Ocean acidification and climate change: synergies and challenges of addressing both under the UNFCCC

Ocean acidification and climate change are linked by their common driver: CO₂. Climate change is the consequence of a range of GHG emissions, but ocean acidification on a global scale is caused solely by increased concentrations of atmospheric CO₂. Reducing CO₂ emissions is therefore the most effective way to mitigate ocean acidification. Acting to prevent further ocean acidification by reducing CO₂ emissions will also provide simultaneous benefits by alleviating future climate change. Although it is possible that reducing CO₂ emissions to a level low enough to address ocean acidification will simultaneously address climate change, the reverse is unfortunately not necessarily true. Despite the ocean's integral role in the climate system and the potentially wide-ranging impacts on marine life and humans, the problem of ocean acidification is largely absent from most policy discussions pertaining to CO₂ emissions. The linkages between ocean acidification, climate change and the United Nations Framework Convention on Climate Change (UNFCCC) are identified and possible scenarios for developing common solutions to reduce and adapt to ocean acidification and climate change are offered. Areas where the UNFCCC is currently lacking capacity to effectively tackle rising ocean acidity are also highlighted.     

Reversible and irreversible impacts of greenhouse gas emissions in multi-century projections with the NCAR global coupled carbon cycle-climate model

The legacy of historical and the long-term impacts of 21st century greenhouse gas emissions on climate, ocean acidification, and carbon-climate feedbacks are investigated with a coupled carbon cycle-climate model. Emission commitment scenarios with zero emissions after year 2100 and 21st century emissions of 1,800, 900, and 0 gigatons of carbon are run up to year 2500. The reversibility and irreversibility of impacts is quantified by comparing anthropogenically-forced regional changes with internal, unforced climate variability. We show that the influence of historical emissions and of non-CO₂ agents is largely reversible on the regional scale. Forced changes in surface temperature and precipitation become smaller than internal variability for most land and ocean grid cells in the absence of future carbon emissions. In contrast, continued carbon emissions over the 21st century cause irreversible climate change on centennial to millennial timescales in most regions and impacts related to ocean acidification and sea level rise continue to aggravate for centuries even if emissions are stopped in year 2100. Undersaturation of the Arctic surface ocean with respect to aragonite, a mineral form of calcium carbonate secreted by marine organisms, is imminent and remains widespread. The volume of supersaturated water providing habitat to calcifying organisms is reduced from preindustrial 40 to 25% in 2100 and to 10% in 2300 for the high emission case. We conclude that emission trading schemes, related to the Kyoto Process, should not permit trading between emissions of relatively short-lived agents and CO₂ given the irreversible impacts of anthropogenic carbon emissions.     

Can ocean iron fertilization mitigate ocean acidification?

Ocean iron fertilization has been proposed as a method to mitigate anthropogenic climate change, and there is continued commercial interest in using iron fertilization to generate carbon credits. It has been further speculated that ocean iron fertilization could help mitigate ocean acidification. Here, using a global ocean carbon cycle model, we performed idealized ocean iron fertilization simulations to place an upper bound on the effect of iron fertilization on atmospheric CO₂ and ocean acidification. Under the IPCC A2 CO₂ emission scenario, at year 2100 the model simulates an atmospheric CO₂ concentration of 965 ppm with the mean surface ocean pH 0.44 units less than its pre-industrial value of 8.18. A globally sustained ocean iron fertilization could not diminish CO₂ concentrations below 833 ppm or reduce the mean surface ocean pH change to less than 0.38 units. This maximum of 0.06 unit mitigation in surface pH change by the end of this century is achieved at the cost of storing more anthropogenic CO₂ in the ocean interior, furthering acidifying the deep-ocean. If the amount of net carbon storage in the deep ocean by iron fertilization produces an equivalent amount of emission credits, ocean iron fertilization further acidifies the deep ocean without conferring any chemical benefit to the surface ocean.     

