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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.

Nympha Palermo Cartoon 254 by Gemma Grimaldi

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

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.

Figure caption

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.

Cloud tracks caused by ship emissions (NASA image)

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]


  1. 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.
  2. 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.
  3. Marine Cloud Brightening Project Website.
  4. 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.
  5. 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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