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 phenological 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.

Weather and climatic extremes can have serious and damaging effects on human society and infrastructure as well as on ecosystems and wildlife. Thus, they are usually the main focus of attention of the news media in reports on climate. There are some indications from observations concerning how climatic extremes may have changed in the past. Climate models show how they could change in the future either due to natural climate fluctuations or under conditions of greenhouse gas-induced warming. These observed and modeled changes relate directly to the understanding of socioeconomic and ecological impacts related to extremes. This is the first of five papers in the "Understanding Changes in Weather and Climate Extremes" series. The following series of five articles was motivated by a need to develop a more comprehensive assessment of changes in weather and extreme climate events. We were interested not only in the impact of extreme weather and climate events, but whether these events were changing in frequency or intensity along with their impacts. Impacts were viewed in terms of loosely managed ecosystems where wildlife flourishes, as well as socioeconomic systems and more heavily managed ecosystems such as agriculture. From a climate perspective , this included a focus both on the historical record and projections for future change. During the summer of 1998 a group of nearly 30 climate scientists, social scientists, and biologists met for 10 days at the Aspen Global Change Institute to discuss what we now know, and how we could reduce some of our major uncertainties. These articles summarize much of the work during that meeting and new information since the meeting.

This paper reviews recent work on trends during this century in societal impacts (direct economic losses and fatalities) in the United States from extreme weather conditions and compares those with trends of associated atmospheric phenomena. Most measures of the economic impacts of weather and climate extremes over the past several decades reveal increasing losses. But trends in most related weather and climate extremes do not show comparable increases with time. This suggests that increasing losses are primarily due to increasing vulnerability arising from a variety of societal changes, including a growing population in higher risk coastal areas and large cities, more property subject to damage, and lifestyle and demographic changes subjecting lives and property to greater exposure. Flood damages and fatalities have generally increased in the last 25 years. While some have speculated that this may be due in part to a corresponding increase in the frequency of heavy rain events, the climate contribution to the observed impacts trends remains to be quantified. There has been a steady increase in hurricane losses. However, when changes in population, inflation, and wealth are considered, there is instead a downward trend. This is consistent with observations of trends in hurricane frequency and intensity. Increasing property losses due to thunderstorm-related phenomena (winds, hail, tornadoes) are explained entirely by changes in societal factors, consistent with the observed trends in the thunderstorm phenomena. Winter storm damages have increased in the last 10-15 years and this appears to be partially due to increases in the frequency of intense nor'easters. There is no evidence of changes in drought-related losses (although data are poor) and no apparent trend in climatic drought frequency. There is also no evidence of changes in the frequency of intense heat or cold waves.

Societal impacts from weather and climate extremes, and trends in those impacts, are a function of both climate and society. United States losses resulting from weather extremes have grown steadily with time. Insured property losses have trebled since 1960, but deaths from extremes have not grown except for those due to floods and heat waves. Data on losses are difficult to find and must be carefully adjusted before meaningful assessments can be made. Adjustments to historical loss data assembled since the late 1940s shows that most of the upward trends found in financial losses are due to societal shifts leading to ever-growing vulnerability to weather and climate extremes. Geographical locations of the large loss trends establish that population growth and demographic shifts are the major factors behind the increasing losses from weather-climate extremes. Most weather and climate extremes in the United States do not exhibit steady, multidecadal increases found in their loss values. Without major changes in societal responses to weather and climate extremes, it is reasonable to predict ever-increasing losses even without any detrimental climate changes. Recognition of these trends in societal vulnerability to weather-climate extremes suggests that the present focus on mitigating the greenhouse effect should be complemented by a greater emphasis on adaptation. Identifying and understanding this societal vulnerability has great importance for understanding the nation's economy, in guiding governmental policies, and for planning for future mitigative activities including ways for society to adapt to possible effects of a changing climate.

