The economics (or lack thereof) of aerosol geoengineering

Anthropogenic greenhouse gas emissions are changing the Earth’s climate and impose substantial risks for current and future generations. What are scientifically sound, economically viable, and ethically defendable strategies to manage these climate risks? Ratified international agreements call for a reduction of greenhouse gas emissions to avoid dangerous anthropogenic interference with the climate system. Recent proposals, however, call for a different approach: to geoengineer climate by injecting aerosol precursors into the stratosphere. Published economic studies typically neglect the risks of aerosol geoengineering due to (i) the potential for a failure to sustain the aerosol forcing and (ii) the negative impacts associated with the aerosol forcing. Here we use a simple integrated assessment model of climate change to analyze potential economic impacts of aerosol geoengineering strategies over a wide range of uncertain parameters such as climate sensitivity, the economic damages due to climate change, and the economic damages due to aerosol geoengineering forcing. The simplicity of the model provides the advantages of parsimony and transparency, but it also imposes severe caveats on the interpretation of the results. For example, the analysis is based on a globally aggregated model and is hence silent on intragenerational distribution of costs and benefits. In addition, the analysis neglects the effects of learning and has a very simplistic representation of climate change impacts. Our analysis suggests three main conclusions. First, substituting aerosol geoengineering for CO₂ abatement can be an economically ineffective strategy. One key to this finding is that a failure to sustain the aerosol forcing can lead to sizeable and abrupt climatic changes. The monetary damages due to such a discontinuous aerosol geoengineering can dominate the cost-benefit analysis because the monetary damages of climate change are expected to increase with the rate of change. Second, the relative contribution of aerosol geoengineering to an economically optimal portfolio hinges critically on, thus far, deeply uncertain estimates of the damages due to aerosol forcing. Even if we assume that aerosol forcing could be deployed continuously, the aerosol geoengineering does not considerably displace CO₂ abatement in the simple economic optimal growth model until the damages due to the aerosol forcing are rather low. Third, substituting aerosol geoengineering for greenhouse gas emission abatement can fail an ethical test regarding intergenerational justice. Substituting aerosol geoengineering for greenhouse gas emissions abatements constitutes a conscious risk transfer to future generations, in violation of principles of intergenerational justice which demands that present generations should not create benefits for themselves in exchange for burdens on future generations.     

US public opinion on climate change issues: implications for consensus-building and policymaking

While the Conference of the Parties wrangle at an international scale with climate policy, a quiet set of policies and measures is being implemented at a local scale by municipalities across the globe. This study examines the motivation municipalities have for undertaking policies to reduce their greenhouse gas emissions, when the theory of free-ridingwould predict that local administrations should find it difficult to unilaterally reduce their emissions for the benefit of the global climate. Through interviews with officials and/or staff in 23 municipalities in the United States enacting climate policy, data are gathered that suggest local government abatement policies are primarily a top—down decision based on what officials or staff members believe to be “good business” or rational policy choices. They are primarily driven by the potential for realised or perceived cost savings and co-benefits rather than by public pressure. Economic data from some dozen municipal projects are analyzed, finding, while municipalities lack sophisticated accounting techniques, some justification for the often-disputed claim that at least initial reductions in emissions can be made at cost savings. In the United States, with the lack of national abatement policies, it is municipalities that are leading the way in beginning to implement mitigation strategies, even if only for initial reductions.     

Why geoengineering is a public good,even if it is bad

Stephen Gardiner argues that geoengineering does not meet the “canonical technical definition” of a global public good, and that it is misleading to frame geoengineering as a public good. A public good is something that is nonrival and nonexcludable. Contrary to Gardiner’s claims, geoengineering meets both of these criteria. Framing geoengineering as a public good is useful because it allows commentators to draw on the existing economic, philosophical, and social scientific literature on the governance of public goods.     

