Mitchell travels from the US to Canada, where he exchanges 150 US dollars for Canadian dollars. He then spends 20 Canadian dollars, returns to the US, and exchanges the remaining money back to US doll
A rental car company charges $0.50 per mile and a flat rate of $30.00 for any type of sedan. If m represents the number of miles driven and C(m) represents the total cost for renting a sedan, what is

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Acknowledgments: We thank all members of the Moritz lab for their helpful comments and suggestions. C.M.’s research relating to climate change effects has been supported by the National Science Foundation (the Australian Research Council) and the Gordon and Betty Moore Foundation. R.A. is funded by a Spanish postdoctoral fellowship financed by the Ramon Areces Foundation ( fundacionareces/).



Climate Change Impacts on Global Food Security Tim Wheeler1,2* and Joachim von Braun3

Climate change could potentially interrupt progress toward a world without hunger. A robust and coherent global pattern is discernible of the impacts of climate change on crop productivity that could have consequences for food availability. The stability of whole food systems may be at risk under climate change because of short-term variability in supply. However, the potential impact is less clear at regional scales, but it is likely that climate variability and change will exacerbate food insecurity in areas currently vulnerable to hunger and undernutrition. Likewise, it can be anticipated that food access and utilization will be affected indirectly via collateral effects on household and individual incomes, and food utilization could be impaired by loss of access to drinking water and damage to health. The evidence supports the need for considerable investment in adaptation and mitigation actions toward a “climate-smart food system” that is more resilient to climate change influences on food security.

Tackling hunger is one of the greatest chal-lenges of our time (1). Hunger has multi-ple dimensions and causes, ranging from deficiencies in macro- and micro-nutrients, throughshort-term shocks on food access, to chronic short-ages. Causes range from constraints on the supply

of food of sufficient quantity and quality and lack of purchasing power to complex interactions of nutrition with sanitation and infectious diseases leading to poor health. Several of these causes have been addressed in recent decades, and sub- stantial progress has been made in reducing the proportion of the world’s undernourished popu- lation from an estimated 980 million in 1990–92 to about 850 million in 2010–12 (2). However, from other relevant indicators of nutrition, such as child underweight and stunting and health sur- veys, an estimated 2 billion people still suffer from micro-nutrient deficiencies today.

The long-term reduction in the prevalence of undernutrition worldwide has slowed since 2007, as a result of pressures on food prices, economic

1Walker Institute for Climate System Research, Department of Agriculture, University of Reading, Reading RG6 6AR, UK. 2Department for International Development, 22-26 Whitehall, London SW1A 2EG, UK. 3ZEF B: Center for Development Re- search, Department of Economic and Technical Change, Univer- sity of Bonn, Walter-Flex-Strasse 3 53113 Bonn, Germany.

*Corresponding author. E-mail:

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volatilities, extreme climatic events, and changes in diet, among other factors. Furthermore, addi- tional pressures on the global food system are expected to build in the future. For example, de- mand for agricultural products is estimated to increase by about 50% by 2030 as the global pop- ulation increases (3), which will require a shift toward sustainable intensification of food systems (4). The impacts of climate change will have many effects on the global food equation, both for sup- ply and demand, and on food systems at local levels where small farm communities often depend on local and their own production (5). Thus, cli- mate change could potentially slow down or re- verse progress toward a world without hunger.

Here, we offer an overview of the evidence for how climate change could affect global food sec- urity, with particular emphasis on the poorer parts of the world. We deliberately take a broad view of the complex interactions between climate change and global food security, stating what we do know with some degree of confidence, as well as ac- knowledging aspects where there is little or no evi- dence. We end by proposing a number of precepts for those making policy or practical decisions on climate change impacts and food security.

Food Security Together, climate change and food security have multiple interrelated risks and uncertainties for societies and ecologies. The complexity of global

food security is illustrated by the United Nations’ Food and Agricultural Organization (FAO) (6) definition: (i) the availability of sufficient quantities of food of appropriate quality, supplied through domestic production or imports; (ii) access by in- dividuals to adequate resources (entitlements) for acquiring appropriate foods for a nutritious diet; (iii) utilization of food through adequate diet, clean water, sanitation, and health care to reach a state of nutritional well-being where all physio- logical needs are met; and (iv) stability, because to be food secure, a population, household or in- dividual must have access to adequate food at all times.

