Climate engineering

Jane Long makes several important points. Among them is that geoengineering research should not have as its mission the deployment of geoengineering concepts. She cogently argues that “The goal for climate intervention research must be to understand the potential efficacy, advisability, and practicality of various concepts in the context of mitigation and adaptation.” David Keith makes a similar point and provides two guiding principles: that research on solar radiation management should be part of a broader climate research portfolio on mitigation and adaptation action, and that research should be linked to governance and policy work.

We generally think of solar radiation management research in terms of small tests that can define particular parameters, such as the atmospheric residence time, transport, and fate of aerosol scattering particles. As both Long and Keith observe, these tests require thoughtful governance arrangements that may be difficult at present.

Twenty-six years ago there was a large-scale natural experiment in solar radiation management: the eruption of Mount Pinatubo in the Philippines that injected roughly 17 million tons of sulfur dioxide into the middle and lower stratosphere. Sulfate aerosols spread across the Pacific Ocean in a few weeks and around the globe within a year. Spectacular sunsets over the next two years were one indication of the stratosphere residence time of the aerosols. The event produced observed cooling in the Northern Hemisphere of 0.5 degrees to 0.6 degrees Centigrade, equivalent to a reduction in of perhaps 3 watts per square meter. Globally averaged cooling of approximately 0.3 degrees was observed.

Such natural experiments in stratospheric aerosol injection are infrequent. The eruption of Krakatau in 1883 produced a forcing of a little over 3 watts per square meter. There were three eruptions between Krakatau and Pinatubo that produced forcings of 1.5 to 2 watts per square meter and five additional ones of 0.5 to 1 watt per square meter. The average frequency was once every dozen years, although there was a long quiet period from about 1920 to 1963.

It seems both worthwhile and feasible to develop a program to learn from the next such eruption. Much was learned about scientific models from Pinatubo, but as the 2015 National Research Council report Climate Intervention: Reflecting Sunlight to Cool Earth stated, “More work is needed in characterizing these processes in nature (through measurements), and in modeling (through better model treatments and a careful comparison with observed features of aerosols and their precursor gases) before scientists can produce truly accurate models of stratospheric aerosols.” Understanding the chemical reactions, mixing, and particle formation after such an event can help characterize not only solar radiation management but also aerosol- forcing effects on climate. Global observations can help understand the consequences of solar radiation management on precipitation, plant productivity, and carbon uptake, among other effects.

The Climate Intervention report had a short section describing observational requirements for making better use of volcanoes as natural experiments. It points out that “our ability to monitor stratosphere aerosols has deteriorated since [Pinatubo], with the loss of the SAGE II and III satellite-borne instruments.” Both satellite systems and a deployable rapid-response observational task force (that would have other atmospheric science uses to occupy it between eruptions) are suggested.

The creation of an international program to learn from the next Pinatubo could jump-start both needed instrumentation and perhaps governance arrangements in a low-key way that could build trust and indicate whether governance of deliberate solar radiation management experimentation is feasible along the lines that Long and Keith describe.

Jay Apt Co-Director, Carnegie Mellon Electricity Industry Center Professor, Tepper School of Business and Department of Engineering & Public Policy Carnegie Mellon University