Global warming estimates,media expectations,and the asymmetry of scientific challenge

Mass media in the U.S. continue to suggest that scientific consensus estimates of global climate disruption, such as those from the Intergovernmental Panel on Climate Change (IPCC), are “exaggerated” and overly pessimistic. By contrast, work on the Asymmetry of Scientific Challenge (ASC) suggests that such consensus assessments are likely to understate climate disruptions. This paper offers an initial test of the competing expectations, making use of the tendency for science to be self-correcting, over time. Rather than relying in any way on the IPCC process, the paper draws evidence about emerging science from four newspapers that have been found in past work to be biased against reporting IPCC findings, consistently reporting instead that scientific findings are “in dispute.” The analysis considers two time periods — one during the time when the papers were found to be overstating challenges to then-prevailing scientific consensus, and the other focusing on 2008, after the IPCC and former Vice-President Gore shared the Nobel Prize for their work on climate disruption, and before opinion polls showed the U.S. public to be growing more skeptical toward climate science once again. During both periods, new scientific findings were more than twenty times as likely to support the ASC perspective than the usual framing of the issue in the U.S. mass media. The findings indicate that supposed challenges to the scientific consensus on global warming need to be subjected to greater scrutiny, as well as showing that, if reporters wish to discuss “both sides” of the climate issue, the scientifically legitimate “other side” is that, if anything, global climate disruption may prove to be significantly worse than has been suggested in scientific consensus estimates to date.     

Global warming mitigation by sulphur loading in the stratosphere: dependence of required emissions on allowable residual warming rate

An approach to mitigate global warming via sulphur loading in the stratosphere (geoengineering) is studied, employing a large ensemble of numerical experiments with the climate model of intermediate complexity IAP RAS CM. The model is forced by the historical+SRES A1B anthropogenic greenhouse gases+tropospheric sulphates scenario for 1860–2100 with additional sulphur emissions in the stratosphere in the twenty-first century. Different ensemble members are constructed by varying values of the parameters governing mass, horizontal distribution and radiative forcing of the stratospheric sulphates. It is obtained that, given a global loading of the sulphates in the stratosphere, among those studied in this paper latitudinal distributions of geoengineering aerosols, the most efficient one at the global basis is that peaked between 50°N and 70°N and with a somewhat smaller burden in the tropics. Uniform latitudinal distribution of stratospheric sulphates is a little less efficient. Sulphur emissions in the stratosphere required to stop the global temperature at the level corresponding to the mean value for 2000–2010 amount to more than 10 TgS/year in the year 2100. These emissions may be reduced if some warming is allowed to occur in the twenty-first century. For instance, if the global temperature trend S _g in every decade of this century is limited not to exceed 0.10 K/decade (0.15 K/decade), geoengineering emissions of 4–14 TgS/year (2–7 TgS/year) would be sufficient. Even if the global warming is stopped, temperature changes in different regions still occur with a magnitude up to 1 K. Their horizontal pattern depends on implied latitudinal distribution of stratospheric sulphates. In addition, for the stabilised global mean surface air temperature, global precipitation decreases by about 10%. If geoengineering emissions are stopped after several decades of implementation, their climatic effect is removed within a few decades. In this period, surface air temperature may grow with a rate of several Kelvins per decade. The results obtained with the IAP RAS CM are further interpreted employing a globally averaged energy–balance climate model. With the latter model, an analytical estimate for sulphate aerosol emissions in the stratosphere required climate mitigation is obtained. It is shown that effective vertical localisation of the imposed radiative forcing is important for geoengineering efficiency.     

International governance of a possible geoengineering intervention to combat climate change

This article explores international governance issues related to a possible future use of geoengineering techniques. Despite the serious arguments against geoengineering, policy-makers may start to take an interest in it in the medium term. The article identifies non-technical characteristics of geoengineering which might influence governance models, and then discusses three broad approaches: through the United Nations, by one state unilaterally, and through a consortium of states. An examination of international legal instruments reveals none that would pose an insuperable barrier to geoengineering. Finally, the article argues for early exploration of the technological, environmental, political and regulatory issues raised by geoengineering, to maximize the chances of good, science-based multilateral decision making if and when geoengineering’s day arrives.     

Geoengineering: Could— or should— we do it?

