National Climate Assessment: We're In Trouble
“We, the people, still believe that our obligations as Americans are not just to ourselves, but to all posterity. We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations. (Applause.) Some may still deny the overwhelming judgment of science, but none can avoid the devastating impact of raging fires and crippling drought and more powerful storms.
“The path towards sustainable energy sources will be long and sometimes difficult. But America cannot resist this transition, we must lead it. We cannot cede to other nations the technology that will power new jobs and new industries, we must claim its promise. That’s how we will maintain our economic vitality and our national treasure — our forests and waterways, our crop lands and snow-capped peaks. That is how we will preserve our planet, commanded to our care by God. That’s what will lend meaning to the creed our fathers once declared.”
President Barack Obama, January 21st, 2013, Presidential Inauguration Speech
Some were surprised by President’s argument that we must tackle global warming soon. However anyone who has read the governments most recent report on climate change would have little doubt that American and the world must tackle climate change now. The most important U.S. climate assessment, the Third Draft of the National Climate Assessment and Development Advisory Committee involving over 240 authors and is 400 pages long was released last Friday for public comment to a local media that largely ignored the report even though the report provides detailed local information (see Table below) . However, being ignored is not something new to this report. President Clinton signed legislation that made the report mandatory every four years. The first report was presented in 2000. President Bush blocked the report during his Administration. President Obama followed the law and a new report was released in 2009. The reports conclusions are consistent with what other recent reports have said which paint a dire picture in both the short-term and the long-term.
1. Global climate is changing, and this is apparent across the U.S. in a wide range of observations. The climate change of the past 50 years is due primarily to human activities, predominantly the burning of fossil fuels.
U.S. average temperature has increased by about 1.5°F since 1895, with more than 80% of this increase occurring since 1980. The most recent decade was the nation’s warmest on record. Because human-induced warming is superimposed on a naturally varying climate, rising temperatures are not evenly distributed across the country or over time.
2. Some extreme weather and climate events have increased in recent decades, and there is new and stronger evidence that many of these increases are related to human activities.
Changes in extreme events are the primary way in which most people experience climate change. Human-induced climate change has already increased the frequency and intensity of some extremes. Over the last 50 years, much of the U.S. has seen an increase in prolonged
stretches of excessively high temperatures, more heavy downpours, and in some regions more severe droughts.
3. Human-induced climate change is projected to continue and accelerate significantly if emissions of heat-trapping gases continue to increase.
Heat-trapping gases already in the atmosphere have committed us to a hotter future with more climate-related impacts over the next few decades. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, now and in the future.
4. Impacts related to climate change are already evident in many sectors and are expected to become increasingly challenging across the nation throughout this century and beyond.
Climate change is already affecting human health, infrastructure, water resources, 25 agriculture, energy, the natural environment, and other factors – locally, nationally, and internationally. Climate change interacts with other environmental and societal factors in a variety of ways that either moderate or exacerbate the ultimate impacts. The types and magnitudes of these effects vary across the nation and through time. Several populations – including children, the elderly, the sick, the poor, tribes and other indigenous people – are especially vulnerable to one or more aspects of climate change. There is mounting evidence that the costs to the nation are already high and will increase very substantially in the future, unless global emissions of heat- trapping gases are strongly reduced.
5. Climate change threatens human health and well-being in many ways, including impacts from increased extreme weather events, wildfire, decreased air quality, diseases transmitted by insects, food, and water, and threats to mental health.
Climate change is increasing the risks of heat stress, respiratory stress from poor air quality, and the spread of waterborne diseases. Food security is emerging as an issue of concern, both within the U.S. and across the globe, and is affected by climate change. Large-scale changes in the environment due to climate change and extreme weather events are also increasing the risk of the emergence or reemergence of unfamiliar health threats.
6. Infrastructure across the U.S. is being adversely affected by phenomena associated with climate change, including sea level rise, storm surge, heavy downpours, and extreme heat.
Sea level rise and storm surges, in combination with the pattern of heavy development in coastal areas, are already resulting in damage to infrastructure such as roads, buildings, ports, and energy facilities. Infrastructure associated with military installations is also at risk from climate change impacts. Floods along the nation’s rivers, inside cities, and on lakes following heavy downpours, prolonged rains, and rapid melting of snowpack are damaging infrastructure in towns and cities, farmlands, and a variety of other places across the nation. Extreme heat is damaging transportation infrastructure such as roads, rail lines, and airport runways. Rapid warming in Alaska has resulted in infrastructure impacts due to thawing of permafrost and the loss of coastal sea ice that once protected shorelines from storms and wave-driven coastal erosion.
