The perils of solar radiation management


Overwhelmed by the rapidity of human-caused climate change, the dire implications of feedback loops like methane and CO2 release from thawing permafrost, and the utter failure of world governments to make the steep greenhouse gas emissions cuts necessary to keep global warming below 2 degrees C, scientists are paying more attention to a group of climate scientists proposing geoengineering techno fixes to cool the planet. For decades, solar geoengineering was not discussed by scientists. It was considered science fiction and a dangerous distraction from efforts to reduce carbon emissions. But in 2006 the Nobel-winning atmospheric chemist Paul Crutzen broke the silence with his paper, “Albedo Enhancement by Stratospheric Sulfur Injections,” calling for serious study of sulfate aerosol spraying. Although many climate scientists still agree with Pierrehumbert, lead author of the third report of the Intergovernmental Panel on Climate Change, that solar geoengineering is “wildly, utterly, howlingly, barking mad,” research on it is going ahead. Research has been endorsed by the UK Royal Society, and the US National Academy. Research projects are planned or under way at Beijing Normal University, the German Research Foundation, and Harvard.

The Harvard University Solar Radiation Management (SRM) Project

David Keith and Frank Keutsch are the scientists who launched the Harvard University Solar Radiation Management (SRM) project in March of 2017. This year, 2018, they hope to spray particles of water and aluminum oxide into the stratosphere from a high-altitude balloon over Tucson. The initial experiments will be small, designed to test the delivery system, discover what kind of particles are the most effective at reflecting sunlight, how the particles clump or disperse, whether they will disrupt the ozone layer, and how they interact with other atmospheric gases. The scientists are working with a Tucson, Arizona balloon company to engineer the balloon and equip it with sensors. The initial phase of the $20 million project is funded by Harvard, Bill Gates (a major contributor), the Hewlett Foundation, and the Alfred P. Sloan foundation.

What is Solar Radiation Management?

 Solar geoengineering was conceived by climate scientists worried about what we could do to protect ourselves if we reached a climate tipping point. Could a reflective shield in the stratosphere cool the planet in the way that dust particles from volcanic eruptions had done in the past? When Mt. Pinatubo in the Philippines erupted in 1991 nearly a million tons of sulfur dioxide were injected into the stratosphere and dispersed around the world causing global temperatures to drop by about 0.5C. The violent eruption of Mt. Tambora in Indonesia in 1815 cast a veil of sulfate around the earth, dropping temperatures an astounding three degrees C and causing crop failures and famine. 1816 was long remembered as “the year without a summer”. Scientists began to think that seeding the stratosphere with reflective particles, like sulfuric acid droplets, could enhance the earth’s albedo (reflectivity) and mimic the cooling effects of a volcanic eruption.

The small group of climate scientists involved in solar geoengineering believe that mimicking the cooling effects of a volcanic eruption by injecting reflective particles into the stratosphere is technically easy, can be applied immediately, and is relatively cheap. They describe it as a temporary stopgap measure to give the world additional time to transition to renewable energy. David Keith is enthusiastic about its potential, “This single technology could increase the productivity of ecosystems across the planet and stop global warming;” he wrote in his 2014 book, A Case For Climate Engineering. “It could increase crop yields, particularly in the hottest and poorest parts of the world.”

SRM Does Not Halt Climate Change

Keith acknowledges that solar radiation management does not stop the fossil fuel emissions that are causing climate change; it does not reduce the cumulative greenhouse gases that are warming our planet; and it does nothing to stop ocean acidification. It merely throws a veil between the earth and the sun, temporarily deflecting enough sunlight to cool the planet. We would still need a crash program to complete the necessary transition from fossil fuels to renewable energy, he emphasizes.

Scientists speculate that they could seed the stratosphere using a small fleet of aircraft, similar to a top- of- the- line business jet, capable of carrying 13 tons or more to altitudes of 50,000 + feet or they could pump material through a gigantic hose suspended in the stratosphere by balloons. Millions of tons of particles a year would have to be transported to the stratosphere for full deployment. Once reflective particles are deployed, seeding would have to be kept up constantly for years (or for centuries.)

