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Vol 458|30 April 2009 COMMENTARY Overshoot, adapt and recover We will probably overshoot our current climate targets, so policies of adaptation and recovery need much more attention, say Martin Parry, Jason Lowe and Clair Hanson. I policy would mean continual 3% year-on-year With the same 3%-per-year long-term emissions reductions that could, after several emissions reductions but a slower start, peak centuries, lead to greenhouse gas concentra- temperatures would rise substantially and the tion of about 350 parts per million (p.p.m.) of overshoot would extend. For example, delaying carbon dioxide equivalents. A new and useful mitigative action by ten years and so reversapproach for quantifying long-term emission ing emissions trends by 2025 would raise peak targets is presented in two new pieces of work median temperature by about 2.5 °C; delaying published in this issue (pages 1158 and 1163). by a further ten years (a 2035 downturn) would We have simulated the outcomes of this mean a rise of about 3 °C, with much longer 3%-per-year reduction strategy with a sim- recovery. ple Earth system model2 and The damage from these levels have plotted them on a table of warming could be substan“We should be of projected effects that we tial, placing billions more peoplanning to adapt constructed, with other Interple at risk of water shortage and governmental Panel on Climillions more at risk of coastal to at least 4 °C of mate Change Working Group flooding. To avoid such damage warming.” II authors, for the IPCC 2007 will require massive investment assessment3 (Fig. 2). Our story in adaptation, such as improvline — immediate implementation, achieving ing water supply and storage, and protecting peak emissions in 2015 and 3% global emis- low-lying settlements from rising seas. But how sions cuts annually thereafter — leaves an even much adaptation should we plan for? chance of exceeding 2 °C of warming. TemIt will be very expensive to protect against peratures would probably peak around 2065 warming at the upper end of the uncertainty just above a 2 °C rise, but with about a 20% range. We therefore will need to make a judgechance of exceeding a 2.5 °C rise. If the same ment about what damage is worth avoiding rate of year-on-year emissions reductions was completely and what we will have to bear. maintained over the next century, temperatures Looking at the median projected warming for would slowly recover to about 1 °C of warm- different peak emissions dates, with ranges ing by 2300. This would be a considerable of uncertainty of climate response (shown by challenge, however, because it would require horizontal bars in Fig. 2), one can predict that substantial reductions in fossil fuel use and damages to the right of the median should deforestation and, in the long term, major and probably be avoided by mitigation, while those much more difficult reductions of emissions to the left would probably need to be borne or be adapted to. from agriculture. If we make some simple assumptions about the amount of risk we wish to cover, we can Peak emissions at 2015 Peak emissions at 2025 Peak emissions at 2035 identify how much we need to adapt. For examPeak temperatures Peak temperatures Peak temperatures circa 2065 circa 2080 circa 2100 ple, we might assume that small amounts of 4.5 adaptation would cover at least 10% of the risk 4.0 of harm; moderate amounts might cover half; 3.5 and much larger amounts could cover 90%. 3.0 The timing and stringency of emissions 2.5 reduction will also influence the scale of 2.0 potential damage, which would affect how 1.5 much adaptation is needed: slower and lower 1.0 reductions would lead to larger effects. Thus, 0.5 if we wished to adapt to 90% of the risk implied 0 2000 2100 2200 2300 2400 2000 2100 2200 2300 2400 2000 2100 2200 2300 2400 by delaying mitigative action until 2035, we should be planning to adapt to at least 4 °C of Temperature uncertainty range warming. Given the severity of the mitigation 10th percentile 90th percentile 50th percentile challenge that we have described, this seems at present to be a wise precaution. Figure 1 | Temperature scenarios. Global average surface temperature scenarios for peak emissions at What would be the cost of such adaptation? three different dates (2015, 2025 and 2035) with 3%-per-year reductions in greenhouse-gas emissions. The UN Framework Convention on Climate Global mean temperature change (°C) f policy-makers are to reach international agreement on greenhouse-gas emissions at the United Nations Framework Convention on Climate Change conference in Copenhagen in December, they need to be optimistic that their decisions could have swift and overwhelmingly positive effects on climate change. The reality is less certain, but no less urgent. Even the most restrictive emissions policies proposed to date leave a sizeable chance that significant climate change will occur over the next several decades, probably surpassing the 2 °C warming target adopted by the European Union and held by many as a dangerous limit beyond which we should not pass1. We must therefore complement a strong emissions policy with a plan to adapt to major environmental, social and economic changes in the lengthy period during which we will overshoot safe levels of climate change. This will require much more investment in adaptation than is currently planned. The stringent greenhouse-gas-reduction policies posed at the G8 summit in 2007, for example, assume a reduction of around 50% in emissions by 2050. The storyline we develop here assumes that everyone agrees to this target at the Copenhagen talks in December and that