Long-term effects of anthropogenic CO<Subscript>2</Subscript> emissions simulated with a complex earth system model

A new complex earth system model consisting of an atmospheric general circulation model, an ocean general circulation model, a three-dimensional ice sheet model, a marine biogeochemistry model, and a dynamic vegetation model was used to study the long-term response to anthropogenic carbon emissions. The prescribed emissions follow estimates of past emissions for the period 1751–2000 and standard IPCC emission scenarios up to the year 2100. After 2100, an exponential decrease of the emissions was assumed. For each of the scenarios, a small ensemble of simulations was carried out. The North Atlantic overturning collapsed in the high emission scenario (A2) simulations. In the low emission scenario (B1), only a temporary weakening of the deep water formation in the North Atlantic is predicted. The moderate emission scenario (A1B) brings the system close to its bifurcation point, with three out of five runs leading to a collapsed North Atlantic overturning circulation. The atmospheric moisture transport predominantly contributes to the collapse of the deep water formation. In the simulations with collapsed deep water formation in the North Atlantic a substantial cooling over parts of the North Atlantic is simulated. Anthropogenic climate change substantially reduces the ability of land and ocean to sequester anthropogenic carbon. The simulated effect of a collapse of the deep water formation in the North Atlantic on the atmospheric CO₂ concentration turned out to be relatively small. The volume of the Greenland ice sheet is reduced, but its contribution to global mean sea level is almost counterbalanced by the growth of the Antarctic ice sheet due to enhanced snowfall. The modifications of the high latitude freshwater input due to the simulated changes in mass balance of the ice sheet are one order of magnitude smaller than the changes due to atmospheric moisture transport. After the year 3000, the global mean surface temperature is predicted to be almost constant due to the compensating effects of decreasing atmospheric CO₂ concentrations due to oceanic uptake and delayed response to increasing atmospheric CO₂ concentrations before.     

Dangerous anthropogenic interference, dangerous climatic change, and harmful climatic change: non-trivial distinctions with significant policy implications

Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) calls for stabilization of greenhouse gas (GHG) concentrations at levels that prevent dangerous anthropogenic interference (DAI) in the climate system. However, some of the recent policy literature has focused on dangerous climatic change (DCC) rather than on DAI. DAI is a set of increases in GHGs concentrations that has a non-negligible possibility of provoking changes in climate that in turn have a non-negligible possibility of causing unacceptable harm, including harm to one or more of ecosystems, food production systems, and sustainable socio-economic systems, whereas DCC is a change of climate that has actually occurred or is assumed to occur and that has a non-negligible possibility of causing unacceptable harm. If the goal of climate policy is to prevent DAI, then the determination of allowable GHG concentrations requires three inputs: the probability distribution function (pdf) for climate sensitivity, the pdf for the temperature change at which significant harm occurs, and the allowed probability (“risk”) of incurring harm previously deemed to be unacceptable. If the goal of climate policy is to prevent DCC, then one must know what the correct climate sensitivity is (along with the harm pdf and risk tolerance) in order to determine allowable GHG concentrations. DAI from elevated atmospheric CO₂ also arises through its impact on ocean chemistry as the ocean absorbs CO₂. The primary chemical impact is a reduction in the degree of supersaturation of ocean water with respect to calcium carbonate, the structural building material for coral and for calcareous phytoplankton at the base of the marine food chain. Here, the probability of significant harm (in particular, impacts violating the subsidiary conditions in Article 2 of the UNFCCC) is computed as a function of the ratio of total GHG radiative forcing to the radiative forcing for a CO₂ doubling, using two alternative pdfs for climate sensitivity and three alternative pdfs for the harm temperature threshold. The allowable radiative forcing ratio depends on the probability of significant harm that is tolerated, and can be translated into allowable CO₂ concentrations given some assumption concerning the future change in total non-CO₂ GHG radiative forcing. If future non-CO₂ GHG forcing is reduced to half of the present non-CO₂ GHG forcing, then the allowable CO₂ concentration is 290–430 ppmv for a 10% risk tolerance (depending on the chosen pdfs) and 300–500 ppmv for a 25% risk tolerance (assuming a pre-industrial CO₂ concentration of 280 ppmv). For future non-CO₂ GHG forcing frozen at the present value, and for a 10% risk threshold, the allowable CO₂ concentration is 257–384 ppmv. The implications of these results are that (1) emissions of GHGs need to be reduced as quickly as possible, not in order to comply with the UNFCCC, but in order to minimize the extent and duration of non-compliance; (2) we do not have the luxury of trading off reductions in emissions of non-CO₂ GHGs against smaller reductions in CO₂ emissions, and (3) preparations should begin soon for the creation of negative CO₂ emissions through the sequestration of biomass carbon.     

