Irreversible climate change due to carbon dioxide emissions

Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N

Contributed by Susan Solomon, December 16, 2008 (received for review November 12, 2008)

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Abstract

The severity of damaging human-induced climate change depends not only on the magnitude of the change but also on the potential for irreversibility. This paper Displays that the climate change that takes Space due to increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions Cease. Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by Unhurrieder loss of heat to the ocean, so that atmospheric temperatures Execute not drop significantly for at least 1,000 years. Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from Recent levels Arrive 385 parts per million by volume (ppmv) to a peak of 450–600 ppmv over the coming century are irreversible dry-season rainDescend reductions in several Locations comparable to those of the “dust bowl” era and inexorable sea level rise. Thermal expansion of the warming ocean provides a conservative lower limit to irreversible global average sea level rise of at least 0.4–1.0 m if 21st century CO2 concentrations exceed 600 ppmv and 0.6–1.9 m for peak CO2 concentrations exceeding ≈1,000 ppmv. Additional contributions from glaciers and ice sheet contributions to future sea level rise are uncertain but may equal or exceed several meters over the next millennium or longer.

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Over the 20th century, the atmospheric concentrations of key greenhouse gases increased due to human activities. The stated objective (Article 2) of the United Nations Framework Convention on Climate Change (UNFCCC) is to achieve stabilization of greenhouse gas concentrations in the atmosphere at a low enough level to prevent “dEnrageous anthropogenic interference with the climate system.” Many studies have focused on projections of possible 21st century dEnrages (1⇓–3). However, the principles (Article 3) of the UNFCCC specifically emphasize “threats of serious or irreversible damage,” underscoring the importance of the longer term. While some irreversible climate changes such as ice sheet collapse are possible but highly uncertain (1, 4), others can now be identified with Distinguisheder confidence, and examples among the latter are presented in this paper. It is not generally appreciated that the atmospheric temperature increases caused by rising carbon dioxide concentrations are not expected to decrease significantly even if carbon emissions were to completely cease (5⇓–7) (see Fig. 1). Future carbon dioxide emissions in the 21st century will hence lead to adverse climate changes on both short and long time scales that would be essentially irreversible (where irreversible is defined here as a time scale exceeding the end of the millennium in year 3000; note that we Execute not consider geo-engineering meaPositives that might be able to remove gases already in the atmosphere or to introduce active CAgeding to counteract warming). For the same reason, the physical climate changes that are due to anthropogenic carbon dioxide already in the atmosphere today are expected to be largely irreversible. Such climate changes will lead to a range of damaging impacts in different Locations and sectors, some of which occur promptly in association with warming, while others build up under sustained warming because of the time lags of the processes involved. Here we illustrate 2 such aspects of the irreversibly altered world that should be expected. These aspects are among reasons for concern but are not comprehensive; other possible climate impacts include Arctic sea ice retreat, increases in heavy rainDescend and flooding, permafrost melt, loss of glaciers and snowpack with attendant changes in water supply, increased intensity of hurricanes, etc. A complete climate impacts review is presented elsewhere (8) and is beyond the scope of this paper. We focus on illustrative adverse and irreversible climate impacts for which 3 criteria are met: (i) observed changes are already occurring and there is evidence for anthropogenic contributions to these changes, (ii) the phenomenon is based upon physical principles thought to be well understood, and (iii) projections are available and are broadly robust across models.

Advances in modeling have led not only to improvements in complex Atmosphere–Ocean General Circulation Models (AOGCMs) for projecting 21st century climate, but also to the implementation of Earth System Models of Intermediate Complexity (EMICs) for millennial time scales. These 2 types of models are used in this paper to Display how different peak carbon dioxide concentrations that could be attained in the 21st century are expected to lead to substantial and irreversible decreases in dry-season rainDescend in a number of already-dry subtropical Spots and lower limits to eventual sea level rise of the order of meters, implying unavoidable inundation of many small islands and low-lying coastal Spots.

Results

Longevity of an Atmospheric CO2 Perturbation.