Schemes to modify large-scale environment systems or to control climate have been seriously proposed for over 50 years, some to (1) increase temperatures in high latitudes, (2) increase precipitation, (3) decrease sea ice, (4) create irrigation opportunities or to offset potential global warming by spreading dust in the stratosphere to reflect away an equivalent amount of solar energy. These and other proposed geoengineering schemes are briefly reviewed from a historical perspective. More recently, many such schemes to advertently modify climate have been proposed as cheaper methods to counteract inadvertent climatic modifications than conventional mitigation techniques such as carbon taxes or pollutant emissions regulations. Whereas proponents argue cost effectiveness, critics of geoengineering argue that there is too much uncertainty to either (1) be confident that any geoengineering scheme would work as planned, or (2) that the many decades of international political stability and cooperation needed for the continuous maintenance of such schemes to offset century long inadvertent efforts is problematic. Moreover, there is potential for transboundary conflicts should negative climatic events occur during geoengineering activities since, given all the large uncertainties, it could not be assured to victims of such events that the schemes were entirely unrelated to their damages. Nevertheless, although I believe it would be irresponsible to implement any large-scale geoengineering scheme until scientific, legal and management uncertainties are substantially narrowed, I do agree that, given the potential for large inadvertent climatic changes now being built into the earth system, more systematic study of the potential for geoengineering is probably needed.     

Global climate policy: will cities lead the way?

While the Conference of the Parties wrangle at an international scale with climate policy, a quiet set of policies and measures is being implemented at a local scale by municipalities across the globe. This study examines the motivation municipalities have for undertaking policies to reduce their greenhouse gas emissions, when the theory of free-riding would predict that local administrations should find it difficult to unilaterally reduce their emissions for the benefit of the global climate. Through interviews with officials and/or staff in 23 municipalities in the United States enacting climate policy, data are gathered that suggest local government abatement policies are primarily a top–down decision based on what officials or staff members believe to be “good business” or rational policy choices. They are primarily driven by the potential for realised or perceived cost savings and co-benefits rather than by public pressure. Economic data from some dozen municipal projects are analyzed, finding, while municipalities lack sophisticated accounting techniques, some justification for the often-disputed claim that at least initial reductions in emissions can be made at cost savings. In the United States, with the lack of national abatement policies, it is municipalities that are leading the way in beginning to implement mitigation strategies, even if only for initial reductions.     

太阳辐射管理地球工程对海洋酸化影响的模拟研究

研究地球工程对海洋酸化的影响对于评估地球工程对全球气候和环境的影响有重要意义。文中使用中等复杂程度的地球系统模式,模拟了典型CO₂高排放情景RCP8.5下,实施太阳辐射管理地球工程对海洋表面的pH和文石（碳酸钙的一种亚稳形态）饱和度的影响,并定量分析了各环境因子对海洋酸化影响的机理。模拟结果表明,在RCP8.5情景下,到2100年,相对于工业革命前水平,全球海洋表面平均pH下降了0.43,文石饱和度下降了1.77。相对于RCP8.5情景,2100年地球工程情景下全球海洋表面平均pH增加了0.003,而文石饱和度降低了0.16。地球工程通过改变溶解无机碳、碱度、温度等环境因子影响海洋酸化。相对于RCP8.5情景,实施地球工程引起的溶解无机碳浓度的增加使pH和文石饱和度均减小,碱度的增加使pH和文石饱和度均增大,温度的降低使pH增大而使文石饱和度减小。总体而言,太阳辐射管理地球工程可以降低全球温度,但无法减缓海洋酸化。     

Does black carbon abatement hamper CO 2 abatement?

Concerns have been raised that near-term black carbon abatement strategies for global warming mitigation would interfere with the longer-term CO₂ abatement efforts. In response, we put forward a “combined target and metric approach”, a theoretical framework, in which the time horizon of the metric is linked to the specific target of the climate policy. In this approach, a shorter time horizon for the metric is justified only when the overall climate policy is tightened; the lower the target level of the climate policy, the earlier the year of the target. Employing a consistent time perspective for the metric and target means that enhanced near-term reduction of short-lived climate forcers does not reduce the importance of the CO₂ abatement, since the overall climate target is stricter.     

