The least harmful means to cool the planet

The energy infrastructure that powers our civilisation must be rebuilt, replacing fossil fuels with carbon-free sources such as solar or nuclear. But even then, zeroing out emissions will not cool the planet. This is a direct consequence of the single most important fact about climate change: Warming is proportional to the cumulative emissions over the industrial era. Eliminating emissions by about 2050 is a difficult but doable goal. Suppose it is achieved. Average temperatures will stop increasing when emissions stop, but cooling will take thousands of years as greenhouse gases slowly dissipate from the atmosphere. Because the world will be a lot hotter by the time emissions reach zero, heat waves and storms will be worse than they are today. And while the heat will stop getting worse, sea level will continue to rise for centuries as polar ice melts in a warmer world. This July was the hottest month ever recorded, but it is likely to be one of the coolest Julys for centuries after emissions reach zero.

Stopping emissions stops making the climate worse. But repairing the damage, insofar as repair is possible, will require more than emissions cuts. To cool the planet in this century, humans must either remove carbon from the air or use solar geoengineering, a temporary measure that may reduce peak temperatures, extreme storms and other climatic changes. Humans might make the planet Earth more reflective by adding tiny sulfuric acid droplets to the stratosphere from aircraft, whitening low-level clouds over the ocean by spraying sea salt into the air or by other interventions. Yes, this is what it comes down to: carbon removal or solar geoengineering or both. At least one of them is required to cool the planet this century.

There are no other options. Carbon removal would no doubt trounce geoengineering in a straw poll of climate experts. Removal is riding a wave of support among centrist environmental groups, governments and industry. Solar geoengineering is seen as such a desperate gamble that it was dropped from the important “summary for policymakers” in the United Nations’ latest climate report. Yet if I were asked which method could cut mid-century temperatures with the least environmental risk, I would say geoengineering.

Lest you dismiss me, I founded Carbon Engineering, one of the most visible companies developing technology to capture carbon directly from the air and then pump it underground or use it to make products that contain carbon dioxide. The company’s interests could be hurt if geoengineering were seen as an acceptable option. I was also an early proponent for burning biofuels like wood waste, capturing the resulting carbon at the smokestack and storing it underground. I am proud to be a part of the community developing carbon removal. These approaches can help manage hard-to-abate emissions, and it is the only way to reduce the long-run climate risks that will remain when net emissions reach zero. But the problem with these carbon removal technologies is that they are inherently slow because the carbon that has accumulated in the atmosphere since the Industrial Revolution must be removed ton by ton. Still, the technology provides a long-term cure.

Geoengineering, on the other hand, is cheap and acts fast, but it cannot deflate the carbon bubble. It is a Band-Aid, not a cure. The trade-off between geoengineering and carbon removal depends on one’s time horizon. The sooner cooling is pursued, the greater the environmental and social impacts of carbon removal. Suppose emissions were under control and you wanted to cool the planet an additional degree by mid-century. How would removal and geoengineering compare?

Carbon removal could work. But it will require an enormous industry. Trees are touted as a natural climate solution, and there are some opportunities to protect natural systems while capturing carbon by allowing deforested landscapes to regrow and pull in carbon dioxide as they do. But cooling this fast cannot be achieved by letting nature run free. Ecosystems would need to be manipulated using irrigation, fire suppression or genetically modified plants whose roots are resistant to rot. This helps to increase the build-up of carbon in soils. To cool a degree by mid-century, this ecological engineering would need to happen at a scale comparable with that of global agriculture or forestry, causing profound disruption of natural ecosystems and the too-often-marginalised people who depend on them. Industrial removal methods have a much smaller land footprint; a single carbon capture facility occupying a square mile of land could remove a million tons of carbon from the air a year. But building and running this equipment would require energy, steel and cement from a global supply chain. And removing the few hundred billion tons required to cool a degree by mid-century requires a supply chain that might be smaller than what feeds the construction industry but larger than what supports the global mining industry. The challenge is that a carbon removal operation — industrial or biological — achieves nothing the day it starts, but only cumulatively, year upon year. So, the faster one seeks that one degree of cooling, the faster one must build the removal industry, and the higher the social costs and environmental impacts per degree of cooling. Geoengineering could also work. The physical scale of intervention is — in some respects — small. Less than two million tons of sulfur per year injected into the stratosphere from a fleet of about a hundred high-flying aircraft would reflect away sunlight and cool the planet by a degree. The sulfur falls out of the stratosphere in about two years, so cooling is inherently short term and could be adjusted based on political decisions about risk and benefit.

Adding two million tons of sulfur to the atmosphere sounds reckless, yet this is only about one-twentieth of the annual sulfur pollution from today’s fossil fuels. Geoengineering might worsen air pollution or damage the global ozone layer, and it will certainly exacerbate some climate changes, making some regions wetter or drier even as it cools the world. While limited, the science so far suggests that the harms that would result from shaving a degree off global temperatures would be small compared with the benefits. Air pollution deaths from the added sulfur in the air would be more than offset by declines in the number of deaths from extreme heat, which would be 10 to 100 times larger. Cooling the planet to reduce human suffering in this century will require carbon removal or solar geoengineering or both. The trade-offs between them are uncertain because little comparative research has been done. The fact that one or both are taboo in some green circles is a dreadful misstep of contemporary environmentalism. Climate justice demands fast action to cut emissions and serious exploration of pathways to a cooler future.

Dr. Keith is a professor of applied physics and of public policy at Harvard. NYT©2021