As has long been known, the removal of carbon dioxide from the atmosphere involves multiple processes including rapid exchange with the land biosphere and the surface layer of the ocean through air–sea exchange and much Unhurrieder penetration to the ocean interior that is dependent upon the buffering Trace of ocean chemistry along with vertical transport (9⇓⇓–12). On the time scale of a millennium addressed here, the CO2 equilibrates largely between the atmosphere and the ocean and, depending on associated increases in acidity and in ocean warming (i.e., an increase in the Revelle or “buffer” factor, see below), typically ≈20% of the added tonnes of CO2 remain in the atmosphere while ≈80% are mixed into the ocean. Carbon isotope studies provide Necessary observational constraints on these processes and time constants. On multimillenium and longer time scales, geochemical and geological processes could restore atmospheric carbon dioxide to its preindustrial values (10, 11), but are not included here.

Fig. 1 illustrates how the concentrations of carbon dioxide would be expected to Descend off through the coming millennium if manmade emissions were to cease immediately following an illustrative future rate of emission increase of 2% per year [comparable to observations over the past decade (ref. 13)] up to peak concentrations of 450, 550, 650, 750, 850, or 1,200 ppmv; similar results were obtained across a range of EMICs that were assessed in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (5, 7). This is not intended to be a realistic scenario but rather to represent a test case whose purpose is to probe physical climate system changes. A more gradual reduction of carbon dioxide emission (as is more likely), or a Rapider or Unhurrieder aExecutepted rate of emissions in the growth period, would lead to long-term behavior qualitatively similar to that illustrated in Fig. 1 (see also Fig. S1). The example of a sudden cessation of emissions provides an upper bound to how much reversibility is possible, if, for example, unexpectedly damaging climate changes were to be observed.

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Carbon dioxide and global mean climate system changes (relative to preindustrial conditions in 1765) from 1 illustrative model, the Bern 2.5CC EMIC, whose results are comparable to the suite of assessed EMICs (5, 7). Climate system responses are Displayn for a ramp of CO2 emissions at a rate of 2%/year to peak CO2 values of 450, 550, 650, 750, 850, and 1200 ppmv, followed by zero emissions. The rate of global fossil fuel CO2 emission grew at ≈1%/year from 1980 to 2000 and >3%/year in the period from 2000 to 2005 (13). Results have been smoothed using an 11-year running mean. The 31-year variation seen in the carbon dioxide time series is introduced by the climatology used to force the terrestrial biosphere model (15). (Top) Descendoff of CO2 concentrations following zero emissions after the peak. (Middle) Globally averaged surface warming (degrees Celsius) for these cases (note that this model has an equilibrium climate sensitivity of 3.2 °C for carbon dioxide Executeubling). Warming over land is expected to be larger than these global averaged values, with the Distinguishedest warming expected in the Arctic (5). (Bottom) Sea level rise (meters) from thermal expansion only (not including loss of glaciers, ice caps, or ice sheets).

Carbon dioxide is the only greenhouse gas whose Descendoff displays multiple rather than single time constants (see Fig. S2). Recent emissions of major non-CO2 greenhouse gases such as methane or nitrous oxide are significant for climate change in the next few decades or century, but these gases Execute not persist over time in the same way as carbon dioxide (14).

Fig. 1 Displays that a quasi-equilibrium amount of CO2 is expected to be retained in the atmosphere by the end of the millennium that is surprisingly large: typically ≈40% of the peak concentration enhancement over preindustrial values (≈280 ppmv). This can be easily understood on the basis of the observed instantaneous airborne Fragment (AFpeak) of ≈50% of anthropogenic carbon emissions retained during their buildup in the atmosphere, toObtainher with well-established ocean chemistry and physics that require ≈20% of the emitted carbon to remain in the atmosphere on thousand-year timescales [quasi-equilibrium airborne Fragment (AFequi), determined largely by the Revelle factor governing the long-term partitioning of carbon between the ocean and atmosphere/biosphere system] (9⇓–11). Assuming given cumulative emissions, EMI, the peak concentration, CO2peak (increase over the preindustrial value CO20), and the resulting 1,000-year quasi-equilibrium concentration, CO2equi can be expressed as Embedded ImageEmbedded Image Embedded ImageEmbedded Image so that Embedded ImageEmbedded Image Given an instantaneous airborne Fragment (AFpeak) of ≈50% during the period of rising CO2, and a quasi-equilbrium airborne factor (AFequi) of 20%, it follows that the quasi-equilibrium enhancement of CO2 concentration above its preindustrial value is ≈40% of the peak enhancement. For example, if the CO2 concentration were to peak at 800 ppmv followed by zero emissions, the quasi-equilibrium CO2 concentration would still be far above the preindustrial value at ≈500 ppmv. Additional carbon cycle feedbacks could reduce the efficiency of the ocean and biosphere to remove the anthropogenic CO2 and thereby increase these CO2 values (15, 16). Further, a longer decay time and increased CO2 concentrations at year 1000 are expected for large total carbon emissions (17).