Circumspection, reciprocity, and optimal carbon prices

Assessments of the benefits of climate change mitigation—and thus of the appropriate stringency of greenhouse gas emissions abatement—depend upon ethical, legal, and political economic considerations. Global climate change mitigation is often represented as a repeated prisoners’ dilemma in which the net benefits of sustained global cooperation exceed the net benefits of uncooperative unilateral action for any given actor. Global cooperation can be motivated either by circumspection—a decision to account for the damages one’s own actions inflict upon others—or by the expectation of reciprocity from others. If the marginal global benefits of abatement are approximately constant in total abatement, the domestically optimal price approaches the global cooperative optimum linearly with increasing circumspection and reciprocity. Approximately constant marginal benefits are expected if climate damages are quadratic in temperature and if the airborne fraction of carbon emissions is constant. If, on the other hand, damages increase with temperature faster than quadratically or carbon sinks weaken significantly with increasing CO₂ concentrations, marginal benefits will decline with abatement. In this case, the approach to the global optimum is concave and less than full circumspection and/or reciprocity can lead to optimal domestic abatement close to the global optimum.     

Climate Strategy with Co2 Capture from the Air

It is physically possible to capture CO₂ directly from the air and immobilize it in geological structures. Air capture differs from conventional mitigation in three key aspects. First, it removes emissions from any part of the economy with equal ease or difficulty, so its cost provides an absolute cap on the cost of mitigation. Second, it permits reduction in concentrations faster than the natural carbon cycle: the effects of irreversibility are thus partly alleviated. Third, because it is weakly coupled to existing energy infrastructure, air capture may offer stronger economies of scale and smaller adjustment costs than the more conventional mitigation technologies. We assess the ultimate physical limits on the amount of energy and land required for air capture and describe two systems that might achieve air capture at prices under 200 and 500 $/tC using current technology. Like geoengineering, air capture limits the cost of a worst-case climate scenario. In an optimal sequential decision framework with uncertainty, existence of air capture decreases the need for near-term precautionary abatement. The long-term effect is the opposite; assuming that marginal costs of mitigation decrease with time while marginal climate change damages increase, then air capture increases long-run abatement. Air capture produces an environmental Kuznets curve, in which concentrations are returned to preindustrial levels.     

Cutting with both arms of the scissors: the economic and political case for restrictive supply-side climate policies

Proponents of climate change mitigation face difficult choices about which types of policy instrument(s) to pursue. The literature on the comparative evaluation of climate policy instruments has focused overwhelmingly on economic analyses of instruments aimed at restricting demand for greenhouse gas emissions (especially carbon taxes and cap-and-trade schemes) and, to some extent, on instruments that support the supply of or demand for substitutes for emissions-intensive goods, such as renewable energy. Evaluation of instruments aimed at restricting the upstream supply of commodities or products whose downstream consumption causes greenhouse gas emissions—such as fossil fuels—has largely been neglected in this literature. Moreover, analyses that compare policy instruments using both economic and political (e.g. political “feasibility” and “feedback”) criteria are rare. This article aims to help bridge both of these gaps. Specifically, the article demonstrates that restrictive supply-side policy instruments (targeting fossil fuels) have numerous characteristic economic and political advantages over otherwise similar restrictive demand-side instruments (targeting greenhouse gases). Economic advantages include low administrative and transaction costs, higher abatement certainty (due to the relative ease of monitoring, reporting and verification), comprehensive within-sector coverage, some advantageous price/efficiency effects, the mitigation of infrastructure “lock-in” risks, and mitigation of the “green paradox”. Political advantages include the superior potential to mobilise public support for supply-side policies, the conduciveness of supply-side policies to international policy cooperation, and the potential to bring different segments of the fossil fuel industry into a coalition supportive of such policies. In light of these attributes, restrictive supply-side policies squarely belong in the climate policy “toolkit”.     

Reexamining the economics of aerosol geoengineering

In this paper, we extend the work of Goes, Tuana, and Keller (Climatic Change 2011; GTK) by reexamining the economic benefit, of aerosol geoengineering. GTK found that a complete substitution of geoengineering for CO₂ abatement fails a cost-benefit test over a wide range of scenarios regarding (i) the probability that such a program would be aborted and (ii) the economic damages caused by geoengineering itself. In this paper, we reframe the conditions under which GTK assumed geoengineering would/could be used. In so doing, we demonstrate that geoengineering may pass a cost-benefit test over a wide range of scenarios originally considered by GTK.     