It is extremely challenging to assess precise- ly the current status of global food security from such a broad concept. However, the big picture is clear: About 2 billion of the global population of over 7 billion are food insecure because they fall short of one or several of FAO’s dimensions of food security. Enormous geographic differ- ences in the prevalence of hunger exist within this global estimate, with almost all countries in the most extreme “alarming” category situated in sub-Saharan Africa or South Asia (7) (Fig. 1).

Nevertheless, it is important to note that the current numbers for undernourished people are rough estimates at best and are seriously defi- cient in capturing the access, utilization, and stability dimensions of food security. First, the methods used to make these estimates only cap-

ture longer-term trends, not the short-term changes that can be an important consequence of climate variability. The most recent data are averages for the period 2010–2012 (2), so they do not cap- ture a specific year, let alone shorter-term shocks, be they climate-related or otherwise. Second, they estimate calorie shortage only and do not cover other dietary deficiencies and related health ef- fects that can impair physical and mental capac- ities. Third, they are derived from aggregate data, not actual household or individual-level food de- ficiencies, which hinders analyses of distribu- tional effects of climate and other shocks. The FAO methodology was recently improved (8), but the above shortcomings could not be addressed within the framework of the current method, and thus, current analyses of climate change impacts on food security are incomplete. An overhaul of data-gathering methods that encompasses food deficiencies at household levels, as well as nu- tritional status, is needed.

Climate Change There is a substantial body of evidence that shows that Earth has warmed since the middle of the 19th century (9–14). Global mean tem- perature has risen by 0.8°C since the 1850s, with the warming trend seen in three independent tem- perature records taken over land and seas and in ocean surface water (15). Climate change can result from natural causes, from human activities

Fig. 1. Global distribution of hunger as quantified by the 2012 Global Hunger Index. TheWelthungerhilfe, IFPRI, andConcernWorldwideHungermap2012 calculated a Global Hunger Index (7) for 120 countries by using the proportion of

people who are undernourished, the proportion of children under 5 who are under- weight, and the mortality rate of children younger than age 5, weighted equally. [Reproduced with permission fromWelthungerhilfe, IFPRI, and ConcernWorldwide (7)] SCIENCE VOL 341 2 AUGUST 2013 509


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through the emission of greenhouse gases such as carbon dioxide and methane, and from changes in land use. Carbon dioxide (CO2) levels in the atmosphere have increased from about 284 ppm in 1832 to 397 ppm in 2013 (16), and there is a theoretical link between the levels of such “green- house” gases in the atmosphere and global warm- ing. Three independent reviews have found strong evidence for human causes for the observed tem- perature warming mainly caused by the burning of fossil fuels, with smaller contributions from land-use changes (15–18).

Thus, climate change is expected to bring warmer temperatures; changes to rainfall pat- terns; and increased frequency, and perhaps sever- ity, of extreme weather. By the end of this century, the global mean temperature could be 1.8° to 4.0°C warmer than at the end of the pre- vious century (15). Warming will not be even across the globe and is likely to be greater over land compared with oceans, toward the poles, and in arid regions (15). Recent weather records also show that land surface temperatures may be increasing more slowly than expected from cli- mate models, potentially because of a higher level of absorption of CO2 by deep oceans (19). Sea-level rises will increase the risk of flooding of agricultural land in coastal regions. Changes in rainfall patterns, particularly over tropical land, are less certain, partly because of the inability of the current models to represent the global hy- drological cycle accurately (20). In general, it is expected that the summer Asian monsoon rainfall may increase, while parts of North and southern Africa could become drier (15). How will these regional changes in climate affect food security?

Research Biases Agriculture is inherently sensitive to climate var- iability and change, as a result of either natural causes or human activities. Climate change caused by emissions of greenhouse gases is expected to directly influence crop production systems for food, feed, or fodder; to affect livestock health; and to alter the pattern and balance of trade of food and food products. These impacts will vary with the degree of warming and associated changes in rain- fall patterns, aswell as from one location to another.

