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Temperature Extremes, Health, and Human Capital Temperature Extremes, Health, and Human Capital Joshua Graff Zivin and Jeffrey Shrader Summary The extreme temperatures expected under climate change may be especially harmful to children. Children are more vulnerable to heat partly because of their physiological features, but, perhaps more important, because they behave and respond differently than adults do. Children are less likely to manage their own heat risk and may have fewer ways to avoid heat; for example, because they don’t plan their own schedules, they typically can’t avoid activity during hot portions of the day. And very young children may not be able to tell adults that they’re feeling heat’s effects. Joshua Graff Zivin and Jeffrey Shrader zero in on how rising temperatures from global warming can be expected to affect children. They review evidence that high temperatures would mean more deaths, especially among fetuses and young children (as well as the elderly). When combined with other conditions—such as high humidity, diseases, or pollution—heat can be even deadlier. Even when it doesn’t kill, high heat directly causes heat-related illnesses such as heat exhaustion; worsens other conditions, such as asthma, by increasing smog and ozone pollution; and harms fetuses in the womb, often with long-term consequences. High temperatures can also make learning more difficult, affecting children’s adult job prospects. What can we do to protect children from a hotter climate? Graff Zivin and Shrader discuss a range of policies that could help. Such policies include requiring air conditioning in schools; heat wave warning systems coupled with public infrastructure that helps people stay indoors and stay cool; and readjusting schedules so that, for example, children are mostly indoors during the hottest time of day or the hottest season of the year. www.futureofchildren.org Joshua Graff Zivin is a professor of economics at the University of California, San Diego. Jeffrey Shrader is a PhD candidate in economics at the University of California, San Diego. Alan Barreca of Tulane University reviewed and critiqued a draft of this article VOL. 26 / NO. 1 / SPRING 2016 31 Joshua Graff Zivin and Jeffrey Shrader By burning fossil fuels and are likely to cause more death and illness thereby releasing carbon and to diminish children’s ability to learn dioxide and other gases, we are and adults’ ability to perform mental tasks.2 reshaping the global climate.1 Children, including fetuses in the womb, The changes we expect globally will likely suffer especially severe effects will both increase average temperatures and from climate change because they are shift the climate toward greater and more more sensitive to temperature and rely on frequent extreme temperatures. Already, others to adapt. For a variety of reasons, the average Americans experience more hot days negative effects of more heat will outweigh per year than they did 60 years ago, and the the benefits of reduced exposure to cold, and number is expected to rise dramatically in the heat is thus the focus of this article. coming decades (see figure 1). In this article, we assess a warming climate’s Rising temperatures and the increasing likely effects on child wellbeing, limiting frequency of extremely high temperatures our attention to temperature’s direct effects Figure 1. Projected number of summer days above 90°F in four US cities Figure 1. Projected number of summer days above 90˚F in four US cities 9900 F ° 90 8800 n F a0˚ er thhan 9 7700 Figure 1. Projected number of summer days above 90°F in four US cities tt oter 6600 ht 90 days ys hot 5500 n 90°F 80 mmer mer da 4400 otter tha 6700 m h mber of suumber of su 23320000 of summer days 345000 NuN mber 20 1100 u N 10 00 0 SeSaeattttlele Seattle NeNweNw eYYwoo rYkr okrk KaKnsaaKsn aCsnisatayss CCityi tHyouston HHoouusstotnon 22000011-–21001 0 22004466-–250555 22009900-2–09999 2001-2010 2046-2055 2090-2099 Note: Each projectioNn oiste t:h Eea ecnh spermojebcleti oanv eisr athgee eonfs beumsbinlee asvse-raasg-eu souf ablu sscineensas-raios- fuosrueacl asscetsn aforiro t he continental United Stat es. forecasts for the continental United States. Sources: Katherine Hayhoe et al., “Development and Dissemination of a High-Resolution National Climate Change Note: Each projection is the ensemble average of business-as-usual scenario Dataset,” Final RepoSrotu frocre Us:nited States Geological Survey, USGS G10AC00248 (2013); Anne M. K. Stoner et al., “An foAsryenccahrsotnso ufso Rr etghioe n acl oRengtriensesinont aMlo Udenl fiotre Sdta Stisttaictael sD.o wnscaFliinnagl oRfe Dpoarilty fCorli mUnaittee dV aSrtiaatbesle s,” International Journal Geologic aKla Stuhrevreiny,e Hayhoe et al., "Dev elopment and Dissemination of a High-‐ of Climatology 33 (2R0e1s3o)l:u 2ti4o7n3 N–a9t4io; nMael lCinlidmaa Ste. CDhaaltnogne a Dnadt aSsoent,y"a A. Jones, comps., Southeast Regional Assessment SPoruojreccte fosr: the National Climate Chan gUeISn aGtneSrd Gn W1a0tiilAodnClia0fel0 J2So4cui8ren(na2cl0 eo1 fC3 Ce)l;ni mAtenarnt, oeUl o.MSg..y KG.e Sotloongeicr aelt Saul.r, v"Ae–yn (Reston, VA: U.S. Geological Survey, 2A0s1y0n)c.hronous Regional Regression Model forS SotuaFtthisietnaicsaat lRl D eRogiweonnpasocl aArlsitns egfs osomfr De Unatil nPy riotjeecdt States Cfolrim thaet eN Vaatiroianballe Csl,i"m ate Change and Wildlife Science Cen 3te3r ,( U20.S1. 