Adjustment of the natural ocean carbon cycle to negative emission rates

Carbon dioxide removal (CDR) is the only geoengineering technique that allows negative emissions and the reduction of anthropogenic carbon in the atmosphere. Since the time scales of the global carbon cycle are largely driven by the exchanges with the natural oceanic stocks, the implementation of CDR actions is anticipated to create outgassing from the ocean that may reduce their efficiency. The adjustment of the natural carbon cycle to CDR was studied with a numerical Earth System Model, focusing on the oceanic component and considering two idealized families of CDR policies, one based on a target atmospheric concentration and one based on planned negative emissions. Results show that both actions are anticipated to release the anthropogenic carbon stored in the surface ocean, effectively increasing the required removal effort. The additional negative emissions are expected to be lower when the CDR policy is driven by planned removal rates without prescribing a target atmospheric CO₂ concentration.     

Colonial contexts and the feasibility of mitigation through transition: A study of the impact of historical processes on the emissions dynamics of nation-states

It has long been accepted that the relative affluence and technological efficiency of nations are important contributors to their rate of emissions. These associations have, in turn, driven questions about the feasibility of mitigating anthropogenic greenhouse gas emissions through incremental transition to “business as usual” policy structures in variant social contexts. Here, I explore the extent to which the historical context of colonial relations impacts the feasibility of a nation mitigating emissions per capita, emissions per dollar, and total emissions under current development logics. To do so I examine the structure of variation for 152 nations during the 1960–2018 period. Subsequently, I examine how being situated as an extractive colony in the past serves to moderate the association of GDP per capita with CO ₂ emissions per capita, CO ₂ emissions per dollar, and total CO ₂ emissions in the present. I find that roughly 11% of cross-national variation in CO ₂ emissions per capita and CO ₂ emissions per dollar, as well as nearly 6% of variation in total CO ₂ emissions between 1960 and 2018 is attributable to having been historically subjected to extractive colonial processes. These findings suggest that mitigation of emissions through transition of “business as usual” policy structures appears significantly less feasible for nations positioned as extractive colonies in the past, relative to all others.     

Overlooked ocean strategies to address climate change

The U.N. Framework Convention on Climate Change’s (UNFCCC’s) Paris Agreement—which aims to limit climate change and increase global resilience to its effects—was a breakthrough in climate diplomacy, committing its Parties to develop and update national climate plans. Yet the Parties to the Agreement have largely overlooked the effect of climate change on ocean-based communities, economies, and ecosystems—as well as the role that the ocean can play in mitigating and adapting to climate change. Because the ocean is an integral part of the climate system, stronger inclusion of ocean issues is critical to achieving the Agreement’s goals. Here we discuss four ocean-climate linkages that suggest specific responses by Parties to the Agreement connected to 1) accelerating climate ambition, including via sustainable ocean-based mitigation strategies; 2) focusing on CO₂ emissions to address ocean acidification; 3) better understanding ocean-based mitigation; and 4) pursuing ocean-based adaptation. These linkages offer a more complete perspective on the reasons strong climate action is necessary and inform a systematic approach for addressing ocean issues under the Agreement to strengthen climate mitigation and adaptation.     

Estimates of anthropogenic CO<Subscript>2</Subscript> uptake in a global ocean model

A global ocean general circulation model (L30T63) is employed to study the uptake and distribution of anthropogenic CO₂ in the ocean. A subgrid-scale mixing scheme called GM90 is used in the model. There are two main GM90 parameters including isopycnal diffusivity and skew (thickness) diffusivity. Sensitivities of the ocean circulation and the redistribution of dissolved anthropogenic CO₂ to these two parameters are examined. Two runs estimate the global oceanic anthropogenic CO₂ uptake to be 1.64 and 1.73 Pg C yr^-1 for the 1990s, and that the global ocean contained 86.8 and 92.7 Pg C of anthropogenic CO₂ at the end of 1994, respectively. Both the total inventory and uptake from our model are smaller than the data-based estimates. In this presentation, the vertical distributions of anthropogenic CO₂ at three meridional sections are discussed and compared with the available data-based estimates. The inventory in the individual basins is also calculated. Use of large isopycnal diffusivity can generally improve the simulated results, including the exchange flux, the vertical distribution patterns, inventory, storage, etc. In terms of comparison of the vertical distributions and column inventory, we find that the total inventory in the Pacific Ocean obtained from our model is in good agreement with the data-based estimate, but a large difference exists in the Atlantic Ocean, particularly in the South Atlantic. The main reasons are weak vertical mixing and that our model generates small exchange fluxes of anthropogenic CO₂ in the Southern Ocean. Improvement in the simulation of the vertical transport and sea ice in the Southern Ocean is important in future work.     