7. Reliability of water supplies is being reduced by climate change in a variety of ways that affect ecosystems and livelihoods in many regions, particularly the Southwest, the Great 17 Plains, the Southeast, and the islands of the Caribbean and the Pacific, including the state of Hawai`i.
Surface and groundwater supplies in many regions are already stressed by increasing demand for water as well as declining runoff and groundwater recharge. In many regions, climate change increases the likelihood of water shortages and competition for water among agricultural, municipal, and environmental uses. The western U.S. relies heavily on mountain snowpack for water storage, and spring snowpack is declining in most of the West. There is an increasing risk of seasonal water shortages in many parts of the U.S., even where total precipitation is projected to increase. Water quality challenges are also increasing, particularly sediment and contaminant concentrations after heavy downpours.
8. Adverse impacts to crops and livestock over the next 100 years are expected.
Over the 29 next 25 years or so, the agriculture sector is projected to be relatively resilient, even 30 though there will be increasing disruptions from extreme heat, drought, and heavy downpours. U.S. food security and farm incomes will also depend on how agricultural systems adapt to climate changes in other regions of the world. Near-term resilience of U.S. agriculture is enhanced by adaptive actions, including expansion of irrigated acreage in response to drought, regional shifts in crops and cropped acreage, continued technological advancements, and other adjustments. By mid-century, however, when temperature increases and precipitation extremes are further intensified, yields of major U.S. crops are expected to decline, threatening both U.S. and international food security. The U.S. food system also depends on imports, so food security and commodity pricing will be affected by agricultural adaptation to climate changes and other conditions around the world.
9. Natural ecosystems are being directly affected by climate change, including changes in biodiversity and location of species. As a result, the capacity of ecosystems to moderate the consequences of disturbances such as droughts, floods, and severe storms is being diminished.
In addition to climate changes that directly affect habitats, events such as droughts, floods, wildfires, and pest outbreaks associated with climate change are already disrupting ecosystem structures and functions in a variety of direct and indirect ways. These changes limit the capacity of ecosystems such as forests, barrier beaches, and coastal- and freshwater-wetlands to adapt and continue to play important roles in reducing the impacts of these extreme events on infrastructure, human communities, and other valued resources.
10. Life in the oceans is changing as ocean waters become warmer and more acidic.
Warming ocean waters and ocean acidification across the globe and within U.S. marine territories are broadly affecting marine life. Warmer and more acidic waters are changing the distribution of fish and other mobile sea life, and stressing those, such as corals, that cannot move. Warmer and more acidic ocean waters combine with other stresses, such as overfishing and coastal and marine pollution, to negatively affect marine-based food production and fishing communities.
11. Planning for adaptation (to address and prepare for impacts) and mitigation (to reduce 20 emissions) is increasing, but progress with implementation is limited.
In recent years, climate adaptation and mitigation activities have begun to emerge in many sectors and at all levels of government; however barriers to implementation of these activities are significant. The level of current efforts is insufficient to avoid increasingly serious impacts of climate change that have large social, environmental, and economic consequences. Well-planned and implemented actions to limit emissions and increase resilience to impacts that are unavoidable can improve public health, economic development opportunities, natural system protection, and overall quality of life.
Regional Observations of Climate Change
Region Effects Northeast Heat waves, coastal flooding due to sea level rise and storm surge, and river flooding due to more extreme precipitation events are affecting
communities in the region.
Southeast Decreased water availability, exacerbated by population growth and land-use change, is causing increased competition for water; risks associated with extreme events like hurricanes are increasing. Midwest Longer growing seasons and rising carbon dioxide levels are increasing yields of some crops, although these benefits have already been offset in some instances by occurrence of extreme events such as heat waves, droughts, and floods. Great Plains Rising temperatures are leading to increased demand for water and energy and impacts on agricultural practices. Southwest Drought and increased warming have fostered wildfires and increased competition for scarce water resources for people and ecosystems. Northwest Changes in the timing of streamflow related to earlier snowmelt have already been observed and are reducing the supply of water in summer, causing far-reaching ecological and socioeconomic consequences. Alaska Summer sea ice is receding rapidly, glaciers are shrinking, and permafrost is thawing, causing damage to infrastructure and major changes to
ecosystems; impacts to Alaska native communities are increasing.