3G)e:o 2lo4g7i3cal9 S4u; rvey Geologic aKla StuhrevMreeinlyin,ed aH S.a Dyalhtoon ean ed tS oanly.a, A".D Joenve se, cloomppms, ent and Dissemination of a High-‐ R3e2s oTlHuEt FioUTnU RNE aOtFi oCHnIaLDl RCElNimate Change Dataset," (Reston, VA: U.S. Geological Survey, 2010). UISnGteSr Gn1a0tiAonCa0l0 J2o4u8rn(a2l0 o1f3 C)l;i mAnantoel oMg.y K. Stoner et al., "A–n Asynchronous Regional Regression Model forS Sotuatthisetaicsat lR Deogiwonnascl aAlsinsegs somf Denatil Py roject Cfolrim thaet eN Vaatiroianballe Csl,i"m ate Change and Wildlife Science Cen 3te3r ,( U20.S1. 3G)e:o 2lo4g7i3cal9 S4u; rvey Melinda S. Dalton and Sonya A. Jones, comps, (Reston, VA: U.S. Geological Survey, 2010). Temperature Extremes, Health, and Human Capital on health. Direct effects include physical or individual can use to create economic value. mental impairment from heat stress on the Most of heat’s effects on wellbeing are acute; body. Examples include death caused by heat heat-related deaths, for example, happen stroke, cardiovascular disease, respiratory within hours of a heat wave. Heat’s effects impairment, or malformed fetal brains, but aren’t limited to acute events, however. we also consider more general bodily stress Research increasingly shows that exposure to caused by heat. We largely ignore indirect warmer days in the womb or during infancy health effects that could arise from a hotter can cause long-lasting, often lifelong harm.5 climate due to food insecurity, natural We have compelling evidence that short-lived disasters, or increased global conflict; those temperature shocks can damage a child’s matters are covered by other authors in this health, but the key policy issue regarding issue.4 When possible, we rely on studies of the health effects of climate change is how the temperature–health relationship that a permanent shift in the overall distribution have focused explicitly on children. But such of weather would play out. A sustained shift studies leave substantial gaps in the evidence, toward more extreme temperatures could so we also make use of studies about the have even more profound impacts than general population to help complete the current estimates indicate. On the other picture. To ensure that our conclusions hand, if the effects of climate change move are relevant for policy, we concentrate on slowly, we have a better chance to limit their studies that focus on the causal relationships impact through adaptation. Some recent between temperature and human wellbeing evidence suggests that we may indeed be (see box 1). able to adapt, calling into question the We’ve organized our discussion around generalized conclusions often drawn from three broad impacts on health: death, illness, studies that rely only on short-term variations and human capital. Human capital involves in weather to predict the long-term effects the skills, knowledge, and abilities that an of heat. Laboratory studies have particular What is Causal Analysis? Imagine that you’re a parent trying to decide whether to enroll your child in a charter school, or that you’re a doctor deciding whether to prescribe a certain medicine for a patient, or that you’re a senator deciding whether to lower the tax rate. In each case, you want an answer to the question: Would my action actually cause an improvement in outcomes? If my child goes to a charter school, would she have a better chance of attending a good college? Would the medicine make my patient healthier? Would lowering taxes make the economy grow faster? In each case, it’s not enough just to know that there’s an association or a correlation between an action being considering and the outcome of the action. Two events may be likely to occur at the same time or place, but if we don’t know how one influences the other, then the fact that they’re associated doesn’t tell us whether our action would do some good. Causal analysis sheds light on questions like those by determining when an action is simply associated with an outcome versus when the action indeed causes the outcome. A simple example serves to illustrate the difference between correlation and causation: Death often occurs after a hospital stay. Therefore, a hospital stay is correlated or associated with dying. But, in general, hospital stays don’t cause death. Rather, some very sick people seek care