SRM Is Far Cheaper Than Other Methods of Mitigating Global Warming

 In 2014, David Keith estimated that the total cost of large scale solar geoengineering would be about one billion dollars a year and the cost of geoengineering the entire planet for a decade would be less than $6 billion. A study using existing aircraft to seed the stratosphere published in Environmental Research Letters put the cost at a mere $1 to $3 billion per year. It is cheap, far cheaper than the cost of cutting carbon emissions. It is inexpensive enough that a small country, or even a rogue billionaire could finance it. In contrast, other climate geoengineering projects, such as carbon capture and sequestration (CCS) and biomass energy carbon capture and storage (BECCS), will require decades of research before they become operational, will take many more decades to have an effect on cumulative CO2 levels and will be tremendously expensive.

Solar Geoengineering Could Affect the Planet in Unexpected Ways

Because little is known about the functioning of the planetary ecosystem as a whole, solar geoengineering could affect the planet in unexpected ways. “How can you engineer a system whose behavior you don’t understand?” asked Ron Prin, Professor of Atmospheric Science at M.I.T. Intensive research will be necessary to better understand the general atmosphere and ocean circulation, especially research on ocean heat transport and the complicated relationship of ocean-atmospheric heat exchange. Sulfuric acid can cause chemical changes in ozone. Would large scale deployment of sulfuric acid particles contribute to the depletion of the ozone layer? How do reductions in solar radiation affect the hydrological cycle? Some of the most alarming results of computer simulations show that SRM initiated in the Arctic or at the equator could disrupt the Asian and African monsoons. How would the diffuse sunlight in an SRM altered world affect the growth rate of plants? And, finally, if unexpected consequences make it necessary to end the program, would the Earth warm so abruptly that it would cause ecosystem collapse?

Keith looks at all of these potential dangers as problems to be solved : The impact of geoengineering particles on the ozone layer, for example, depends on what kind of particles we use, and also on how much chlorine is in the stratosphere when we inject the particles. The danger of abrupt warming when an SRM program is ended can be controlled by gradually reducing the number of reflective particles we deploy each year. Yes, solar geoengineering could reduce precipitation in some areas, counteracting, in part, the increase in precipitation caused by climate change. But the location where the reflective particles are deployed and the amount of particles deployed could be adjusted to minimize changes in precipitation.

Nearly all the work on SRM is done by computer modeling, but Keith believes we need to move to experiments in the real world to find ways we can avoid negative outcomes. Experiments that seek to understand large-scale climate response will be difficult or impossible because they cannot be limited to a specific geographic region – the atmosphere has no boundaries. “I don’t think you can actually measure a climate response without a forcing that is large enough to make it practically equivalent to deployment.” says MacMarten, a scientist engaged in solar geoengineering research at Cornell University. Computer models show that trying to limit SRM to the Arctic, as has been suggested to preserve Arctic sea ice, is not successful. Studies done by Robock (2008), MacCracken (2013), and others have found that sulfate aerosols injected to cool the Arctic spread southward, affecting precipitation patterns in Asia and Africa.

As research advances, the risks and uncertainties of SRM will be evaluated by comparison with the risks and uncertainties of climate change at the time the evaluation takes place. “For me,” says Daniel Schrag, professor of environmental science and engineering at Harvard, “solar geoengineering is terrifying. We’re talking about an engineering project that could affect every living thing on this planet. The possibility that something could go wrong is really scary.” But, he adds, “It could take decades or longer for the world’s energy system to shift from fossil fuels to renewables—and by then climate impacts could be devastating. The evidence is becoming clearer and clearer that not doing geoengineering and letting climate change proceed might be worse, as an alternative.”

This article will be continued in the April issue of Works in Progress.