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The Science of Change The Science of

Michael Oppenheimer and Jesse K. Anttila-Hughes

Summary Michael Oppenheimer and Jesse Anttila-Hughes begin with a primer on how the works, how we know that is rapidly getting warmer, and how we know that the recent warming is caused by human activity. They explain the sources of scientific knowledge about climate change as well as the basis for the models scientists use to predict how the climate will behave in the future. Although they acknowledge the large degree of uncertainty that surrounds predictions of what will happen decades or even centuries in the future, they also emphasize the near certainty that climate change has the potential to be extremely harmful to children. Most children around the world will face hotter, more extreme temperatures more frequently. Higher temperatures will directly affect children’s health by increasing the rates of heatstroke, heat exhaustion, and heat-related mortality. Excessive heat is also likely to affect children indirectly by disrupting agricultural systems, driving up prices, and increasing food scarcity. Many of the world’s children may see local demand for water outstrip supply, as shifting patterns dry out some regions of the world, make other regions wetter, and increase the frequency of both unusually dry periods and unusually severe rains. Mountain glaciers will recede further, significantly reducing storage of winter snows and thus springtime runoff, which has traditionally been used to water fields and recharge reservoirs. Melting ice will also raise sea levels, triggering direct physical threats to children through flooding and erosion and indirect threats through migration and expensive adaptation. Climate change is also expected to make weather-based disasters more frequent and more damaging. This is particularly worrisome for children, not only because of the physical peril disasters pose but also because disasters can have debilitating long-term indirect effects on children. Damage to from climate change may also harm children; for example, acidification the world’s oceans will reduce food supplies, and disease-carrying insects will invade new areas in response to changing rains and temperatures. In the face of such dire forecasts, Oppenheimer and Anttila-Hughes argue, climate change forces us to directly confront the value we put on future children’s wellbeing. Fortunately, we have reason for hope as well as for concern: “History,” they write, “has demonstrated time and again that humans can tackle uncertain threats in times of need.”

www.futureofchildren.org

Michael Oppenheimer is the Albert G. Milbank Professor of Geosciences and International Affairs in the Woodrow Wilson School of Public and International Affairs and the Department of Geosciences at ; he is also the director of the Program in Science, Technology and Environmental Policy at the Woodrow Wilson School and a faculty associate of the Atmospheric and Ocean Sciences Program, Princeton Environmental Institute, and the Princeton Institute for International and Regional Studies. Jesse K. Anttila- Hughes is an assistant professor in the Department of Economics at the University of San Francisco. Geoffrey Heal of Columbia University reviewed and critiqued a draft of this article.

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Michael Oppenheimer and Jesse K. Anttila-Hughes

nderstanding how humanity’s costs of inaction are high. At the same accumulated greenhouse time, enormous uncertainty surrounds any gas emissions will alter forecast of specific outcomes of climate Earth’s climate over the change. Which regions will be affected next few centuries requires and in what ways, how quickly changes will Ua broad perspective, so climate change occur, and how humans will respond are all is usually discussed as a global issue. But impossible to know with certainty, given the understanding how climate change will complex natural and social forces involved. affect children who live through it requires From a risk management perspective, the a narrower focus—one that pushes directly possibility of extremely negative outcomes against the limitations of that global view. means climate change has much in common Geographic variation in climate change’s with other large-scale global threats such as effects over time, uncertainty stemming conflict between nuclear powers, wherein from scientific complexity, and, more than the potential for highly undesirable and anything, the inherent impossibility of irreversible outcomes is real but very difficult forecasting future human behavior combine to predict with precision. We will return to to make climate change’s eventual impacts this theme many times. on children both very different from place to place and extraordinarily difficult to Origins of Understanding predict with any certainty. Climate change The greenhouse effect is a prerequisite will influence children’s lives in few “global” for life as we know it because without it, ways. Rather, during the coming decades, Earth would be much colder (by about 32° children will face myriad interactions Celsius, or 57.6° Fahrenheit) and drier: between changes in the climate and social, a frozen desert. Nobel Prize–winning economic, and cultural forces. Swedish chemist Svante Arrhenius laid out the greenhouse “problem” in an 1896 A defining theme of this article is the need paper. He showed that a rise in atmospheric to balance high uncertainty in some areas concentrations of carbon dioxide—a by- with relative certainty in others. As we will product of combustion, caused by burning show, we now have overwhelming evidence coal as an energy source in the emergent that human emission of greenhouse gases industrialized countries—would make the has already begun to change the climate and planet warmer, although he saw that warming that it will continue to do so unless emissions as beneficial rather than problematic. Other are halted; hence we call this climate change notable nineteenth- and early-twentieth- anthropogenic, from the Greek for human century scientists also contributed to our influenced. Moreover, ample evidence understanding by linking earlier, natural indicates that we can expect many changes changes in atmospheric carbon dioxide to the in the weather and the climate that will fall comings and goings of ice ages. outside the range of human experience. Unless we reduce emissions drastically, After Arrhenius, interest in the problem those changes are expected to have pervasive lagged until the 1950s, when a few scientists impacts worldwide, including, in some began exploring in detail how carbon dioxide cases, the destabilization or destruction traps infrared radiation. They provided the of ecological and social systems. Thus the first credible estimates of the fraction of

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The Science of Climate Change emissions that remain in the atmosphere and so forth that we experience during any rather than dissolving in the ocean. The given day, week, month, or year. Climate, advent of modern computing advanced on the other hand, refers to the behavior weather forecasting and led to an interest of weather over longer periods, such as in modeling the general circulation of the decades, from a statistical perspective (for atmosphere. An offshoot of those studies example, annual mean temperature or mean examined the effect of carbon dioxide and, daily maximum temperature, averaged in the 1960s, produced the first computer- for a geographic region). Climate change based models for estimating future climate thus refers to an increase in average . By the 1970s, scientists had come temperature, along with all of the ways such to understand that the cooling effect of an increase affects the characteristics of particulate matter, which is a by-product of climate and weather. dirty, combustion techniques, had been substantially offsetting the warming Failure to differentiate between effect of carbon dioxide. The roles played by weather and climate can lead to serious water vapor, clouds, and minor atmospheric misunderstandings. We easily recall gases other than carbon dioxide were also weather, and that readiness of perception elaborated in great detail. By the late 1980s, (or availability, as psychologists call it) the scientific consensus that carbon emissions often dominates our assessment of risk: If would warm the climate was sufficient to this winter is cold, what happened to global become a major political issue, leading to warming? If this summer is hot, we’d better the 1992 negotiation of the hurry up and fix the problem! Obviously, Framework Convention on Climate Change such misunderstandings can be manipulated treaty, which was dedicated to stabilizing to fit political agendas, and we must act to concentrations in the decouple our understanding of the larger, atmosphere “at a level that would prevent global problem from the random weather dangerous anthropogenic interference with experienced on any given day. the climate system.” Today, carbon dioxide The Physical Problem has increased by more than 40 percent from its preindustrial level because of the Concern about climate change has grown mining and burning of fossil fuels, the cutting over the past 25 years. Today, thousands and burning of forests, certain agricultural of climatological scientists and researchers practices that emit greenhouse gases, and across related fields are conducting the output of certain industries, such as on topics ranging from the specifics of those that produce cement and halocarbon obscure climate processes to the likely refrigerants. impacts of climate change on everything from alpine ecosystems to financial markets. The terms climate and weather are The pace of discovery and the growth in sometimes confused with each other, and understanding have been sufficiently rapid, that confusion can have serious implications. the breadth of impacts sufficiently wide, and Weather denotes the actual behavior of the implications of social concern sufficiently Earth’s oceans and atmosphere over a given broad that a major international organization short period; the term weather refers to the was created to synthesize scientific evidence temperature, precipitation, wind, storminess, on climate change. The Intergovernmental