Hawaii Increasingly constrained freshwater supplies, coupled with increased temperatures, are stressing both people and ecosystems, and decreasing
food and water security.
Coasts Coastal lifelines, such as water supply infrastructure and evacuation routes, are increasingly vulnerable to higher sea levels and storm surges,
inland flooding, and other climate-related changes.
Oceans The oceans are currently absorbing about a quarter of human-caused carbon dioxide emissions to the atmosphere and over 90% of the heat
associated with global warming, leading to ocean acidification and the alteration of marine ecosystems.
Mainstream Media Response
At the national level reporting was spotty as well. One reason may be that there are only twelve environmental reporters left at the countries top five newspapers.
Report Executive Summary
Climate change is already affecting the American people. Certain types of weather events have become more frequent and/or intense, including heat waves, heavy downpours, and, in some regions, floods and droughts. Sea level is rising, oceans are becoming more acidic, and glaciers and Arctic sea ice are melting. These changes are part of the pattern of global climate change, which is primarily driven by human activity. Many impacts associated with these changes are important to Americans’ health and livelihoods and the ecosystems that sustain us. These impacts are the subject of this report. The impacts are often most significant for communities that already face economic or health-related challenges, and for species and habitats that are already facing other pressures. While some changes will bring potential benefits, such as longer growing seasons, many will be disruptive to society because our institutions and infrastructure have been designed for the relatively stable climate of the past, not the changing one of the present and future. Similarly, the natural ecosystems that sustain us will be challenged by changing conditions. Using scientific information to prepare for these changes in advance provides economic opportunities, and proactively managing the risks will reduce costs over time. Evidence for climate change abounds, from the top of the atmosphere to the depths of the oceans. This evidence has been compiled by scientists and engineers from around the world, using satellites, weather balloons, thermometers, buoys, and other observing systems. The sum total of this evidence tells an unambiguous story: the planet is warming.
U.S. average temperature has increased by about 1.5°F since 1895; more than 80% of this increase has occurred since 1980. The most recent decade was the nation’s hottest on record. Though most regions of the U.S. are experiencing warming, the changes in temperature are not uniform. In general, temperatures are rising more quickly at higher latitudes, but there is considerable observed variability across the regions of the U.S. U.S. temperatures will continue to rise, with the next few decades projected to see another 2°F to 4°F of warming in most areas. The amount of warming by the end of the century is projected to correspond closely to the cumulative global emissions of greenhouse gases up to that time: roughly 3°F to 5°F under a lower emissions scenario involving substantial reductions in emissions after 2050 (referred to as the “B1 scenario”), and 5°F to 10°F for a higher emissions
scenario assuming continued increases in emissions (referred to as the “A2 scenario”).
The chances of record-breaking high temperature extremes will continue to increase as the climate continues to change. There has been an increasing trend in persistently high nighttime temperatures, which have widespread impacts because people and livestock get no respite from the heat. In other places, prolonged periods of record high temperatures associated with droughts contribute to conditions that are driving larger and more frequent wildfires. There is strong evidence to indicate that human influence on the climate has already roughly doubled the probability of extreme heat events like the record-breaking summer of 2011 in Texas and Oklahoma.
Human-induced climate change means much more than just hotter weather. Increases in ocean and freshwater temperatures, frost-free days, and heavy downpours have all been documented. Sea level has risen, and there have been large reductions in snow-cover extent, glaciers, permafrost, and sea ice. Winter storms along the west coast and the coast of New England have increased slightly in frequency and intensity. These changes and other climatic changes have affected and will continue to affect human health, water supply, agriculture, transportation, energy, and many other aspects of society.
Some of the changes discussed in this report are common to many regions. For example, very heavy precipitation has increased over the past century in many parts of the country. The largest increases have occurred in the Northeast, Midwest, and Great Plains, where heavy downpours have exceeded the capacity of infrastructure such as storm drains, and have led to flooding events and accelerated erosion. Other impacts, such as those associated with the rapid thawing of permafrost in Alaska, are unique to one U.S. region. Some impacts that occur in one region have more wide-ranging effects. For example, the dramatic decline of summer sea ice in the Arctic – a loss of ice cover roughly equal to half of the continental U.S. – exacerbates global warming by reducing the reflectivity of Earth’s surface and increasing the amount of heat the Arctic absorbs. There is some evidence that this affects
weather patterns farther south in the United States. Similarly, wildfires in one region can trigger poor air quality in far-away regions, and new evidence suggests the particulate matter in the atmosphere affects global circulation, leading to more persistent periods of anomalous weather. Major storms that hit the Gulf Coast affect the entire country through their cascading effects on oil and gas production and distribution.
