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A March 2021 report from the National Academies of Sciences, Engineering, and Medicine (NASEM) brought renewed attention to a long-simmering question among scientists, policy makers, ethicists, and others: Should researchers pursue approaches to “solar engineering” as a way to temper the effects of climate change and, if so, what research strategies would be most appropriate? Solar engineering refers to an array of theorized but largely untested approaches to cooling the Earth, either by adding small reflective particles to the upper atmosphere or by influencing Earth’s cloud cover. It is one of two major categories of “geoengineering”—the large-scale manipulation of the planetary environment to counteract the impacts of climate change—essentials of which are described below. The NASEM report concluded that the United States should indeed create a focused research program on this topic. But it also emphasized the many scientific, social, and political risks of any such efforts and called for ongoing attention to more widely accepted—and more likely effective—ways of countering climate change, primarily by reducing emissions of Earth-warming greenhouse gasses.

What it is

  • Royal Society’s definition: “Deliberate, large-scale manipulation of the planetary environment to counteract anthropogenic [human-generated] climate change” (generally synonymous with “climate engineering,” “climate intervention,” or—somewhat pejoratively—“climate hacking”).

  • Geoengineering has been proposed by some as a supplement to the two current approaches to moderating the impacts of climate change: mitigation (reducing greenhouse gas buildup) and adaptation (adjusting to changing conditions).

How it might work

  • Two general classes of geoengineering have been proposed: removal of carbon dioxide (CO2) from the atmosphere, and modifying the atmosphere to allow the escape to space of more of the thermal radiation emitted by Earth.
  • Possible approaches to removing CO2: chemical capture and underground storage of CO2; increasing forest cover to absorb CO2 through photosynthesis; fertilization of oceans with iron or by other means to spur growth of photosynthetic marine organisms.
  • Possible approaches to reducing the amount of solar absorption relative to the amount of radiation emitted: inject reflective aerosols into the stratosphere; use sea spray or other approaches to increase marine cloud brightness and reflectivity; thin cirrus clouds to reduce their heat-trapping capacity; develop space-based reflectors.

Potential benefits and risks

  • Every approach has its own mix of potential benefits and risks. For example, solar radiation management with stratospheric aerosols does not directly reduce CO2 levels so would not by itself prevent ongoing acidification of oceans caused by the conversion of ocean-absorbed CO2 to carbonic acid – an important consequence of current greenhouse gas buildup; also, chemical models have suggested that sulfur-based aerosols (the most commonly proposed type) could harm the Earth’s protective ozone layer.
  • No geoengineering experiments have been conducted on a scale large enough to have a measurable impact on climate. As a result, and because the climate system is so complex, there remains considerable uncertainty about the overall benefits or costs of various approaches, or—in the case of solar radiation management—the potential risks, such as unexpected or unwanted local meteorological impacts.
  • Predictions are based on three indirect sources of data: small-scale experiments; observations of “experiments of nature” such as volcanic eruptions, which can block solar radiation; and computer models that attempt to take into account the very large number of biological, physical, and chemical variables that would influence outcomes.

What if?

  • One widely agreed upon prediction: any significant unilateral effort to manage solar radiation would likely raise geopolitical tensions, since no internationally agreed-upon system of governance exists that defines acceptable such research or activity.
  • The Convention on Biological Diversity, ratified by all United Nations member states except the United States, allows for small-scale geoengineering research studies in controlled settings but calls for a ban on larger activities likely to affect biodiversity, pending an “adequate scientific basis on which to justify such activities” and “appropriate consideration of associated risks.”
  • Several other international agreements have relevance to geoengineering, including the United Nations Framework Convention on Climate Change, which specifically calls for commitments to enhance removal of CO2 from the atmosphere.

What it would take

  • Current technologies and deployment capacities cannot produce the amounts of solar radiation management or CO2 absorption needed to have a well-managed, short-term impact on climate change, though it is possible that such capacities could be developed in time to make a difference in the long term.
  • Cost estimates vary considerably, but preliminary analyses have concluded that well-designed solar radiation management systems could operate on budgets ranging from hundreds of millions of dollars (for regional efforts) to tens of billions of dollars (for global efforts) per year.

What’s next

Some scientists have proposed what they call a stratospheric controlled perturbation experiment (SCoPEx), a small-scale balloon-based experiment that would gather preliminary data on one approach to solar engineering. As proposed, the experiment would break not only scientific ground but procedural and governance ground as well, as it would voluntarily undergo an independent risk assessment and a public approval process before moving forward. A test flight to assess technical issues but without actually attempting to alter solar radiation is currently planned for June 2021.

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  1. Benefits, risks, and costs of stratospheric geoengineering, 2009 (Geophysical Research Letters) offers a detailed summary of potential approaches, benefits, risks, and costs of geoengineering.

  2. Sulfur injections for a cooler planet, 2017 (Science magazine) concluded that under one scenario, climate normalization would require annual atmospheric injections of the amount of sulfur spewed by the 1991 eruption of Mount Pinatubo, for 160 years. That volume would require thousands of flights per day, would require currently nonexistent technology, and would cost about $20 billion a year.

  3. Climate Intervention Reports, 2015 (National Academy of Sciences) concluded that “Climate intervention is no substitute for reductions in carbon dioxide emissions and adaptation efforts aimed at reducing the negative consequences of climate change.” The report acknowledged geoengineering’s potential to help in the long run, but warned that considerable research is needed to better assess risks and costs.

  4. Towards a comprehensive climate impacts assessment of solar geoengineering, 2016 (Earth’s Future; American Geophysical Union) offers a comprehensive climate impacts assessment of solar geoengineering. Related articles can be found here:

  5. Eastern Pacific Emitted Aerosol Cloud Experiment, 2013 (Bulletin of the American Meteorological Society) describes the small-scale, 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) that involved cloud “brightening” through the use of smoke generators on ocean-going ships, concluding that 15% brightening of clouds by aerosol-generators on ships on their trans-oceanic voyages could render those ships carbon-neutral.

  6. Two thorough fact sheets from the Institute for Advanced Sustainability Studies in Potsdam, Germany: Carbon Dioxide Removal; Solar Radiation Management.

  7. Stratospheric controlled perturbation experiment: a small-scale experiment to improve understanding of the risks of solar engineering, 2014 (Philosophical Transactions of the Royal Society A) describes the proposed SCoPEx study to examine risks to the Earth’s protective ozone layer posed by certain geoengineering approaches.

  8. Code of Conduct for Responsible Geoengineering Research was published in October 2017 by the Geoengineering Research Governance Project at the University of Calgary, Canada. It is a proposed code of conduct designed as a voluntary instrument but is based on existing international law and policies, and was released as an interim document meant to stimulate discussion about potential regulatory and governance frameworks for geoengineering research.

  9. The International Legal Framework for Climate Engineering (Working Paper) offers a primer on international law as it relates to geoengineering, and describes some of the specific relevant international conventions and other agreements that would likely be invoked to help settle disagreements about the deployment of climate engineering technologies.