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Vol 439|26 January 2006 SPECIAL REPORT The costs of global warming

Efforts to forecast how Earth’s future climate will affect us must consider the economic growth of both rich and poor nations. But there are doubts over the theories being used, as Quirin Schiermeierexplains.

iscussions of climate change tend to reflect how lifestyle and energy demand in both involve uncertainties, and most climate rich and poor countries are likely to change. Dresearchers have come to accept the Climate researchers are familiar with the inherent unknowns of their business. After all, problem. “Some emissions scenarios are per- the climate models they use to project the haps already demonstrably wrong,” says Erich course of global warming are generally seen as Roeckner, a climate modeller at the Max Planck the best that science can offer. But there is a Institute for Meteorology in Hamburg, Ger- growing feeling that the economic assumptions many, who has modelled three of them for the on which their work is based are outdated and IPCC (see “Early results”). “It is possible that all unreliable. And this could have serious impli- of them are wrong.” But most feel that econom- cations for assessments of climate change. ics is a field they are not qualified to assess. The Intergovernmental Panel on Climate Change (IPCC), which coordinates efforts Ridiculous assumption? to predict the effects of global warming, is One key criticism is the assumption that the currently finalizing its fourth assessment economies of poor countries will quickly catch report. It has asked 15 climate groups to run up with those of rich nations. “It is ridiculous to their models using output from a range of dif- assume, as the IPCC does, that rich and poor Translating temperature changes into impacts ferent ‘scenarios’, representing various assump- countries will economically converge as rapidly on society is beset with unknowns. tions about energy use, economic development as the European Union has done over the past and population increase up to 2100. 40 years,” says Richard Tol, an economist at responds to fixed concentrations of green- For the different models to be compared, all Princeton University in New Jersey. house gas. In the simplified view of science it groups will run two of the 40 available scenar- A particular worry about convergence was does not matter how the gas gets into the ios — A1B, which describes a world with large first raised by David Henderson, former head atmosphere, only how much of it is there.” economic growth and a balance between fossil of economics at the Organisation for Economic But Tol argues that when it comes to trans- and renewable energy sources, and B1, which Co-operation and Development, and Ian lating those temperature predictions into assumes less economic growth and a large shift Castles, former head of Australia’s bureau of impacts on society, how the carbon got there towards renewable energy. Each group will also statistics, after the IPCC’s third assessment matters very much indeed. Many problems run a selection of other scenarios. The results report. They pointed out that the IPCC used relating to climate change, such as the distribu- will be fed into the report of the IPCC’s Work- market exchange rates to compare the wealth of tion of malaria and water-borne diseases, are ing Group I, which projects how temperatures different countries, instead of what a certain highly sensitive to development and wealth, may rise over the next century. amount of money will buy in each place. This says Tol. “You can’t use flawed economic sce- Working Group II will translate this into exaggerates the differences between countries, narios for any meaningful analysis of the likely effects on society, income, food security they argue, and overestimates how much poor impacts of climate change.” and disease, and Working Group III will assess countries will develop in coming decades, and In terms of human welfare, some even assert the options for limiting greenhouse-gas emis- how much their carbon emissions will increase. that the difference between the economic sions and mitigating climate change. Tol says that even if certain economic effects of the various scenarios is more signifi- The scenarios were described in the IPCC’s assumptions are wrong, the predictions of cant than the predicted effects of climate “Special Report on Emissions Scenarios” global warming itself may not be far off. The change. For example, changes in malaria inci- (SRES) for the panel’s 2001 third assessment errors may even cancel each other out: if eco- dence that result from people becoming rich report. In June 2003, the IPCC decided that nomic development is slower than thought, enough to have mosquito nets outweigh any there was no time to develop new scenarios for countries may also switch to renewable energy changes in the geographical spread of malaria its fourth assessment report, so the same ones more slowly. caused by global warming. are being used again. Susan Solomon, who chairs the IPCC’s Development of the SRES scenarios was But many economists are vehemently ques- Working Group I, emphasizes this point. overseen by Nebojsa Nakicenovic, an energy tioning the assumptions on which the SRES “Existing scenarios cover a very wide range of economist at Vienna Technical University in scenarios are based. They say the scenarios emissions trajectories,” she says. “They are per- Austria. He says he is aware of the criticisms, rely on outdated economic theories and fail to fectly suited to physical tests of how the climate but argues that the scenarios were designed to