Climate change could have a range of direct and indirect effects on all four dimensions of food sec- urity. How is the evidence base distributed across the dimensions of food security? We undertook a bib- liographic analysis of peer-reviewed journal papers on food security and climate change since the pub- lication of the first Intergovernmental Panel on Cli- mate Change (IPCC) report in 1990 (21). That report was ground-breaking for the climate science that it reviewed, but agriculture was entirely absent. Our analysis shows that a small peak of papers with climate change and food security in the title or abstract were published in the mid-1990s, followed by a lull then a sharp increase in papers published with these terms from 2008 onward.

The distribution of the evidence across the four dimensions of food security is, however, heavily skewed toward food availability within 70% of the publications. Access, utilization, and stability di- mensions of food security are represented by only 11.9, 13.9, and 4.2% of the total publications on food security and climate change, respectively.

Why is the evidence based on climate change impacts so unevenly distributed across the four dimensions of food security? There are several possibilities. Research has largely concentrated on the direct effects of climate change, such as those on crop growth and on the distribution of agricultural pests and diseases. Also, studies have understandably focused on areas that can be easily investigated, often through analyzing single-factor changes, and have avoided the com-

plex and multilayered features of food security that require integrations of biophysical, econom- ic, and social factors. Clearly, current knowledge of food security impacts of climate change is dra- matically lacking in coverage across all dimen- sions of food security. Nevertheless, where there is good evidence, what are the broad conclusions?

Food Availability Rosenzweig and Parry (22) produced the first global assessment of the potential impacts of climate change scenarios on crops. They used numerical crop models of wheat, rice, maize, and soybeans to simulate yields at 112 locations in 18 countries, in the current climate and under climate change using the output of three climate models. These point-based estimates of change

Fig. 2. Global impacts of climate change on crop productivity from simulations published in 1994 and 2010. (Top) The 1994 study (22) used output from the GISS GCM (in this example) with twice the baseline atmospheric CO2 equivalent concentrations as input to crop models for wheat, maize, soybean, and rice that were run at 112 sites in 18 countries. Crop model outputs were aggregated to a national level using production statistics. (Bottom) The 2010 study (27) simulated changes in yields of 11 crops for the year 2050, averaged across three greenhouse emission scenarios and five GCMs. [Reprinted by permission from (top) Macmillan Publishers Ltd. (22); (bottom) World Bank Publishers (27)]

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were then scaled-up to country level by using national crop production statistics. Future climate simulations under both present-day and doubled- CO2 concentrations were used. They found that enhanced concentrations of atmospheric CO2 in- crease the productivity of most crops through increasing the rate of leaf photosynthesis and improving the efficiency of water use. However, more recent research has proposed that the CO2 yield enhancement in crop models is too large compared with observations of crop experiments under field conditions (23). If true, these revised estimates will affect the magnitude of the pre- vious global crop yield changes but not the spatial distribution of impacts. Even if there is some debate on the magnitude of CO2 effects, higher concentrations of CO2 in the atmosphere are already having noticeable continental level effects on plant growth in sub-Saharan Africa (24).

The simulations of Rosenzweig and Parry (22) showed that there is a large degree of spatial var- iation in crop yields across the globe. Both the sign and magnitude of the projected changes in crop yield vary with alternative climate models and from one country to another. In general, yields increased in Northern Europe, but decreased across Africa and South America (22) (Fig. 2). Inevitably, there were methodological weaknesses in this study, including the use of only just over one hundred points to represent global crop production, the absence of any change in the areas suitable for crop production in future climates, limitations on how each of the model points is representative of their surrounding regions, and assumptions about varieties in the crop model parameters themselves. Nevertheless, as the first example of global im- pacts of climate change on crop production, these simulations are remarkable.