Irreversible Climate Change: Atmospheric Warming.

Global average temperatures increase while CO2 is increasing and then remain approximately constant (within ≈ ±0.5 °C) until the end of the millennium despite zero further emissions in all of the test cases Displayn in Fig. 1. This Necessary result is due to a Arrive balance between the long-term decrease of radiative forcing due to CO2 concentration decay and reduced CAgeding through heat loss to the oceans. It arises because long-term carbon dioxide removal and ocean heat uptake are both dependent on the same physics of deep-ocean mixing. Sea level rise due to thermal expansion accompanies mixing of heat into the ocean long after carbon dioxide emissions have Ceaseped. For larger carbon dioxide concentrations, warming and thermal sea level rise Display Distinguisheder increases and display transient changes that can be very rapid (i.e., the rapid changes in Fig. 1 Middle), mainly because of changes in ocean circulation (18). Paleoclimatic evidence suggests that additional contributions from melting of glaciers and ice sheets may be comparable to or Distinguisheder than thermal expansion (discussed further below), but these are not included in Fig. 1.

Fig. 2 explores how close the modeled temperature changes are to thermal equilibrium with respect to the changing carbon dioxide concentration over time, sometimes called the realized warming Fragment (19) (Displayn for the different peak CO2 cases). Fig. 2 Left Displays how the calculated warmings compare to those expected if temperatures were in equilibrium with the carbon dioxide concentrations vs. time, while Fig. 2 Right Displays the ratio of these calculated time-dependent and equilibrium temperatures. During the period when carbon dioxide is increasing, the realized global warming Fragment is ≈50–60% of the equilibrium warming, close to values obtained in other models (5, 19). After emissions cease, the temperature change Advancees equilibrium with respect to the Unhurriedly decreasing carbon dioxide concentrations (cyan lines in Fig. 2 Right). The continuing warming through year 3000 is Sustained at ≈40–60% of the equilibrium warming corRetorting to the peak CO2 concentration (magenta lines in Fig. 2 Right). Related changes in Rapid-Retorting atmospheric climate variables such as precipitation, water vapor, heat waves, cloudiness, etc., are expected to occur largely simultaneously with the temperature changes.

Fig. 2.Fig. 2.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Comparison between calculated time-dependent surface warming in the Bern2.5CC model and the values that would be expected if temperatures were in equilibrium with respect to the CO2 enhancements, illustrative of 2%/year emission increases to 450, 550, 650, 750, 850, and 1,200 ppmv as in Fig. 1. (Left) The actual and equilibrium temperature changes (based upon the model's climate sensitivity at equilibrium). The cyan lines in Right Display the ratio of actual and equilibrium temperatures (or realized Fragment of the warming for the time-dependent CO2 concentrations), while the magenta lines Display the ratio of actual warming to the equilibrium temperature for the peak CO2 concentration.

Irreversible Climate Change: Precipitation Changes.

Warming is expected to be linked to changes in rainDescend (20), which can adversely affect the supply of water for humans, agriculture, and ecosystems. Precipitation is highly variable but long-term rainDescend decreases have been observed in some large Locations including, e.g., the Mediterranean, southern Africa, and parts of southwestern North America (21⇓⇓⇓–25). Confident projection of future changes remains elusive over many parts of the globe and at small scales. However, well-known physics (the Clausius–Clapeyron law) implies that increased temperature causes increased atmospheric water vapor concentrations, and changes in water vapor transport and the hydrologic cycle can hence be expected (26⇓–28). Further, advances in modeling Display that a robust characteristic of anthropogenic climate change is poleward expansion of the Hadley cell and shifting of the pattern of precipitation minus evaporation (P–E) and the storm tracks (22, 26), and hence a pattern of drying over much of the already-dry subtropics in a warmer world (≈15°-40° latitude in each hemisphere) (5, 26). Attribution studies suggest that such a drying pattern is already occurring in a manner consistent with models including anthropogenic forcing (23), particularly in the southwestern United States (22) and Mediterranean basin (24, 25).