at hospitals and then die. It’s the fact that sick people seek care at hospitals that creates the relationship, and a policy maker would be making a grave misjudgment by closing all hospitals in an effort to reduce the number of citizen deaths. Numerous statistical and scientific techniques have been developed to distinguish causality from correlation, and applying and refining those techniques is a major component of empirical research in the social sciences. VOL. 26 / NO. 1 / SPRING 2016 33 Joshua Graff Zivin and Jeffrey Shrader limitations, because they don’t consider how the body and causing a heat stroke. Studies adaptive actions could limit negative effects that examine specific causes of death often over the long run. focus on cardiovascular impairment, which is generally more relevant for the elderly than It’s clear that public policy can and must it is for children.7 Among infants, evidence help minimize the damage to children’s from heat waves suggests that in addition to health caused by climate change. The cardiovascular illness, blood disorders and immediate and compounding impacts of failures of the digestive system are leading harm to children should carry great weight causes of death.8 As temperatures rise in when it comes to encouraging action to cut developing countries, so does the prevalence greenhouse gas emissions. Public information of gastrointestinal illnesses.9 Heat-related campaigns and alert systems will also deaths among children are also increased become increasingly important in helping by many of the indirect impacts of rising us minimize exposure and take action to temperatures, including disease transmission avoid exposure in the first place. Investing by insects like mosquitoes. more in emergency response can also help us minimize the damage when we do One of the main ways heat kills is by experience extreme temperatures. Children limiting the body’s ability to regulate its are vulnerable, and they have little choice own temperature. When body temperature about how and where they spend their time, rises as a result of ambient heat, physical so policies expressly designed to protect them exertion, or fever, more blood flows to the and the adults who care for them may be skin and we sweat to dissipate body heat. If especially important. the ambient temperature is too high, those mechanisms can’t cool the body efficiently Extreme Temperatures Kill and may even work to warm the body Although humans are very well adapted further. Adverse weather conditions like high to high temperatures, heat can and does humidity magnify that effect by reducing kill. It does so through direct effects such cardiovascular efficiency.10 as heat stroke, cardiovascular failure, or Children are a special case. Infants’ and other physical disease and through indirect children’s vulnerability to cold temperatures effects such as starvation after crop loss or is well established, and humans have the spread of infectious disease.6 Research evolved physical features to combat indicates that children are especially that susceptibility, including extra fatty vulnerable to heat-related death—partly tissue in infancy and reduced sweating because of their physiological features, during childhood. Children may also be but, perhaps more important, because they physiologically more vulnerable to high behave and respond differently than adults temperature because they regulate their body do. temperature less efficiently, although the How Heat Kills evidence here is less conclusive.11 Generally, heat directly causes death through At least one mechanism by which heat kills cardiovascular failure—often made worse is unique to the very young. In a series of by respiratory disease—or by overheating studies starting in the early 1990s, high 34 THE FUTURE OF CHILDREN Temperature Extremes, Health, and Human Capital heat has been linked to sudden infant death children. A recent review of public health syndrome (SIDS).12 Because the infant body studies concluded that children younger than is relatively more vulnerable to heat loss, it 15 years have a higher risk of dying from compensates with higher fat and reduced heat than adults do; infants and children sweating. An infant who is wrapped or younger than five years are particularly at covered heavily might be unable to shed risk.14 The review estimates that in developed excess heat, producing brain trauma. Brain countries, adults experience a 2 to 3 percent trauma can also occur when infants sleep in increase in mortality with every 1˚C (1.8˚F) the same bed as a warm adult, or when they rise in temperature above a threshold of 27˚C suffer from fever, or when they’re exposed to (80.6˚F) to 29˚C (84.2˚F). For children, the high ambient air temperature. mortality rate is estimated to be 50 to 100 percent higher than for adults.15 Behavior, too, may contribute to the disproportionate number of