Efforts to block sunlight get boost from prominent scientists

By Corbin Hiar | 02/27/2023 07:00 AM EST

A letter by dozens of top scientists could increase momentum for a climate intervention that some academics consider too dangerous to study.

James Hansen sits for a portrait in his home in New York on April 12, 2018.

Climate scientist James Hansen is among dozens of researchers calling for more attention to geoengineering. AP Photo/Marshall Ritzel

Former NASA director James Hansen, who first warned Congress about the dangers of global warming in 1988, is among more than 60 leading scientists who are calling for dramatic increases in research on ways to limit sunlight so it won’t overheat the planet.

Their open letter published Monday expresses support for studies and field experiments that would spray reflective aerosols in the stratosphere and alter the density of certain clouds — an area of climate research described by advocates as “solar radiation modification.” Critics, which include hundreds of other academics, deride such atmospheric interventions as “geoengineering.”

The letter comes as heat waves, wildfires, floods and other climate-related disasters grow increasingly common and severe. Average temperature increases are also inching dangerously close to levels that climate scientists fear could trigger catastrophic, irreversible changes to ice sheets, ecosystems and vulnerable communities.


Those sobering realities have prompted some international governments, top universities and wealthy philanthropists to begin considering a last-ditch means to cool the planet. The letter, which stops short of fully embracing solar radiation modification, or SRM, could provide new momentum for thoroughly researching a type of climate intervention some still consider too dangerous to study.

“Since decisions on whether or not to implement SRM are likely to be considered in the next one to two decades, a robust international scientific assessment of SRM approaches is needed as rapidly as possible,” the letter says.

It was mainly written and organized by Sarah Doherty, an atmospheric sciences professor at the University of Washington, with input from Philip Rasch, the chief climate scientist at the Pacific Northwest National Laboratory.

The letter emphasizes the need to slash emissions “starting immediately” and argues that the jury is still out on the viability of SRM as a climate fix. “Uncertainties in how SRM implementation would play out in the climate system are presently too large to support implementation,” it says.

It obliquely rebukes Make Sunsets, a venture capital-backed startup that aims to profit from selling “cooling credits.” Last year, Make Sunsets began releasing aerosol-dispersing weather balloons before the Mexican government effectively shut down the unauthorized venture (Climatewire, Jan. 19).

“Since SRM does not address the cause of climate change, nor all of the effects of increased greenhouse gas concentrations, it likely will never be an appropriate candidate for an open market system of credits and independent actors,” the letter says.

One of the main critiques of trying to block sunlight is the geopolitical nightmare that could result from such an effort.

“The current global governance system is unfit to develop and implement the far-reaching agreements needed to maintain fair, inclusive, and effective political control over solar geoengineering deployment,” says a separate letter posted online in January 2022. “Technocratic governance based on expert commissions cannot adjudicate complex global conflicts over values, risk allocation and differences in risk acceptance that arise within the context of solar geoengineering.”

Last year’s letter calls for an “international non-use agreement on solar geoengineering” that would ban state support for research, development and testing of means to limit heating from the sun. It was initiated by Frank Biermann, a sustainable development professor at Utrecht University in the Netherlands, and 15 other scholars and now has more than 370 signatories from over 50 countries.

The letter released Monday attempts to preempt those governance concerns.

“Research must be undertaken independently, so that research findings and assessments are protected from political influence, business interests, and public pressure,” it says. “Where possible, governments, philanthropists and the scientific community must seek ways to expand scientific capacity for Global South researchers to both engage in and direct research on SRM.”

The letter was also signed by several prominent researchers in middle-income countries: Govindasamy Bala of the Indian Institute of Science, Paulo Artaxo from Brazil’s Universidade de Sao Paulo and Heri Kuswanto of the Institut Teknologi Sepuluh Nopember in Indonesia.

The primary goal of the letter is to add some nuance to the fractious policy debate over SRM, according to Doherty, its lead author.

“We think it’s important that the public understand what is motivating scientists to support research on this topic, how it fits in with climate research more broadly, and to make the important distinction between supporting objective research to rigorously evaluate climate interventions versus supporting actively using them,” she told E&E News. “We do hope to correct the claims that all scientists are opposed to research in this area.”