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Michael Oppenheimer and Jesse K. Anttila-Hughes

Panel on Climate Change, or IPCC, operates a consistent picture of carbon’s current and under the auspices of the United Nations future warming effects on the planet. Environment Programme and the World Meteorological Organization. Every six years Scientists are nearly certain that climate or so, the panel publishes assessment reports change is occurring and has the potential to be extremely harmful. Climate change that summarize the state of the research on nonetheless has several unique characteristics climate change science, impacts, and policy.1 that combine to present a very challenging Many other organizations, too, have assessed mix for policy makers. Climate changes— aspects of the problems inherent in climate both those already observed and those change, resulting in projects ranging from the anticipated—will affect different countries 2007 Stern Review—a UK government study and different regions very differently. But, emphasizing the economic benefits of early eventually, the changes will affect humans in action against climate change—to the 2014 every nation on the planet; in no place will philanthropically funded American Climate climate remain unchanged. Moreover, every Prospectus, which summarizes the expected country’s carbon dioxide emissions affect economic risks of climate change in the the climate in every other country because United States.2 carbon dioxide’s long lifetime means that it Perhaps the most important point about achieves a nearly uniform distribution in the the science of climate change is that our atmosphere. Thus climate change is a global knowledge arises from four very different commons problem at the largest conceivable sources: direct observations of the climate scale; the atmosphere is an easily damaged, system and changes within it, including open-access resource whose preservation will demand increasingly active coordination everything from almanac records to satellite- across the full complexity of human social based imaging; paleoclimate evidence of interactions. Climate change’s global nature Earth’s climate in the distant past—for thus distinguishes it from almost every other example, what we can deduce by examining major environmental policy problem— air bubbles trapped in the Antarctic except, perhaps, the effects of ozone by snow that fell hundreds of thousands depletion or large-scale . of years ago, or by analyzing the chemical composition of fossilized marine animal Another implication of carbon dioxide’s very shells trapped in sedimentary layers at the long lifetime is that a significant fraction sea bottom for tens of millions of years; (about 25 percent) of today’s emissions will laboratory studies of the chemical and remain airborne even a millennium from now physical processes that take place in the unless we invent a technology to affordably atmosphere; and—perhaps most important capture and bury the carbon dioxide, for forecasting—numerical, computer-based meaning that many expected changes are models of climate circulation and other effectively irreversible. Furthermore, the climate properties, which in many respects huge mass of the oceans is absorbing a large are similar to the meteorological models portion of the climate’s thermal energy as used for generating weather forecasts. Our Earth warms, and the resulting thermal understanding of climate change is based on inertia means that the effects of today’s all four of these sources, which together paint emissions will take several decades to appear.

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The Science of Climate Change

Even if we could eliminate emissions entirely suffering opprobrium) for the outcomes. It’s today, enough greenhouse gases have already no wonder that many leaders have resisted been released to gradually warm the planet grappling with climate change—all the more for the rest of the current century and so because of the potential costs of reducing beyond. .

A natural question is whether all of climate Policy makers will need an change’s impacts on children’s wellbeing must unusual degree of foresight, necessarily be bad. Generally speaking, most scientists say climate changes will disrupt and extraordinary powers of damage both natural and human systems in judgment, and a willingness most places around the world; the IPCC, for to act without getting credit example, acknowledges eight risks associated for the outcomes. with climate change, ranging from increases in rates of death and illness during periods of extreme heat to loss of rural livelihoods.3 Climate change science is also rife with Certain regions are predicted to be more uncertainty. Even though scientists are mildly affected, and cooler countries closer increasingly certain about the general to the poles, such as Canada and Russia, characteristics of global climate changes may actually see a variety of benefits under under certain emissions scenarios, extensive climate change (at least temporarily), thanks uncertainties remain when it comes to principally to longer growing seasons and details of how the climate will respond at milder winters. However, those beneficial time and spatial scales relevant to humans. effects are expected to be dwarfed by a The answers to such questions as how fast variety of negative impacts around the the sea level will rise are so uncertain that world, particularly in poorer countries, and scientists can offer policy makers only a very especially after factoring in certain indirect limited basis for making decisions, much effects of the increased stress that climate less tell them with confidence how high to change will exert on socioeconomic systems. build a seawall. When combined with the fact that, in the coming years, humans will Scholars have made strides in understanding change their emissions behaviors in response how social and economic systems respond to changes in energy supply and economic to climate changes, often using variability in development, uncertainty about what will historical weather patterns to provide insights happen becomes daunting. into what future climate change might mean The combination of universality; effective for human society. Readers who want to learn irreversibility; lags between emissions, more about such research should consult a policy actions, and system responses; and recent review by economists Melissa Dell, general uncertainty means that policy makers Benjamin Jones, and Benjamin Olken in the will need an unusual degree of foresight, open-access Journal of Economic Literature extraordinary powers of judgment, and a or the Impacts, Adaptation, and Vulnerability willingness to act without getting credit (or section of the most recent IPCC report.4

