# David Keith

Lee Miller and David Keith. 10/4/2018. “Observation-based solar and wind power capacity factors and powerdensities.” Environmental Research Letters, 13. Publisher's VersionAbstract

Power density is the rate of energy generation per unit of land surface area occupied by an energy system. The power density of low-carbon energy sources will play an important role in mediating the environmental consequences of energy system decarbonization as the world transitions away from high power-density fossil fuels. All else equal, lower power densities mean larger land and environmental footprints. The power density of solar and wind power remain surprisingly uncertain: estimates of realizable generation rates per unit area for wind and solar power span 0.3–47Wem−2 and 10–120Wem−2 respectively. We refine this range using US data from 1990–2016. We estimate wind power density from primary data, and solar power density from primary plant-level data and prior datasets on capacity density. The mean power density of 411 onshore wind power plants in 2016 was 0.50Wem−2. Wind plants with the largest areas have the lowest power densities. Wind power capacity factors are increasing, but that increase is associated with a decrease in capacity densities, so power densities are stable or declining. If wind power expands away from the best locations and the areas of wind power plants keep increasing, it seems likely that wind’s power density will decrease as total wind generation increases. The mean 2016 power density of 1150 solar power plants was 5.4Wem−2. Solar capacity factors and (likely) power densities are increasing with time driven, in part, by improved panel efficiencies. Wind power has a 10-fold lower power density than solar, but wind power installations directly occupy much less of the land within their boundaries. The environmental and social consequences of these divergent land occupancy patterns need further study.

Lee Miller and David Keith. 10/4/2018. “Climatic Impacts of Wind Power.” Joule, 2. Publisher's VersionAbstract

We find that generating today’s US electricity demand (0.5 TWe) with wind power would warm Continental US surface temperatures by 0.24C. Warming arises, in part, from turbines redistributing heat by mixing the boundary layer. Modeled diurnal and seasonal temperature differences are roughly consistent with recent observations of warming at wind farms, reflecting a coherent mechanistic understanding for how wind turbines alter climate. The warming effect is: small compared with projections of 21st century warming, approximately equivalent to the reduced warming achieved by decarbonizing global electricity generation, and large compared with the reduced warming achieved by decarbonizing US electricity with wind. For the same generation rate, the climatic impacts from solar photovoltaic systems are about ten times smaller than wind systems. Wind’s overall environmental impacts are surely less than fossil energy. Yet, as the energy system is decarbonized, decisions between wind and solar should be informed by estimates of their climate impacts.

Peter J. Irvine, David W. Keith, and John Moore. 7/27/2018. “Brief communication: Understanding solar geoengineering's potential to limit sea level rise requires attention from cryosphere experts.” The Cryosphere, 12, Pp. 2501-2513. Publisher's VersionAbstract
Stratospheric aerosol geoengineering, a form of solar geoengineering, is a proposal to add a reflective layer of aerosol to the stratosphere to reduce net radiative forcing and so to reduce the risks of climate change. The efficacy of solar geoengineering at reducing changes to the cryosphere is uncertain; solar geoengineering could reduce temperatures and so slow melt, but its ability to reverse ice sheet collapse once initiated may be limited. Here we review the literature on solar geoengineering and the cryosphere and identify the key uncertainties that research could address. Solar geoengineering may be more effective at reducing surface melt than a reduction in greenhouse forcing that produces the same global-average temperature response. Studies of natural analogues and model simulations support this conclusion. However, changes below the surfaces of the ocean and ice sheets may strongly limit the potential of solar geoengineering to reduce the retreat of marine glaciers. High-quality process model studies may illuminate these issues. Solar geoengineering is a contentious emerging issue in climate policy and it is critical that the potential, limits, and risks of these proposals are made clear for policy makers.
Joshua B. Horton, Jesse L. Reynolds, Holly Jean Buck, Daniel Callies, Stefan Schäfer, David W. Keith, and Steve Rayner. 6/28/2018. “Solar Geoengineering and Democracy.” Global Environmental Politics, Pp. 5-24. Publisher's VersionAbstract
Some scientists suggest that it might be possible to reflect a portion of incoming sunlight back into space to reduce climate change and its impacts. Others argue that such solar radiation management (SRM) geoengineering is inherently incompatible with democracy. In this article, we reject this incompatibility argument. First, we counterargue that technologies such as SRM lack innate political characteristics and predetermined social effects, and that democracy need not be deliberative to serve as a standard for governance. We then rebut each of the argument’s core claims, countering that (1) democratic institutions are sufficiently resilient to manage SRM, (2) opting out of governance decisions is not a fundamental democratic right, (3) SRM may not require an undue degree of technocracy, and (4) its implementation may not concentrate power and promote authoritarianism. Although we reject the incompatibility argument, we do not argue that SRM is necessarily, or even likely to be, democratic in practice.
David W. Keith, Geoffrey Holmes, David St. Angelo, and Kenton Heidel. 6/7/2018. “A Process for Capturing CO2 from the Atmosphere.” Joule. Publisher's VersionAbstract