Sea level rise, combined with coastal storms, has increased the risk of erosion, storm-surge damage, and flooding for coastal communities, especially along the Gulf of Mexico, the Atlantic seaboard, and Alaska. In the Southeast, coastal infrastructure including roads, rail lines, energy infrastructure, and port facilities including naval bases, are at risk from storm surge that is exacerbated by rising sea level. Over the past century, global sea level has risen by about 8 inches. Since 1992, the rate of global sea level rise measured by satellites has been roughly twice the rate observed over the last century. Sea level is projected to rise by another 1 to 4 feet in this century. A wider range of scenarios, ranging from 8 inches to 6.6 feet of rise by 2100, has been suggested for use in risk-based analyses. In general, higher emissions scenarios that lead to more warming would be expected to lead to sea level rise toward the upper end of the projected range.
The stakes are high, as nearly five million Americans live within four feet of the local high-tide level. In addition to changing climate, carbon dioxide from fossil fuel burning has a direct effect on the world’s oceans. Carbon dioxide interacts with ocean water to form carbonic acid, lowering the ocean’s pH. Ocean surface waters have become 30% more acidic as they have absorbed large amounts of carbon dioxide from the atmosphere. This ocean acidification reduces the capacity of marine organisms with shells or skeletons made of calcium carbonate (such as corals, krill, oysters, clams, and crabs) to survive, grow, and reproduce, which in turn will affect the entire marine food chain.
Climate change produces a variety of stresses on society, affecting human health, natural ecosystems, built environments, and existing social, institutional, and legal agreements. These stresses interact with each other and with other non-climate stresses, such as habitat fragmentation, pollution, increased consumption patterns, and biodiversity loss. Addressing these multiple stresses requires the assessment of composite threats as well as tradeoffs among the costs, benefits, and risks of available response options.
Climate change will influence human health in many ways; some existing health threats will intensify, and new health threats will emerge. Some of the key drivers of health impacts include: increasingly frequent and intense extreme heat, which causes heat-related illnesses and deaths and over time, worsens drought and wildfire risks, and intensifies air pollution; increasingly frequent extreme precipitation and associated flooding that can lead to injuries and increases in marine and freshwater-borne disease; and rising sea levels that intensify coastal flooding and storm surge. Certain groups of people are more vulnerable to the range of climate change-related health impacts, including the elderly, children, the poor, and the sick. Others are vulnerable because of where they live, including those in floodplains, coastal zones, and some urban areas.
In fact, U.S. population growth has been greatest in coastal zones and in the arid Southwest, areas that already have been affected by increased risks from climate change. Just as some choices can make us more vulnerable, other choices can make us more resilient. Maintaining a robust public health infrastructure will be critical to managing the potential health impacts of climate change.
Climate change affects the entire living world, including people, through changes in ecosystems and biodiversity. Ecosystems provide a rich array of benefits to humanity, including fisheries, drinking water, fertile soils for growing crops, buffering from climate impacts, and aesthetic and cultural values. These benefits are not always easy to quantify, but they translate into jobs, economic growth, health, and human well-being. Climate change-driven perturbations to ecosystems that have direct human impacts include reduced water supply and quality, the loss of iconic species and landscapes, distorted rhythms of nature, and the potential for extreme events to eliminate the capacity of ecosystems to provide benefits.
Climate change and other human modifications of ecosystems and landscapes often increase their vulnerability to damage from extreme events while at the same time reducing their natural capacity to modulate the impacts of such events. Salt marshes, reefs, mangrove forests, and barrier islands defend coastal ecosystems and infrastructure, including roads and buildings, against storm surges; their losses from coastal development, erosion, and sea level rise increase the risk of catastrophic damage during or after extreme weather events. Floodplain wetlands, although greatly reduced from their historical extent, absorb floodwaters and reduce the effects of high flows on river-margin lands. Extreme weather events that produce sudden increases in water flow, often carrying debris and pollutants, can decrease the natural capacity of ecosystems to process pollutants.