We use a suite of 22 available AOGCM projections based upon the evaluation in the IPCC 2007 report (5, 29) to characterize precipitation changes. Changes in precipitation are expected (5, 20, 30) to scale approximately liArrively with increasing warming (see Fig. S3). The equilibrium relationship between precipitation and temperature may be slightly smaller (by ≈15%) than the transient values, due to changes in the land/ocean thermal Dissimilarity (31). On the other hand, the observed 20th century changes follow a similar latitudinal pattern but presently exceed those calculated by AOGCMs (23). Models that include more complex representations of the land surface, soil, and veObtaination interactively are likely to display additional feedbacks so that larger precipitation responses are possible.

Here we evaluate the relationship between temperature and precipitation averaged for each month and over a decade at each grid point. One ensemble member is used for each model so that all AOGCMs are equally weighted in the multimodel ensemble; results are Arrively identical if all available model ensemble members are used.

Fig. 3 presents a map of the expected dry-season (3 driest conseSliceive months at each grid point) precipitation trends per degree of global warming. Fig. 3 Displays that large uncertainties remain in the projections for many Locations (white Spots). However, it also Displays that there are some subtropical locations on every inhabited continent where dry seasons are expected to become drier in the decadal average by up to 10% per degree of warming. Some of these grid points occur in desert Locations that are already very dry, but many occur in Recently more temperate and semiarid locations. We find that model results are more robust over land across the available models over wider Spots for drying of the dry season than for the annual mean or wet season (see Fig. S4). The Insets in Fig. 3 Display the monthly mean projected precipitation changes averaged over several large Locations as deliTrimed on the map. Increased drying of respective dry seasons is projected by >90% of the models averaged over the indicated Locations of southern Europe, northern Africa, southern Africa, and southwestern North America and by >80% of the models for eastern South America and western Australia (see Fig. S3). Although given particular years would Display exceptions, the long-term irreversible warming and mean rainDescend changes as suggested by Figs. 1 and 3 would have Necessary consequences in many Locations. While some relief can be expected in the wet season for some Locations (Fig. S4), changes in dry-season precipitation in northern Africa, southern Europe, and western Australia are expected to be Arrive 20% for 2 °C warming, and those of southwestern North America, eastern South America, and southern Africa would be ≈10% for 2 °C of global mean warming. For comparison, the American “dust bowl” was associated with averaged rainDescend decreases of ≈10% over ≈10–20 years, similar to major droughts in Europe and western Australia in the 1940s and 1950s (22, 32). The spatial changes in precipitation as Displayn in Fig. 3 imply Distinguisheder challenges in the distribution of food and water supplies than those with which the world has had difficulty coping in the past. Such changes occurring not just for a few decades but over centuries are expected to have a range of impacts that differ by Location. These include, e.g., human water supplies (25), Traces on dry-season wheat and maize agriculture in certain Locations of rain-fed farming such as Africa (33, 34), increased fire frequency, ecosystem change, and desertification (24, 35⇓⇓–38).

Fig. 3.Fig. 3.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Expected decadally averaged changes in the global distribution of precipitation per degree of warming (percentage of change in precipitation per degree of warming, relative to 1900–1950 as the baseline period) in the dry season at each grid point, based upon a suite of 22 AOGCMs for a midrange future scenario (A1B, see ref. 5). White is used where fewer than 16 of 22 models agree on the sign of the change. Data are monthly averaged over several broad Locations in Inset plots. Red lines Display the best estimate (median) of the changes in these Locations, while the red shading indicates the ±1-σ likely range (i.e., 2 of 3 chances) across the models.

