud layer by
convection to increase cloud brightness by 5% to 10%.[2] This is a relatively low cost activity.[2] The target clouds are stratocumulus clouds at altitudes of just 1-2 kilometers.[2] It's estimated that an additional 300 to 400 particles per
cubic centimeter are needed in these clouds for the intended cooling effect.[2]
Liquid droplets from 120-400
nanometers in
diameter are needed for creation of optimally-sized dry salt
crystals by spraying seawater, which is difficult to do
efficiently.[2] Conventional
spray nozzles, in which water is forced through a narrow
orifice, produce droplet sizes of tens of micrometers to several
millimeters.[2] Other spray techniques are being investigated, including
electrospray in which
electric charge is induced in the droplets, and
Coulomb repulsion between charged droplets inhibits their
coalescence into larger droplets.[2] Electrospray technology can produce a spray having nearly all the droplets in the desired size range, and the final droplet size can be tuned via the
electric field to tens of nanometers with a tight size
distribution.[2]
Further research is required to determine whether marine cloud brightening is both
safe and efficacious.[2] Of course, the best approach to combat global warming is to curtail
greenhouse gas emissions. Since it appears that current measures to limit such emissions are not stringent enough, this method might be a way to mitigate the impact of these emissions. The project has submitted their experimental plans for review by the
U.S. National Oceanic and Atmospheric Administration.[2]
References:
- Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance, National Academies of Sciences, Engineering, and Medicine. March 25, 2021, p. 17. doi:10.17226/25762.
- Kate Murphy, Gary Cooper, Sarah Doherty, and Rob Wood, "Here's How We Could Brighten Clouds to Cool the Earth," IEEE Spectrum, September 7, 2021.
- Marine Cloud Brightening Project Website.
- J. Latham, K. Bower, T. Choularton, H. Coe, P. Connolly , G. Cooper ,T. Craft, J. Foster, A. Gadian, L. Galbraith, H. Iacovides, D. Johnston, B. Launder, B. Leslie, J. Meyer, A. Neukermans, B. Ormond, B. Parkes, P. J. Rasch, J. Rush, S. Salter, T. Stevenson, H. Wang, Q. Wang, and R. Wood, "Marine Cloud Brightening," Phil Trans Roy. Soc., vol. A370 (2012), pp. 4217-4262, doi: 10.1098/rsta.2012.0086.
- P. J. Connolly, G. B. McFiggans, and R. Wood, "Factors determining the most efficient spray distribution for marine cloud brightening," Phil. Trans. R. Soc., vol. A372 (2014), article no. 20140056, http://dx.doi.org/10.1098/rsta.2014.0056.
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