CMIP6二氧化碳移除模式比较计划（CDRMIP）概况与评述

地球工程是指通过人工方法在大规模尺度上干预气候系统,使地球气候降温的一种手段。CO₂移除是地球工程的主要途径之一。文中概述了CO₂移除地球工程的科学背景、各种CO₂移除方法的利弊、大尺度应用CO₂移除方法可能产生的气候效应和风险,以及CO₂移除方案在未来气候情景研究中的现状。在第六次国际耦合模式比较计划（CMIP6）框架下开展的CO₂移除模式比较计划（CDRMIP),为深入研究CO₂移除地球工程减缓气候变化的效能和气候系统对其的响应提供了统一试验方案和平台。CDRMIP的开展,对地球工程和气候变化研究具有重要的促进作用。     

Why geoengineering is not a ‘global public good’, and why it is ethically misleading to frame it as one

In early policy work, climate engineering is often described as a global public good. This paper argues that the paradigm example of geoengineering—stratospheric sulfate injection (hereafter ‘SSI’)—does not fit the canonical technical definition of a global public good, and that more relaxed versions are unhelpful. More importantly, it claims that, regardless of the technicalities, the public good framing is seriously misleading, in part because it arbitrarily marginalizes ethical concerns. Both points suggest that more clarity is needed about the aims of geoengineering policy—and especially governance—and that this requires special attention to ethics.     

Fast versus slow response in climate change: implications for the global hydrological cycle

Recent studies have shown that changes in global mean precipitation are larger for solar forcing than for CO₂ forcing of similar magnitude. In this paper, we use an atmospheric general circulation model to show that the differences originate from differing fast responses of the climate system. We estimate the adjusted radiative forcing and fast response using Hansen’s “fixed-SST forcing” method. Total climate system response is calculated using mixed layer simulations using the same model. Our analysis shows that the fast response is almost 40% of the total response for few key variables like precipitation and evaporation. We further demonstrate that the hydrologic sensitivity, defined as the change in global mean precipitation per unit warming, is the same for the two forcings when the fast responses are excluded from the definition of hydrologic sensitivity, suggesting that the slow response (feedback) of the hydrological cycle is independent of the forcing mechanism. Based on our results, we recommend that the fast and slow response be compared separately in multi-model intercomparisons to discover and understand robust responses in hydrologic cycle. The significance of this study to geoengineering is discussed.     

Carbon dioxide sequestration: how much and when?

Carbon dioxide (CO₂) sequestration has been proposed as a key component in technological portfolios for managing anthropogenic climate change, since it may provide a faster and cheaper route to significant reductions in atmospheric CO₂ concentrations than abating CO₂ production. However, CO₂ sequestration is not a perfect substitute for CO₂ abatement because CO₂ may leak back into the atmosphere (thus imposing future climate change impacts) and because CO₂ sequestration requires energy (thus producing more CO₂ and depleting fossil fuel resources earlier). Here we use analytical and numerical models to assess the economic efficiency of CO₂ sequestration and analyze the optimal timing and extent of CO₂ sequestration. The economic efficiency factor of CO₂ sequestration can be expressed as the ratio of the marginal net benefits of sequestering CO₂ and avoiding CO₂ emissions. We derive an analytical solution for this efficiency factor for a simplified case in which we account for CO₂ leakage, discounting, the additional fossil fuel requirement of CO₂ sequestration, and the growth rate of carbon taxes. In this analytical model, the economic efficiency of CO₂ sequestration decreases as the CO₂ tax growth rate, leakage rates and energy requirements for CO₂ sequestration increase. Increasing discount rates increases the economic efficiency factor. In this simple model, short-term sequestration methods, such as afforestation, can even have negative economic efficiencies. We use a more realistic integrated-assessment model to additionally account for potentially important effects such as learning-by-doing and socio-economic inertia on optimal strategies. We measure the economic efficiency of CO₂ sequestration by the ratio of the marginal costs of CO₂ sequestration and CO₂ abatement along optimal trajectories. We show that the positive impacts of investments in CO₂ sequestration through the reduction of future marginal CO₂ sequestration costs and the alleviation of future inertia constraints can initially exceed the marginal sequestration costs. As a result, the economic efficiencies of CO₂ sequestration can exceed 100% and an optimal strategy will subsidize CO₂ sequestration that is initially more expensive than CO₂ abatement. The potential economic value of a feasible and acceptable CO₂ sequestration technology is equivalent – in the adopted utilitarian model – to a one-time investment of several percent of present gross world product. It is optimal in the chosen economic framework to sequester substantial CO₂ quantities into reservoirs with small or zero leakage, given published estimates of marginal costs and climate change impacts. The optimal CO₂ trajectories in the case of sequestration from air can approach the pre-industrial level, constituting geoengineering. Our analysis is silent on important questions (e.g., the effects of model and parametric uncertainty, the potential learning about these uncertainties, or ethical dimension of such geoengineering strategies), which need to be addressed before our findings can be translated into policy-relevant recommendations.     