policy is implemented immediately, thus ensuring the start of a downturn in global emissions — currently increasing at about 3% per year — by 2015 (Fig. 1). Implementing the 1102 Climate Crunch OPINION NATURE|Vol 458|30 April 2009 Global mean change from pre-industrial temperatures (°C) 0.5 1.5 2.5 3.5 Peak temp. 2080 Peak temp. 2100 More water available in moist tropics and high latitudes Decreasing water availability and more drought in mid-latitudes and semi-arid low latitudes 0.4–1.7 billion 1.0–2.0 billion More coral bleaching Most coral bleached Increasing species range shifts and wildfire risk Food Crop productivity Additional people with increased water stress 1.1–3.2 billion Major extinctions around the globe About 20–30% species at increasingly high risk of extinction More amphibian extinction Ecosystems 5.5 Peak temp. 2065 Peak emissions 2015 Peak emissions 2025 Peak emissions 2035 Water 4.5 Widespread coral mortality Terrestrial biosphere tends towards a net carbon source, as: ~40% of ecosystems affected ~15% of ecosystems affected Low latitudes: decreases for some cereals Mid to high latitudes: increases for some cereals All cereals decrease Decrease in some regions More damage from floods and storms About 30% loss of coastal wetlands Coast Additional people at risk of coastal flooding each year 0–3 million 2–15 million Increasing burden from malnutrition, diarrhoea, cardiorespiratory disease and infections Health More morbidity and mortality from heatwaves, floods and droughts Changed distribution of some disease vectors Singular events Local retreat of ice in Greenland and West Antarctic Substantial burden on health services Long-term commitment to several metres of sea-level rise due to ice sheet loss Leading to reconfiguration of coastlines worldwide and inundation of low-lying areas Ecosystem changes due to weakening of the meridional overturning circulation Adaptation needed Adaptation needed to cover 10% to cover 50% of risks of risks Adaptation needed to cover 90% of risks Figure 2 | Expected effects on a range of global sectors for different global mean temperature increases from preindustrial levels. Plotting peak temperatures from the three scenarios explained in Fig. 1 shows the range of projected damaging effects and the adaptation needs for 10%, 50% and 90% coverage of impact risk, and examples of these effects for a range of global sectors (source, ref. 4). Change (UNFCCC) has estimated that between US$50 billion and US$170 billion per year (in current values) will be needed by the year 2030 (ref. 4). This is only a twentieth of current spending on development of new infrastructure globally and a tenth the expected cost of emissions reduction5. Such a low figure for the cost of adaptation might lead one to doubt whether climate change poses much of a challenge. The ultimate cost will probably be several times the UNFCCC estimate, however, and much more than this if emissions reduction is delayed or if we wish to protect against high-end uncertainty3. Additionally, much adaptation may not be physically possible or economically worthwhile. One estimate is that this impracticable adaptation would amount to two-thirds of all damages — about $1 trillion per year in 2030 (ref. 6), or ten times the UNFCCC estimate for total adaptation funding. This includes damages to irreplaceable biological systems such as coral reefs or the costs of continuing to irrigate for farming in drying regions. A final concern is that the multi-century- long recovery process, from peak temperatures this century to roughly 1 °C of warming far in the future, may be far from linear. Sea levels, for example, may continue to rise for some decades after land areas have begun to cool; and there is also the possibility that extended melting in the Arctic would reduce albedo and increase methane release, pushing the warming peak higher and further into the future7. The window of opportunity for beginning effective long-term action on climate change is extraordinarily narrow. Urgent and major emissions reductions are essential to avoid the most severe effects. Yet even the most prompt and stringent action still risks overshooting a target of 2 °C, and it will require centuries to achieve a roughly stable climate with tolerably low amounts of warming. The consequent demands on adaptation will be enormous, many times those currently envisaged. We should therefore give policies of adaptation much more urgent attention. ■ Martin Parry was co-chair of the IPCC’s 2007 Working Group II assessment and is now at the Grantham Institute for Climate Change and Centre for Environmental Policy, Imperial College London, London SW7 2AR, UK. Jason Lowe is head of mitigation advice at the Met Office, Exeter EX1 3PB, UK. Clair Hanson was deputy director of the 2007 IPCC Working Group II technical support unit and is at the University of East Anglia, Norwich NR4 7TJ, UK. e-mail: [email protected] See also Editorial, page 1077, and www.nature.com/climatecrunch 1. European Commission Limiting Global Climate Change to 2 degrees Celsius (2007). 2. Lowe, J. A. et al. Environ. Res. Lett. 4, 014012 (2009). 3. IPCC Climate Change 2007: Impacts, Adaptation and Vulnerability (eds. Parry, M. L., Canziani, O. F., Palutikof, J. P., van der Linden, P. J. & Hanson, C. E.) (2007). 4. UNFCCC Investment and Financial Flows to Address Climate Change (2007). 5. Stern, N. The Economics of Climate Change: The Stern Review (Cambridge Univ. Press, 2007). 6. Ackerman, F., Stanton, E. A., Hope, C. & Alberth, S. Energy Policy (in the press). 7. Parry, M. L., Lowe, J. A., Palutikof, J. & Hanson, C. E. Nature Reports Climate Change doi:10.1038/climate.2008.50 (2008). 1103