Since 1994, knowledge of the effects of cli- mate on crop plant physiology has improved, the skill of simulation methods for climate change impact studies has increased, and better com- puting power and data sets to run global simu- lations have become available. Landmark studies since 1994 include those by Parry and colleagues (25), Cline (26), and, most recently, the World Bank (27) (Fig. 2). Specific projections vary with the climate model scenario used, the simulations methods, and the time scale over which the pro- jections are done. However, the broad-scale pat- tern of climate change impacts on crop productivity and production has remained consistent across all of these global studies spanning almost 20 years of research. Crop yields are more nega- tively affected across most tropical areas than at higher latitudes, and impacts become more se- vere with an increasing degree of climate change. Furthermore, large parts of the world where crop productivity is expected to decline under climate change (Fig. 2) coincide with countries that cur- rently have a high burden of hunger (Fig. 1). We conclude that there is a robust and coherent pattern on a global scale of the impacts of climate change

on crop productivity and, hence, on food availa- bility and that climate change will exacerbate food insecurity in areas that already currently have a high prevalence of hunger and undernutrition.

A recent systematic review of changes in the yields of the major crops grown across Africa and South Asia under climate change found that average crop yields may decline across both regions by 8% by the 2050s (28). Across Africa, yields are predicted to change by –17% (wheat), –5% (maize), –15% (sorghum), and –10% (millet) and, across South Asia, by –16% (maize) and –11% (sorghum) under climate change. No mean change in yield was detected for rice. Knox et al. (28) concluded that evidence for the impact of climate change on crop productivity in Africa and South Asia is robust for wheat, maize, sorghum, and millet, and inconclusive, absent, or contradic- tory for rice, cassava, and sugarcane.

Global-scale climate change impacts at a grid scale of 200 to 250 km can provide useful infor- mation on shifts in production zones and perhaps guide the focus of global crop improvement pro- grams seeking to develop better-adapted crop varieties. However, much of the adaptation of ag- ricultural practice to climate change will be driven by decisions at the farm and farm-enterprise scale. These decisions need much finer resolution infor- mation than that shown in Fig. 2. At much finer grid scales of 5 to 20 km there are even greater limits to the skill of predictive crop science than at the global scale. Additional uncertainties arise from how the output of global-scale climate mod- els is down-scaled, whether input data (such as crop, soil, topographic, and management information) are available across the domain for crop simula- tion at this scale, as well as questions as to how skillful the simulation methods are across a fine- scale domain. Recent attempts to harmonize mod- eling approaches for wheat simulations under climate change found considerable variation in projected impacts among models owing to differ- ences in model structures and parameter values (29). It is not surprising that the sheer complexity of food production systems at a very fine scale is difficult to reproduce in numerical models. How- ever, there is a real need for studies that test how well fine-scale simulations compare with observa- tions in the current climate, as a necessary test of their utility in future climates.

Although the evidence for direct climate change effects on crop productivity is reasonable, impor- tant limitations remain for impacts on food avail- ability more broadly. First, models that adequately capture expected climate change effects on crops are only available for the major cereals, groundnut, and some roots and tubers. Impacts on other im- portant crops (such as vegetables, pulses, and lo- cally important, but globally minor, crops) are often inferred based on similar plant characteristics, rather than studied explicitly. Second, changes in grassland productivity and grazing quality and the quality of crops for livestock feed (30) have hardly

been captured, which limits the understanding of climate change–livestock linkages. Last, many crop studies capture the impacts of mean changes in climate, but are less accurate for changes in weather extremes, which can have even more important consequences for crop yields (31).

Food Access Food access (and utilization) connects to climate change through indirect, but well-known, pathways. Access to food is largely a matter of household and individual-level income and of capabilities and rights. Food access issues have been studied through two types of approaches: top-down by models that attempt to link macro-shocks to household level responses and adaptation outcomes; and by community- and household-level studies that try to assess climate change effects from the bottom up.

The macro-modelsare often composed of interlinked models—including climate, crop, and economic models. In this approach, outcomes from a climate model feed into the crop model to simulate crop yields under different climate scenarios. The simulated yields are then used to make economic forecasts for the impact of climate change on prices, incomes, trade, and such like. The macro-models can either be constructed fol- lowing a partial equilibrium approach, i.e., studying the impacts only in one specific sector, such as agriculture, or as general equilibrium models seek- ing to capture the impacts on the whole economy. The weakness of this approach is that it barely captures climate adaptations. In contrast, micro- level studies are often based on detailed household surveys and usually better account for adaptation by households and communities to climate change.