Context & Scale
An industrial process for large-scale capture of atmospheric CO2 (DAC) serves two roles. First, as a source of CO2 for making carbon-neutral hydrocarbon fuels, enabling carbon-free energy to be converted into high-energy-density fuels. Solar fuels, for example, may be produced at high-insolation low-cost locations from DAC-CO2 and electrolytic hydrogen using gas-to-liquids technology enabling decarbonization of difficult-to-electrify sectors such as aviation. And second, DAC with CO2 sequestration allows carbon removal.

The feasibility of DAC has been disputed, in part, because publications have not provided sufficient engineering detail to allow independent evaluation of costs. We provide an engineering cost basis for a commercial DAC system for which all major components are either drawn from well-established commercial heritage or described in sufficient detail to allow assessment by third parties. This design reflects roughly 100 person-years of development by Carbon Engineering.

Summary
We describe a process for capturing CO2 from the atmosphere in an industrial plant. The design captures ∼1 Mt-CO2/year in a continuous process using an aqueous KOH sorbent coupled to a calcium caustic recovery loop. We describe the design rationale, summarize performance of the major unit operations, and provide a capital cost breakdown developed with an independent consulting engineering firm. We report results from a pilot plant that provides data on performance of the major unit operations. We summarize the energy and material balance computed using an Aspen process simulation. When CO2 is delivered at 15 MPa, the design requires either 8.81 GJ of natural gas, or 5.25 GJ of gas and 366 kWhr of electricity, per ton of CO2 captured. Depending on financial assumptions, energy costs, and the specific choice of inputs and outputs, the levelized cost per ton CO2 captured from the atmosphere ranges from 94 to 232 $/t-CO2. Sebastian D. Eastham, Debra K. Weisenstein, David W. Keith, and Steven R. H. Barrett. 5/25/2018. “Quantifying the impact of sulfate geoengineering on mortality from air quality and UV-B exposure.” Atmospheric Environment. Publisher's VersionAbstract Sulfate geoengineering is a proposed method to partially counteract the global radiative forcing from accumulated greenhouse gases, potentially mitigating some impacts of climate change. While likely to be effective in slowing increases in average temperatures and extreme precipitation, there are known side-effects and potential unintended consequences which have not been quantified. One such consequence is the direct human health impact. Given the significant uncertainties, we take a sensitivity approach to explore the mechanisms and range of potential impacts. Using a chemistry-transport model, we quantify the steady-state response of three public health risks to 1 °C global mean surface cooling. We separate impacts into those which are “radiative forcing-driven”, associated with climate change “reversal” through modification of global radiative forcing, and those “direct impacts” associated uniquely with using sulfate geoengineering to achieve this. We find that the direct (non-radiative forcing driven) impact is a decrease in global mortality of ∼13,000 annually. Here the benefits of reduced ozone exposure exceed increases in mortality due to UV and particulate matter, as each unit of injected sulfur incurs 1/25th the particulate matter exposure of a unit of sulfur emitted from surface sources. This reduction is exceeded by radiative forcing-driven health impacts resulting from using sulfate geoengineering to offset 1 °C of surface temperature rise. Increased particulate matter formation at these lower temperatures results in ∼39,000 mortalities which would have been avoided at higher temperatures. As such we estimate that sulfate geoengineering in 2040 would cause ∼26,000 (95% interval: −30,000 to +79,000) early deaths annually relative to the same year without geoengineering, largely due to the loss of health benefits associated with CO2-induced warming. These results account only for impacts due to changes in air quality and UV-B flux. They do not account for non-mortality impacts or changes in atmospheric dynamics, and must be considered in the wider context of other climate change impacts such as heatwave frequency and sea level rise. Jordan P. Smith, John Dykema, and David Keith. 4/2/2018. “Production of Sulfates Onboard an Aircraft: Implications for the Cost and Feasibility of Stratospheric Solar Geoengineering.” Earth and Space Science. Publisher's VersionAbstract Injection of sulfate aerosols into the stratosphere, a form of solar geoengineering, has been proposed as a means to reduce some climatic changes by decreasing net anthropogenic radiative forcing. The cost and technical feasibility of forming aerosols with the appropriate size distribution are uncertain. We examine the possibility of producing the relevant sulfur species, SOor SO3, by in situ conversion fromelemental sulfur onboard an aircraft. We provide afirst-order engineering analysis of an open cycle chemicalplant for in situ sulfur to sulfate conversion using a Brayton cycle combustor and a catalytic converter. We find that such a plant could have sufficiently low mass that the overall requirement for mass transport to the lower stratosphere may be reduced by roughly a factor of 2. All else equal, this suggests that—for a given radiative forcing—the cost of delivering sulfate aerosols may be nearly halved. Beyond reducing cost, the use of elemental sulfur reduces operational health and safety risks and should therefore reduce environmental side effects associated with delivery. Reduction in cost is not necessarily beneficial as it reduces practical barriers to deployment, increasing the urgency of questions concerningthe efficacy, risks, and governance of solar geoengineering. Douglas G. MacMartin, Katharine L. Ricke, and David W. Keith. 