The Tuvalu Syndrome

Geoengineering research has historically been inhibited by fears that the perceived availability of a technological fix for climate change, such as the deployment of space-based deflectors, may undermine greenhouse gas abatement efforts. I develop a game theoretic model to show that the credible threat of unilateral geoengineering may instead strengthen global abatement and lead to a self-enforcing climate treaty with full participation. A ‘rogue nation’ may wish to unilaterally geoengineer if it faces extreme climate damages (as with Tuvalu), or if there are minimal local side effects from geoengineering, such as hydrological cycle disruption or stratospheric ozone depletion. However, the costly global side effects of geoengineering may make it individually rational for other countries to reduce emissions to the level where this rogue nation no longer wishes to unilaterally geoengineer. My results suggest a need to model the impacts of a “selfish geoengineer” intent only on maximizing net domestic benefits, as well as a “benevolent geoengineer” out to restore global mean temperature and minimize global side effects.     

Greenhouse-gas versus aerosol forcing and African climate response

There are many indicators that human activity may change climate conditions all around the globe through emissions of greenhouse gases. In addition, aerosol particles are emitted from various natural and anthropogenic sources. One important source of aerosols arises from biomass burning, particularly in low latitudes where shifting cultivation and land degradation lead to enhanced aerosol burden. In this study the counteracting effects of greenhouse gases and aerosols on African climate are compared using climate model experiments with fully interactive aerosols from different sources. The consideration of aerosol emissions induces a remarkable decrease in short-wave solar irradiation near the surface, especially in winter and autumn in tropical West Africa and the Congo Basin where biomass burning is mainly prevailing. This directly leads to a modification of the surface energy budget with reduced sensible heat fluxes. As a consequence, temperature decreases, compensating the strong warming signal due to enhanced trace gas concentrations. While precipitation in tropical Africa is less sensitive to the greenhouse warming, it tends to decrease, if the effect of aerosols from biomass burning is taken into account. This is partly due to the local impact of enhanced aerosol burden and partly to modifications of the large-scale monsoon circulation in the lower troposphere, usually lagging behind the season with maximum aerosol emissions. In the model equilibrium experiments, the greenhouse gas impact on temperature stands out from internal variability at various time scales from daily to decadaland the same holds for precipitation under the additional aerosol forcing. Greenhouse gases and aerosols exhibit an opposite effect on daily temperature extremes, resulting in an compensation of the individual responses under the combined forcing. In terms of precipitation, daily extreme events tend to be reduced under aerosol forcing, particularly over the tropical Atlantic and the Congo basin. These results suggest that the simulation of the multiple aerosol effects from anthropogenic sources represents an important factor in tropical climate change, hence, requiring more attention in climate modelling attempts.     

Effects of Greenhouse Gas Emissions on World Agriculture, Food Consumption, and Economic Welfare

Because of many uncertainties, quantitative estimates of agriculturally related economic impacts of greenhouse gas emissions are often given low confidence. A major source of uncertainty is our inability to accurately project future changes in economic activity, emissions, and climate. This paper focuses on two issues. First, to what extent do variable projections of climate generate uncertainty in agriculturally related economic impacts? Second, to what extent do agriculturally related economic impacts of greenhouse gas emissions depend on economic conditions at the time of impacts? Results indicate that uncertainty due to variable projections of climate is fairly large for most of the economic effects evaluated in this analysis. Results also indicate that economic conditions at the time of impact influence the direction and size of as well as the confidence in the economic effects of identical projections of greenhouse gas impacts. The economic variable that behaves most consistently in this analysis is world crop production. Increases in mean global temperature, for example, cause world crop production to decrease on average under both 1990 and improved economic conditions and in both instances the confidence with respect to variable projections of climate is medium (e.g.,67%) or greater. In addition and as expected, CO₂ fertilization causesworld crop production to increase on average under 1990 and improved economic conditions. These results suggest that crop production may be a fairly robust indicator of the potential impacts of greenhouse gas emissions.A somewhat unexpected finding is that improved economic conditions are not necessarily a panacea to potential greenhouse-gas-induced damages, particularly at the region level. In fact, in some regions, impacts of climate change or CO₂ fertilization that are beneficial undercurrent economic conditions may be detrimental under improved economic conditions (relative to the new economic base). Australia plus New Zealand suffer from this effect in this analysis because under improved economic conditions they are assumed to obtain a relatively large share of income from agricultural exports. When the climate-change and CO₂-fertilization scenariosin this analysis are also included, agricultural exports from Australia plus New Zealand decline on average. The resultant declines in agricultural income in Australia plus New Zealand are too large to be completely offset by rising incomes in other sectors. This indicates that regions that rely on agricultural exports for relatively large shares of their income may be vulnerable not only to direct climate-induced agricultural damages, but also to positive impacts induced by greenhouse gas emissions elsewhere.     

