As Earth heats up, the world must consider sunlight reflection

From suffocating heat in India to rampaging wildfires in New Mexico, the planet is sending us a message: the long-dreaded climate emergency is now. The world is poised to blow past the Paris Agreement temperature targets, with devastating consequences. Faced with this clear and present danger, the United States and other nations must consider expanding their strategies for managing climate risk to include sunlight reflection. Also known as solar geoengineering, it would entail reflecting a tiny percentage of incoming sunlight back into space to limit solar radiation’s heating effect as humanity tackles the massive, protracted task of decarbonization.

Sunlight reflection has long been the third rail of climate politics, but that is starting to change as the gravity of global warming becomes increasingly obvious. A year ago, the prestigious U.S. National Academies of Science advocated its enhanced study. On April 27, the Council on Foreign Relations released its own report, “Reflecting Sunlight to Reduce Climate Risk,” examining the logic and feasibility of such climate intervention and the requirements for its effective international governance.

Humanity currently has three approaches for reducing climate risk: emissions reductions, carbon removal and adaptation. Unfortunately, all are lagging or have inherent limitations. Emissions, which must be halved by 2030 to limit warming to 1.5 degrees Celsius, are on track to rise more than 16 percent. Carbon removal technologies could take decades to scale up. Adaptation and resilience, while critical, are enormously expensive and will fail to prevent significant suffering. 

In short, the world confronts a high-stakes timing predicament. We know what we have to do but are not doing it fast enough to prevent soaring temperatures, melting icecaps and rising seas. Sunlight reflection thus offers a tempting bridging option, a possible way to “shave the peak” of global warming during the transition to net zero carbon emissions.

The two most straightforward methods to reflect sunlight are stratospheric aerosol injection (SAI) and marine cloud brightening (MCB). The former would involve dispersing particles like sulfates or calcites into the stratosphere, notionally from customized aircraft or balloons. It would mimic the cooling effect of volcanic eruptions such as the 1991 explosion of Mount Pinatubo, which temporarily reduced global temperatures by about 0.6 degrees Celsius over 15 months. MCB would entail spraying sea salt crystals into low-lying clouds from ocean-based vessels or platforms.

Sunlight reflection is not a “solution” to climate change since it would not affect emissions or eliminate atmospheric carbon. What it offers is a technologically plausible, potentially rapid and relatively cheap way to slow or even reverse global warming and its attendant hazards, buying time for more ambitious mitigation efforts to take hold. It could be remarkably cost-effective: models suggest that SAI could cost as little as $10 billion per year, a minute fraction of the estimated $275 trillion price tag for decarbonizing the global economy by 2050. It thus deserves genuine consideration as another arrow in humanity’s quiver of strategies to manage climate risk. Given the stakes, it would be irresponsible for leaders not to evaluate its viability and possible consequences.

Nevertheless, sunlight reflection is not without critics. Some fear its very possibility could create a so-called moral hazard, giving nations, corporations and consumers a get-out-of-jail-free card to continue their polluting ways. Others raise practical objections, worrying that such planetary intervention could have unanticipated, negative consequences like disrupting rainfall patterns.

These risks merit careful review and scrutiny. However, they should be assessed not in isolation, as if we inhabited a perfect world, but alongside the known dangers, tensions and inequities inherent in the ongoing, if unwitting, experiment we are already running by pumping greenhouse gases into the atmosphere. The relevant question is: Would we be better or worse off by adding sunlight reflection to our mix of climate responses?

Unfortunately, the world is in no position to answer this crucial question, given scientific uncertainty about sunlight reflection’s potential efficacy and repercussions and the lack of multilateral rules governing intentional manipulation of Earth’s climate system. This dual vacuum is untenable. It leaves policymakers flying blind, unable to make informed, responsible choices. It also increases the danger, as the National Intelligence Council has warned, that a single power could take matters into its own hands, with potentially destabilizing geopolitical and environmental impacts.

The Biden administration and Congress should fill the first, knowledge gap by launching a robust, transparent U.S. research program on sunlight reflection science, building on modest provisions included in the 2022 consolidated appropriations act. To fill the second, governance gap, the United States should help negotiate a multilateral framework in which national governments can jointly assess the feasibility and likely consequences of alternative approaches and take collective decisions about any future deployment.

Confronting a future of dramatic warming, humanity must consider all its options, no matter how unsettling they might first appear. Sunlight reflection carries risks, but it could also provide humanity the breathing space needed for more durable solutions to manifest. The United States has both the opportunity and the responsibility to help forge reasonable global rules for its research and use — and, given what’s at stake, it can ill afford to remain on the sidelines.

Stewart Patrick is senior fellow at the Council on Foreign Relations and author of the report “Reflecting Sunlight to Reduce Climate Risk: Priorities for Research and International Governance.”

Sherri Goodman is the secretary general of the International Military Council on Climate & Security and senior fellow at the Woodrow Wilson International Center for Scholars, and she chaired the advisory committee for the report.