Possibility of geoengineering stabilization of global temperature in the 21st century using the stratospheric aerosol and estimation of potential negative effects

The climate model of atmospheric and oceanic circulation is used to assess a potential of the geoengineering to stabilize the global temperature at the level of +2°C relative to the average for the 20th century. An anthropogenic forcing was set in accordance with the RCP8.5 scenario. The injection of H₂S into the stratosphere transformed afterwards into the sulfate aerosol starts when the temperature reaches a threshold of +2°C. The intensity of the injection is chosen so that the estimated global temperature remains close to the threshold. It is demonstrated that the stabilization of temperature by geoengineering is possible within +(2 ± 0.11)°C during the 21st century. The stabilization of temperature by the end of the 21st century needs the yearly injection of 4.5 Mt S in the form of H₂S. The specific efficiency of the method is about 0.09°C/Mt of aerosol. It was found that the stabilization of global temperature does not provide the stabilization of mean global precipitation. The maximum influence of aerosol is in the equatorial zone where its specific density in the atmosphere will reach 0.074 g/m² by the end of the 21st century. Carried out is a comparison of regional features of temperature and precipitation fields with and without geoengineering. It is shown that the geoengineering will decrease significantly the regional anomalies in the most part of regions and will not increase them in the rest part. Estimated is an effect of the rapid growth in global temperature at the dramatic cessation of geoengineering impacts. Considered is a variant of the gradual decrease in geoengineering intensity, when the negative effects will be smoothed.     

The radiative forcing benefits of “cool roof” construction in California: quantifying the climate impacts of building albedo modification

Exploiting surface albedo change has been proposed as a form of geoengineering to reduce the heating effect of anthropogenic increases in greenhouse gases (GHGs). Recent modeling experiments have projected significant negative radiative forcing from large-scale implementation of albedo reduction technologies (“cool” roofs and pavements). This paper complements such model studies with measurement-based calculations of the direct radiation balance impacts of replacement of conventional roofing with “cool” roof materials in California. This analysis uses, as a case study, the required changes to commercial buildings embodied in California’s building energy efficiency regulations, representing a total of 4300 ha of roof area distributed over 16 climate zones. The estimated statewide mean radiative forcing per 0.01 increase in albedo (here labeled RF01) is ?1.38 W/m². The resulting unit-roof-area mean annual radiative forcing impact of this regulation is ?44.2 W/m². This forcing is computed to counteract the positive radiative forcing of ambient atmospheric CO₂ at a rate of about 41 kg for each square meter of roof. Aggregated over the 4300 ha of cool roof estimated built in the first decade after adoption of the State regulation, this is comparable to removing about 1.76 million metric tons (MMT) of CO₂ from the atmosphere. The point radiation data used in this study also provide perspective on the spatial variability of cool roof radiative forcing in California, with individual climate zone effectiveness ranging from ?37 to ?59 W/m² of roof. These “bottom-up” calculations validate the estimates reported for published “top down” modeling, highlight the large spatial diversity of the effects of albedo change within even a limited geographical area, and offer a potential methodology for regulatory agencies to account for the climate effects of “cool” roofing in addition to its well-known energy efficiency benefits.