4/2/2018. “Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target.” Philosophical Transactions of the Royal Society, 376, 2119.Abstract Solar geoengineering refers to deliberately reducing net radiative forcing by reflecting some sunlight back to space, in order to reduce anthropogenic climate changes; a possible such approach would be adding aerosols to the stratosphere. If future mitigation proves insufficient to limit the rise in global mean temperature to less than 1.5°C above preindustrial, it is plausible that some additional and limited deployment of solar geoengineering could reduce climate damages. That is, these approaches could eventually be considered as part of an overall strategy to manage the risks of climate change, combining emissions reduction, net-negative emissions technologies and solar geoengineering to meet climate goals. We first provide a physical science review of current research, research trends and some of the key gaps in knowledge that would need to be addressed to support informed decisions. Next, since few climate model simulations have considered these limited-deployment scenarios, we synthesize prior results to assess the projected response if solar geoengineering were used to limit global mean temperature to 1.5°C above preindustrial in an overshoot scenario that would otherwise peak near 3°C. While there are some important differences, the resulting climate is closer in many respects to a climate where the 1.5°C target is achieved through mitigation alone than either is to the 3◦C climate with no geoengineering. This holds for both regional temperature and precipitation changes; indeed, there are no regions where a majority of models project that this moderate level of geoengineering would produce a statistically significant shift in precipitation further away from preindustrial levels. This article is part of the theme issue ‘The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels’. Sebastian D. Eastham, David W. Keith, and Steven R. H. Barrett. 3/9/2018. “Mortality tradeoff between air quality and skin cancer from changes in stratospheric ozone.” Environmental Research Letters, 13, 3. Publisher's VersionAbstract Skin cancer mortality resulting from stratospheric ozone depletion has been widely studied. Similarly, there is a deep body of literature on surface ozone and its health impacts, with modeling and observational studies demonstrating that surface ozone concentrations can be increased when stratospheric air mixes to the Earth's surface. We offer the first quantitative estimate of the trade-off between these two effects, comparing surface air quality benefits and UV-related harms from stratospheric ozone depletion. Applying an idealized ozone loss term in the stratosphere of a chemistry-transport model for modern-day conditions, we find that each Dobson unit of stratospheric ozone depletion results in a net decrease in the global annual mortality rate of ~40 premature deaths per billion population (d/bn/DU). The impacts are spatially heterogeneous in sign and magnitude, composed of a reduction in premature mortality rate due to ozone exposure of ~80 d/bn/DU concentrated in Southeast Asia, and an increase in skin cancer mortality rate of ~40 d/bn/DU, mostly in Western Europe. This is the first study to quantify air quality benefits of stratospheric ozone depletion, and the first to find that marginal decreases in stratospheric ozone around modern-day values could result in a net reduction in global mortality due to competing health impact pathways. This result, which is subject to significant methodological uncertainty, highlights the need to understand the health and environmental trade-offs involved in policy decisions regarding anthropogenic influences on ozone chemistry over the 21st century. Zhen Dai, Debra Weisenstein, and David Keith. 1/2018. “Tailoring Meridional and Seasonal Radiative Forcing by Sulfate Aerosol Solar Geoengineering.” Geophysical Research Letters, 45. Publisher's VersionAbstract We study the possibility of designing solar radiation management schemes to achieve a desired meridional radiative forcing (RF) profile using a two-dimensional chemistry-transport-aerosol model. Varying SO2 or H2SO4 injection latitude, altitude, and season, we compute RF response functions for a broad range of possible injection schemes, finding that linear combinations of these injection cases can roughly achieve RF profiles that have been proposed to accomplish various climate objectives. Globally averaged RF normalized by the sulfur injection rate (the radiative efficacy) is largest for injections at high altitudes, near the equator, and using emission of H2SO4 vapor into an aircraft wake to produce accumulation-mode particles. There is a trade-off between radiative efficacy and control as temporal and spatial control is best achieved with injections at lower altitudes and higher latitudes. These results may inform studies using more realistic models that couple aerosol microphysics, chemistry, and stratospheric dynamics. David W. Keith, Gernot Wagner, and Claire L. Zabel. 9/1/2017. “Solar geoengineering reduces atmospheric carbon burden.” Nature Climate Change, 7, Pp. 617–619. Publisher's VersionAbstract Solar geoengineering is no substitute for cutting emissions, but could nevertheless help reduce the atmospheric carbon burden. In the extreme, if solar geoengineering were used to hold radiative forcing constant under RCP8.5, the carbon burden may be reduced by ~100 GTC, equivalent to 12–26% of twenty-first-century emissions at a cost of under US$0.5 per tCO2.

