What Impacts Weather & Climate Extremes ? One of the major concerns with a potential change in climate is that an increase in extreme events will occur. Results of observational studies suggest that in many areas that have been analyzed, changes in total precipitation are amplified at the tails, and changes in some temperature extremes have been noticed. It is clear from the observed record that there has been an increase in the global mean temperature of about 0.6°C since the start of the 20th century, and that this increase is associated with a stronger warming in daily minimum temperatures than in maximums, leading to a reduction in the diurnal temperature range. Land surface precipitation has also increased over the same period in the mid- to high latitudes, but shows a decrease in the tropics and subtropics. Given these changes, it is expected that there would also be changes in what are now considered extreme events. Therefore, if there were indeed identifiable trends in certain extreme climatic events, such as extremes in temperature or precipitation, it would add to the body of evidence that there is a discernable human effect on the climate, and potentially have important consequences on society and natural systems. Model output has been analyzed that shows changes in extreme events for future climates, such as increases in extreme high temperatures, decreases in extreme low temperatures, and increases in intense precipitation events. In addition, the societal infrastructure is becoming more sensitive to weather and climate extremes, which would be exacerbated by climate change. In wild plants and animals, climate-induced extinctions, distributional and phonological changes, and species' range shifts are being documented at an increasing rate. Several apparently gradual biological changes are linked to responses to extreme weather and climate events. Climate Extreme Atmospheric extremes--which include floods, droughts, severe heat and cold, and storms--have resulted in steady increases in economic costs and lives lost around the world. Shifts in the frequency and intensity of severe weather (events lasting hours or days) and climate extremes (events persisting for months or years) could exacerbate this growing problem. Growths of population and wealth, as well as demographic shifts to coastal areas and to expanding metropolitan areas, have collectively increased the vulnerability of the world to losses from weather extremes. Extremes now have an impact on all levels of government and on the insurance industry. More than 90% of the nation's natural disasters are a result of weather or climate extremes. Extreme Policy Policies related to extremes have followed two approaches, proactive and reactive, and emphasis on each evolved as society changed, new technologies developed, and government leadership shifted. Policies relating to atmospheric extremes underwent major changes after World War II. Costly proactive structural policies to prevent losses during disasters were finally recognized as unable to control all losses. A "nonstructural" philosophy emerged in national policies: move people out of hazardous areas, seek improvements in building codes and encourage use of crop and flood insurance. Rapid improvements in storm forecasting coupled with local policies led to the development of thousands of community warning systems after 1960. In addition to these proactive measures, other policies have provided assistance to those with losses. One of the major concerns with a potential change in climate is that an increase in extreme events will occur. Results of observational studies suggest that in many areas that have been analyzed, changes in total precipitation are amplified at the tails, and changes in some temperature extremes have been observed. Model output has been analyzed that shows changes in extreme events for future climates, such as increases in extreme high temperatures, decreases in extreme low temperatures, and increases in intense precipitation events. In addition, the societal infrastructure is becoming more sensitive to weather and climate extremes, which would be exacerbated by climate change. In wild plants and animals, climate-induced extinctions, distributional and phonological changes, and species' range shifts are being documented at an increasing rate. Several apparently gradual biological changes are linked to responses to extreme weather and climate events. Impact and Review of Extremes There is general agreement that changes in the frequency or intensity of extreme weather and climate events would have profound impacts on both human society and the natural environment. Recent years have seen a number of weather events cause large losses of life as well as a tremendous increase in economic losses from weather hazards. These life and property losses helped raise alarm over the possibility that the recent increases were due to a shifting climate. Are these increases merely a function of decadal fluctuations, or are they indicative of longer-term trends related to anthropogenic-induced climate change? Here, we review climate extremes focusing on four areas: (i) What the observational record can tell us about past changes; (ii) The potential effects of enhanced radioactive forcing on climate extremes through climate modeling; (iii) The potential impacts of climate extremes on society, focusing on the United States; and (iv) The sensitivities of natural systems to climate change and climate extremes. Types of Climate Extremes Climate extremes can be placed into two broad groups: (i) Those based on simple climate statistics, which include extremes such as a very low or very high daily temperature, or heavy daily or monthly rainfall amount, that occur every year; and (ii) More complex event-driven extremes, examples of which include drought, floods, or hurricanes, which do not necessarily occur every year at a given location. Because a change in climate extremes is expected with anthropogenic-induced climate change, it is important to keep in mind the difference between the detection of a change, and being able to attribute that change to some identifiable climate forcing factor. The detection of changes in extremes on the basis of climate statistics is much more likely than detection of event-driven extremes. This also holds true in attempting to attribute a detected change to some forcing factor. Currently, climate models are the main source of quantitative estimates of changes in the bid to attribute some detected change in climate, such as an increase in extreme temperatures, to some climate forcing, such as increasing greenhouse gases (GHGs). Without some