How does framing affect policy support for emissions mitigation? Testing the effects of ocean acidification and other carbon emissions frames

Public support for carbon emissions mitigation is crucial to motivate action to address global issues like climate change and ocean acidification (OA). Yet in the public sphere, carbon emissions mitigation policies are typically discussed in the context of climate change and rarely in the context of OA or other global change outcomes. In this paper, we advance research on OA and climate change perceptions and communication, by (i) examining causal beliefs about ocean acidification, and (ii) measuring support for mitigation policies from individuals presented with one of five different policy frames (climate change, global warming, carbon pollution, air pollution, and ocean acidification). Knowledge about OA causes and consequences is more widespread than we anticipated, though still generally low. Somewhat surprisingly, an “air pollution” mitigation frame elicits the highest degree of policy support overall, while “carbon pollution” performs no better than “climate change” or “global warming.” Framing effects are in part contingent on prior knowledge and attitudes, and mediated by concern. Perhaps due to a lack of OA awareness, the OA frame generates the least support overall, although it seems to close the gap in support associated with political orientation: the OA frame increases support among those (few) conservatives who report having heard of OA before the survey. These findings complement previous work on climate change communication and suggest the need for further research into OA as an effective way to engage conservatives in carbon emissions mitigation policy. Potentially even more promising is the air pollution framing.     

Emission scenarios and global climate protection

This paper evaluates the effectiveness of a wide range of emission scenarios in protecting climate (where ‘protecting climate’ Is used here to mean minimizing ‘dangerous anthropogenic interference with the climate system’ which results in impacts to society and the natural environment). Under baseline (no action) conditions there is a significant Increase in emissions, temperature and climate impacts. Controlling only CO₂ emissions (ie freezing emissions in year 2000 at 1990 levels, and decreasing them afterwards at 1%/yr) and only in Annex I countries, does not significantly reduce the impacts observed under the baseline scenario. However, impacts are substantially reduced when emissions are controlled in both Annex I and non-Annex I countries, and when both CO₂ and non-CO₂ emissions are controlled. It was also found that stabilizing CO₂ in the atmosphere below 450 ppm substantially reduces climate impacts. But in order to follow the pathway to stabilization at 450 ppm specified by the IPCC, global emissions can only slightly increase in the coming decades, and then must be sharply reduced. On the other hand, stabilizing CO₂ in the atmosphere above 450 ppm can have significant impacts, which indicates that stabilization of greenhouse gases in the atmosphere will not necessarily provide a high level of climate protection. Results from these and other scenarios are synthesized and related to climate protection goals through a new concept — ‘safe emission corridors’. These corridors indicate the allowable range of near-term global emissions (equivalent CO₂) which complies with specified short- and long-term climate goals. For an illustrative set of climate goals, the allowable anthropogenic global emissions in 2010 are computed to range from 7.3 to 14.5 GtC/yr equivalent CO₂ (1990 level = approximately 9.6 GtC/ yr); when these limits are set twice as strict (ie divided by two), the allowable range becomes 7.6 to 9.3 GtC/yr. To fall within this lower corridor, global emissions must be lower in 2010 than in 1990.     