heat-related Economists Olivier Deschênes (one of the deaths among children. Children are less editors of this issue) and Michael Greenstone likely to manage their own heat risk and may show that death rates increase dramatically have fewer ways to avoid heat than adults do. at temperatures above 32˚C (89.6˚F): a For instance, children playing sports often day above 32˚C sees triple the death rate don’t hydrate sufficiently.13 Also, because of a day at 26˚C (78.8˚F) to 32˚C. As in the children don’t usually choose their daily review of public health studies, they found schedule, they typically can’t avoid activity that infants suffer the second-highest heat during hot portions of the day. We’ll return mortality rate among all age groups, after the to that issue in more detail when we discuss adaptation. elderly. Deschênes and Greenstone predict that, given the current understanding of how Estimating the Relationship between climate change will unfold, infants constitute Heat and Death the age group likely to experience the greatest increase in mortality rates.16 Much of the research on heat’s direct health impacts has focused on death, partly because Very few studies examine how heat interacts death is an important topic and partly with other climatic and atmospheric because it’s easy to measure. The abundant conditions, and therefore we know research documenting the relationship little about those potentially important between temperature and death provides relationships. One exception is Tulane good estimates of the overall effect of the University economist Alan Barreca’s work relationship and explores in detail the on the interaction of heat with humidity.17 mechanisms by which heat kills. Moreover, in Barreca shows that high humidity contrast to studies of most other temperature independently leads to more deaths and that effects on health, many studies of heat deaths humidity interacts with heat, making high explicitly examine impacts on children, which heat even deadlier. Barreca also shows that lets us clearly compare them with adults. humidity levels are associated with increases Studies that classify their results by age group in infant deaths: three additional days of high tend to find that death rates are highest humidity increase the average monthly infant among the elderly and second highest among mortality rate by 1.1 percent. VOL. 26 / NO. 1 / SPRING 2016 35 Joshua Graff Zivin and Jeffrey Shrader High heat can also harm health when it because crop failure arising from excessive interacts with air pollution. Such interaction heat leads to losses of real income. Urban isn’t generally that important, but the residents in India experience temperature- formation of ozone pollution depends on related mortality rates similar to rates among high temperatures, and high ozone levels residents of the United States, suggesting, have been proposed as one explanation for first, that deaths caused directly by heat may the particularly deadly European heat wave be relatively few in number compared with of 2003.18 Ozone is also a significant predictor those caused by heat’s effects on productivity of childhood asthma.19 and, second, that the possibilities for adaptation are extensive across the globe.22 In contrast to records of the deaths of infants and children, fetal mortality isn’t In developed countries, infant and child well recorded, so the extent of heat-related mortality from heat is higher among low- deaths in the womb is not clear. The fact income groups, suggesting that wealthier that exposing unborn children to extreme households can make investments that temperatures has measurable impacts on offset at least some of the damage from their health after birth, which we discuss in high heat.23 Death rates associated with the next section, lends credence to the idea high temperatures are also higher in the that heat can also kill the unborn. northern United States than in the southern Heat and Death in Developing United States—likely because of the Countries adoption of air conditioning in areas that routinely experience high heat.24 This gap Outside the United States and Europe, has narrowed, however, because over time, estimates of deaths caused by heat are heat-related mortality rates have fallen faster sparse. In a review of 36 epidemiological in the northern states relative to those in the studies, only eight contained any data from South.25 We return to that issue later, when countries outside the Organisation for we discuss adaptation and avoidance. Economic Co-operation and Development, In summary, high temperatures are strongly and of those eight studies, most were from a associated with increases in death rates single country: Brazil.20 Developing-country among all age groups, and the very young are studies can shed light on how income and particularly at risk. Heat kills in many ways, infrastructure help mitigate