water scarcity is already a major global issue that carries heavy adverse health sequelae , and climate change will further destabilize access to fresh water . one : in the north china plain , where half of china s wheat is grown , the water table is falling by as much as 3 m per year . two : certain states in india are using half of their electricity budget to pump water from depths as deep as 1 km to irrigate crops . three : roughly 300 million chinese and indians are eating food grown on  fossil  water that is not replenished . four : in the middle east and north africa , current rates of freshwater use are equivalent to 115% of total renewable runoff . given such unsustainable use around the world , the number of people living in water - scarce countries is expected to rise sixfold from 470 million to 3 billion between 1990 and 2025 . rapid human population growth combined with economic development begets increased water demand from homes , industry , and agriculture . in agriculture alone , the amount of water required to keep pace with global food demand this is water that , in many parts of the world , simply is not available . the availability of adequate potable water is a pillar of public health without which human well - being falters . in addition to hindering food production , inadequate access to water and sanitation has direct effects on human health . today , half of the urban populations of africa , asia , latin america , and the caribbean suffer from diseases associated with inadequate access to water and sanitation , and roughly 1.7 million people die every year from these diseases . scientists predict that the hydrologic cycle will be altered with , generally speaking , wet areas becoming wetter while dry areas become dryer . heavy precipitation events and associated runoff may make water supplies less reliable and may also make conditions favorable to outbreaks of water - borne disease . in the united states , for example , over 50% of water - borne disease outbreaks in the second half of the 20th century were preceded by precipitation events that were above the 90th percentile . warmer temperatures will also increase evapotranspiration , increasing water requirements for agriculture . at the same time , the availability of water during the height of the growing season may be less , owing to earlier melting of winter snowpack in the spring , leaving less water for summer irrigation . agricultural systems dependent on glacial melt , such as the andean nations of south america or much of asia that receives water from the tibetan plateau , may suffer such a fate as glaciers melt away . as these glaciers dwindle , they will provide ever less dry - season water flow into many of the world s great rivers . the indus river , for example , receives 4050% of its dry - season flow from glaciers that are rapidly receding . finally , sea - level rise and more extreme storms will lead to coastal inundation and intrusion of salt water into freshwater aquifers , further reducing coastal freshwater supply . perhaps even more problematic to human welfare in the coming century than a further diminution in the availability of fresh water is the impact of climatic disruption on the supply of food . as with water , the number of people suffering from protein - calorie malnutrition exceeded 1 billion after reaching a low of around 830 million in the 1990s ( figure 1 ) . in poor countries around the world , malnutrition underlies roughly one - third of the entire burden of disease , and roughly 2 to 3 billion people  almost half the human population  already suffer from micronutrient deficiencies . looking forward , farmers around the world will need to double agricultural production by 2050 in order to keep up with demand from a growing and more affluent human population that aspires to a more meat - based diet . this doubling of output will need to occur despite headwinds that already strain agricultural productivity . water scarcity , as discussed above , is a major constraint to increasing agricultural production . in addition , roughly one - third of the earth s land surface suffers from land degradation from the combined effects of soil erosion , salinization , nutrient depletion , and desertification . finally , the rise of the biofuels industry  evidenced in the diversion of one - quarter of the us grain harvest to biofuel production  is generating enormous demand for grain . by increasing grain demand and , as a consequence , demand for arable land and irrigation , growing grains as biofuel feedstock pits human food needs against biofuel production . amid rapidly rising demand for food , increasing environmental pressures on food production , and growing human malnutrition , climate change additionally compromises both agricultural yields and the nutritional quality of the crops produced . observational , longitudinal , and modeling studies all confirm that a 1c rise in temperature corresponds to roughly a 10% reduction in yield of the major grains [ 3 - 5 ] . as temperatures rise 26c over the next century , the reduction in agricultural yield will depend , in part , on our capacity to adapt and , in part , on how temperature variability changes ; but , in general , yields are expected to drop . as discussed above , climate change is expected to alter the timing and quantity of water available for agriculture while increasing the needs of plants as temperatures rise . increased production of ground - level ozone curtails agricultural yields , as ozone is a potent plant toxin . when ozone concentrations reach 3045 ppb ( parts per billion ) , yield losses for the major grains approach 1040% . by 2030 , mean annual ozone concentrations in south asia are expected to exceed 50 ppb . increasingly intense tropical cyclones , sea level rise , more frequent forest fires , droughts , and floods will also conspire to diminish local harvests . despite strong evidence that climate change will increase the risk of numerous challenges to agricultural production , the net effect of climate change on global food production is difficult to quantify . little is known , for example , about how climate change may alter the relationships between plants and their pests and pathogens , though several worrying examples indicate that climate