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In general, the fact that climate change’s climate is enough to seriously influence impacts are expected to be mostly negative human wellbeing; strong El Niño events are reflects the speed and intensity with which associated with punishing and heavy human activity is expected to change the floods throughout the world, including in climate. Although the climate is constantly major agricultural regions like California and in flux, natural variations on such a large eastern Australia. One vivid albeit imperfect scale normally occur many times more way of conceptualizing climate change’s slowly than the current rate of change. magnitude would be to think of a permanent The rapid pace of anthropogenic climate shift in the global climate regime several change limits our ability to respond smoothly times stronger than El Niño, though at a and gradually to changes in risk, and it much slower pace. hampers the efficacy of slow-moving policy options for mitigating —such as We can extend our understanding of the improving infrastructure or developing new climate further into the past by analyzing technologies—thereby potentially forcing data related to the paleoclimate. Air bubbles populations and food systems to change at trapped in ice that froze millennia ago, speeds far faster than normal. tree rings that capture growing-season conditions, microscopic fossils millions of What Can Past Tell Us years old buried beneath the ocean floor, and about Climate Change? plentiful other data let scientists infer what Natural climate variation has arisen from (1) the atmosphere and climate were like in ages a host of small changes in the amount of light past and to chart climate history. Scientists the emits, (2) fluctuations in the amount now know that the causes of natural, of volcanic dust in the atmosphere (which preindustrial climate changes included very cools Earth by reflecting sunlight), and (3) a gradual shifts in Earth’s orbit and axis of spectrum of other variations, including some rotation relative to the sun over tens and that are chaotic and therefore unpredictable. hundreds of thousands of years. Those cycles Taken together, these factors have caused alter the pattern of sunlight that reaches global average temperature to vary by a Earth’s surface and thereby affect the level few tenths of a degree Celsius through the of photosynthesis, the melting of ice sheets, decades—less than the current level of and many other processes that determine human-influenced warming. both the amount of greenhouse gases in the One lesson science can draw from the recent atmosphere and, ultimately, the behavior of past stems from the effects of the El Niño Earth’s climate. In the past million years, at Southern Oscillation, a suite of climatological the climatic minimums of such cycles—which changes tied to an increase in the surface we call ice ages—glaciers covered much temperature of the eastern tropical Pacific of the Northern Hemisphere, and global Ocean that occurs every three to seven years. surface temperature averaged around 5°C El Niño and similar oscillations are associated (9°F) below its value during periods of peak with changes in weather patterns around the warmth, called interglacials. The entirety of world, including changes of a few tenths of human , starting at the dawn of a degree in the global average temperature. agriculture, has taken place during the most Even that small a variation in the global recent interglacial.

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The Science of Climate Change

Much earlier, about 65 million years ago, by satellite-based observations of sea surface when the age of the dinosaurs came to an height beginning around 1990. end, temperatures averaged 8–10°C (14.4– 18°F) higher than today. About 55 million Together, our climatic records indicate that years ago, during the Eocene, global average Earth’s average temperature has gradually temperature jumped relatively rapidly, to increased during the past century and a half by 0.85°C, or about 1.5°F. That increase has 12°C (21.6°F) higher than today, possibly been uneven, with alternating intervals of because of unusually high atmospheric levels one to three decades of above-average or (as of methane, a potent greenhouse gas. During was the case for the most recent 16 years) the period of sustained warmth 50 million to below-average warming or even complete 55 million years ago, the Arctic latitudes were cessation (1940–70), a natural consequence home to alligators, tapirs, and rain forests.5 In of the climate system’s highly complex other words, the projected warming for this and variable nature. In the inland areas of century is modest in terms of the very long continents, the warming observed so far has span of climate history but is comparable been greater than the global average because in magnitude to changes of the past million coastal areas experience the moderating years that remade Earth’s surface; in our effect of the oceans. Warming has also been case, however, changes are occurring much, greater than average in the northern polar much more quickly than the natural rate. regions, where melting increases the Sea level also varies naturally, but the trend oceans’ absorption of the sun’s rays. Global associated with global warming, about 6–8 mean sea level, meanwhile, has risen about inches of over the past century, 15–20 centimeters (6–8 inches) during the now exceeds natural variations. Eight inches past century. Warming has melted the land may not seem like much, but it is sufficient to ice of mountain glaciers and polar ice sheets erode and permanently submerge about 60 and simultaneously caused the thermal feet landward from the typical US East Coast expansion of water already in the oceans; beach tide line. both factors have raised the oceans’ height.

Observed Global Changes Such changes in mean temperatures Earth’s average temperature since the and sea levels are already worrisome. To mid nineteenth century is known with fair provide context, a further 1°C increase precision. By that point, enough ground- in global average temperature above and ship-based thermometers were in place today’s levels, which many scientists say is and readings were being reported with already inevitable, would put Earth clearly sufficient reliability that scientists today outside the range of global temperature can retrospectively establish a credible experienced in the entire 10,000-year record of global average temperature by history of civilization. In addition—and using modern analytic techniques; that critically important when considering record is supplemented by satellite-based impacts on humans and infrastructure—are measurements beginning around 1980. changes in climatic extremes, which are Similarly, global sea level measurements expected to increase as the planet warms. using tide gauges go back to the late The frequency of extremely hot days and nineteenth century and are supplemented nights has already surpassed the historical

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Michael Oppenheimer and Jesse K. Anttila-Hughes record, as have the frequency and duration climate change, although they say changes of heat waves. Very cold days have become are likely to occur in the future. less common. Because more heat means more evaporation of water from the ocean How Do We Know Humans Are surface to drive the hydrologic cycle, more Responsible? land areas are seeing increases rather than A variety of evidence establishes that humans decreases in the frequency and intensity are the primary culprits causing climate of extreme precipitation. When the excess change. Humans emit 35 billion metric ocean vapor encounters conditions under tons of carbon dioxide into the atmosphere which precipitation would normally occur, it per year. Under natural conditions, Earth’s adds to the moisture available for storms; and ocean and land areas, including organic heavy rainfall, which causes damaging inland and inorganic material, emit about 20 times flooding, only becomes heavier. In addition, that amount, and they also naturally absorb even minor changes in average sea level can an almost equal amount via dissolution in produce major changes in the likelihood of the ocean and photosynthesis. Without coastal flooding, dangerously high tides, and human interference, the gains and losses in storm surges, all of which have increased. For the carbon cycle would be more or less in example, in the mid nineteenth century, a balance, and the amount of carbon dioxide flood level of about four feet occurred about in the atmosphere would vary very slowly once every 10 years in New York Harbor. over thousands of years. Human additions Since then, the local sea level has risen 1.3 to the cycle can be absorbed only so fast, feet. That seemingly small shift in average however, making it fairly straightforward to sea level means that the 10-year flood level connect the recent, rapid buildup of carbon now reaches 6.4 feet, topping the seawall that dioxide to human activity. The isotopes of 6 protects much of lower Manhattan. atmospheric carbon dioxide (that is, heavy Scientists have documented many other and light forms of carbon dioxide that carry phenomena in the past few decades different numbers of neutrons in their consistent with unusual climate changes, carbon atoms) carry distinctive fossil carbon ranging from rapid loss of mountain glaciers signatures, making it easy to demonstrate and ice caps known to be thousands of years the amount of carbon in the atmosphere that old to migrations of species toward cooler comes from fossil fuels versus the amount climates, to changes in annual ecological that comes from natural processes. Legal cycles such as the flowering and fruiting of records for the major fossil fuel extraction plants. Many of these changes are subtle for companies dating back more than a century now, but together they paint a consistent make total emissions from a supplier’s picture of a planet that’s warming with perspective easy to calculate. Even the nearly unprecedented speed. At the same time, uniform distribution of carbon dioxide in the scientists still can’t prove that some of the atmosphere is broken slightly by a pattern of climate’s more complex behaviors—such as geographic variation that can be traced to the the rate of formation and the intensity of distribution of emission sources around the tropical cyclones or large-scale oscillations world. In sum, there is no doubt that humans such as El Niño—have been altered by have radically altered the carbon cycle.