damage from varying with location, level of development, heat. A recent study of weather-related infant and degree of acclimatization. Without mortality in a group of African countries investment in infrastructure to protect people showed that in poorer countries, temperature from heat, we can expect child mortality can kill through many channels. In particular, associated with high temperatures to rise as malaria infection as a result of increased the globe warms. Also, because the majority mosquito activity was one of the main sources of developing countries already have warmer of temperature-based infant deaths.21 climates than the United States and Europe A study of temperature-related deaths do, we should expect to see fewer gains from in India found that heat’s average effect reduced low temperatures in those countries, on mortality there is more than 10 times making the net effect of climate change there greater than in the United States, largely even more severe. To learn more about heat’s 36 THE FUTURE OF CHILDREN Temperature Extremes, Health, and Human Capital effects on children in developing countries, high humidity and behaviors like exercise or see the article by Rema Hanna and Paulina wearing inappropriately heavy clothing, high Oliva elsewhere in this issue. temperatures can cause heart attack, stroke, and respiratory failure.28 Medical conditions Illness that impede the circulatory or respiratory system—such as asthma, heart disease, or a Even when it doesn’t kill, high temperature previous stroke—increase the likelihood of can cause illnesses or worsen existing medical those acute episodes.29 conditions to the point that sufferers must be hospitalized. Compared with studies of heat- Pregnant women and their fetuses are related deaths, however, we’ve seen fewer especially vulnerable to high temperatures studies of how temperature affects illness— for several reasons, including higher core partly because of data limitations. For temperature because of the pregnant example, detailed hospitalization data can woman’s increased fat deposition, her be difficult to obtain, and data on physician diminished capacity to sweat, and the and pharmacy visits are even scarcer.26 additional thermal stress associated with Moreover, many heat-related illnesses fetal maintenance.30 Warmer temperatures may be socially costly without producing increase both the proportion of preterm doctor or emergency room visits, because births and the incidence of low-birth- the symptoms are treatable at home—for weight babies.31 Shocks to the fetus often instance, through rest and hydration. Access have lifelong consequences.32 And the to health insurance coverage (in places negative relationship between fetal health like the United States, where coverage is and temperature may mean that early life not universal) may also limit doctor and exposure to extreme temperatures can have pharmacy visits, making it hard to infer how long-lasting effects, discussed next. income shapes the relationship. Linking Heat and Illness How Heat Impairs Health The vast majority of evidence on heat and Heat exhaustion and heat stroke are the most ill health focuses on adults. For example, serious illnesses caused directly by heat. They for each 1˚C above 29˚C (84.2˚F), adult generally result from dehydration associated hospitalizations for respiratory disease rise with exposure to or physical activity during by about 3 percent. Cardiovascular illness periods of high heat. As it regulates heat, rates also rise in many cases, with effects the body loses water and salt in the form of concentrated among older people.33 Recent sweat. If the water and salt don’t get replaced research from Germany showed that hospital over time, the body can overheat, leading to admissions rise by up to 20 percent on hot the dizziness, muscle cramps, and fever that days, although, again, the effects occur characterize heat exhaustion. At the extreme, predominantly among people older than 60 heat exhaustion becomes heat stroke, which, years.34 even when it doesn’t kill, generally leads to The evidence on how heat affects children’s permanent neurological damage.27 health is more nuanced. A study from High heat also strains the heart and can Spain found that hospital admissions in make breathing difficult. Combined with general rose dramatically above 34˚C–36˚C VOL. 26 / NO. 1 / SPRING 2016 37 Joshua Graff Zivin and Jeffrey Shrader (93.2˚F–96.8˚F) and that children younger concluded that the adverse effects of high than nine years were slightly more likely temperature were consistently stronger for to be admitted than were adults aged 18 to birth weight than for early birth.38 Another 44 years. Moreover, admissions of children study compared infants’ birth weights with appeared to be higher during periods when the temperatures their mothers were exposed high heat was combined with elevated