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.     

对IPCC第五次评估报告中有关淡水资源相关结论的解读

IPCC第五次评估报告指出,与淡水资源相关的气候变化风险随着温室气体浓度增加而显著增加。气候变化已经导致区域降水发生显著变化;多年冻土、冰川持续萎缩,积雪不断减少;降雪区春季最大径流量逐渐提前,夏季干旱不断加剧。预估结果表明：21世纪温室气体排放将加剧淡水资源相关风险。如显著减少亚热带干旱地区的地表水和地下水资源,加剧行业之间用水竞争;极端事件（如极端降水）明显影响原水水质,威胁用水安全;气候变化同时将导致农业灌溉用水量增加、能源生产效率降低等不利影响。报告指出需采取硬性基础设施建设和软性制度措施建设相结合的适应措施,加强水资源管理,克服气候变化的负面影响,减少损失。     

Avoiding Dangerous Anthropogenic Interference with the Climate System

The UN Framework Convention on Climate Change calls for the avoidance of “dangerous anthropogenic interference with the climate system”. Among the many plausible choices, dangerous interference with the climate system may be interpreted as anthropogenic radiative forcing causing distinct and widespread climate change impacts such as a widespread demise of coral reefs or a disintegration of the West Antarctic ice sheet. The geological record and numerical models suggest that limiting global warming below critical temperature thresholds significantly reduces the likelihood of these eventualities. Here we analyze economically optimal policies that may ensure this risk-reduction. Reducing the risk of a widespread coral reef demise implies drastic reductions in greenhouse gas emissions within decades. Virtually unchecked greenhouse gas emissions to date (combined with the inertia of the coupled natural and human systems) may have already committed future societies to a widespread demise of coral reefs. Policies to reduce the risk of a West Antarctic ice sheet disintegration allow for a smoother decarbonization of the economy within a century and may well increase consumption in the long run.     

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.     

Climate change in California: scenarios and approaches for adaptation

Even with aggressive global action to reduce greenhouse gas emissions, the climate will continue to change for decades due to previous emissions and the inertia in biogeophysical and social systems. Therefore, as a complement to mitigation actions, society must also focus on enhancing its capacity to adapt to the unavoidable impacts of climate change that we are already experiencing and will continue to experience over the next few decades. Resource managers, regional planners, and government agencies need to consider climate risks in their planning. We provide an overview of climate change scenarios for California and suggestions on the use of climate projections in state and regional planning efforts in the future.     

A GCM study of future climate response to aerosol pollution reductions

We use the global atmospheric GCM aerosol model ECHAM5-HAM to asses possible impacts of future air pollution mitigation strategies on climate. Air quality control strategies focus on the reduction of aerosol emissions. Here we investigate the extreme case of a maximum feasible end-of-pipe abatement of aerosols in the near term future (2030) in combination with increasing greenhouse gas (GHG) concentrations. The temperature response of increasing GHG concentrations and reduced aerosol emissions leads to a global annual mean equilibrium temperature response of 2.18 K. When aerosols are maximally abated only in the Industry and Powerplant sector, while other sectors stay with currently enforced regulations, the temperature response is 1.89 K. A maximum feasible abatement applied in the Domestic and Transport sector, while other sectors remain with the current legislation, leads to a temperature response of 1.39 K. Increasing GHG concentrations alone lead to a temperature response of 1.20 K. We also simulate 2–5% increases in global mean precipitation among all scenarios considered, and the hydrological sensitivity is found to be significantly higher for aerosols than for GHGs. Our study, thus highlights the huge potential impact of future air pollution mitigation strategies on climate and supports the need for urgent GHG emission reductions. GHG and aerosol forcings are not independent as both affect and are influenced by changes in the hydrological cycle. However, within the given range of changes in aerosol emissions and GHG concentrations considered in this study, the climate response towards increasing GHG concentrations and decreasing aerosols emissions is additive.     