An important example is the International Food Policy Research Institute (IFPRI) Interna- tional Model for Policy Analysis of Agricultural Commodities and Trade (IMPACT) model, which connects climate change scenarios with food sup- ply effects and market and price outcomes, and traces the economic consequences of food avail- ability drivers to access and utilization of food, that is, food energy consumption and children’s nutritional situation (32, 33). Specific findings are heavily dependent on the assumptions made about future income and population growth but, in general, show clear linkages between economic growth and resilience to climate change (32).

A host of studies is emerging that analyzes what happens to communities and households when they are exposed to climate shocks (34–37). These approaches tend to capture more adapta- tion capabilities than macro-models, such as asset draw-down, job-switching, migration, social policy responses, and collective action for adaptation and assistance. But it is difficult to appropriately capture with micro-level studies the covariate risks of climate change that cut across broad regions.

Climate change could transform the ability to produce certain products at regional and interna- tional levels. If it turns out, for example, that the SCIENCE VOL 341 2 AUGUST 2013 511


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geography of biomass production shifts at a global scale (38), this will have production implications for all bio-based products—whether food, feed, fuels, or fiber—and will impinge on food trade flows, with implications for (farm) incomes and access to food (39). Similar changes have been observed in the geography and relative productivity of certain ocean species, such as shifts between anchovy and sardine regimes in the Pacific Ocean (40).

Thus, macro-modeling and micro-level analy- ses of climate change linkages to food security are complementary. The prices of the basic resources, such as land and water, are formed by long-term expectations (41, 42), and these prices encompass expectations of climate change, such as revalua- tion of land with access to water. Structural conse- quences can emerge, particularly when property rights are lacking and traditional land and water rights are not protected, as is the case in many developing countries with food security problems (43–45); these structural problems lead to ero- sion of the assets of the poor, as seen during “land grabbing” by external and foreign interests (46).

Food Utilization Food utilization, to attain nutritional well-being, depends upon water and sanitation and will be affected by any impact of climate change on the health environment. Little research has been done on this dimension of food and nutrition se- curity. Links with drinking water may be obvious, when climate variability stresses clean drinking water availability (47, 48). Hygiene may also be affected by extreme weather events causing flood- ing or drought in environments where sound san- itation is absent (49–51). In addition, uptake of micronutrients is adversely affected by the prev- alence of diarrheal diseases, which in turn is strongly correlated with temperature (52).

Climate change can also impinge on diet qual- ity, and increased costs may result from measures required to avoid food contamination stemming from ecological shifts of pests and diseases of stored crops or food (53, 54). Science and in- novation have a role to play here, and in recent years there has been good progress made in im- proving food utilization through fortification and biofortification (55, 56). Vulnerability to food secu- rity shocks needs further research, as do ways to strengthen adaptive capacities under climate change, (57) as public policy responses depend on such insights. For example, appropriate de- sign of programs transferring income to the poor, employment-related transfer programs, and early childhood nutrition actions (58–60) may all need expanding to respond to climate-related volatilities.

New nutritional stresses are emerging, and the most striking example has been the recent “nutrition transition,” i.e., the process by which globalization, urbanization, and changes in life- style are linked to excess caloric intake, poor- quality diets, and low physical activity. Together, these factors have led to rapid rises in the inci-

dence of obesity and chronic diseases, even among the poor, in developing countries (61). The nutri- tion transition will unfold in parallel with cli- mate change in coming decades, but very little research on the potentially reinforcing effects of these phenomena has been done.

Stability of the Food System The stability of whole food systems may be at risk under climate change, as climate can be an important determinant for future price trends (32), as well as the short-term variability of prices. Since 2007, the world food equation has been at a precariously low level and, consequently, even small shocks on the supply or demand side of the equation will have large impacts on prices, as experienced in 2008 (62). Food security of the poor is strongly affected by staple food prices, as a large part of an impoverished family’s income has to be spent on staple foods.

Climate change is likely to increase food market volatility for both production and supply [see (63) for the supply side]. Food system sta- bility can also be endangered by demand shocks, for instance, when aggressive bioenergy subsidies and quota policies were applied by the political economy (64). These sorts of policy shifts, made in the past decade by the United States and the European Union, have been motivated in part by energy security concerns and partly by climate mitigation objectives (65–67). The resulting de- stabilization of food markets, which contributed to major food security problems, was therefore partly related to climate change (policy).