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It’s harder to ascribe responsibility for radiation reaching Earth, nor changes in the changes in temperature and precipitation amount of volcanic particulates in the upper because human forcing is only one of many atmosphere, which can cool the planet after things that influence the climate’s complex eruptions, have produced anything but small behavior. On a grand scale, observed average effects on the planet’s temperature. global temperatures have been increasing in time with emissions and in line with Projecting Future Climate and our understanding of climate. But that Scientific Uncertainty average state masks wide-ranging variability. To the best of our understanding, climate Although scientists say they’re certain that change’s impacts on humans have so far been we’re changing the climate overall, it’s hard small and subtle compared with variations to show that any specific climatic event in other environmental factors that affect happens “because of” climate change. To human welfare. Under business-as-usual infer that climate change bears some of the scenarios whereby we continue to emit responsibility for a specific event or shift in vast quantities of carbon dioxide, however, the climate involves sophisticated statistical the impact of climate change is expected optimal-fingerprinting techniques, which to grow markedly, eventually becoming compare observed geographic distribution a significant drag on human wellbeing all of warming, precipitation, and other factors over the planet. To understand the full with climate models that either include scope of the problem, we need to predict or exclude the buildup of anthropogenic climate change decades into the future. greenhouse gases and particulate matter. The The most reliable tools for such predictions optimal-fingerprinting method estimates the are climate-modeling computer programs effect of an increase in greenhouse gases, called atmosphere-ocean general circulation thereby enabling scientists to calculate the models (AOGCMs). These models solve odds that certain events, such as an unusual complex systems of equations embodying , would not have occurred in the known physical and chemical laws that the absence of climate change. Simpler describe how the atmosphere and the oceans techniques compare the time series of behave under the influence of sunlight, observed warming with a model projection Earth’s rotation, and changes in the chemical method that yields best estimates of how composition of the climate system, including climate variables would have changed emission of greenhouse gases. AOGCMs continent by continent or region by region. take as input the historical record of Earth’s In both cases, models that account for climate and make predictions subject to past increasing amounts of greenhouse gases constraints, thereby producing a long-term substantially agree with what we’ve actually climate forecast not unlike weather forecasts observed, whereas models that don’t include provided daily by the world’s meteorological rising greenhouse gases do not agree. organizations. Earth system models expand Moreover, direct observations since about on AOGCMs by adding descriptions of how 1980 have ruled out the possibility that other the ocean, atmosphere, and climate interact factors might be responsible for climate with surface vegetation. change; compared with anthropogenic factors, neither variations in the sun’s activity, Even the most advanced models can which can slightly alter the amount of solar only approximate the climate’s behavior,

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Michael Oppenheimer and Jesse K. Anttila-Hughes and they often disagree about specific that make Earth more or less reflective can aspects. That uncertainty stems from two influence warming. The clearest example sources. First, our understanding of the involves ice: land-based glaciers and ice physical and biological world is incomplete sheets—in particular, Arctic sea ice—reflect and must be approximated in ways that more light back into space than do the compromise accuracy. Second, the surfaces underlying them. As the planet equations that underpin AOGCMs must warms and surface ice coverage shrinks, be solved numerically on computers with Earth will absorb more sunlight, thereby finite capacity, resulting in low (but rapidly warming the planet further still and melting improving) spatial and temporal resolutions even more ice. on even the fastest computers. Together, those uncertainties mean that most models Clouds, too, make predictions more agree fairly well about large changes over difficult. Sunlight is reflected from the tops long periods of time, but they disagree of clouds, especially opaque clouds from about smaller-scale changes. For example, which precipitation falls, thereby altering projections of how mean temperature will . But clouds—especially cirrus clouds, change in an area the size of half of North which are high and thin—can also absorb America can be taken as fairly defensible— infrared radiation, much like greenhouse unlike projections of specific changes in a gases. Because we poorly understand many small area and a short time frame, such as aspects of cloud formation, it’s hard to say the intensity of windstorms in Beijing in the how, on balance, cloud changes feed back winter of 2051. into warming. As a result, each Differences in how models project global represents cloud processes in a distinct mean temperature arise from a variety of way and thus produces a level of cloud sources, the most influential which is the feedback different from that of other models. modeling of feedback factors—complex Differences in cloud feedback are the main responses to warming that either amplify cause of disagreement among the models or dampen the heat-trapping effect of when it comes to projecting global mean greenhouse gases. For example, water temperature. However, there is consensus vapor is a potent greenhouse gas, but it’s that cloud feedback would at least modestly so abundant in the air that direct human amplify warming rather than help lessen it. emissions don’t alter its concentrations. However, the indirect effect of ocean surface The uncertainty that various kinds of warming that results from cause in climate models, dominated causes more evaporation from the oceans and by the uncertainty in cloud feedback, has an even greater greenhouse effect, leading been summarized by a gross property of each to increased warming, or a positive feedback. model called its , or the Similarly, about 30 percent of the sunlight amount of warming the model predicts if that strikes Earth is reflected back into space carbon dioxide concentrations were to double under natural conditions without being from their preindustrial levels. The range of absorbed—an effect called albedo. Changes model sensitivities is 1.5–4.5°C (2.7–8.1°F); in albedo lead to changes in the amount of that is, average projected future global solar energy that Earth absorbs, so changes warming is 3°C (5.4°F), with uncertainty

