Facing Global Warming
- SCIENCE ESSAY
||6 / 2001
||Stephen H. Schneider And Kristin Kuntz-duriseti
Stephen H. Schneider is professor of environmental biology and
global change at Stanford University. He has studied and
written widely in areas related to policy, science, and
technology at the broad human interface with environment and
global climate. Kristin Kuntz-Duriseti is in the department of
political science at the University of Michigan and is also
associated with the Institute for International Studies and
Biological Sciences at Stanford University.
Given the mounting evidence supporting the view that
the "fingerprints" of human activities are becoming visible
amid the noisy climate data, should we care, or can we do
anything about it? |
The 1990s were the warmest decade in a thousand years. Glaciers receded around the world, plants bloomed and birds laid eggs a week or two earlier than a few decades ago, and damages from catastrophic events such as floods and hurricanes were up 800 percent since the 1970s. Are these the "canaries" in the climate coal mine, or just a long list of coincidental or random phenomena unrelated to global warming? How compelling are these events as evidence of climate change driven by human activities? Certainly, alternate explanations can be proposed. These include land-cover changes resulting from human activities, natural climate variability, the natural end of the previous "mini ice age," and the effect of sunspots or flares.
Given the potentially profound social impact around the world of changes in the climate, the United Nations has initiated the Intergovernmental Panel on Climate Change (IPCC), an international body commissioned by over 100 governments to assess the causes and implications of global climate change. In the IPCC's Second Assessment Report, released in 1995, the 100 plus lead authors of the Working Group I report on the science of climate change were fully aware of proposed alternate explanations of the observed patterns of change in recent decades, as well as the broad range of expert opinions. The authors' central concern was to make the best possible determination of the causes of the century-long warming trend at Earth's surface. After opening their assessment with several caveats on remaining uncertainties, the author scientists concluded their Summary for Policymakers with the now-famous "discernible statement": "Nevertheless, the balance of evidence suggests that there is a discernible impact of human activities on global climate." Their statement had been approved word for word by the participating governments--an arduous task.
The reasons for adopting this widely quoted subjective judgment were many, including a well-validated theoretical case for the natural greenhouse effect, validation tests of both the structure and performance of climate models against contemporary and paleoclimatic data, and several lines of circumstantial evidence supporting the patterns of global warming predicted by coupled atmosphere-ocean models. These "fingerprints" linking human activity and global warming were seen as evidence that the long-anticipated "anthropogenic signal" (i.e., detectable human-induced effects) was finally beginning to show up in the century-long climate record. The difficulty has always been in identifying the emerging signal of global warming against the noisy background of natural climate variability. Although more research is needed to strengthen the confidence with which cause-and-effect claims can be made, enough is already known to warrant a request by the 100 governments for further study of climate issues. They asked specifically for assessments of the possible impacts of projected global warming (the task of Working Group II of the IPCC) and of the relative merits of alternative actions toward mitigating emissions or reducing the costs of adaptations (the role of Working Group III).
This article discusses recent advances in climate change science and reviews suggestions about how a policy aimed at mitigating global warming might be implemented. It emphasizes the differences between the objectives of researchers and policymakers that confound policy analysis. It concludes by separating what is currently known with high confidence regarding climate change from what is more speculative.
Since the Second Assessment Report (SAR) was published, there have been major advances in science's ability to assess the nature and trends of global warming, its possible impacts on environment and society, and the probable costs and effectiveness of mitigation strategies. We touch on several advances here but suggest that readers check out for themselves the conclusions of the three Working Groups in the forthcoming Third Assessment Report (TAR) for a comprehensive discussion (all will be published in the summer of 2001 by Cambridge University Press; summaries can be obtained at http://www.ipcc.ch). In the Working Group I TAR on the science of climate change, five main conclusions in our opinion will emerge as paramount:
1. The global warming of the past century is likely evidence of an anthropogenic signal. Surface temperature measures indicate continued warming, with 1998 being the warmest year in at least 1,000 years. The current best estimate of the increase in globally averaged near-surface temperature since the late nineteenth century is 0.6*C ü 0.2*C (1*F ü 0.4*F) , which is approximately 0.1*C (0.2*F) higher than the estimate given five years ago in the SAR. Evidence comes not only from the current instrumental record but from proxy records (i.e., paleoclimatic data) from tree rings, boreholes, coral reefs, and ice cores. The finding of consistent results across multiple, independent measures increases confidence in the validity of the trend data. Furthermore, confidence in measuring the rate of surface warming has grown.