The 2008 food crisis stemmed from a com- bination of a general reduction of agricultural productivity and acute policy failures, exacer- bated by export restrictions applied by many countries, a lack of transparency in markets, and poor regulation of financial engagement in food commodity markets (68, 69). A broad set of risks needs to be considered, of which climate change is an increasingly important one, that can ripple out to destabilize food systems, resulting in high and volatile food prices that temporarily limit poor people’s food consumption (70–73), finan- cial and economic shocks that lead to job loss and credit constraints (74), and risks that polit- ical disruptions and failed political systems cause food insecurity (75). This complex system of risks can assume a variety of patterns that could po- tentially collide in catastrophic combinations.

What We Need to Know—Research and Evidence Gaps Despite a burgeoning literature over the past 5 years or so, much remains unknown about many food security impacts of climate change. Getting better evidence will help to some extent. For example, uncertainties in understanding the underlying science, social science, and econom- ics of climate change impacts will reduce as the evidence base expands with more research. How-

ever, other uncertainties will always remain as they arise from projections of climate change, sources of natural variability in climate, and future path- ways of emissions of greenhouse gases.

Four broad priorities for future research emerge from our review: (i) gathering evidence on the effects of climate change impacts on the food ac- cess, utilization, and stability dimensions in order to achieve a more holistic understanding of food security; (ii) understanding the indirect impacts of climate change on food security requires more comprehensive analytical approaches and sophis- ticated modeling, including links to the political economy; (iii) improving projections of regional climate change effects on food security at country level and on smaller scales that are crucial for decision-making for adaptation of food systems; (iv) better integrating of human dimensions of cli- mate change impacts into food security planning— because food systems are ultimately driven by people and their behavioural responses to real and perceived changes in their local climate—that will be central to the adaptation to climate change and actions to address hunger.

What We Know We Know—Messages for Decision-Makers Decisions still need to be taken by policy-makers and practitioners confronted with the prospect of climate change impacts on food security, despite very real uncertainties in current knowledge and future trends. For those making decisions, we pro- pose, with a fair degree of confidence from the existing evidence, six precepts for the impacts of climate change on food security:

1) Climate change impacts on food security will be worst in countries already suffering high levels of hunger and will worsen over time.

2) The consequences for global undernutri- tion and malnutrition of doing nothing in response to climate change are potentially large and will increase over time.

3) Food inequalities will increase, from local to global levels, because the degree of climate change and the extent of its effects on people will differ from one part of the world to another, from one community to the next, and between rural and urban areas.

4) People and communities who are vulner- able to the effects of extreme weather now will become more vulnerable in the future and less resilient to climate shocks.

5) There is a commitment to climate change of 20 to 30 years into the future as a result of past emissions of greenhouse gases that neces- sitates immediate adaptation actions to address global food insecurity over the next two to three decades.

6) Extreme weather events are likely to become more frequent in the future and will increase risks and uncertainties within the global food system.

All of these precepts support the need for considerable investment in adaptation and mit-

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igation actions to prevent the impacts of climate change from slowing progress in eradicating global hunger and undernutrition. A wide range of potential adaptation and resilience options exist and more are being developed. These need to address food security in its broadest sense and to be integrated into the development of agricul- ture worldwide. Building agricultural resilience, or “climate-smart agriculture,” through improve- ments in technology and management systems is a key part of this, but will not be sufficient on its own to achieve global food security. The whole food system needs to adjust to climate change, with strong attention also to trade, stocks, and to nutrition and social policy options. We need to work toward what could be termed a climate- smart food system that addresses climate change impacts on all dimensions of food security.

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Acknowledgments: We thank H. Li, Q. Zhang, and F. Zhang of CABI for conducting the bibliographic analysis. T.W. was partly supported by the UK-China Sustainable Agriculture Innovation Network programme of the UK Department for Environment, Food and Rural Affairs, International Sustain- able Development Fund (DC09-07).

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Climate Change Impacts on Global Food Security Tim Wheeler and Joachim von Braun

DOI: 10.1126/science.1239402 (6145), 508-513.341Science



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