Academic Publications

Working Paper
Gernot Wagner and Richard J. Zeckhauser. Working Paper. “Confronting Deep and Persistent Climate Uncertainty.” Harvard Kennedy School, Faculty Research Working Paper Series, 16-025. Publisher's Version Abstract

Deep-seated, persistent uncertainty is a pernicious feature of climate change. One key parameter, equilibrium climate sensitivity, has eluded almost all attempts at pinning it down more precisely than a ‘likely’ range that has stalled at 1.5–4.5°C for over thirty-five years.

The marginal damages due to temperature increase rise rapidly. Thus, uncertainty in climate sensitivity significantly raises the expected costs of climate change above what they would be if the temperature increases were known to be close to a mean value 3.0°C. The costs of this uncertainty are compounded given that the distribution of possible temperature changes is strongly skewed toward higher values.

2016
David Keith, Debra Weisenstein, John Dykema, and Frank Keutsch. 12/12/2016. “Stratospheric Solar Geoengineering without Ozone Loss.” PNAS. Publisher's Version Abstract

Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of −1 W⋅m−2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y−1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.

Elizabeth T. Burns, Jane A. Flegal, David W. Keith, Aseem Mahajan, Dustin Tingley, and Gernot Wagner. 11/1/2016. “What do people think when they think about solar geoengineering? A review of empirical social science literature, and prospects for future research.” Earth's Future. Publisher's Version Abstract

Public views and values about solar geoengineering should be incorporated in science-policy decisions, if decision makers want to act in the public interest. In reflecting on the past decade of research, we review around 30 studies investigating public familiarity with, and views about, solar geoengineering. A number of recurring patterns emerge: (1) general unfamiliarity with geoengineering among publics; (2) the importance of artifice versus naturalness; (3) some conditional support for certain kinds of research; and (4) nuanced findings on the “moral hazard” and “reverse moral hazard” hypotheses, with empirical support for each appearing under different circumstances and populations. We argue that in the coming decade, empirical social science research on solar geoengineering will be crucial, and should be integrated with physical scientific research.

John Dykema, David Keith, and Frank Keutsch. 7/30/2016. “Improved aerosol radiative properties as a foundation for solar geoengineering risk assessment.” Geophysical Research Letters. Publisher's Version Abstract

Side effects resulting from the deliberate injection of sulfate aerosols intended to partially offset climate change have motivated the investigation of alternatives, including solid aerosol materials. Sulfate aerosols warm the tropical tropopause layer, increasing the flux of water vapor into the stratosphere, accelerating ozone loss, and increasing radiative forcing. The high refractive index of some solid materials may lead to reduction in these risks. We present a new analysis of the scattering efficiency and absorption of a range of candidate solid aerosols. We utilize a comprehensive radiative transfer model driven by updated, physically consistent estimates of optical properties. We compute the potential increase in stratospheric water vapor and associated longwave radiative forcing. We find that the stratospheric heating calculated in this analysis indicates some materials to be substantially riskier than previous work. We also find that there are Earth-abundant materials that may reduce some principal known risks relative to sulfate aerosols.

Lee Miller, Vaclav Smil, Gernot Wagner, and David Keith. 7/18/2016. “Establishing practical estimates for city-integrated solar PV and wind.” Science eLetter. Publisher's Version
Pete Irvine, Ben Kravitz, Mark Lawrence, and Helene Muri. 7/2016. “An overview of the Earth system science of solar geoengineering.” Wiley Interdisciplinary Reviews: Climate Change. Publisher's Version Abstract

Solar geoengineering has been proposed as a means to cool the Earth by increasing the reflection of sunlight back to space, for example, by injecting reflective aerosol particles (or their precursors) into the lower stratosphere. Such proposed techniques would not be able to substitute for mitigation of greenhouse gas (GHG) emissions as a response to the risks of climate change, as they would only mask some of the effects of global warming. They might, however, eventually be applied as a complementary approach to reduce climate risks. Thus, the Earth system consequences of solar geoengineering are central to understanding its potentials and risks. Here we review the state-of-the-art knowledge about stratospheric sulfate aerosol injection and an idealized proxy for this, ‘sunshade geoengineering,’ in which the intensity of incoming sunlight is directly reduced in models. Studies are consistent in suggesting that sunshade geoengineering and stratospheric aerosol injection would generally offset the climate effects of elevated GHG concentrations. However, it is clear that a solar geoengineered climate would be novel in some respects, one example being a notably reduced hydrological cycle intensity. Moreover, we provide an overview of nonclimatic aspects of the response to stratospheric aerosol injection, for example, its effect on ozone, and the uncertainties around its consequences. We also consider the issues raised by the partial control over the climate that solar geoengineering would allow. Finally, this overview highlights some key research gaps in need of being resolved to provide sound basis for guidance of future decisions around solar geoengineering.