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The Science of Climate Change ranging from 50 percent below to 50 percent offers strong evidence that the models above that value. perform reasonably well for conditions not so different from today’s—that is, when Differences in how models project global greenhouse gas concentrations in the mean temperature arise from a variety of atmosphere range from 280 to 400 parts sources, the most influential which is the per million. For concentrations beyond that modeling of feedback factors—complex range, paleoclimate data enhance the models’ responses to warming that either amplify credibility; such data include correlations or dampen the heat-trapping effect of between atmospheric temperatures and greenhouse gases. Those differences mean greenhouse gas concentrations that we can that estimated uncertainty increases when infer from ancient ice cores retrieved from we make predictions that are regional deep under the Antarctic and Greenland rather than global, sometimes producing ice sheets. Not only are the correlations high geographic variability. For example, consistent with our understanding of how a moderate emissions scenario predicts geophysical and climatological processes that by the last two decades of this century, have evolved over time, but the magnitude of the globe will warm 1.2–2.7°C (2.2–4.9°F) the changes is consistent with model-based compared with recent temperatures; the estimates of how large the temperature same model predicts average warming in difference should be between cold, glacial the broad range of 1.7–4°C (3.1–7.2°F) periods and warm, interglacial periods in central North America and Asia, with a (like our current epoch). The warming that narrower range in Africa and South America. followed the most recent glaciation, which Predictions of mean precipitation increases substantially remade Earth’s surface, was vary even more, ranging from 0 percent about 3–5°C (5.4–9°F), comparable to the to 3 percent and 3 percent to 9 percent, higher end of projections for warming by the respectively, for North America and Asia, to year 2100. minus 9 percent to plus 9 percent for Africa. The uncertainties make projections more We’ve shown that the climate’s complexity or less meaningless for areas smaller than makes prediction difficult. An even bigger about 1,000 square kilometers (386 square problem is uncertainty about future miles, or about the size of San Diego). The emissions. To accurately estimate emissions uncertainties also affect shorter time scales. A would involve an unimaginable degree 4°C (7.2°F) increase in average temperature of foresight about future technologies, in an American Midwestern state like Kansas economies, cultures, and policies, including would shift the temperature distribution emission abatement policies. Science’s enough to lead to dozens more days per year answer has been to create hypothetical of dangerously high temperatures exceeding scenarios in the form of estimates of 35°C (95°F), but trying to predict how such different, plausible ways that humanity local-scale changes would evolve from year to might choose to increase or decrease carbon year is simply too complex a task for current emissions over the next several decades— models. generally guided by economic, technical, and political experts. The highest emissions The comparison of observed warming with scenario is usually characterized as the reconstructed weather data, discussed earlier, likely outcome of business as usual, wherein

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Michael Oppenheimer and Jesse K. Anttila-Hughes countries carry on with using carbon- Regime Shifts in Planetary Systems intensive fuel sources for decades. The Scientists see a significant chance that lowest emissions scenario represents a world certain changes in the physical climate with strict climate policies and rapid attempts system could be so rapid, and their impact to drastically reduce emissions and prevent so widely distributed geographically, that further changes. The differences between they would radically alter human society. those two scenarios are sufficiently large Examples include a multi-meter sea level that they have a far greater influence on the rise from the melting of ice sheets; a rapid uncertainty of future temperature predictions release of methane (a potent greenhouse than do model uncertainties themselves. gas) from melting Arctic ocean sediments Put differently, models disagree about the and , that would in turn produce difference between temperature predictions several extra degrees of warming; a shift in low-emissions scenarios by a little more from moist tropical forest to savannah in the than one degree over this century, but the Amazon, causing large losses of ecosystems difference between projected temperature and species and substantial warming in any one model between low- and high- feedback from release of carbon dioxide from emissions scenarios is on the order of three soils and biota; and shifts in precipitation degrees. and temperature large enough to drastically The many sources of uncertainty in projecting reduce agricultural productivity. the future climate could mean huge These possibilities are relatively less likely differences in the eventual impact on human than other, less extreme changes. But should lives. If change is relatively modest, then this they occur, their impact will be high. We century’s warming would increase the global average temperature by about 2°C (3.6°F). likely won’t face them in this century, but Under this scenario, a child born in the United they are nonetheless plausible outcomes of States in 2080 would experience a climate extreme warming that policy makers should markedly different from the one children take into account. Low-probability but high- born today experience; 2080 would see hotter impact risks, such as those stemming from summers, more extreme precipitation, and cancer-causing chemicals, nuclear accidents, various other changes outlined later in this or geopolitical missteps are often viewed as article. But those effects pale in comparison threatening enough to require major shifts to what we can expect if climate changes are in policy. While the risk of a 4°C rise in substantial. A child born into a 2080 world global average temperature is low, it is not that is 4°C (7.2°F) warmer would experience zero, and some estimates put the likelihood a global average temperature higher than of even a 6°C rise in temperature at greater anything seen in the past several million than one percent by the end of the century years of Earth’s history. That scenario would under a business-as-usual scenario. From a produce a climate radically different from the risk management perspective, the threat of one we currently live in. Serious droughts, less likely but extremely damaging regime extreme heat waves, and rising sea levels shifts may thus be even more important than would expose children to a range of risks the threat of more likely but less damaging unprecedented in human experience. outcomes.

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The Science of Climate Change

How Will Children Be Vulnerable systems. The indirect effects, however, will to Climate Change? include everything from changes to the global The many climate changes expected to food system that threaten to raise prices and occur in the coming century are expected induce shortages, to geopolitical changes that to threaten children’s wellbeing in a variety occur because climate change destabilizes of both overt and subtle ways. Of particular social relations, thereby increasing conflict concern are changes in environmental risk and migration. Moreover, children will that could influence children’s development experience the indirect effects of climate both directly—through increasing levels of change as people and institutions respond exposure to a given hazard—and indirectly: not only to actual changes but also to climate- through intermediate effects on social driven risks—from governments’ decisions and economic systems. For example, an about urban development to families’ increase in the number of heat waves decisions about where to rear children. threatens children directly by exposing them Such adaptive choices are difficult to predict to higher temperatures, increasing their because they will be influenced by complex risk of heatstroke and other heat-related political, economic, and social factors.7 illnesses, and making it harder to learn, play, and exercise outdoors. Heat waves’ Poverty and development add more indirect effects are more subtle. More heat complexity. Children in poor countries waves will make crop failures more likely, are particularly vulnerable and exposed driving up prices in market economies and to climate-driven threats such as crop potentially depriving children of food in failures, heat waves, and tropical storms, rural parts of the world. Heat waves also and they won’t be able to draw on the interact with emissions from local industry more sophisticated adaptation mechanisms and transportation systems to increase available to children in rich countries. atmospheric concentrations of gases like Moreover, in developing countries, ozone (the central component of smog) families tend to rely more directly on that harm children’s health. And high the environment for their livelihoods— temperatures increase rates of interpersonal particularly through agriculture, meaning violence such as murder and abuse, as well as that climate change may cause serious harm group violence such as war. to family livelihoods. In their article in this issue, Rema Hanna and Paulina Oliva cover Climate change’s indirect effects are in the threats that climate change poses to many ways more worrisome than the children in developing countries.8 direct ones because so much of children’s wellbeing is conditioned by social and Wherever children live, climate change is economic factors. The climate’s influence on likely to affect their development in ways a child’s life doesn’t occur in isolation but, that last well into later life. In recent years, rather, in combination with specific social researchers such as Douglas Almond and circumstances. For example, a middle-class Janet Currie, who is one of the editors of this child in the Midwestern United States might issue, have amassed evidence demonstrating be well insulated from many of climate that even relatively mild disturbances to a change’s direct effects by technologies such child’s developmental trajectory may have as air conditioning and modern sanitation effects that last into adulthood, particularly