quantitative sense of what expected changes in climate extremes are likely to occur with increasing GHGs, it is impossible to attribute any change detected in the observed record to observed increases in GHGs. Extreme course of action In a global context, U.S. policies for dealing with extremes have been more complex and sophisticated than those in most other nations. Most other nations rely on relief assistance, often under girded by U.S. funds, for dealing with losses. The United States has also exported its policy of structural approaches to assist many developing nations. Canada passed comprehensive emergency preparedness legislation in 1988, defining government actions in natural disasters, and further established an emergency preparedness agency to promote mitigation and coordination during disasters. The United States and Japan have established a panel to deal jointly with problems associated with high winds, typhoons, and seismic disturbances. How and when these extremes occurred, as well as the impacts they created, have helped define recent policy reactions. Mitigation as opposed to recovery payments emerged as a policy theme. Future Policy Directions Endeavors to control damage and to provide assistance should continue to be a part of government policy, but some structural policies, like those that once required continuing construction of electric power systems by utilities, have ended, and others should change. Agricultural policies should continue to encourage adaptation (wise land use, better seed varieties, etc.); water policy should de-emphasize construction and resource control (reservoirs, diversions, dams, etc.) and policies to protect society must emphasize warning systems and mitigate actions (stronger buildings, better insulation, and shelters). Natural Hazard Reduction Program (NHRP) The NHRP of the Clinton Administration represented a major move to mitigate losses and to reduce disaster costs, and to help accomplish this goal; the government has joined with the property insurance industry to improve disaster reduction. In an effort to address problems caused by temperature extremes, government policies have also focused on various issues such as helping to fund energy costs for the poor, establishing public shelters for protection during extremes, setting energy price controls, and encouraging conservation practices. An emerging excellent thrust is "energy efficiency," which has the added benefit of minimizing emissions of carbon dioxide to lessen the potential for global warming. In a global context, U.S. policies for dealing with extremes have been more complex and sophisticated than those in most other nations. Most other nations rely on relief assistance, often under girded by U.S. funds, for dealing with losses. The United States has also exported its policy of structural approaches to assist many developing nations. The United States and Japan have established a panel to deal jointly with problems associated with high winds, typhoons, and seismic disturbances. Future assistance should not rely on political decisions about which events deserve assistance and how much relief is needed. Such policies also will need to be enforced by the insurance industry and should involve a partnership of local, state, and federal entities and the private sector. The Subcommittee on Natural Disaster Reduction identified major issues that need scientific attention to help reduce future losses, and these include studies to better estimate losses, to more effectively adapt new technologies that mitigate losses, to improve prediction of weather hazards like hurricanes, to define the effect of global change on hazards, to define impacts of disasters on natural ecosystems, and to assess the vulnerability of critical infrastructures. Most loss of life during recent extremes has been attributed either to location in inadequate facilities or questionable personal actions after receiving a warning such as driving a vehicle into a heavily flooded highway. Education is needed to create greater awareness of dangers and individual responsibility. Insurance coverage related to extremes should become a requirement for those deciding to live in high-risk areas. If extremes increase with time owing to changes in climate, society and its systems will have to adapt. Regardless of a change in climate, population growth and increasing vulnerability of the nation's infrastructure mean that losses will continue to increase, a clarion call for mitigate efforts. The property insurance industry fears the potential for massive, financially crippling losses, and "market-based" approaches and new federal policies are likely. One obvious need is for the design of systems and structures having greater flexibility and to reduce infrastructure vulnerability. This includes using diverse and better-adapted crop strains, more efficient irrigation, floating docks in major harbors, stronger homes and structures, and new infrastructure, particularly in aging urban areas. Highly vulnerable infrastructures include communications, electricity and natural gas supply systems, water supply and sewage treatment systems, and transportation. Sustainability and wise land use have been largely ignored in most public policies dealing with natural hazards. Policies with incentives for "doing the right thing" are necessary. Impacts of Extremes on Natural Systems Recent documentation of systematic change across a broad range of species spread over many continents now provides convincing evidence that 20th-century climate trends have impacted natural systems. Global warming scenarios predicted many of the observed biotic changes more than a decade ago. However, most of these studies relate mean climate trends to averaged biotic trends, with little analyses of more detailed linkages. Thus, it is well documented that a gradual change in climate, as well as local or regional climate characteristics, can affect population abundance, species' distribution, morphology, and behavior, ultimately impacting community structure as well. Much less studied are the mechanistic links between small- and large-scale processes, and the relative roles in these processes of climate means as compared with climatic variability or extreme events. In spite of these gaps, knowledge from basic ecological and physiological research provides clear evidence that natural systems should be strongly influenced by extremes of weather and climate. One of the very first such studies dates back to the last century. In the late 1800s, Bumpus documented that a severe winter storm over Lake Michigan, in the United States, disproportionately killed off both the largest and the