Although such data suggest strongly that real global warming is under way, they cannot prove with a high degree of confidence that human activities are driving it. Rather, the "noise" in the record of natural climate variability still complicates detection of an emerging signal of anthropogenic global warming.
Another complication is the inconsistency over the two decades since 1979 (when satellite records began) between temperature trends measured at the surface and those measured by weather balloons and inferred from satellite-derived temperatures of the lower atmosphere, the troposphere. From the 1950s to 1980, the readings from the surface and from balloons were in rough agreement about trends; after that, the inconsistency arose. While all three data sets indicate some warming in the troposphere and at the surface, the balloon and satellite data show a warming of approximately 0.05*C (0.09*F) per decade for the past 20 years, even as the surface measures indicate a warming about three times as high during this period. The discrepancy raises questions about whether the "disparity is real or an artifact of the observations" (Working Group I TAR). This has been an extremely contentious issue, especially since initially, an error in the analysis of the remotely sensed satellite data exaggerated the cooling inferred from satellite sensors. When the error was corrected, the satellite-inferred trends switched from cooling to slightly positive. Despite correction of the error, however, a discrepancy between the surface data and satellite data remains.
Hypotheses to explain the inconsistency include spatial differences in measurement, the effects of stratospheric (middle atmosphere) cooling from ozone depletions, errors in extrapolating trends from short-term records, peculiarities of the satellite temperature-reconstruction formulas, a temporary disconnection between surface and atmospheric temperatures (perhaps due to the strong El Niľo cycles in the 1980s and '90s), or some not-yet-understood phenomena. All these possibilities require further exploration, as has been noted in a recent National Academy of Science report and by the IPCC. Regardless, there is virtually no dispute that the global surface temperature record is one of clear global warming since the middle of the nineteenth century.
2. Climatic fingerprints are clearer. Patterns of climate change, including cooling in the stratosphere, warming near the surface, and changes in the day/night and winter/summer temperature differences (both of the latter are reduced, which is predicted by climate models), can be considered to be "fingerprints" of anthropogenic climatic signals. The IPCC has strengthened its assessment by concluding in 2001: "There is now stronger evidence for a human influence on global climate than at the time of the SAR, and it is likely that increasing concentrations of greenhouse gases have contributed substantially to the observed global warming over the last 50 years."
Data from the paleoclimatic record support the inference that it is statistically unlikely that the increase in surface temperatures over the last hundred years is due solely to natural causes. Coupled ocean-atmosphere general circulation models similarly support this conclusion. Specifically, simulations of climate variability apart from human influences do not predict a high temperature as extreme as is observed, even when the simulation includes such natural forcing factors as solar influx and the particles and gases emitted from volcanoes. While it is still impossible to differentiate precisely between the anthropogenic and natural factors forcing climate change, the evidence strongly supports the conclusion that some human component is necessary to explain the observed climate changes.
Additional modeling results since the SAR have confirmed the importance of the effects of sulfate aerosols on observed temperatures. Including the cooling effects of aerosols dramatically improves the fit between model predictions and observed temperature records. Furthermore, the agreement between the prediction and the observed record is too close to have occurred solely by chance.
3. Updated climate projections raise the projected threat. In the SAR, projected warming, based on scenarios run on eight climate models, ranged from about 1 to 3.5*C (1.8--6.3* F) by 2100. In the TAR, the updated predicted range, based on the same basic climate models, is from 1.4 to 5.8*C (2.5--10.5*F) by 2100. This dramatic upward revision results primarily from social and technological scenarios based on changes in the assumptions about the emissions of sulfur and carbon dioxide (CO2). Specifically, unlike those in the SAR, some of the TAR model runs assumed substantial reductions in sulfur emissions, whereas other runs assumed larger CO2 emissions than were considered in the SAR. The projection of reduced future sulfur emissions follows predictions that emissions will decline dramatically due to concern about the health damages and acid rain of sulfur dioxide--polluted air. Just as the now-developed countries cut air pollution as they grew wealthier after the Industrial Revolution, China and India, the new scenarios assume, are expected to gain control of their toxic emissions. Thus, the sulfate aerosol cooling that offsets a substantial fraction of the projected warming in the SAR has been reduced in the TAR.
