Solar Geoengineering May Fail to Prevent Catastrophic Warming in the Long Run

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Seeding the air with aerosols might not prevent high concentrations of atmospheric carbon dioxide (CO₂) from destabilizing low-lying clouds, which could lead to increased warming.

The process of pumping aerosols into the air to reflect sunlight, which cools Earth, is one desperate technique to mitigate climate change. A new study from the California Institute of Technology (Caltech) reports, however, that solar geoengineering might fail to avoid catastrophic warming in the long run.

Solar geoengineering has gained attention as it can be achieved with present-day technology, states Tapio Schneider, Theodore Y. Wu Professor of Environmental Science and Engineering and a senior research scientist at JPL, which is managed by Caltech for NASA.

Obviously, there are governance and ethical questions about who controls Earth’s thermostat, beyond that, our research shows that solar geoengineering ultimately may not fix the problem if high greenhouse gas emissions continue for more than a century.

Tapio Schneider, Theodore Y. Wu Professor of Environmental Science and Engineering, JPL

Schneider and his collaborators Colleen M. Kaul and Kyle G. Pressel of Pacific Northwest National Laboratories in Washington used high-resolution computer simulations to demonstrate that very high concentrations of CO₂ would still disintegrate low-lying stratocumulus clouds, and this cloud destruction could activate possibly abrupt warming of about 10 °F.

A study describing their results was published in the Proceedings of the National Academy of Sciences on November 16^th, 2020.

Broadly speaking, solar geoengineering functions like this: if Earth is becoming too warm due to rising concentrations of greenhouse gases, which warm Earth by absorbing infrared radiation, Earth can be cooled by blocking some sunlight.

According to Schneider and his collaborators, while that could possibly hold in the short term, it does not consider the bigger picture of how clouds work.

Earth is cooled by the low-lying stratocumulus clouds by reflecting sunlight back into space. Moreover, they radiate infrared radiation upward from their cloud tops, thus cooling the air present in the clouds and driving it downward toward the surface of the planet. This creates a link between the clouds and their moisture supply at the surfaces of oceans on Earth.

The computer simulations developed by the researchers illustrate that the occurrence of high concentrations of greenhouse gases like CO₂ efficiently forms an infrared blanket over the clouds, which inhibits them from radiating energy upward. This can result in the disintegration of clouds, leading to strong warming—which would happen even if a portion of the incoming sunlight is blocked by geoengineering measures.

The results may also have implications for climates early in Earth’s history, when the sun was fainter yet Earth was relatively warm. But most of all, they show that by continuing to emit greenhouse gases, with or without geoengineering, humans are perturbing an incredibly complex system that may hold climate surprises for us.

Tapio Schneider, Theodore Y. Wu Professor of Environmental Science and Engineering, JPL

This study was financially supported by Eric and Wendy Schmidt upon the recommendation of the Schmidt Futures program; the Earthrise Alliance; Mountain Philanthropies; the Paul G. Allen Family Foundation; Caltech senior trustee Charles Trimble (BS '63, MS '64); and the National Science Foundation.

Journal Reference:

Schneider, T., et al. (2020) Solar geoengineering may not prevent strong warming from direct effects of CO₂ on stratocumulus cloud cover. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2003730117.

Source: https://www.caltech.edu/