The letter:

https://climate-intervention-research-letter.org/

The letter’s authors argue for research on tiny reflective particles injected in the atmosphere to reduce global warming, including small scale outdoor experiments.  We agree this should be a high priority.

Reflective Earth is a 501(c)(3) non-profit with a mission to promote research into methods for increasing Earth’s reflectivity to reduce global warming and to promote those methods where the benefits outweigh the risks, including consideration of costs. We do not yet know if injecting tiny particles to the atmosphere will be cost-beneficial or will outweigh the risks. However, there are other methods of Solar Radiation Management where the risks are low, and they should be pursued now when there are enough side benefits to make them cost-beneficial.

The letter’s authors argue that the state of scientific knowledge about Solar Radiation Management is currently insufficient for it to be included as part of a climate credit system or other commercial offering.  This statement is overbroad and therefore wrong.  While the statement may be true for injecting particles in the atmosphere, it is not true for methods of increasing solar reflectivity of surfaces at ground level.

For example, many water reservoirs and canals can benefit from covers to reduce evaporation and provide shade to minimize unwanted algae blooms to improve water quality and reduce maintenance.  Considering only these benefits, such covers are cost-beneficial for only a few reservoirs or canals.  However, subsidizing placement and maintenance of reflective covers that reduce evaporation and reduce bio growth will also reflect large amounts of solar energy out to space where that energy would otherwise be absorbed.  Such a subsidy will make it cost-beneficial to put such covers on many more reservoirs and canals.  The subsidies can most effectively be marshalled by selling reflection carbon offset credits.  

For example, the Egyptian government has been studying the feasibility of covering 2000 square mile Lake Nasser to reduce evaporation.  Covering 60% of the lake, which has a reflectivity potential of 186 watts per square meter,^[1] for a reflectivity gain from about .1 to .85 would reflect about 435 gigawatts.^[2]  This would be worth encouraging with a subsidy from reflection carbon offset credits. Covering reservoirs with floating solar photovoltaic panels may be a better use for many reservoirs near urban areas or hydropower dams,^[3] but even if all these opportunities are pursued, there will still be large reservoirs and canals where a reflective cover is the best option.

[1]  https://www.reflectiveearth.org/map?&latitude=22.458775409072004&longitude=31.76027956554867&zoom=6.977216106481924&bearing=0&pitch=0

[2] 5,250 km2 = 5.2 x 10^9 m2 x .75 = 3.9 x 10^9 m2 x 186 w/m2 x .6 = 725 x 10^9 = 435gigawatts

[3] https://www.nature.com/articles/d41586-022-01525-1

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