Carbon dioxide capture and storage: a status report

Abstract

Fossil fuel combustion is the largest source of anthropogenic greenhouse gas (GHG) emissions. As a result of combustion, essentially all of the fuel carbon is emitted to the atmosphere as carbon dioxide (CO₂), along with small amounts of methane and, in some cases, nitrous oxide. It has been axiomatic that reducing anthropogenic GHG emissions requires reducing fossil-fuel use. However, that relationship may no longer be as highly coupled in the future. There is an emerging understanding that CO₂ capture and storage (CCS) technology offers a way of using fossil fuels while reducing CO₂ emissions by 85% or more. While CCS is not the ‘silver bullet’ that in and of itself will solve the climate change problem, it is a powerful addition to the portfolio of technologies that will be needed to address climate change. The goal of this Commentary is to describe CCS technology in simple terms: how it might be used, how it might fit into longer term mitigation strategies, and finally, the policy issues that its emergence creates. All of these topics are discussed in much greater detail in the recently published Intergovernmental Panel on Climate Change (IPCC) Special Report on Carbon Dioxide Capture and Storage (SRCCS) (IPCC, 2005).     

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.     

Is aerosol scattering in the stratosphere a safety technology preventing global warming?

In accordance with numerous investigations, global climate warming due to the increased greenhouse gas content in the atmosphere can significantly influence the environment already in the near decades. In order to mitigate or prevent possible adverse consequences of this warming the technologies on reducing greenhouse gas emissions as well as a deliberate interference with climate, including its control, are under consideration. Let us analyze the present investigations on the estimate of the influence of a simultaneous increase in the atmospheric CO₂ concentration and in the stratospheric aerosol on the global and regional climate, ozone layer, and World Ocean acidification. It is noted that the production and subsequent maintenance of the artificial aerosol layer in the stratosphere could, in principle, eliminate or retard climate warming, but it would be accompanied by a decrease in the global precipitation, especially in the tropical zone. Furthermore, the stratospheric aerosol screen does not solve the problem of the atmospheric CO₂ increase, which in turn results in the further World Ocean acidification, and thus has an adverse effect on the marine part of the biosphere. Political and ethic issues connected with the deliberate global man interference with the natural environment are also under considerations.     

Ocean dynamics determine the response of oceanic CO2 uptake to climate change

The increase of atmospheric CO₂ concentrations due to anthropogenic activities is substantially damped by the ocean, whose CO₂ uptake is determined by the state of the ocean, which in turn is influenced by climate change. We investigate the mechanisms of the ocean’s carbon uptake within the feedback loop of atmospheric CO₂ concentration, climate change and atmosphere/ocean CO₂ flux. We evaluate two transient simulations from 1860 until 2100, performed with a version of the Max Planck Institute Earth System Model (MPI-ESM) with the carbon cycle included. In both experiments observed anthropogenic CO₂ emissions were prescribed until 2000, followed by the emissions according to the IPCC Scenario A2. In one simulation the radiative forcing of changing atmospheric CO₂ is taken into account (coupled), in the other it is suppressed (uncoupled). In both simulations, the oceanic carbon uptake increases from 1 GT C/year in 1960 to 4.5 GT C/year in 2070. Afterwards, this trend weakens in the coupled simulation, leading to a reduced uptake rate of 10% in 2100 compared to the uncoupled simulation. This includes a partial offset due to higher atmospheric CO₂ concentrations in the coupled simulation owing to reduced carbon uptake by the terrestrial biosphere. The difference of the oceanic carbon uptake between both simulations is primarily due to partial pressure difference and secondary to solubility changes. These contributions are widely offset by changes of gas transfer velocity due to sea ice melting and wind changes. The major differences appear in the Southern Ocean (?45%) and in the North Atlantic (?30%), related to reduced vertical mixing and North Atlantic meridional overturning circulation, respectively. In the polar areas, sea ice melting induces additional CO₂ uptake (+20%).     

Reducing emissions from deforestation—The “combined incentives” mechanism and empirical simulations

Despite accounting for 17–25% of anthropogenic emissions, deforestation was not included in the Kyoto Protocol. The UN Convention on Climate Change is considering its inclusion in future agreements and asked its scientific board to study methodological and scientific issues related to positive incentives to reduce emissions from deforestation. Here we present an empirically derived mechanism that offers a mix of incentives to developing countries to reduce emissions from deforestation, conserve and possibly enhance their ecosystem's carbon stocks. We also use recent data to model its effects on the 20 most forested developing countries. Results show that at low CO₂ prices (～US$ 8/t CO₂) a successful mechanism could reduce more than 90% of global deforestation at an annual cost of US$ 30 billion.     