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Michael Oppenheimer and Jesse K. Anttila-Hughes when the disturbances occur during whereas oceans will heat more gradually— pregnancy and infancy.9 Economists Sharon much like the pattern that has already been Maccini and Dean Yang, for example, have observed. Similarly, the world as a whole demonstrated that women in rural Indonesia will be wetter because of evaporation from who were born during wetter rainy seasons the warmer ocean surface, but the excess are taller, better educated, richer, and in moisture will be unevenly distributed and better health than their counterparts born generally restricted to high latitudes and during drier rainy seasons.10 parts of the tropics. Broad areas at the historically arid horse latitudes (belts of Lastly, some of the most psychologically high pressure roughly 30–35 degrees north important losses that children can expect and south of the equator) are expected to to incur from climate change involve become even drier. Precipitation overall the destruction of aesthetic and cultural will become more variable: wet areas and heritage. Although such losses are difficult periods will generally become wetter, and to quantify, climate change is expected to dry areas and periods drier, especially in the submerge islands and coastlines, eradicate middle of continents. Ice will continue to or permanently change a number of melt worldwide; melting will reduce drinking ecosystems, threaten many traditional ways water sources for areas like Lima, Peru, that of life, and combine with other human depend in part on mountain glaciers for social forces to lead the world through their water supplies, and it will increase sea what many biologists say is already the sixth level rise. Extremes of heat, precipitation, mass of species in Earth’s history. coastal flooding, and are all likely or Many of the changes will be irreversible, very likely to continue to increase, and the potentially leaving this century’s children strongest tropical cyclones (that is, hurricanes a world bereft of a host of iconic species, and typhoons) are more likely than not to delicate ecosystems, and culturally relevant grow even more intense. All of these factors sites. Cultural practices that depend on can be expected to influence children’s the environment—such as skiing, camping, welfare over the next century in a variety of hunting, and fishing—are likely to be ways. permanently altered in many areas, and they may disappear entirely from certain areas. Changes in Temperature Distribution Climate change will thus reshape the very The increase in average temperatures, cultural fabric in which children develop, including the higher likelihood of extremely albeit in ways we can’t yet know for certain. hot days, is one of the most direct ways that children will be affected by climate change. What Will Changes Relevant to Regional forecasts vary, but most children Children Look Like? around the world will face hotter, more Uncertainties and caveats aside, a variety extreme temperatures more frequently in a of changes in the climate are expected to variety of forms, ranging from heat waves to influence social and economic outcomes higher nighttime temperatures to warmer that are particularly relevant for children. winters. Assuming that future population Climate models agree that at high latitudes centers don’t radically shift, a typical and in the interiors of continents, warming American family will experience 45–96 days will be greater than the global mean change, above 35°C (95°F) each year, on average, if

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The Science of Climate Change emissions don’t abate during this century; physiological effects of high temperatures that’s somewhere from four to eight times on aggressive behavior. In his article in as many as we’ve experienced in the past 30 this issue, Richard Akresh reviews that years.11 The higher temperatures will directly research.14 More generally, many studies affect children’s health and physiology in have found that the combined influence of potentially serious ways, increasing the rates higher temperatures on everything from crop of heatstroke, heat exhaustion, and heat- productivity to the human body’s ability to related mortality and reducing children’s work means that economies grow less quickly basic ability to enjoy the outdoors. Health than they otherwise would, which reduces economists Joshua Graff Zivin and Jeffrey GDP growth, especially in poorer countries. Shrader examine heat’s effects on children’s If that’s true, then climate change will likely health and human capital in their article in mean that children around the world will be this issue.12 less prosperous than they otherwise would.

Heat will also affect children indirectly in a Hydrologic Stress variety of ways. For example, many crops are A second defining aspect of climate change vulnerable to high temperatures, and even that will influence children’s welfare is a relatively small increases in heat exposure global increase in hydrologic stress. Even can cause huge reductions in crop health without climate change, many areas of the above certain threshold temperatures. Under world already face serious water shortages business-as-usual warming scenarios, by the because of rapid population growth, end of the century the United States may migration into cities, increasing , produce more than 50 percent less of such and other processes that have hugely key crops as corn.13 Higher temperatures increased global demand for water. Climate will likely disrupt food systems, drive up change will worsen the situation in three prices, and increase scarcity, particularly major ways. First, it will shift precipitation when combined with increased stress on patterns around the world, drying out water supply due to population growth and certain regions and making others wetter. drought. The changes may be particularly Second, it will increase the variability of damaging in developing countries, where precipitation in many places, making both poor growing-season conditions can cause unusually dry periods and unusually severe marked increases in death and illness among rains more likely. Third, it will reduce the children. mass of mountain glaciers in ranges such as the Himalayas, the Rockies, and the Andes, Heat has other indirect effects that may significantly reducing storage of winter be more subtle but are no less worrisome. snows and thus springtime runoff, which has Scientists from a range of disciplines have traditionally been used to water fields and shown that increased temperatures and recharge reservoirs. more-variable rains are broadly associated with increased rates of violent conflict, both As climate change interacts with increasing interpersonally and societywide. A variety of future water needs, much of the world’s mechanisms seem to explain those results, population may see local demand for ranging from heat-wave-induced crop water outstrip supply. Municipal water failures that lead to poverty and unrest to the and sanitation systems will be increasingly