HALT CALLED ON SINGLE- DRUG ANTIMALARIAL PRESCRIPTIONS Irresponsible treatments could render artemisinin ineffective. www.nature.com/news

of considering an ever wider range of possibil- change. They estimate that the cost of stabiliz- ities, the IPCC needs to rethink its scenarios ing emissions at 450 parts per million — a and come up with a selection of likely options value it is hoped will be sufficient to avoid dan-

based on more up-to-date economic theory. gerous climate change — would be just 0.4% PRINSLOO of AP/K. Tol believes that the composition of the the world’s present gross product (O. Eden- SRES teams was too narrow, and that future hofer et al. Energy J., in the press). efforts should have a wide range of experts. As And this week, William Nordhaus, an econ- well as convergence, they will need to consider omist at Yale University in New Haven, Con- how future societies will operate, how fast the necticut, published results from a model that population will grow, and how technologi- combines data on historical climate, economic cal progress will change things. “All these activity and country-specific temperature pre- questions are at the heart of dictions to estimate the eco- economics,” agrees Ottmar “You can’t use flawed nomic impact of a temperature Edenhofer, an economist at the economic scenarios rise of 3 C. He concludes that Potsdam Institute for Climate for any meaningful such a rise would cause a 1–3% Impact Research in Germany. decline in global income — Edenhofer says that econo- analysis of the impacts significantly larger than previ- mists must also routinely check of climate change. ” ous estimates (W. D. Nordhaus their ideas against historical Proc. Natl Acad. Sci. USA data, and test their predictions against those of doi:10.1073/pnas.0509842103; 2006). other teams, as climate scientists do. The way The IPCC is starting to consider economists’ natural scientists compare their models “is a concerns. In September 2005 it set up a group totally new thing in economics”, says Edenhofer. to look at scenario development led by Bert “We can learn a lot from climate modellers.” Metz, a chemist at the Netherlands Environ- The aim is to develop flexible theories, in mental Assessment Agency and co-chair of which variables such as gross domestic product Working Group III. Metz says he will recom- and technical progress influence one another, mend the development of completely new to test the effects of policy interventions — emissions scenarios, including a more sophis- emissions caps or trading, for example — and ticated treatment of convergence and technol- work out the cost of different policies. ogy. The IPCC will decide on whether to go cover a range of possibilities, and include one A few attempts have already been com- ahead with Metz’s plan at a meeting in April on group that assumes little convergence. pleted. In 2003, economists Michael Grubb of Mauritius. But even if work starts now, the sce- Martin Parry, co-chair of Working Group II, Imperial College London and Carlo Carraro of narios won’t be ready for use until the IPCC’s which met in Merida, Mexico, last week to con- the University of Venice, along with John fifth assessment report, due in 2013 or later. sider an early draft of the fourth assessment Schellnhuber, research director at the Tyndall For critics such as Tol and Edenhofer, that’s report, adds that he and his colleagues also take Centre for Climate Change Research in Nor- not a moment too soon. “Economists must other projections into account, for example wich, UK, launched a project to investigate learn to bring their knowledge to bear in the those from the World Bank or the Millennium how technological progress affects emissions discussion, and climate scientists need to real- Ecosystem Assessment. But which ones are strategies and mitigation costs. ize the significance of economics,” says Eden- likely to be right isn’t for them to judge, says Their comparison of models incorporating hofer. Rather than arguing about temperature Parry. “The IPCC summarizes current knowl- data on the economy, energy use and the envi- predictions, “the future battle will be about cost edge — it does not have a view.” ronment suggests that climate policies such as and damages”. ■ But economists such as Tol say that instead the Kyoto Protocol will trigger technological See Editorial on page 370.

Early results TEMPERATURE CHANGE RELATIVE TO 1961–1990 Global temperatures are experiments, requiring will become ice-free during 5 likely to rise by 2.5–4 C 400,000 computing hours, summer by 2090, says Observations 4 by 2100, according to the using their atmospheric Erich Roeckner, who heads A2 — 800 p.p.m. CO2 by 2100 A1B — 700 p.p.m. CO by 2100 latest calculations by general circulation model the group. The global sea 3 2 B1 — 550 p.p.m. CO2 by 2100 scientists at the Max Planck ECHAM5. level will rise by up to 30 Simulated past Institute for Meteorology in They looked at three centimetres as water warms °C2 Hamburg, Germany. emissions scenarios, and expands, and by an 1 The institute is one representing carbon dioxide additional 10 centimetres as of 15 asked by the concentrations of 550, part of Greenland’s ice sheet 0 Intergovernmental Panel 700 and 800 parts per melts. The scientists also –1 on Climate Change to million (p.p.m.) by 2100 expect a weakening — but 1850 1900 1950 2000 2050 2100 run extended climate (see graph). Even under not a shut-down — of the Year simulations for its fourth the most optimistic Atlantic ocean circulation. the tropics, and less rainfall Extreme precipitation and assessment report. The assumptions, the model There will be more rain and in Mediterranean and extreme drought are likely researchers ran six parallel suggests that the Arctic snow at high latitudes and in subtropical regions. to increase worldwide.Q.S.