levels to during each trimester of pregnancy. of particulate matter, although that finding It found that in all trimesters, high is not universal.35 In contrast, a recent study temperatures were associated with reduced in New York about how hot weather affects birth weight but that the effect was slightly hospital admissions due to cardiovascular larger during the third trimester.39 Birth and respiratory diseases found no significant weight is a proxy measure of fetal health that link between high temperatures and either can be linked to illness in childhood and later cardiovascular or respiratory diseases for in life.40 people from birth to 19 years old. In fact, in the New York study, the estimated magnitude The evidence for heat effects on children’s ill of temperature effects for that age group health is less conclusive than in the case of was the lowest among all groups.36 But the childhood mortality, but the impacts on fetal wide age range in the study could be masking health appear to be unambiguously negative. larger effects on the very young by averaging Children appear to be less susceptible than them in with older children and adolescents. adults or the elderly to some of the more dramatic heat-induced illnesses, perhaps In contrast to heat’s effect on fetal mortality, because they tend not to suffer from the its health effects on fetuses have been better combinations of conditions that can lead studied. A study that summarized several to adult hospitalizations. On the other investigations into how temperature affects hand, children may be more at risk for heat mammalian fetuses found that a wide range exhaustion and related illnesses because of less than fatal outcomes could occur. The they are less able to monitor and respond to study’s authors cautioned that even though signs of their own dehydration. But because healthy thermal ranges vary among other parents or caregivers can treat children’s mammalian species and are not always dehydration fairly easily, we may be less likely reliable models for human biology, we can to see those effects in the data. Studies of learn some general lessons from them. For heat-related illness thus likely understate the example, elevating fetal temperature by true rate among children. 2˚C–2.5˚C for as little as one hour can cause moderate to severe damage to the nervous Heat and Human Capital system and impede neural development. In addition to causing illness and death, Physical exertion combined with elevated extreme temperatures may also make it ambient temperature could trigger harder to learn and thus may limit children’s potentially dangerous internal temperature educational attainment and economic spikes, although a stationary, healthy woman prospects in the long run. Though we have would likely be able to avoid them.37 little direct evidence of such a relationship, Research on humans has focused on preterm studies of the fetal period establish the births and birth weight. A recent study linkage indirectly—through impacts on birth 38 THE FUTURE OF CHILDREN Temperature Extremes, Health, and Human Capital outcomes that prove important for education only 2 percent of the mass of a typical and work later in life. body, the brain generates 20 percent of the body’s heat.48 The body is generally able How Heat Hinders Human Capital to efficiently discard heat created by the Formation brain, but when the weather is warm and humid or when we engage in heavy physical Excess heat in the womb can produce activity, our bodies can struggle to regulate physical defects, delay brain development, temperature, leading to spikes in brain and lead to a host of central nervous system temperature of up to 2.5˚C.49 problems that make it harder to accumulate human capital in the long run.41 As we’ve Evidence on the Heat–Human Capital seen, low birth weight is a common proxy Relationship measure for fetal setbacks and has been shown to significantly affect education and Experimental evidence supports the notion work outcomes later in life. Reduced fetal that heat can directly impair cognitive nutrition may be one important mechanism function. For example, soldiers exposed to behind that relationship.42 hot environments perform worse on complex cognitive tasks, are more prone to error, and In addition to how temperature affects are less able to carry out physical tasks.50 birth weight and central nervous system Other studies show that heat exposure functioning, it may also play an important reduces performance on multitracked role in gene expression. We now know that tasks that mimic real-world school and environmental stress can affect how genetic office duties, impairs working memory, and code gets translated into observable human lowers test scores.51 A review of a number traits and that gestation is a particularly of studies of office workers’ productivity susceptible period for such effects.43 estimated