Ilissa B. Ocko, Steven P. Hamburg, Daniel J. Jacob, David W. Keith, Nathaniel O. Keohane, Michael Oppenheimer, Joseph D. Roy-Mayhew, Daniel P. Schrag, and Stephen W. Pacala. 5/5/2017. “Unmask temporal trade-offs in climate policy debates.” Science, 356, 6337, Pp. 492-493. Publisher's VersionAbstract

Global warming potentials (GWPs) have become an essential element of climate policy and are built into legal structures that regulate greenhouse gas emissions. This is in spite of a well-known shortcoming: GWP hides trade-offs between short- and long-term policy objectives inside a single time scale of 100 or 20 years (1). The most common form, GWP100, focuses on the climate impact of a pulse emission over 100 years, diluting near-term effects and misleadingly implying that short-lived climate pollutants exert forcings in the long-term, long after they are removed from the atmosphere (2). Meanwhile, GWP20 ignores climate effects after 20 years. We propose that these time scales be ubiquitously reported as an inseparable pair, much like systolic-diastolic blood pressure and city-highway vehicle fuel economy, to make the climate effect of using one or the other time scale explicit. Policy-makers often treat a GWP as a value-neutral measure, but the time-scale choice is central to achieving specific objectives (2–4).

David W. Keith and Peter J. Irvine. 11/30/2016. “Solar geoengineering could substantially reduce climate risks — A research hypothesis for the next decade.” Earth's Future, 4, Pp. 549–559. Publisher's VersionAbstract

We oﬀer a hypothesis that if solar geoengineering (SG) were deployed to oﬀset half of the increase in global-mean temperature from the date of deployment using a technology and deployment method chosen to approximate a reduction in the solar constant then, over the 21st century, it would (a) substantially reduce the global aggregate risks of climate change, (b) without making any country worse oﬀ, and (c) with the aggregate risks from side-eﬀects being small in comparison to the reduction in climate risks. We do not set out to demonstrate this hypothesis; rather we propose it with the goal of stimulating a strategic engagement of the SG research community with policy-relevant questions. We elaborate seven sub-hypotheses on the eﬀects of our scenario for key risks of climate change that could be assessed in future modeling work. As an example, we provide a defence of one of our sub-hypotheses, that our scenario of SG would reduce the risk of drought in dry regions, but also identify issues that may undermine this sub-hypothesis and how future work could resolve this question. SG cannot substitute for emissions mitigation but it may be a useful supplement. It is our hope that scientiﬁc and technical research over the next decade focuses more closely on well-articulated variants of the key policy-relevant question: could SG be designed and deployed in such a way that it could substantially and equitably reduce climate risks?