In a recent article James Hansen of NASA notes that emissions from burning fossil fuels include not only CO2 (which induces warming) but also aerosols that induce both warming (i.e., black soot from fires and diesel exhaust) and cooling (i.e., light-colored sulfate aerosols). Hence, Hansen argues, the most cost-effective approach to reducing human contributions to global warming may be to cut non-CO2 emissions (especially methane) first. This study is flawed, however, in that it does not take full advantage of the latest revisions to the projected sulfur emissions. Thus, although their controversial suggestion to reduce other greenhouse gases or aerosols, in conjunction with or prior to reductions in carbon dioxide emissions, is not unreasonable, the advantages are yet to be quantitatively demonstrated.
4. Climate variability and extremes are more likely, but large uncertainties remain. In the climate-change debate, recognition of the critical importance of climate variability and extreme events is rising. Unfortunately, much of the impacts-assessment literature is based on scenarios in which extreme events and climate variability are not adequately taken into account. Variability in precipitation, most notably associated with an increase in high-intensity rainfall, is projected to increase. In addition, the El Niľo/Southern Oscillation (ENSO) could well continue the trend of the past two decades and become a more recurrent and intense phenomenon. Projections for tropical cyclones (known locally as hurricanes, typhoons, or cyclones), tornadoes, and severe storms are more controversial. Even so, the TAR assesses the increase in hurricane intensities from projected warming as "likely."
Although a causal relationship has not been demonstrated, recent research links increases in atmospheric carbon dioxide and other air pollutants affecting Earth's radiation balance to changes in climate variability and an increase in the number and severity of extreme events. Theoretical models that track the development of hurricanes suggest an increase in tropical storm intensity with warming. The financial services sector, especially the reinsurance industry, has taken particular note of the potential losses from climate change. Losses from weather-related disasters in the last decade were eight times higher than in the 1960s. Moreover, although there is no clear evidence that hurricane frequency has changed over the past few decades (or will change in the next few decades), there is overwhelming evidence that damage from such storms has increased astronomically. Attribution of this trend to changes in socioeconomic factors (e.g., economic growth, population growth, and other demographic changes, as well as increased penetration of insurance coverage) or an increase in the occurrence or intensity of extreme weather events, as a result of global climate change, is uncertain and controversial.
Since tropical cyclones cause the greatest damages of all the top-intensity storms, the credibility of projections of increased hurricane, cyclone, or typhoon intensity due to warming is critical to policymakers. The climate record is currently too noisy to detect a clear signal of increased hurricane intensities, but the theoretical understanding of the driving forces behind hurricanes strongly suggests that peak intensities should be higher in a warmed world.
5. Abrupt nonlinear events ("surprises") are possible, but specifics are speculative. Since 1995, climate-change researchers have been more willing to examine the potential for changes in climatic extremes or even irreversible, potentially catastrophic shifts (for example, a change in the "conveyor belt" circulation in the North Atlantic Ocean, a disintegration of the West Antarctic ice sheet, or a change in the frequency or intensity of severe storms associated with climate regimes like ENSO or the North Atlantic Oscillation). In addition, there is an acknowledgment that climate change can occur quite rapidly, on the order of decades, which is very rapid compared to most sustained changes on a global scale over geological history. Damage from changes that are either very large (greater than 5*C (9*F)) or very rapid may approach the catastrophic if they increase the likelihood of large-scale abrupt changes.