Robert E. Kopp, Rachael Shwom, Gernot Wagner, and Jiacan Yuan. 7/2016. “Tipping elements and climate-economic shocks: Pathways toward integrated assessment.” Earth's Future. Publisher's Version Abstract

The literature on the costs of climate change often draws a link between climatic ‘tipping points’ and large economic shocks, frequently called ‘catastrophes’. The phrase ‘tipping points’ in this context can be misleading. In popular and social scientific discourse, ‘tipping points’ involve abrupt state changes. For some climatic ‘tipping points,’ the commitment to a state change may occur abruptly, but the change itself may be rate-limited and take centuries or longer to realize. Additionally, the connection between climatic ‘tipping points’ and economic losses is tenuous, though emerging empirical and process-model-based tools provide pathways for investigating it. We propose terminology to clarify the distinction between ‘tipping points’ in the popular sense, the critical thresholds exhibited by climatic and social ‘tipping elements,’ and ‘economic shocks’. The last may be associated with tipping elements, gradual climate change, or non-climatic triggers. We illustrate our proposed distinctions by surveying the literature on climatic tipping elements, climatically sensitive social tipping elements, and climate-economic shocks, and we propose a research agenda to advance the integrated assessment of all three.

David Keith, Gernot Wagner, and Juan Moreno-Cruz. 6/24/2016. “Modeling the effects of climate engineering.” Science, 6293, 352: 1526-1527. Publisher's Version
Lee Miller, Vaclav Smil, Gernot Wagner, and David Keith. 6/20/2016. “Stated estimates for city-integrated wind and solar PV are too high.” Science eLetter. Publisher's Version

Proposed large-scale intentional interventions in natural systems in order to counter climate change, typically called “climate engineering” or “geoengineering,” stand to dramatically alter the international politics of climate change and potentially much more. There is currently a significant and growing literature on the international politics of climate engineering. However, it has been produced primarily by scholars from outside the discipline of International Relations (IR). We are concerned that IR scholars are missing a critical opportunity to offer insights into, and perhaps help shape, the emerging international politics of climate engineering. To that end, the primary goal of this paper is to call the attention of the IR community to these developments. Thus, we offer here an overview of the existing literature on the international politics of climate engineering and a preliminary assessment of its strengths and lacunae. We trace several key themes in this corpus, including problem structure, the concern that climate engineering could undermine emissions cuts, the potentially “slippery slope” of research and development, unilateral implementation, interstate conflict, militarization, rising tensions between industrialized and developing countries, and governance challenges and opportunities. The international politics of climate engineering is then considered through the lenses of the leading IR theories (Realism, Institutionalism, Liberalism, and Constructivism), exploring both what they have contributed and possible lines of future inquiry. Disciplinary IR scholars should have much to say on a number of topics related to climate engineering, including its power and transformational potentials, the possibility of counter-climate engineering, issues of institutional design, international law, and emergent practices. We believe that it is incumbent on the IR community, whose defining focus is international relations, to turn its attention to these unprecedented technologies and to the full scope of possible ramifications they might have for the international system.

Katie Dagon and Daniel P. Schrag. 1/27/2016. “Exploring the Effects of Solar Radiation Management on Water Cycling in a Coupled Land-Atmosphere Model.” Journal of Climate, 29: 2635-2650. Publisher's Version Abstract

Solar radiation management (SRM) has been proposed as a form of geoengineering to reduce the climate effects of anthropogenic greenhouse gas emissions. Modeling studies have concluded that SRM, through a reduction in total solar irradiance by approximately 2%, roughly compensates for global mean temperature changes from a doubling of carbon dioxide concentrations. This paper examines the impact of SRM on the terrestrial hydrologic cycle using the Community Land Model, version 4, coupled to the Community Atmosphere Model, version 4, with reductions in solar radiation relative to simulations with present-day and elevated CO2 concentrations. There are significant global and regional impacts due to vegetation–climate interactions that are not compensated when reductions in total solar irradiance of 1%, 2%, and 3% are imposed on top of a doubling of present-day CO2 concentrations. Water cycling slows down under SRM, including decreases in global mean precipitation and evapotranspiration. Changes in runoff and soil moisture are spatially and temporally variable, with implications for local water availability. In the tropics, evapotranspiration decreases because of increases in vegetation water use efficiency. In northern midlatitudes, soil moisture increases when evapotranspiration decreases, with some exceptions during boreal summer. Changes in soil evaporation influence water cycling in the southern subtropics, rather than changes in transpiration. The hydrologic response to SRM is nonlinear, with global mean decreases greater than expected. These results imply that SRM may not compensate for higher greenhouse gas concentrations when one considers land–atmosphere interactions.