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Michael Oppenheimer and Jesse K. Anttila-Hughes stressed, increasing the cost of access to The increased potential for large disasters clean water for consumption and sanitation. is particularly worrisome for children, not Agricultural systems already threatened by only because of the physical peril they more frequent extreme heat will see damage pose but also because a growing number exacerbated by insufficient water, particularly of studies have found that disasters can in areas where crops are fed by rain rather have debilitating long-term indirect effects than irrigation. will threaten a on children through everything from variety of water-intensive industrial processes households’ ability to earn a living and such as power generation and, in the long feed their children, to urban planning and run, may put serious pressure on people to infrastructure investment decisions that may migrate out of drier regions. fundamentally determine children’s living environments. In her article in this issue, Technological advances such as heat-resistant Carolyn Kousky reviews the expected impacts genetically modified crops, cheaper ways to of increased natural hazards on children. remove salt from seawater, and improved Sea Level Rise efficiency of water use could help avert those difficulties. But such technologies may not Rising seas will increase both (1) long-term come to fruition fast enough. land loss, thus reducing the amount of land available for settlement, and (2) episodic Changes in Extreme Events and coastal inundation. At current rates of sea Hazards level rise, for example, the portion of New Climate change is expected to alter the York City at risk for a one-hundred-year flood behavior of hydrometeorological and will double under high emissions scenarios climatological disasters, partly because of from just over 10 percent of the city today 15 the increased variability of precipitation to 20 percent by 2100. Rising sea levels and temperatures. More frequent extreme are also expected to increase erosion and to interact with tropical and extratropical rainfalls will bring more flooding to cyclones to worsen storm surges, all of which many parts of the world, while in other pose direct threats to children’s wellbeing. areas, higher temperatures combined Less directly, sea level rise will affect children with decreased rainfall will raise the risk by forcing coastal settlements to adopt of drought. Extreme temperatures and expensive adaptive urban planning systems hydrologic stress will cause more , and infrastructure such as seawalls. Sea and more-intense rains will cause more level rise will also increase the likelihood of landslides in mountain areas. There is no large-scale migration and extremely costly consensus as to whether tropical cyclones— relocation of urban centers. the large storms we call cyclones, typhoons, or hurricanes, depending on the ocean Damage to Ecosystems basin—will occur any more or less frequently, Damage to ecosystems is itself a result of and the physics behind them is complex. But climate change and in turn poses threats they will more likely than not increase in to children. Climate change is expected average intensity over the coming decades, to reduce or fundamentally alter major with stronger, more damaging storms services provided by the planet, becoming more common in some areas. such as regeneration of soil, pollination of

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The Science of Climate Change crops, and regulation of erosion. Many of ranging from human food sources like those effects will have the potential to harm lobsters and clams all the way down the children’s wellbeing indirectly—for example, food chain to the zooplankton that form by working with other factors to reduce the foundation of the ocean ecosystem. agricultural yields or by increasing the cost Coral reefs, which are home to much of the of access to clean water. In less-developed oceans’ and a critical habitat countries, where more people depend on for commercially fished species, are at risk ecosystem services, the impacts promise from both acidification and warming, as well to be more devastating than in wealthy as from several nonclimate threats. Unless countries. we reduce emissions, more than half of fish species are expected to be harmed by ocean caused by climate change acidification alone during this century. will present further indirect threats to children’s wellbeing. Biodiversity makes Climate-driven changes in Earth’s ecosystems ecosystems resilient, and the stress that are also expected to influence key aspects rapid climate change places on animal and of the complex disease interaction between plant species will further reduce ecosystem humans and the natural environment. services such as pollination and pest control. Disease vectors such as the Anopheles More broadly, loss of biodiversity poses a mosquito are expected to move to new serious threat to cultural heritage for children areas in response to changing rains and in many countries. Many threatened species temperatures, which would expose new with high aesthetic, cultural, patriotic, or populations to diseases ranging from malaria religious value, such as polar bears or coral to dengue to chikungunya. Changes in the reefs, will face increased risk of extinction, distribution and migration behaviors of potentially depriving future generations.16 birds and other animals are potentially more worrisome because these animals serve as One source of ecosystem damage that frequent sources of diseases passed on to deserves special mention is the gradual humans. influenzas, for example, acidification of the world’s oceans, which has are believed to occur when different already begun under climate change. The influenza viruses recombine in the same host; oceans naturally absorb carbon dioxide from some evidence suggests that flu the atmosphere as part of the global carbon may be sparked partly by climate-driven cycle. Carbon dioxide forms a mild acid, shifts in migratory bird patterns. called carbonic acid, when dissolved in water, and adding anthropogenic carbon dioxide to Pollution Earth’s climate has slowly begun to acidify Air pollutants such as carbon monoxide and the oceans. Acidification poses a major threat ozone have such harmful effects that the to the many invertebrates, including coral, World Health Organization has named air that calcium dissolved in seawater pollution as the single greatest environmental to form their shells. If it isn’t slowed and health risk, and children are more vulnerable eventually stabilized or reversed, the gradual than adults. The major sources of greenhouse increase in acidity would reduce calcium gas emissions typically also emit common concentrations sufficiently to threaten air pollutants known to damage health; populations of many ocean invertebrates, moreover, temperature and precipitation

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Michael Oppenheimer and Jesse K. Anttila-Hughes affect whether and how those emissions some nations with growing emissions, such become smog. Economist Matthew Neidell as , remain hesitant. The international and research analyst Allison Larr, in their agreement at the December 2015 Paris article in this issue, review the pollution Climate Conference provides at least some impacts of climate change.17 promise that key emitter nations will take meaningful steps over the next five to ten The Policy Response years. Scientists and policy makers broadly agree that without large-scale international At this writing, there is relatively little cooperation, economic development and indication that world leaders are considering technological progress on their own will not world carbon emission trajectory changes slow emissions enough to save us from large of the size needed to achieve a two-degree changes in the global climate, which creates target; economist Joseph Aldy, in his article a clear need for active international climate in this issue, reviews the political aspects of policies. Unfortunately, for many reasons, we climate change. We have nonetheless seen haven’t yet seen an adequate global policy substantial progress in the broader field of response. The uneven global impacts of climate policy. Policy makers and researchers climate change and the unequal emission generally divide the social response to histories of developed versus developing climate change into two complementary nations produce political divides that have halves: mitigation and adaptation. Mitigation made it hard to find common ground on policies seek to insulate society from issues ranging from who should begin climate change by preventing it via emission reducing emissions first to how much rich reductions—for example, by replacing fossil countries should pay poor countries not to fuels with or by reversing increase (a secondary source of deforestation. Adaptation policies seek carbon dioxide emissions). to protect society from climate changes that have already occurred or will occur; The long delay between emissions and their such policies can consist of anything from eventual impact on the climate means that improving disaster response to making effective climate policy must simultaneously agricultural systems more drought resistant. satisfy a wide variety of global stakeholders today while maintaining a point of view Most experts agree that limiting warming to sufficiently farsighted to incur nontrivial no more than 2°C (3.6°F)—governments’ costs that will not show benefits for decades. chosen benchmark of danger—is Uncertainty and scientific complexity make technologically feasible and would likely the problem difficult for policy makers to serve to avoid many types of disruptive deal with and the public to understand. changes. That agreement implies that there’s Attempts to reach binding agreements, most a limit on how much additional carbon notably the 1997 , have had can be emitted—that is, a carbon budget mixed results at best. Recent moves by the for the planet—before the 2°C target is leaders of the United States, China, and exceeded. If humans stay within the carbon certain other main greenhouse-gas-emitting budget, adaptation will be feasible, although nations indicate that those leaders have potentially costly. If the carbon budget is begun to see the matter as more pressing, but exceeded and if climate changes become

