The goal of last year’s Paris Agreement to limit global warming to 2C, if not 1.5C, are admirable, but it’s unlikely that this aspirational goal can be reached with voluntary greenhouse gas emission reductions alone. Already, we are nearing the 1.5C global warming level, with predictions for reaching 2C not far into the future. The implications of global warming are recognized widely, both in short-term events like coastal inundation and extreme weather, and long-term in the form of permanently shifting climate zones and higher sea level. The range of our actions, however, is not limited to greenhouse gas generation only.

Building on humanity’s remarkable history of engineering approaches to overcoming challenges, climate engineering techniques should be included as viable solutions for reducing the impacts of global warming.

Building on humanity’s remarkable history of engineering approaches to overcome challenges—from early use of fire to create stronger tools, to modern manufacturing and construction—climate engineering techniques should be included as viable solutions for reducing the impacts of global warming. Investigations of geoengineering approaches have been around for several decades, but have grown especially since the 2006 publication of Paul Crutzen’s essay on reducing solar influence, called “Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma?”

Climate engineering takes two approaches: (1) Carbon dioxide removal (CDR), and (2) solar radiation management (SRM). CDR addresses the cause of climate warming by removing greenhouse gas from the atmosphere (“treat the illness”). SRM offsets the warming effects of greenhouse gases by allowing Earth to absorb less solar radiation (“treat the symptoms”). Reduction of greenhouse gas emissions, as proposed in the Paris Agreement, is desirable, but is not a prerequisite for climate engineering. Among the range of techniques, SRM, the focus of Crutzen’s essay, is the main source of professional and public anxiety and has mostly remained taboo. There are concerns about unintended consequences, local applications with global consequences, runaway effects, and even climate warfare.

Given that climate engineering remains highly controversial, a set of thoughtful research papers and scientific commentaries have been published on this topic in AGU’s open-access journal Earth’s Future, introduced by Boettcher and Schäfer (2017). The thematic set of papers entitled Crutzen +10: Reflecting upon 10 years of geoengineering research, examines the techniques and risks of climate engineering, from specific methodologies to sociopolitical dimensions. The contributions highlight our much improved understanding of the environmental, political, and societal risks and benefits of climate engineering, but they also recognize that the current state of our knowledge is insufficient for reliable deployment. Computer modeling and integrated assessments have advanced the positive and negative aspects of various techniques, allowing for an informed public debate and eventual decision-making. Some nations more than others are advancing this understanding and are considering some implementation. However, more extensive scientific efforts and social study that includes real-world, outdoor experimentation will be needed to adequately assess near-term deployments and their impact.

Climate engineering has unquestionable potential to limit global warming when coupled with currently available technologies, but the scientific, social and ethical dimensions of implementation are not sufficiently examined. Given the worldwide impact of most deployment approaches, planning should occur on a global scale, involving all nations, both rich and poor, and not be limited to a few technologically advanced, wealthy stakeholders. We know we must limit the impacts of global warming, but we also know that warming will continue for decades or centuries even with radical reductions in greenhouse gas emissions. This situation generates an urgent need to invest in research and impact analysis of climate engineering approaches. Judging by the resilience of today’s human society to global environmental change, ignoring the potential of climate engineering solutions does not seem prudent nor realistic.

—Ben van der Pluijm, Department of Earth & Environmental Sciences, University of Michigan; email: [email protected]; and Guy Brasseur, Max Planck Institute for Meteorology; National Center for Atmospheric Research; email: guy[email protected]