that performance declines rapidly Exposure to high temperatures has been when temperatures go above or below implicated as a source of environmental 21˚C–22˚C (69.8˚C–71.6˚F). For instance, at stress in a wide range of plants and animals.44 27˚C (80.6˚F), office workers’ performance Vertebrates’ brains and other central nervous declines by 5 percent relative to their system structures are particularly sensitive to such sources of environmental stress, performance at 21˚C.52 suggesting that cognitive development may Though heat appears important to human be vulnerable to such exposures.45 capital formation and productivity, its real- Once outside the womb, the developing world effects on those areas have been brain is still sensitive to heat on chemical and poorly documented, likely because it’s hard electrical levels.46 Rapid brain development, to observe the relevant outcomes. But the which can be disrupted by extreme heat, gains from such studies would be immense. continues through early childhood.47 In more The effects are likely to be substantially mature children, ambient temperature, important, and low-level temperature as well as the heat generated by the brain impacts are, by definition, much more itself, can impede mental processes, widespread than the extreme temperatures thereby creating the potential for heat to that lead to the majority of hospitalizations hinder learning. Although it represents and deaths. Thus, from the standpoint of VOL. 26 / NO. 1 / SPRING 2016 39 Joshua Graff Zivin and Jeffrey Shrader total welfare, human capital effects might overstate the true damage associated with be more important than the relatively well- long-run temperature changes. On the other understood, direct temperature effects we hand, multiple acute responses could have discussed in previous sections. substantial long-term impacts by hampering children’s acquisition of skills, an issue we Studies of pollution’s impacts have analyzed discuss later in the context of adaptation.57 school attendance, and we could do the same for temperature.53 Researchers The study that found a link between have documented a link between school temperature and test scores also offers attendance and indoor air pollution, outdoor evidence of climate’s long-term effects on air pollution, and ventilation rates.54 We human capital formation. In particular, the aren’t aware of any studies by economists that authors found that longer-term weather show a similar link between temperature and averages are not strongly associated with school attendance, although the data needed student performance. The authors suggest to do so are relatively available. We know that that compensatory adaptive behaviors— high temperatures reduce the adult labor particularly, additional investments in supply, so it’s certainly plausible that heat also learning that are made after heat events— increases school absenteeism.55 might be mitigating the short-term effects.58 But we have evidence that temperature can Student test scores offer a more direct have long-term effects on national wealth. way to assess the human capital effects of We don’t know whether that result is driven temperature. Economists Joshua Graff Zivin by changes in human capital or by something (one of the authors of this article), Solomon else, but the big differences in aggregate Hsiang, and Matthew Neidell have shown output that are associated with differences that a temperature above 26˚C (78.8˚F) on in climate suggest that high heat may cause the day of a math test can diminish students’ at least some loss of human capital or some performance.56 decline in productivity.59 Heat and Human Capital in the Long Run Short-run shocks can also be linked to long- run outcomes through fetal exposures that The studies and hypotheses we’ve reduce birth weight. For example, low-birth- discussed focus largely on acute responses weight children in Britain were significantly to temperature. Temporarily impaired less likely to pass standardized tests as brain function that leads to absenteeism teenagers.60 A sample of Californians found or reductions in test scores, however, similar effects on school attainment and doesn’t necessarily affect either cognition adult poverty.61 Studies of twins, which let or human capital attainment in the long researchers control for genetic endowments term. Do the acute effects we’ve identified and other family characteristics, have also translate into poorer outcomes in school consistently found that low birth weight or work later in life? When it comes to diminishes educational attainment, IQ, and estimating the impact of climate change, even earnings.62 the question of long-term effects becomes even more important. Simple aggregation Put simply, it seems that temperature of short-lived, acute-impact estimates might extremes can impair cognitive functioning in 40 THE FUTURE OF CHILDREN