As climate change and its impacts are becoming more prevalent, Americans face choices. As a result of past emissions of heat-trapping gases, some amount of additional climate change and related impacts is now unavoidable. This is due to the long-lived nature of many of these gases, the amount of heat absorbed and retained by the oceans, and other responses within the climate system. However, beyond the next few decades, the amount of climate change will still largely be determined by choices society makes about emissions. Lower emissions mean less future warming and less severe impacts; higher emissions would mean more warming and more severe impacts. The choices about emissions pathway fall into a category of response options usually referred to as “mitigation” – ways to reduce the amount and speed of future climate change by reducing emissions of heat-trapping gases.
The other major category of response options is known as “adaptation” and refers to changes made to better respond to new conditions, thereby reducing harm or taking advantage of opportunity. Mitigation and adaptation are linked, in that effective mitigation reduces the need for adaptation. Both are essential parts of a comprehensive response strategy. The threat of irreversible impacts makes the timing of mitigation efforts particularly critical. This report includes chapters on Mitigation, Adaptation, and Decision Support that offer an overview of the kinds of options and activities being planned or implemented around the country as governments at local, state, federal, and tribal levels, businesses, other organizations, and individuals begin to respond to climate change.
Large reductions in global emissions, similar to the lower emissions scenario (B1) analyzed in this assessment, would be necessary to avoid some of the worst impacts and risks of climate change. The targets called for in international agreements would require even larger reductions than those outlined in scenario B1. Meanwhile, global emissions are still rising, and are on track to be even higher than the high emissions scenario (A2) analyzed in this report. The current U.S. contribution to global emissions is about 20%. Voluntary efforts, the recent shift from coal to natural gas for electricity generation, and governmental actions in city, state, regional, and federal programs under way and have contributed to reducing U.S. emissions in the last few years. Some of these actions are motivated by climate concerns, sometimes with non-climate co-benefits, while others are motivated primarily by non-climate objectives. These U.S. actions and others that might be undertaken in the future are described in the Mitigation chapter of this report; at present they are not sufficient to reduce total U.S. emissions to a level that would be consistent with scenario B1 or the targets in international agreements.
With regard to adaptation, the pace and magnitude of observed and projected changes emphasize the need for being prepared for a wide variety and intensity of climate impacts. Because of the influence of human activities, the past climate is no longer a sufficient indicator of future conditions. Planning and managing based on the climate of the last century means that tolerances of some infrastructure and species will be exceeded. For example, building codes and landscaping ordinances will likely need to be updated not only for energy efficiency, but also to conserve water supplies, protect against insects that spread disease, reduce susceptibility to heat stress, and improve protection against extreme events. The knowledge that climate change is real and accelerating points to the need to develop and refine approaches that enable decision-making and increase flexibility, robustness, and resilience in the face of ongoing and future impacts. Being prepared for such events paves the way for economic opportunities.
Adaptation considerations include local, state, regional, national, and international jurisdictional issues. For example, in managing water supplies to adapt to a changing climate, the implications of international treaties should be considered in the context of managing the Great Lakes, the Columbia River, and the Colorado River to deal with increased drought risk. Both “bottom up” community planning and “top down” national strategies may help regions deal with impacts such as increases in electrical brownouts, heat stress, floods, and wildfires. Such a mix of approaches will require cross-boundary coordination at multiple levels as operational agencies integrate adaptation planning into their programs.
Proactively preparing for climate change can reduce impacts, while also facilitating a more rapid and efficient response to changes as they happen. The Adaptation chapter in this report highlights efforts at the federal, regional, state, tribal, and local levels, as well as initiatives in the corporate and non-governmental sectors to build adaptive capacity and resilience towards climate change. This report identifies a number of areas for which improved scientific information or understanding would enhance the capacity to estimate future climate change impacts. For example, knowledge of the mechanisms controlling the rate of ice loss in Greenland and Antarctica is limited, making it difficult for scientists to narrow the range of future sea level rise. Research on ecological responses to climate change is limited, as is understanding of social responses and how ecological and social responses will interact. There is also a section on creating a sustained climate assessment process to more efficiently collect and synthesize the rapidly evolving science and to help supply timely and relevant information to decision-makers. Results from all of these efforts will continue to build our understanding of the interactions of human and natural systems in the context of a changing climate.
Crosscutting Themes and Issues
There are several themes that run throughout the assessment. These include: the “multiple stresses context” in which climate change impacts must be interpreted; the effects of socioeconomic and cultural decisions on vulnerabilities to climate change; and the importance of considering climate-change impacts on the U.S. in an international context.