A serious possibility exists that human activities in the twenty-first century may trigger irreversible events in centuries to come. One example could be the possible collapse of the oceanwide, thermohaline circulation driven by the sinking of salt-dense water at the terminus of the Gulf Stream in the North Atlantic. Such a dire consequence has been suggested in the literature of the past five years as a possible consequence from a large accumulation of carbon dioxide in the atmosphere (greater than a doubling) or a rapid rate of buildup of carbon dioxide (more than a 0.5 percent per year increase). Although we agree with the IPCC that collapse of the West Antarctic ice sheet or thermohaline circulation is unlikely in the twenty-first century, most current analyses do not convey to decision makers the possibility that human activities in this century could initiate a chain of events with irreversible consequences stretching for centuries beyond. Most researchers are still very cautious about offering much confidence in projections of increased "surprise" events, at least for this century, although they do acknowledge that business as usual in greenhouse-gas buildup is likely to increase chances of abrupt climatic disruptions beyond the twenty-first century (acceded in both the SAR and TAR).
Part of researchers' reluctance to assess low-probability or singular events stems from professional tradition. By training and temperament, natural scientists are inclined to express cautious statements regarding the uncertainty-punctuated science of climate change. The scientific community rightly places great credence in acquiring knowledge through experimental testing and control. Understandably, scientists are reluctant to discuss issues until there is a substantial body of data to validate components of the theory as well as behavior of the overall system. It is not surprising that the scientists have struggled with ways to both alert the public to these more damaging possibilities and at the same time not appear overconfident in projecting outcomes that cannot be directly validated by the data available now or in the foreseeable future.
Decision makers, on the other hand, often prefer to hedge against a potentially damaging event rather than wait for it to happen and get blamed for having done nothing. According to the New York Times, "Global warming is a classic example of the persistent mismatch between the language of science and the needs of policy. ... Policy makers, always eager for black and white, have once again found science offering shades of gray" (Sept. 10, 2000). Translating the scientists' conclusions, framed as they are in carefully hedged language, into credible messages interpretable by the decision-making community is an ongoing challenge.
Cobenefits of mitigation
A major obstacle to implementing a coherent international policy to mitigate climate change is that the costs and benefits of climate change are distributed across diverse economic sectors and geographic regions. IPCC Working Group II concluded in the TAR that although certain regions and sectors may experience slight benefits from a global warming of up to a few degrees (since, for example, increased carbon dioxide enhances plant productivity), most people affected by global warming will likely experience net losses. Greatest losses, they project, are likely to be felt in regions and sectors that are already economically disadvantaged or possess warm climates--typically developing countries. Vulnerability is especially great for societies with limited resources, which lack the infrastructure or financial institutions that could moderate the losses.
The stresses from global warming are compounded by other pressures, including increasing population, stressed urbanization, limited or low-quality land and water, and environmental pollution. Policy responses to global warming, including both mitigating emissions and adapting to climate changes, are more likely to succeed if they are linked to or integrated with policies designed to address nonclimatic stresses. Thus, any assessment of possible responses to global warming should weigh competing risks and priorities against the costs of climate policy options and consider how policies to address competing objectives may complement each other.
In addition to their primary benefits, climate policies are likely to produce important cobenefits, secondary benefits that also meet other policy objectives, such as reducing air and water pollution. Identifying both primary and secondary benefits introduces the possibility of realizing "synergies," or linkages between climate policies and other policies with different objectives. For example, a low greenhouse gas emission scenario could result from a sustainable development policy. Forest preservation is a particularly important, contemporary example of how accounting for cobenefits affects the value of policy options.
By current estimates, the burning of tropical forests accounts for 20 to 30 percent of carbon emissions. Clearly, protecting primary forests is a "first-best solution" to climate change at the global level. For example, it may well behoove the international community to support conservation efforts in tropical rain forests because of the tremendous global economic (and intrinsic) value of these forests. Paying national constituencies to preserve their local forests could result in safeguarding a valuable carbon sink at a relatively low cost compared to paying carbon taxes on conventional energy systems to accomplish the same abatement level. Thus, paying for preserving forests could well be seen as a viable global-warming mitigation policy that brings the added benefits of protecting biodiversity and ecosystem services inherent in these natural systems.