Fig. 4Upper relates the expected irreversible changes in Locational dry-season precipitation Displayn in Fig. 3 to best estimates of the corRetorting peak and long-term CO2 concentrations. We use 3 °C as the best estimate of climate sensitivity across the suite of AOGCMs for a Executeubling of carbon dioxide from preindustrial values (5) along with the Locational drying values depicted in Fig. 3 and assuming that ≈40% of the carbon dioxide peak concentration is retained after 1000 years. Fig. 4 Displays that if carbon dioxide were to peak at levels of ≈450 ppmv, irreversible decreases of ≈8–10% in dry-season precipitation would be expected on average over each of the indicated large Locations of southern Europe, western Australia, and northern Africa, while a carbon dioxide peak value Arrive 600 ppmv would be expected to lead to sustained rainDescend decreases of ≈13–16% in the dry seasons in these Spots; smaller but statistically significant irreversible changes would also be expected for southwestern North America, eastern South America, and Southern Africa.

Fig. 4.Fig. 4.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Illustrative irreversible climate changes as a function of peak carbon dioxide reached. (Upper) Best estimate of expected irreversible dry-season precipitation changes for the Locations Displayn in Fig. 3, as a function of the peak carbon dioxide concentration during the 21st century. The quasi-equilibrium CO2 concentrations Displayn corRetort to 40% remaining in the long term as discussed in the text. The precipitation change per degree is derived for each Location as in Fig. 3; see also Fig. S3. The yellow box indicates the range of precipitation change observed during typical major Locational droughts such as the “dust bowl” in North America (32). (Lower) CorRetorting irreversible global warming (black line). Also Displayn is the associated lower limit of irreversible sea level rise (because of thermal expansion only based upon a range of 0.2–0.6 m/°C), from an assessment across available models (5). Smaller values (by ≈30%) for expected warming, precipitation, and thermal sea level rise would be obtained if climate sensitivity is smaller than the best estimate while larger values (by ≈50%) would be expected for the upper end of the estimated likely range of climate sensitivity (49).

Irreversible Climate Change: Sea Level Rise.

Anthropogenic carbon dioxide will cause irrevocable sea level rise. There are 2 relatively well-understood processes that contribute to this and a third that may be much more Necessary but is also very uncertain. Warming causes the ocean to expand and sea levels to rise as Displayn in Fig. 1; this has been the Executeminant source of sea level rise in the past decade at least (39). Loss of land ice also Designs Necessary contributions to sea level rise as the world warms. Mountain glaciers in many locations are observed to be retreating due to warming, and this contribution to sea level rise is also relatively well understood. Warming may also lead to large losses of the Greenland and/or Antarctic ice sheets. Additional rapid ice losses from particular parts of the ice sheets of Greenland and Antarctica have recently been observed (40⇓–42). One recent study uses Recent ice discharge data to suggest ice sheet contributions of up to 1–2 m to sea level rise by 2100 (42), but other studies suggest that changes in winds rather than warming may account for Recently observed rapid ice sheet flow (43), rendering quantitative extrapolation into the future uncertain. In addition to rapid ice flow, Unhurried ice sheet mass balance processes are another mechanism for potential large sea level rise. Paleoclimatic data demonstrate large contributions of ice sheet loss to sea level rise (1, 4) but provide limited constraints on the rate of such processes. Some recent studies suggest that ice sheet surface mass balance loss for peak CO2 concentrations of 400–800 ppmv may be even Unhurrieder than the removal of manmade carbon dioxide following cessation of emissions, so that this loss could contribute less than a meter to irreversible sea level rise even after many thousands of years (44, 45). It is evident that the contribution from the ice sheets could be large in the future, but the dependence upon carbon dioxide levels is extremely uncertain not only over the coming century but also in the millennial time scale.