Correlation between climate sensitivity and aerosol forcing and its implication for the “climate trap”

Climate sensitivity and aerosol forcing are dominant uncertain properties of the global climate system. Their estimates based on the inverse approach are interdependent as historical temperature records constrain possible combinations. Nevertheless, many literature projections of future climate are based on the probability density of climate sensitivity and an independent aerosol forcing without considering the interdependency of such estimates. Here we investigate how large such parameter interdependency affects the range of future warming in two distinct settings: one following the A1B emission scenario till the year 2100 and the other assuming a shutdown of all greenhouse gas and aerosol emissions in the year 2020. We demonstrate that the range of projected warming decreases in the former case, but considerably broadens in the latter case, if the correlation between climate sensitivity and aerosol forcing is taken into account. Our conceptual study suggests that, unless the interdependency between the climate sensitivity and aerosol forcing estimates is properly considered, one could underestimate a risk involving the “climate trap”, an unpalatable situation with a high climate sensitivity in which a very drastic mitigation may counter-intuitively accelerate the warming by unmasking the hidden warming due to aerosols.     

Multi-fingerprint detection and attribution analysis of greenhouse gas, greenhouse gas-plus-aerosol and solar forced climate change

 A multi-fingerprint analysis is applied to the detection and attribution of anthropogenic climate change. While a single fingerprint is optimal for the detection of climate change, further tests of the statistical consistency of the detected climate change signal with model predictions for different candidate forcing mechanisms require the simultaneous application of several fingerprints. Model-predicted climate change signals are derived from three anthropogenic global warming simulations for the period 1880 to 2049 and two simulations forced by estimated changes in solar radiation from 1700 to 1992. In the first global warming simulation, the forcing is by greenhouse gas only, while in the remaining two simulations the direct influence of sulfate aerosols is also included. From the climate change signals of the greenhouse gas only and the average of the two greenhouse gas-plus-aerosol simulations, two optimized fingerprint patterns are derived by weighting the model-predicted climate change patterns towards low-noise directions. The optimized fingerprint patterns are then applied as a filter to the observed near-surface temperature trend patterns, yielding several detection variables. The space-time structure of natural climate variability needed to determine the optimal fingerprint pattern and the resultant signal-to-noise ratio of the detection variable is estimated from several multi-century control simulations with different CGCMs and from instrumental data over the last 136 y. Applying the combined greenhouse gas-plus-aerosol fingerprint in the same way as the greenhouse gas only fingerprint in a previous work, the recent 30-y trends (1966–1995) of annual mean near surface temperature are again found to represent a significant climate change at the 97.5% confidence level. However, using both the greenhouse gas and the combined forcing fingerprints in a two-pattern analysis, a substantially better agreement between observations and the climate model prediction is found for the combined forcing simulation. Anticipating that the influence of the aerosol forcing is strongest for longer term temperature trends in summer, application of the detection and attribution test to the latest observed 50-y trend pattern of summer temperature yielded statistical consistency with the greenhouse gas-plus-aerosol simulation with respect to both the pattern and amplitude of the signal. In contrast, the observations are inconsistent with the greenhouse-gas only climate change signal at a 95% confidence level for all estimates of climate variability. The observed trend 1943–1992 is furthermore inconsistent with a hypothesized solar radiation change alone at an estimated 90% confidence level. Thus, in contrast to the single pattern analysis, the two pattern analysis is able to discriminate between different forcing hypotheses in the observed climate change signal. The results are subject to uncertainties associated with the forcing history, which is poorly known for the solar and aerosol forcing, the possible omission of other important forcings, and inevitable model errors in the computation of the response to the forcing. Further uncertainties in the estimated significance levels arise from the use of model internal variability simulations and relatively short instrumental observations (after subtraction of an estimated greenhouse gas signal) to estimate the natural climate variability. The resulting confidence limits accordingly vary for different estimates using different variability data. Despite these uncertainties, however, we consider our results sufficiently robust to have some confidence in our finding that the observed climate change is consistent with a combined greenhouse gas and aerosol forcing, but inconsistent with greenhouse gas or solar forcing alone. Received: 28 April 1996 / Accepted: 27 January 1997     

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”.     

A transient climate change simulation with greenhouse gas and aerosol forcing: experimental design and comparison with the instrumental record for the twentieth century