The Sierra Club recently commissioned a report with ECONorthwest, an independent economics and financial consulting firm, to consider the value of the U.S. national forests. Its conclusion supports the idea that preserving and restoring the forests is often more valuable than logging. The USDA Forest Service estimates that the total value of the most easily measured goods and services from U.S. national forests, including timber, minerals, forage, recreation, and fish and wildlife, is $145.1 billion, or 2 percent of the U.S. GDP. It estimates timber's value at $3.9 billion, or only 2.7 percent of the total value of the most easily measured goods and services from the U.S. national forests, whereas recreation's value at $108.4 billion and fish and wildlife's at $14.4 billion together account for 84 percent. These estimates neglect the value of additional forest services, such as clean water, flood prevention, carbon sequestration, or aesthetic value.
Such figures suggest the need for national and international regimes to assess the full value of primary forests as both havens for biodiversity and potential carbon stores--not just traditional consumptive uses. Mechanisms to promote preservation of these forests are a high priority for international environmental policy development. The key is providing suitable incentives (and phasing out perverse subsidies) to all relevant actors--landowners, forest dwellers, local and national governments, and business--so that benefits, other than logging, from the forest are realized.
Recent advances in the science of climate change include
l increased confidence in temperature measures indicating global warming;
l an upgrade to an evaluation of "likely" that science is detecting the "discernible impact" of human activities on climate;
l a key revision in the temperature increase projected for 2100, from the 1995 assessment's range of 1--3.5*C (1.8--6.3*F) to the 2001 assessment's range of 1.4--5.8*C (2.5--10.5*F), as a result of a decrease in estimated twenty-first century aerosol emissions and an increase in estimated CO2 emissions;
l high-confidence predictions of increasing frequencies for such extreme events as heat waves and high-intensity rainfall, and medium-confidence predictions of increases in the frequencies of such extreme events as hail and lightning and the intensity of severe storms;
l growing, but still generally low-confidence, scientific acceptance of the view that human actions taken in the twenty-first century could trigger such irreversible changes in later centuries as ocean-current reorientations;
l clarification and growing scientific acceptance of the view that modest temperature increases would likely produce a mix of beneficial and detrimental changes. Harmful effects would be concentrated in the already warm tropics and subtropics, but warming beyond a few degrees would likely produce negative impacts across the board.
Ecosystems, with their specific flora and fauna, are especially vulnerable to negative impact from global warming. Already, human interventions have imposed such extensive disruptions and fragmentations on them that they have lost many of their original adaptive mechanisms, such as long-term, large-scale migration of animal and plant species, or local shifting of the species balance. Finally, we emphasize that the assessments provided by climate-change science and impacts assessment will continue to be accompanied by uncertainty and attendant caveats. Such conditions will quite likely prevail even as the world's nations make and implement policies that will have a major influence on the CO2 emissions of the twenty-first century and beyond.
One major concern is that advocates from many special interests from many sides of the debate will continue to quote out of context potentially beneficial or catastrophic outcomes as if they were the primary likelihood, when the most probable outcomes lie in the middle. Nearly all knowledgeable scientists agree that some global warming is inevitable, that major warming is quite possible, and that for the bulk of humanity the net effects are more likely to be negative than positive. This will hold true particularly if global warming is allowed to increase beyond a few degrees, which is likely to occur by the middle of this century if no policies are undertaken to mitigate emissions. We hope that orderly transitions to reduce greenhouse gas emissions will occur, abetted by a policy designed to provide incentives to develop more cost-effective and "climate friendly" technologies, since rapid replacement of current energy systems before their retirement age is likely to be economically harmful. The key is not to replace them with more of the same. To fail to do so is to tempt the fate of more serious and irreversible climate risks. It is time for the combative level of rhetoric that has marked the climate debate over the past decade to diminish and the quality of the dialogue that can bring out cost-effective actions to increase.
Kenneth Green, "Heated Debate Over a Hot Theory,"
The World & I , January 2001.
Andrew Revkin, "Global Waffling: When Will We Be Sure?"
New York Times, Sept. 10, 2000.
On the Internet
Seeing the Forests for Their Green: The Economic Benefits
of Forest Protection, Recreation, and Restoration
(August 2000 report)
The Third Assessment Report's (forthcoming, summer
2001) summaries for policymakers are available at
Intergovernmental Panel on Climate Change
J.P. Weyant, An Introduction to the Economics of
Climate Change Policy, Pew Center on Global Climate
Change, 2000, Arlington, Virginia.