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The Science of Climate Change sufficiently severe, policies could expand to chemicals is another. Climate change has include geoengineering projects intended much in common with those uncertain but either to reduce the carbon dioxide in very real global threats. We must understand the atmosphere or to reduce the average that scientific uncertainty about the specifics temperature. Such efforts could range of a complex problem can go hand in hand from seeding the atmosphere with sulfate with broad agreement about the overall particles to increase albedo and thus cool the riskiness of an outcome. planet, to injecting billions of tons of carbon dioxide into old oil and gas deposits and At the heart of the climate change problem other geologic formations—a process called lies a tension that forces us to directly carbon capture and sequestration. Albedo confront the value we put on future modification is widely regarded as a concept children’s wellbeing. The long lag between rather than an established technology, and it the emission of a greenhouse gas and its would be risky for several reasons, including eventual warming effect means that costly the potential for unforeseen interactions decisions to reduce emissions today will with Earth’s complex climate system. Many bring benefits largely through reduced experts see albedo modification only as a last harm to future generations born many resort.18 years hence. There is much debate over the best way to approach decisions when costs Climate Change and Future and benefits are distributed over time, and Generations many deep philosophical and ethical issues It’s easy to feel overwhelmed by the scope surrounding how we justify those decisions and scale of climate change as a problem. are not easily settled. In their article in this The uncertainty that stems from our issue, economists William Pizer, Ben Groom, incomplete knowledge about climate and and Simon Dietz review discounting and our inability to forecast future human intergenerational decision making. behavior suggests a practically unknowable future, in which potentially huge losses In the remainder of this issue, leading caused by climate change compete with experts on the social effects of climate technological advances, economic growth, change examine issues relevant to climate and social and cultural shifts to determine change’s impacts on children. In each case, children’s welfare for the rest of the century. readers can find ample cause for concern, as That said, history has demonstrated time well as ample reason for hope that children’s and again that humans can tackle uncertain lives will continue to improve throughout threats in times of need. The insurance the current century as they did during the industry exists to help us manage risks, and previous one. Taken together, these reports businesses in many industries perform risk make it clear that ensuring that children’s analyses and adopt policies to reduce risks. futures are adequately protected from On a larger scale, international frameworks the hazards of climate change will require are in place to manage global safety risks. unprecedented effort, innovation, and International agreements adopted to reduce coordination, suggesting that few of our the risk of nuclear war constitute one such decisions about any other issues will come example; the Montreal Protocol prohibiting close to having as strong an influence on the manufacture of ozone-layer-destroying children’s lives.

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Michael Oppenheimer and Jesse K. Anttila-Hughes

ENDNOTES 1. Intergovernmental Panel on Climate Change (IPCC), Climate Change 2014: Impacts, Adaptation, and Vulnerability (Cambridge: Cambridge University Press, 2014). 2. Nicholas Stern, The Economics of Climate Change: The Stern Review (Cambridge: Cambridge University Press, 2007); Trevor Houser et al., Economic Risks of Climate Change: An American Prospectus (New York: Columbia University Press, 2015). 3. IPCC, Climate Change 2014. 4. Melissa Dell, Benjamin F. Jones, and Benjamin A. Olken, “What Do We Learn from the Weather? The New Climate-Economy Literature,” Journal of Economic Literature 52(2014): 740–98, doi: 10.1257/ jel.52.3.740; IPCC, Climate Change 2014. 5. Jaelyn J. Eberle and David R. Greenwood, “Life at the Top of the Greenhouse Eocene World—A Review of the Eocene Flora and Vertebrate Fauna from Canada’s High Arctic,” Geological Society of America Bulletin 124 (2012): 3–23, doi: 10.1130/b30571.1. 6. Stefan A. Talke, Philip Orton, and David A. Jay, “Increasing Storm Tides in New York Harbor, 1844– 2013,” Geophysical Research Letters 41 (2014): 3149–55, doi: 10.1002/2014gl059574. 7. Michael Oppenheimer, “Climate Change Impacts: Accounting for the Human Response,” Climatic Change 117 (2013): 439–49, doi: 10.1007/s10584-012-0571-9. 8. Rema Hanna and Paulina Oliva, “Implications of Climate Change for Children in Developing Countries,” Future of Children 26, no. 1 (2016): 113–130. 9. Janet Currie and Douglas Almond, “Human Capital Development before Age Five,” in Handbook of Labor Economics, vol. 4B, ed. David Card and Orley Ashenfelter (Amsterdam: North Holland, 2011), 1315–1486, doi: 10.1016/s0169-7218(11)02413-0. 10. Sharon Maccini and Dean Yang, “Under the Weather: Health, Schooling, and Economic Consequences of Early-Life Rainfall,” American Economic Review 99 (2009): 1006–26, doi: 10.1257/aer.99.3.1006. 11. Houser et al., Economic Risks. 12. Joshua Graff Zivin, “Temperature Extremes, Health, and Human Capital,” Future of Children 26, no. 1 (2016): 31–50. 13. Wolfram Schlenker and Michael J. Roberts, “Nonlinear Temperature Effects Indicate Severe Damages to U.S. Crop Yields under Climate Change,” Proceedings of the National Academy of Sciences 106 (2009): 15594–8, doi: 10.1073/pnas.0906865106. 14. Richard Akresh, “Conflict and Climate Change,”Future of Children 26, no. 1 (2016): 51–70. 15. Talke, Orton, and Jay, “Increasing Storm Tides”; Houser et al., Economic Risks. 16. Ove Hoegh-Guldberg and John F. Bruno, “The Impact of Climate Change on the World’s Marine Ecosystems,” Science 328 (2010): 1523–8, doi: 10.1126/science.1189930. 17. Allison Larr and Matthew Neidell, “Pollution and Climate Change,” Future of Children 26, no. 1 (2016): 91–111. 18. National Research Council, Climate Intervention: Reflecting Sunlight to Cool Earth (Washington, DC: National Academies Press, 2015).

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