Joshua Horton, David Keith, and Matthias Honegger. 2016. “Implications of the Paris Agreement for Carbon Dioxide Removal and Solar Geoengineering.” Policy Brief, Harvard Project on Climate Agreements, Belfer Center for Science and International Affairs, Harvard Kennedy School. Publisher's Version
Caitlin G. McCormack, Wanda Born, Peter Irvine, Eric P. Achterberg, Tatsuya Amano, Jeff Ardron, Pru N. Foster, Jean-Pierre Gattuso, Stephen J. Hawkins, Erica Hendy, W. Daniel Kissling, Salvador E. Lluch-Cota, Eugene J. Murphy, Nick Ostle, Nicholas J.P. Owens, R. Ian Perry, Hans O. Pörtner, Robert J. Scholes, Frank M. Schurr, Oliver Schweiger, Josef Settele, Rebecca K. Smith, Sarah Smith, Jill Thompson, Derek P. Tittensor, Mark van Kleunen, Chris Vivian, Katrin Vohland, Rachel Warren, Andrew R. Watkinson, Steve Widdicombe, Phillip Williamson, Emma Woods, Jason J. Blackstock, and William J. Sutherland. 2016. “Key impacts of climate engineering on biodiversity and ecosystems, with priorities for future research.” Journal of Integrative Environmental Sciences, 1-26. Publisher's Version Abstract

Climate change has significant implications for biodiversity and ecosystems. With slow progress towards reducing greenhouse gas emissions, climate engineering (or ‘geoengineering’) is receiving increasing attention for its potential to limit anthropogenic climate change and its damaging effects. Proposed techniques, such as ocean fertilization for carbon dioxide removal or stratospheric sulfate injections to reduce incoming solar radiation, would significantly alter atmospheric, terrestrial and marine environments, yet potential side-effects of their implementation for ecosystems and biodiversity have received little attention. A literature review was carried out to identify details of the potential ecological effects of climate engineering techniques. A group of biodiversity and environmental change researchers then employed a modified Delphi expert consultation technique to evaluate this evidence and prioritize the effects based on the relative importance of, and scientific understanding about, their biodiversity and ecosystem consequences. The key issues and knowledge gaps are used to shape a discussion of the biodiversity and ecosystem implications of climate engineering, including novel climatic conditions, alterations to marine systems and substantial terrestrial habitat change. This review highlights several current research priorities in which the climate engineering context is crucial to consider, as well as identifying some novel topics for ecological investigation.

Joshua Horton and David Keith. 2016. “Solar Geoengineering and Obligations to the Global Poor.” In Climate Justice and Geoengineering: Ethics and Policy in the Atmospheric Anthropocene, edited by Christopher J. Preston. London: Rowman & Littlefield. Publisher's Version
Horton and Keith 2016

Wind turbines generate electricity by removing kinetic energy from the atmosphere. Large numbers of wind turbines are likely to reduce wind speeds, which lowers estimates of electricity generation from what would be presumed from unaffected conditions. Here, we test how well wind power limits that account for this effect can be estimated without explicitly simulating atmospheric dynamics. We first use simulations with an atmospheric general circulation model (GCM) that explicitly simulates the effects of wind turbines to derive wind power limits (GCM estimate), and compare them to a simple approach derived from the climatological conditions without turbines [vertical kinetic energy (VKE) estimate]. On land, we find strong agreement between the VKE and GCM estimates with respect to electricity generation rates (0.32 and 0.37 We m−2) and wind speed reductions by 42 and 44%. Over ocean, the GCM estimate is about twice the VKE estimate (0.59 and 0.29 We m−2) and yet with comparable wind speed reductions (50 and 42%). We then show that this bias can be corrected by modifying the downward momentum flux to the surface. Thus, large-scale limits to wind power use can be derived from climatological conditions without explicitly simulating atmospheric dynamics. Consistent with the GCM simulations, the approach estimates that only comparatively few land areas are suitable to generate more than 1 We m−2 of electricity and that larger deployment scales are likely to reduce the expected electricity generation rate of each turbine. We conclude that these atmospheric effects are relevant for planning the future expansion of wind power.

2015
Jana Sillmann, Timothy M. Lenton, Anders Levermann, Konrad Ott, Mike Hulme, Francois Benduhn, and Joshua Horton. 2015. “Climate Emergencies Do Not Justify Engineering the Climate.” Nature Climate Change. Publisher's Version
Hossein Safaei and David Keith. 2015. “How much bulk energy storage is needed to decarbonize electricity?.” Energy and Environmental Science, 12, 8: 3409-3417. Publisher's Version
Steven R. H. Barrett, Raymond L. Speth, Sebastian D. Eastham, Irene C. Dedoussi, Akshay Ashok, Robert Malina, and David Keith. 2015. “Impact of the Volkswagen emissions control defeat device on US public health.” Environmental Research Letters, 11, 10: 114005. Publisher's Version

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