A blind expert test of contrarian claims about climate data

Although virtually all experts agree that CO₂ emissions are causing anthropogenic global warming, public discourse is replete with contrarian claims that either deny that global warming is happening or dispute a human influence. Although the rejection of climate science is known to be driven by ideological, psychological, and political factors rather than scientific disagreement, contrarian views have considerable prominence in the media. A better understanding of contrarian discourse is therefore called for. We report a blind expert test of contrarian claims about climatological variables. Expert economists and statisticians were presented with representative contrarian statements (e.g., “Arctic ice is recovering”) translated into an economic or demographic context. In that blind test, contrarian claims were found to be misleading. By contrast, mainstream scientific interpretations of the data were judged to be accurate and policy relevant. The results imply that media inclusion of contrarian statements may increase bias rather than balance.     

Factors influencing anthropogenic carbon dioxide uptake in the North Atlantic in models of the ocean carbon cycle

The uptake and storage of anthropogenic carbon in the North Atlantic is investigated using different configurations of ocean general circulation/carbon cycle models. We investigate how different representations of the ocean physics in the models, which represent the range of models currently in use, affect the evolution of CO₂ uptake in the North Atlantic. The buffer effect of the ocean carbon system would be expected to reduce ocean CO₂ uptake as the ocean absorbs increasing amounts of CO₂. We find that the strength of the buffer effect is very dependent on the model ocean state, as it affects both the magnitude and timing of the changes in uptake. The timescale over which uptake of CO₂ in the North Atlantic drops to below preindustrial levels is particularly sensitive to the ocean state which sets the degree of buffering; it is less sensitive to the choice of atmospheric CO₂ forcing scenario. Neglecting physical climate change effects, North Atlantic CO₂ uptake drops below preindustrial levels between 50 and 300 years after stabilisation of atmospheric CO₂ in different model configurations. Storage of anthropogenic carbon in the North Atlantic varies much less among the different model configurations, as differences in ocean transport of dissolved inorganic carbon and uptake of CO₂ compensate each other. This supports the idea that measured inventories of anthropogenic carbon in the real ocean cannot be used to constrain the surface uptake. Including physical climate change effects reduces anthropogenic CO₂ uptake and storage in the North Atlantic further, due to the combined effects of surface warming, increased freshwater input, and a slowdown of the meridional overturning circulation. The timescale over which North Atlantic CO₂ uptake drops to below preindustrial levels is reduced by about one-third, leading to an estimate of this timescale for the real world of about 50 years after the stabilisation of atmospheric CO₂. In the climate change experiment, a shallowing of the mixed layer depths in the North Atlantic results in a significant reduction in primary production, reducing the potential role for biology in drawing down anthropogenic CO₂.     

Historical and future anthropogenic emission pathways derived from coupled climate�Ccarbon cycle simulations

Using a coupled climate?Ccarbon cycle model, fossil fuel carbon dioxide (CO₂) emissions are derived through a reverse approach of prescribing atmospheric CO₂ concentrations according to observations and future projections, respectively. In the second half of the twentieth century, the implied fossil fuel emissions, and also the carbon uptake by land and ocean, are within the range of observational estimates. Larger discrepancies exist in the earlier period (1860?C1960), with small fossil fuel emissions and uncertain emissions from anthropogenic land cover change. In the IPCC SRES A1B scenario, the simulated fossil fuel emissions more than double until 2050 (17 GtC/year) and then decrease to 12 GtC/year by 2100. In addition to A1B, an aggressive mitigation scenario was employed, developed within the European ENSEMBLES project, that peaks at 530 ppm CO₂(equiv) around 2050 and then decreases to approach 450 ppm during the twenty-second century. Consistent with the prescribed pathway of atmospheric CO₂ in E1, the implied fossil fuel emissions increase from currently 8 GtC/year to about 10 by 2015 and decrease thereafter. In the 2050s (2090s) the emissions decrease to 3.4 (0.5) GtC/year, respectively. As in previous studies, our model simulates a positive climate?Ccarbon cycle feedback which tends to reduce the implied emissions by roughly 1 GtC/year per degree global warming. Further, our results suggest that the 450 ppm stabilization scenario may not be sufficient to fulfill the European Union climate policy goal of limiting the global temperature increase to a maximum of 2°C compared to pre-industrial levels.     

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.