1. Climate change should be considered in the context of multiple factors
Climate change and its impacts cannot be adequately assessed in isolation. Rather, they are part of a broader context including many other factors such as: land-use change, local economies, air and water pollution, and rates of consumption of resources. This perspective has implications for
10 assessments of climate change impacts and the design of research questions at the national, regional, and local scales. This assessment begins to explore the consequences of interacting factors by focusing on sets of crosscutting issues in a series of six chapters: Water, Energy, and Land Use; Biogeochemical Cycles; Impacts of Climate Change on Tribal Lands and Resources; Urban Infrastructure and Vulnerability; Land Use and Land Cover Change; and Impacts on Rural Communities. This Assessment also includes discussions of cascading impacts in several chapters (particularly in the Urban Infrastructure and Vulnerability Chapter and the Water, Energy, and Land Use Chapter), and emphasizes that many of the impacts identified in the Assessment will occur in parallel, not in isolation from one another. As illustrated by recent events, this greatly stresses the capacity to respond to a series of climate-related crises that occur simultaneously or soon after one another.
2. Societal choices affect vulnerability to climate change impacts.
Because environmental, cultural and socioeconomic systems are tightly coupled, climate change impacts can either be amplified or reduced by cultural and/or socioeconomic decisions. In the context of the “risk-based framing” for their chapters, the authors of this report were asked to focus on attributes of regions and sectors most likely to experience significant impacts. In many chapters, it is clear that societal decisions have the greatest impact on valued resources. For example, rapid population growth and development in areas that are particularly susceptible to climate change impacts can amplify those impacts. Recognition of these couplings, together with recognition of the multiple-stresses perspective, helps identify the information needs of decision-makers as they manage risk.
3. Importance of the international context
Climate change is a global phenomenon; the causes and the impacts involve energy-use and risk-management decisions across the globe. Impacts, vulnerabilities, and opportunities in the U.S. are related in complex and interactive ways with changes outside the U.S., and vice versa. In order for U.S. concerns related to climate change to be addressed comprehensively, the international context must be considered. U.S. security, foreign assistance, and economic interests are affected by climate changes experienced in other parts of the world. Although there is significantly more work to be done in this area, this report does identify some initial implications of global and international trends that can be more fully investigated in future assessments.
4. Thresholds, Tipping Points, and Surprises
A significant issue in studying and preparing for global climate change is the fact that changes in human, social, and physical systems do not always occur gradually. Same changes may occur in a relatively predictable way, while others involve unexpected break-points or thresholds beyond which there are irreversible changes or changes of higher magnitudes than expected based on previous experience. These “tipping points” are very hard to predict, as there are many uncertainties associated with understanding future conditions. These uncertainties come from a number of sources, including insufficient data associated with low probability/high consequence events, models that are not yet able to represent the interactions of multiple stresses, incomplete understanding of physical climate mechanisms related to tipping points, and a multitude of issues associated with human behavior, risk management, and decision-making.
5. Weather and Climate Extremes
Understanding how climate is changing requires consideration of changes in the average climate as well as changes in “extremes” – weather and climate events like hot spells, heavy rains, periods of drought and flooding, and severe storms. The climate change impacts expected to have the greatest consequences are those involving extremes: changes in the frequency, intensity, timing, duration, and spatial extent of such extremes, as well as through the occurrence of unprecedented extremes. Terms like “weather-extremes,” “climate extremes,” “heat waves,” and “heavy downpours” need to be defined when used in a scientific context. Researchers use different definitions depending on which characteristics of extremes they are choosing to explore at any one time, in the context of the particular issue they are studying. Nevertheless, most of the scientific literature on extremes uses definitions that fall roughly into two categories (IPCC 2012): those related to the probability of occurrence of a certain type of event, and those related to exceeding a particular threshold. For example, common measures of extremes include the number, percentage, or fraction of days in a month, season, or year with maximum (or minimum) temperature above the 90th, 95th, or 28 99th percentile compared to a reference time period (for example, 1961-1990) – or alternatively, how often a threshold temperature (for example, 32°F or 90°F) is exceeded during a given decade. Alternative definitions refer to how often, on average, an event of a specific magnitude occurs (sometimes called the “return period”) – for example, how frequently we might expect to see daily rainfall exceeding two inches in a given region.
by Todd Miller
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