 The Canadian Centre for Climate Modelling and Analysis (CCCma) global coupled model is used to investigate the potential climate effects of increasing greenhouse gas (GHG) concentrations and changes in sulfate aerosol loadings. The forcing scenario adopted closely resembles that of Mitchell et al. for both the greenhouse gas and aerosol components. Its implementation in the model and the resulting changes in forcing are described. Five simulations of 200 years in length, nominally for the years 1900 to 2100, are available for analysis. They consist of a control simulation without change in forcing, three independent simulations with the same greenhouse gas and aerosol changes, and a single simulation with greenhouse gas only forcing. Simulations of the evolution of temperature and precipitation from 1900 to the present are compared with available observations. Temperature and precipitation are primary climate variables with reasonable temporal and spatial coverage in the observational record for the period. The simulation of potential climate change from the present to the end of the twenty-first century, based on projected GHG and aerosol forcing changes, is discussed in a companion paper. For the historical period dealt with here, the GHG and aerosol forcing has changed relatively little compared to the forcing changes projected to the end of the twenty-first century. Nevertheless, the forced climate signal for temperature in the model is reasonably consistent with the observed global mean temperature from the instrumental record. This is true also for the trend in zonally averaged temperature as a function of latitude and for some aspects of the geographical and regional distributions of temperature. Despite the modest change in overall forcing, the difference between GHG+aerosol and GHG-only forcing is discernible in the temperature response for this period. Changes in precipitation, on the other hand, are much less evident in both the instrumental and simulated record. There is an apparent increasing trend in average precipitation in both the observations and the model results over that part of the land for which observations are available. Regional and geographical changes and trends (which are less affected by sampling considerations), if they exist, are masked by the large natural variability of precipitation in both model and observations. Received: 24 September 1998 / Accepted: 8 October 1999     

Safe policies in an uncertain climate: an application of FUND

Various aspects of the role of uncertainty in greenhouse gas emission reduction policy are analyzed with the integrated assessment model FUND. FUND couples simple models of economy, climate, climate impacts, and emission abatement. Probability distribution functions are assumed for all major parameters in the model. Monte Carlo analyses are used to study the effects of parametric uncertainties. Uncertainties are found to be large and grow over time. Uncertainties about climate change impacts are more serious than uncertainties about emission reduction costs, so that welfare-maximizing policies are stricter under uncertainty than under certainty. This is more pronounced without than with international cooperation. Whether or not countries cooperate with one another is more important than whether or not uncertainty is considered. Meeting exogenously defined emission targets may be more or less difficult under uncertainty than under certainty, depending on the asymmetry in the uncertainty and the central estimate of interest. The major uncertainty in meeting emissions targets in each of a range of possible future is the timing of starting (serious) reduction policies. In a scenario aiming at a stable CO₂ concentration of 550 ppm, the start date varies 20 years for Annex I countries, and much longer for non-Annex countries. Atmospheric stabilization at 550 ppm does not avoid serious risks with regard to climate change impacts. At the long term, it is possible to avoid such risks but only through very strict emission control at high economic costs.     

Integrated assessment of abrupt climatic changes

One of the most controversial conclusions to emerge from many of the first generation of integrated assessment models (IAMs) of climate policy was the perceived economic optimality of negligible near-term abatement of greenhouse gases. Typically, such studies were conducted using smoothly varying climate change scenarios or impact responses. Abrupt changes observed in the climatic record and documented in current models could substantially alter the stringency of economically optimal IAM policies. Such abrupt climatic changes — or consequent impacts — would be less foreseeable and provide less time to adapt, and thus would have far greater economic or environmental impacts than gradual warming. We extend conventional, smooth IAM analysis by coupling a climate model capable of one type of abrupt change to a well-established energy–economy model (DICE). We compare the DICE optimal policy using the standard climate sub-model to our version that allows for abrupt change — and consequent enhanced climate damage — through changes in the strength (and possible collapse) of the North Atlantic thermohaline circulation (THC). We confirm the potential significance of abrupt climate change to economically optimal IAM policies, thus calling into question all previous work neglecting such possibilities — at the least for the wide ranges of relevant social and climate system parameters we consider. In addition, we obtain an emergent property of our coupled social–natural system model: “optimal policies” that do consider abrupt changes may, under relatively low discount rates, calculate emission control levels sufficient to avoid significant abrupt change, whereas “optimal policies” disregarding abrupt change would not prevent this non-linear event. However, there is a threshold in discount rate above which the present value of future damages is so low that even very large enhanced damages in the 22nd century, when a significant abrupt change such as a THC collapse would be most likely to occur, do not increase optimal control levels sufficiently to prevent such a collapse. Thus, any models not accounting for potential abrupt non-linear behavior and its interaction with the discounting formulation are likely to miss an important set of possibilities relevant to the climate policy debate.     

Sensitivity of multi-gas climate policy to emission metrics

The Global Warming Potential (GWP) index is currently used to create CO₂-equivalent emission totals for multi-gas greenhouse targets. While many alternatives have been proposed, it is not possible to uniquely define a metric that captures the different impacts of emissions of substances with widely disparate atmospheric lifetimes, which leads to a wide range of possible index values. We examine the sensitivity of emissions and climate outcomes to the value of the index used to aggregate methane emissions using a technologically detailed integrated assessment model. The methane index is varied between 4 and 70, with a central value of 21, which is the 100-year GWP value currently used in policy contexts. We find that the sensitivity to index value is, at most, 10–18 % in terms of methane emissions but only 2–3 % in terms of the maximum total radiative forcing change, with larger regional emissions differences in some cases. The choice of index also affects estimates of the cost of meeting a given end of century forcing target, with total two-gas mitigation cost increasing by 7–9 % if the index is increased, and increasing in most scenarios from 4 to 23 % if the index is lowered, with a slight (1 %) decrease in total cost in one case. We find that much of the methane abatement occurs as the induced effect of CO₂ abatement rather than explicit abatement, which is one reason why climate outcomes are relatively insensitive to the index value. We also find that the near-term climate benefit of increasing the methane index is small.     