change may promote infestations of plant pests , as higher winter minimum temperatures enable insects to overwinter more effectively . also poorly understood is how climate change will impact the extent and pattern of cloud cover which , to some degree , determines the solar radiation available to plants for photosynthesis . in addition to these impacts on the quantity of food produced , it appears that rising concentrations of atmospheric carbon dioxide also affect nutritional quality . grains grown at elevated concentrations of carbon dioxide appear to have reduced concentrations of protein , iron , zinc , and perhaps other nutrients . as with other examples already cited in this article , these potential decrements in plant nutritional content are of particular concern given the current state of affairs : iron and zinc deficiency account for roughly 63 million years of life lost annually , and the major grains are a critical source of these nutrients for many populations around the world . taken together , these additional threats to food production may very well have extensive impacts on human nutrition and global health . population displacement may be the final common pathway for many of the climate change impacts discussed above . regional changes in precipitation leading to increased droughts and flooding , increased incidence of natural disasters like tropical cyclones and forest fires , local crop failures , and severe water scarcity are all likely to force people around the world to abandon their homes ( figure 2 ) . in addition to these threats , coastal vulnerability puts many people at risk for displacement . more than one - third of the human population lives in coastal areas and on small islands that are within 100 km of the shore and less than 50 m above sea level . the combination of sea - level rise , increasingly intense storms , and destruction of coastal barriers ( mangroves , wetlands , vegetated dunes , and coral reefs ) leaves these populations extremely vulnerable . by 2050 an estimated 200 million to 1 billion people may be displaced due to climate change . these estimates are highly uncertain in part because many of the biophysical changes that contribute to displacement remain hard to quantify . despite this uncertainty , a significant fraction of the human population will undoubtedly be displaced in this century , and displaced persons face stark health realities . nonimmune populations migrating into endemic areas are more susceptible to a variety of infectious diseases . poor housing , sanitation , and access to safe drinking water combined with poor nutrition lead to disease epidemics , particularly diarrheal disease , measles , and acute respiratory infections . malnutrition plagues displaced populations , with rates as high as 50% seen in refugee populations in africa . in addition to infectious disease and malnutrition , displaced people suffer from high rates of violence , sexual abuse , and mental illness . post - traumatic stress disorder has been diagnosed in 3075% of resettled refugee children and adolescents . finally , displacement may lead to civil strife and violent conflict as large numbers of people with few resources move into areas where they frequently are not welcome . there is no doubt that climate change will have important impacts on human health , but we are uncertain about what those impacts will be and where and when they will be most severe . the most consequential health effects of climate change will come about from interactions between biophysical changes to the natural environment , demographic trends , and human adaptations ( figure 3 ) . the biophysical changes  such as temperature variability or sea level rise  are difficult to predict with accuracy today , and the capacity for adapting to these changes is largely unknown . but uncertainty about the exact timing , location , or magnitude of climate change impacts is no excuse for complacency . with evidence that climate change is already imposing a hefty health burden , the future climate , particularly if greenhouse gas releases into the atmosphere go unabated , portends health crises for hundreds of millions of people . rather than be used as a rationale for inaction , the uncertainty inherent in climate science should serve as an organizing principle for adaptation to its ill effects . for example , uncertainty about future viable regions and conditions for agriculture requires a variety of new crop strains with traits such as heat and drought resistance . changes in the timing of seasonal flow from melting snowpack or glaciers call for a dramatic increase in water - storage capacity for people depending on these flows for household use or irrigation . insurance schemes allowing different countries or populations to pool their risk of crop failure and food insecurity might be enacted to blunt the toll of regional climatic disruption . surveillance efforts should be designed that allow us to better detect changing distributions of infectious disease , water scarcity , or food insecurity early on so that resources can be efficiently redirected . never before have the consumption patterns of those in the wealthy countries of the world played such an important role in putting the health of the poor at greater risk . how the wealthy world responds to the moral imperative to help the developing world adapt to climate - change vulnerability will be a defining characteristic of this century .
global climate change threatens the health of hundreds of millions of people . while much has been written about the direct impacts of climate change on health as a result of more severe storms , more intense heat stress , changes in the distribution of infectious disease , and reduced air quality , we are concerned that the indirect impacts of a disrupted climate system may be orders of magnitude more important in terms of the human suffering they cause . because these indirect effects will result from changes in biophysical systems , which are inherently complex , 
 there is significant uncertainty about their magnitude , timing , and location . 
 however , the uncertainty that shrouds this issue should not be cause for complacency ; rather it should serve as an organizing principle for adaptation to its ill effects .