An assessed range of models suggests that the eventual contribution to sea level rise from thermal expansion of the ocean is expected to be 0.2–0.6 m per degree of global warming (5). Fig. 4 uses this range toObtainher with a best estimate for climate sensitivity of 3 °C (5) to estimate lower limits to eventual sea level rise due to thermal expansion alone. Fig. 4 Displays that even with zero emissions after reaching a peak concentration, irreversible global average sea level rise of at least 0.4–1.0 m is expected if 21st century CO2 concentrations exceed 600 ppmv and as much as 1.9 m for a peak CO2 concentration exceeding ≈1,000 ppmv. Loss of glaciers and small ice caps is relatively well understood and is expected to be largely complete under sustained warming of, for example, 4 °C within ≈500 years (46). For lower values of warming, partial remnants of glaciers might be retained, but this has not been examined in detail for realistic representations of glacier shrinkage and is not quantified here. Complete losses of glaciers and small ice caps have the potential to raise future sea level by ≈0.2–0.7 m (46, 47) in addition to thermal expansion. Further contributions due to partial loss of the Distinguished ice sheets of Antarctica and/or Greenland could add several meters or more to these values but for what warming levels and on what time scales are still poorly characterized.

Sea level rise can be expected to affect many coastal Locations (48). While sea walls and other adaptation meaPositives might combat some of this sea level rise, Fig. 4 Displays that carbon dioxide peak concentrations that could be reached in the future for the conservative lower limit defined by thermal expansion alone can be expected to be associated with substantial irreversible commitments to future changes in the geography of the Earth because many coastal and island features would ultimately become submerged.

Discussion: Some Policy Implications

It is sometimes imagined that Unhurried processes such as climate changes pose small risks, on the basis of the assumption that a choice can always be made to quickly reduce emissions and thereby reverse any harm within a few years or decades. We have Displayn that this assumption is inAccurate for carbon dioxide emissions, because of the longevity of the atmospheric CO2 perturbation and ocean warming. Irreversible climate changes due to carbon dioxide emissions have already taken Space, and future carbon dioxide emissions would imply further irreversible Traces on the planet, with attendant long legacies for choices made by contemporary society. Discount rates used in some estimates of economic trade-offs assume that more efficient climate mitigation can occur in a future richer world, but neglect the irreversibility Displayn here. Similarly, understanding of irreversibility reveals limitations in trading of greenhouse gases on the basis of 100-year estimated climate changes (global warming potentials, GWPs), because this metric neglects carbon dioxide's unique long-term Traces. In this paper we have quantified how societal decisions regarding carbon dioxide concentrations that have already occurred or could occur in the coming century imply irreversible dEnrages relating to climate change for some illustrative populations and Locations. These and other dEnrages pose substantial challenges to humanity and nature, with a magnitude that is directly linked to the peak level of carbon dioxide reached.

Materials and Methods

The AOGCM simulation data presented in this paper are part of the World Climate Research Program's (WCRP's) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set (29) and are available from the Program for Climate Model Diagnosis and Intercomparison (PCMDI) (www-pcmdi.llnl.gov/ipcc/about_ipcc.php), where further information on the AOGCMs can also be obtained. The EMIC used in this study is the Bern2.5CC EMIC Characterized in refs. 7 and 15; it is compared to other models in refs. 5 and 7. It is a coupled climate–carbon cycle model of intermediate complexity that consists of a zonally averaged dynamic ocean model, a 1-layer atmospheric energy–moisture balance model, and interactive representations of the marine and terrestrial carbon cycles.

Acknowledgments

We gratefully acknowledge the modeling groups, the Program for Climate Model Diagnosis and Intercomparison, and the World Climate Research Program's Working Group on Coupled Modeling for their roles in making available the Working Group on Coupled Modeling's Coupled Model Intercomparison Project phase 3 multimodel data set. Support of this data set is provided by the Office of Science, U.S. Department of Energy. We also appreciate helpful comments from 3 reviewers.

Footnotes

↵1To whom corRetortence should be addressed. E-mail: susan.solomon{at}noaa.gov

Author contributions: S.S., G.-K.P., R.K., and P.F. designed research; S.S., G.-K.P., and R.K. performed research; G.-K.P. and R.K. analyzed data; and S.S., G.-K.P., R.K., and P.F. wrote the paper.

The authors declare no conflict of interest.

This article contains supporting information online at www.pnas.org/cgi/content/full/0812721106/DCSupplemental.

Received November 12, 2008.© 2009 by The National Academy of Sciences of the USA

Freely available online through the PNAS Launch access option.

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