Evaluation Endpoints and Climate Policy: Atmospheric Stabilization, Benefit-Cost Analysis, and Near-Term Greenhouse-Gas Emissions

The Framework Convention on Climate Change calls for stabilizing atmospheric concentrations of carbon dioxide and other greenhouse gases. In contrast, many economists espouse the goal of minimizing the present value of abatement costs and damages. The choice between evaluation endpoints – least-cost stabilization and maximization of benefits less costs – involves a tradeoff between accuracy and relevance. Atmospheric concentrations associated with candidate abatement policies can be more accurately predicted, for any level of confidence, than can the monetary values of climate damages associated with those policies. The monetary value of damages is more relevant to the question of what resources should be devoted to abatement, however, because atmospheric concentrations are of little interest except as they influence climate and its impacts on economic activities, ecosystems, and other elements of human concern. As demonstrated using both analytic and numerical models, the choice between endpoints is not solely a matter of analytic convenience but has substantive implications when comparing near-term abatement policies. For the next few decades, maximization of benefits less costs is likely to require greater abatement than will cost-effective stabilization of atmospheric concentrations.     

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.     

Development of a risk-hedging CO2-emission policy, part I: Risks of unrestrained emissions

A rational global strategy with respect to greenhouse-gas emissions would seek to minimize total risk, which is the sum of the risk of negative impacts due to climatic change associated with a given level of emissions, and the risks associated with the process of achieving that emission level. Given the existence of reducible uncertainties in estimating these risks, and the possibility that an emission target thought to minimize total risk is later found to be not strict enough, a risk-hedging strategy is a more realistic policy objective. This paper is Part I of a two-part series in which these risks are reviewed and an interim risk-hedging emission level is proposed. Here, the risks associated with unrestrained greenhouse-gas emissions are reviewed. In particular, the carbon-cycle response to continuing CO₂ emissions; the heat trapping of projected greenhouse gas increases in comparison to other anthropogenic and natural heating or cooling perturbations; the climatic response to heating perturbations; and the impacts of projected climatic change on global agriculture, forests, coastal regions, coral reefs, water resources, terrestrial species, stratospheric and tropospheric ozone, and human comfort and welfare are critically examined. It is concluded that unrestrained emissions of greenhouse gases pose real and substantial risks to human societies and to ecosystems, and that these risks are likely to grow substantially if the climate warms beyond that associated with a CO₂ doubling. These risks clearly justify some action to limit emissions. The magnitude of emission restraint that is justified depends not only on the risks reviewed here, but also on the risks associated with measures to limit greenhouse-gas emissions, which are reviewed in Part II.     

Understanding public complacency about climate change: adults’ mental models of climate change violate conservation of matter

Public attitudes about climate change reveal a contradiction. Surveys show most Americans believe climate change poses serious risks but also that reductions in greenhouse gas (GHG) emissions sufficient to stabilize atmospheric GHG concentrations can be deferred until there is greater evidence that climate change is harmful. US policymakers likewise argue it is prudent to wait and see whether climate change will cause substantial economic harm before undertaking policies to reduce emissions. Such wait-and-see policies erroneously presume climate change can be reversed quickly should harm become evident, underestimating substantial delays in the climate’s response to anthropogenic forcing. We report experiments with highly educated adults – graduate students at MIT – showing widespread misunderstanding of the fundamental stock and flow relationships, including mass balance principles, that lead to long response delays. GHG emissions are now about twice the rate of GHG removal from the atmosphere. GHG concentrations will therefore continue to rise even if emissions fall, stabilizing only when emissions equal removal. In contrast, most subjects believe atmospheric GHG concentrations can be stabilized while emissions into the atmosphere continuously exceed the removal of GHGs from it. These beliefs – analogous to arguing a bathtub filled faster than it drains will never overflow – support wait-and-see policies but violate conservation of matter. Low public support for mitigation policies may arise from misconceptions of climate dynamics rather than high discount rates or uncertainty about the impact of climate change. Implications for education and communication between scientists and nonscientists (the public and policymakers) are discussed.