The scientist who proposes pumping sulfuric acid into the atmosphere to cool the planet

At this stage of the climate crisis, the need to cut carbon emissions is repeated like a mantra. The goal is to prevent the world from continuing to warm, but in a scenario where emissions continue to rise and temperature records are broken every year, some scientists have a put forward a controversial idea: what if the planet could be cooled artificially? This is what solar geoengineering proposes. The best known of its promoters, the American David Keith, does not believe that this is a magic wand or an alternative to cutting emissions, but a “necessary conversation.” The idea is, on the face of it, simple: pump sulfuric acid into the atmosphere so that it reflects sunlight and lowers temperatures.

Keith, 60, is a professor in the department of geophysical sciences at the University of Chicago, where he has a team dedicated to solar geoengineering research. He was previously a professor of applied physics at Harvard University and, in 2009, he was included in Time magazine’s list of Heroes of the Environment. In 2013, he published the book A Case for Climate Engineering.

The physicist has been writing about solar geoengineering since 1992, although he explains his interest in the subject without a hint of romanticism. “I studied physics and a graduate degree at the Massachusetts Institute of Technology, but I was looking to do something that was more environmental and I found a network of people doing research on climate change, a subject that no one was working on at the time, so I got into it,” he explains to EL PAÍS by video call from Chicago.

The idea of solar geoengineering seeks to mimic the effect of volcanic eruptions, which shoot sulfur dioxide into the atmosphere. When it reaches the stratosphere, it transforms into sulfuric acid and builds up to function as a reflector. According to author Jeff Goodell in his book The Heat Will Kill You First: Life and Death on a Scorched Planet (2023), when the Pinatubo volcano erupted in the Philippines in 1991, it released 15 million tons of sulfuric acid into the atmosphere and lowered temperatures in the region by about 1ºC for a year. Solar geoengineering aims to recreate a similar effect on a planetary scale using aerosols to emit the same substance around the globe.

These are called stratospheric aerosols, small particles that reflect solar radiation, such as sulfuric acid. They can reach the stratosphere naturally, as in the case of volcanic eruptions, but in solar geoengineering the proposal is to inject them artificially to reduce global warming.

Pollution and temperature

Keith is careful with words when talking about his work, and does not deny the risks attached to this experimental method: “Sulfuric acid is an air pollutant, probably the most significant in terms of impact on humans. It kills millions of people a year, so it is an obvious risk. There is also the risk of damaging the ozone layer.”

But even with these dangers, the physicist argues that the benefits may be greater. “Although there are uncertainties, there is a broad consensus, even among the most skeptical, that stratospheric aerosols would reduce temperatures everywhere. And the benefits of temperature reduction are greater, especially in the hottest countries and for poorer people. For me, if there is one ethical reason to take this technology seriously, it’s that.”

For Keith, there are benefits and risks to “almost any intervention” that humans make, which should not be an argument for not doing something. “For example, solar energy is fantastic, the most important thing that has happened in the world of energy, but it also brings environmental damages. There are risks of toxic metals in the production chain. These are real things and we should work on them, but it is not a reason to not use solar energy,” he says.

“The first high-quality paper on stratospheric aerosols is from 1960. So, we know a lot. Thousands of papers have been written over a long period of time. We don’t necessarily have the hardware, but we have the technological capability to do it. So what we can do is compare how much cooling would reduce heat-related deaths, and we can compare that to deaths from air pollution,” Keith argues.

In a recent article in The New York Times on solar geoengineering and Keith’s work, the paper quotes several critics of the science. Environmentalist David Suzuki, for example, called the claim of releasing sulfuric acid into the atmosphere “arrogant and simplistic.” The article mentions that solar geoengineering implies as-yet unknown consequences as well as the danger that the world will embrace these solutions over emissions cuts because they are more convenient for polluting industries. Keith, however, is adamant that he does not believe solar geoengineering is a substitute for other environmental efforts.

Another criticism of solar geoengineering is the frequency with which the aerosols would have to be applied, as sulfuric acid only stays in the stratosphere for around two years. For Keith, however, that is a good thing, because the aerosols could be applied gradually and, if the results are not as expected, they could be stopped, or adjustments made. “That’s a good thing. If you put sulfuric acid in and it stayed there forever and you couldn’t remove it, you’d be permanently changing the climate,” he explains.

Thinking in four dimensions

According to the scientist, there are four ways humans can manage the climate crisis: reducing emissions, removing carbon from the atmosphere, solar geoengineering, and, finally, adaptation methods. “We have to think in four dimensions,” he says.

In addition to solar geoengineering, the physicist has also worked with the second mentioned method: carbon removal. In 2009, Keith founded Carbon Engineering, a company that specializes in a process for removing carbon dioxide from the atmosphere. This technique is known as direct air capture and some oil companies have begun to use it to try to reduce their huge environmental footprint. Among Carbon Engineering’s initial investors was the oil company Chevron. Last year, the company was acquired by Occidental Petroleum, another Texas-based oil company that has made air extraction its flagship project, for a total of $1.1 billion.

Keith no longer has any ties to Carbon Engineering and avoids the subject altogether when asked if he still “promotes” carbon extraction as a viable alternative for the environment. In his statements to The New York Times, however, he said he was “not comfortable” with the company being acquired by an oil company, and that he was considering donating the money he received to a conservation group.

Despite no longer being involved in any carbon removal initiatives, Keith believes there should be as much talk about this method as there is about solar geoengineering and adaptation. “A long time ago, people like Al Gore said we shouldn’t talk about adaptation because it would be immoral to divert attention away from reducing emissions. But I think most people would now agree that, in fact, Gore’s own statement was immoral. People in Bangladesh have the right to try to protect themselves from the harmful effects of climate pollution. That is why there is now much more attention being paid to adaptation.” Keith believes that this interest will also gradually spread to carbon removal and solar geoengineering.

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