smallest sparrows, thereby generating strong natural selection on body size. Many biological processes undergo sudden shifts at particular thresholds for temperature or precipitation. Tolerances to frost and to low levels of precipitation often determine plant and animal range boundaries. Single extreme temperature events can alter physical characteristics. For example, the adult sex of many turtle species (and hence population sex ratio) is determined by the maximum temperature experienced by the growing embryo. Periods of unusually heavy precipitation have been shown to alter breeding systems. Under high-rain, high-resource conditions, the Galapagos mockingbird becomes more polygamous, and in African elephants a few dominant males go into mush and capture all the mating. Single drought years have been shown to affect individual fitness and population dynamics of many insects, causing drastic crashes in some species, while leading to population booms in others. An extended drought in New Mexico in the 1950s caused the boundary between pine and piñon/juniper forest to shift by 2 km, where it remains today. Drought years in the Galapagos, cause evolution of larger beak size in Darwin's finches, while extremely wet years cause evolution of small beak (and body) size. Many studies have related El Niño events to changes in marine biotic systems. Particularly striking were widespread massive coral bleaching events that followed the 1982-1983 intense El Niño. Finally, ecosystem structure and function are impacted by disturbance events, many of which are associated with tornadoes, floods, and tropical storms. It is likely, then, that changes in the proportions of days exceeding species-specific temperature thresholds, or changes in the frequency of droughts or extreme seasonal precipitation, will lead to physical and behavioral changes in a few species and to dramatic changes in the distributions of many other species. For most of the studies of response to climate change, data have been gathered over too short a period, or contain too many temporal gaps, to indicate whether these changes during the past several decades stem from specific climatic events or from longer term response to a gradual shift of mean climate. However, a few studies contain direct observations through time. These cases indicate that the mechanistic basis of many of these gradual long-term biotic changes may indeed lie in responses to a few, brief, extreme events. In western North America, Edith's Checkerspot butterfly has shifted its range northward (by 92 km) and upward (by 124 m) during this century. This closely matches the temperature increase over the same region and time period where mean temperature isotherms shifted 105 km northward and 105 m upward. The mechanism of this shift has been a higher rate of local population extinction in the south (Mexico) than in the north (Canada), and at low elevations compared to high. Previous studies showed that fluctuations in population size were strongly associated with variance of both temperature and precipitation. A diversity of extreme weather events, including drought, "false springs," and midsummer frost, have been directly observed to cause extinction of local populations of this butterfly. Thus, the gradual northward and upward movement of the species' range since 1904 is likely due to the effects of a few extreme weather events on population extinction rates. Changes in oceanic circulation also appear to drive biotic change. The North Atlantic Oscillation (NAO) has been implicated in several trends in northern Europe, with data spanning as far back as 60 years. In British birds, 31% of species since 1971, and 53% of species since 1939, show long-term, significant trends toward earlier breeding, and only one species is nesting later. Among six species of British amphibians, five are breeding significantly earlier since 1978 to 2004. Over the last 20 to 25 years, the shift in breeding has been almost 9 days earlier in birds and up to 7 weeks earlier in amphibians. For the Red Deer in Norway, warm NAO winters have been shown to select for small females and large males. Over the past 40 years, the deer population has gradually shifted in these directions, with the result that the size difference between the sexes has grown larger. All of these trends, in birds, amphibians, and deer, have been linked to the periodicity and severity of NAO. For most other cases, the potential links between biotic and climatic changes must be inferred from more indirect measures of the influence of climate, such as from biogeography or physiological studies. One limitation of such inference is that many of these relationships have been studied with respect to mean climatologically values, even though the underlying mechanisms may involve extreme weather events. Furthermore, predictive power is hindered by the barrage of no climatic anthropogenic forces affecting natural systems--urbanization, land conversion, water diversion, and pollution (129). Thus, not only are scenarios of global climate change predicting nonlinear ties and "surprises" in the climate system, but if we incorporate the complexities of modern, human-dominated environments, then wildlife should also be expected to exhibit novel, unpredictable responses. Interpretation One of the biggest problems in determining whether extreme events have changed in the observed record, and if these changes are consistent with what we may expect from an increase in GHGs in the climate models, is that investigators have often used quite different criteria to define an extreme climate event. This lack of consensus on the definition of extreme events, coupled with other problems, such as a lack of suitable homogeneous data for many parts of the world, likely means that it will be difficult, if not impossible, to say that extreme events in general have changed in the observed record. Although the direct link between societal and biological impacts and climate change is often difficult to make, a growing body of evidence linking climatic and biological changes suggests systematic global increases in both the frequency and impact of extreme weather and climate events. Furthermore, as climate models become better developed, climate simulations will provide a much better idea of the kinds of changes in climate extremes to be expected with increasing GHGs, which will allow the observed record to be examined for further evidence of these kinds of changes. Lastly, it must be kept in mind that the kinds of climate changes discussed here are often nonlinear, and that both temporal and regional variability are associated with any kind of climate change.