In the rapidly expanding literature on the ethics of climate engineering, a lot has been made of the fact that stratospheric aerosol injection would for the first time create a world whose climate had been intentionally shaped by deliberate human decisions. Intention has always mattered in ethics. Due to the importance of intention in assigning culpability for harms, one might expect that the moral responsibility for any harms created during an attempt to reconstruct the global climate using stratospheric aerosols would be considerable. This article investigates such an expectation by making a comparison between the culpability for any unintended harms resulting from stratospheric aerosol injection and culpability for the unintended harms already taking place due to carbon emissions. To make this comparison, both types of unintended harms are viewed through the lens of the doctrine of double effect. The conclusion reached goes against what many might expect. The article closes by suggesting that a good way to read this surprising conclusion is that it points toward the continuing moral importance of prioritizing emission reductions.

Christopher J. Preston's use of the doctrine of double effect to claim that hypothetical climate engineers might very well be less culpable for climate harms than those who continue to emit greenhouse gases is unpersuasive. His argument rests shakily on the ability to determine and quantify climate harms and to distinguish forensically between their causes. He is also largely silent about the distributional effects of these harms and their ethical and political ramifications.

In his article “Carbon Emissions, Stratospheric Aerosol Injection, and Unintended Harms,” Christopher J. Preston compares the culpability of carbon emitters versus that of geoengineers deploying stratospheric aerosol injection (SAI). This comparison relies on a parallel between carbon emitters and SAI deployers that requires both to be agents. However, both are not. While the harms of geoengineering will be caused by culpable agents acting intentionally, the harms connected to climate change emerge out of the uncoordinated actions of billions of people. Taken as a large group, carbon emitters cause harm but do not constitute an agent. Taken individually, carbon emitters are agents but do not cause the harms of climate change. As a result, the parallel collapses, and Preston's “surprising” conclusion is one that he is not entitled to reach.

In order to advance a neatly deductive argument, Christopher J. Preston must make a number of assumptions and framing decisions that exclude important practical points from the scope of his analysis. We do not criticize him for doing so, as these simplifications allow him to advance a concise argument about an ethically complex subject. However, as scholars of politics and law, we are interested in what this ethical argument means—and does not mean—for the messy politics of climate engineering. Accordingly, in our response we unpack the political implications of some of Preston's assumptions and framing decisions in an effort to add a layer of practical richness to the abstraction of Preston's analysis.