The short but intense heat wave in mid-July 1995 caused 830 deaths nationally, with 525 of these deaths in Chicago. Many of the dead were elderly, and the event raised great concern over why it happened. Assessment of causes for the heat wave-related deaths in Chicago revealed many factors were at fault, including an inadequate local heat wave warning system, power failures, questionable death assessments, inadequate ambulance service and hospital facilities, the heat island, an aging population, and the inability of many persons to properly ventilate their residences due to fear of crime or a lack of resources for fans or air conditioning. Heat-related deaths appear to be on the increase in the United States. Heat-related deaths greatly exceed those caused by other life-threatening weather conditions. Analysis of the impacts and responses to this heat wave reveals a need to 1) define the heat island conditions during heat waves for all major cities as a means to improve forecasts of threatening conditions, 2) develop a nationally uniform means for classifying heat-related deaths, 3) improve warning systems that are designed around local conditions of large cities, and 4) increase research on the meteorological and climatological aspects of heat stress and heat waves.

A brief but intense heat wave developed in the central and eastern United States in mid-July 1995, causing hundreds of fatalities. The most notable feature of this event was the development of very high dewpoint temperature (Td) over the southern Great Lakes region and the Upper Mississippi River Basin. At many locations, hourly values of Td set new records. The combination of high air and dewpoint temperatures resulted in daily average apparent temperatures exceeding 36°C over a large area on some days. A comparison with past heat waves shows that this was the most intense short-duration heat wave in at least the last 48 years at some locations in the southern Great Lakes region and Upper Mississippi River Basin. An analysis of historical data for Chicago, where the majority of fatalities occurred, indicates the intensity of this heat wave was exceeded only by a few periods in the 1910s and 1930s. Impacts in the Chicago urban center were exacerbated by an urban heat island that raised nocturnal temperatures by more than 2°C. An analysis of radiosonde data indicates that maximum daytime boundary layer mixing depths were only a few hundred meters in the core region of the heat wave. Simulations using a single-column version of a three-dimensional mesoscale model strongly suggest that this contributed to the very high values of Td since soil moisture in the central United States was near to above average and evapotransporation was likely high, causing a rapid moistening of the shallow boundary layer.

Historical weather records at eight American urban areas of varying size, type, and climate were studied for indications of inadvertent precipitation modification. The six largest cities all had experienced warm seasonal rainfall increases of 9 to 17% during the 1955-70 period. The increases in the Midwest cities occurred largely with cold frontal systems, but in the coastal cities they were largely during air mass (non-frontal) conditions. The Midwest increases also were found to occur as enhancement, not initiation, of moderate to heavy rain days. Significant increases in summer thunder-day frequencies (13 to 41%) and hail-day frequencies (90 to 450%) were found at the six largest cities, and the increases occurred largely in the morning hours. The typical locations of maxima in the Midwest cities were thunder over and near the city, and rain and hail 25 to 55 km downwind. The maxima of all events in coastal cities were in or near the city. Overall, the results suggest that urban precipitation enhancement is related to city size, industrial nuclei generation, and urban thermal effects. The alterations have considerable relevance to urban design, local area forecasting, local water supplies, agricultural production, hydrologic design, and to planned weather modification.

The July 1999 heat wave in the Midwest was an event of relatively long duration punctuated by extreme conditions during its last 2 days. The intensity of the heat wave on 29 and 30 July rivaled that of the 1995 heat wave that killed more than 1000 people in the central United States. In 1999, however, the death toll was about one-fourth of this amount in the same region. The 1999 heat wave 2-day maximum apparent temperature was slightly less than during the 1995 heat wave at most Midwestern first-order stations. In addition, the 2-day peak was preceded by several hot days that allowed some short-term acclimatization to occur prior to the intense final days. In Chicago, conditions during the peak of the 1999 heat wave were very similar to those during the 1995 heat wave peak, especially the extreme nocturnal conditions of temperatures and humidity. Therefore, it seems unlikely that the reduction in the heat wave death toll in Chicago from about 700 in 1995 to 114 in 1999 is due solely to meteorological differences between the two heat waves. In St. Louis, the 1999 heat wave was intense for a much longer duration than the 1995 heat wave, thus partially explaining the increase in heat-related deaths there from the 1995 event to the 1999 event. An examination of heat wave response efforts in both Chicago and St. Louis leads to the conclusion that both cities were quite effective at mitigating their respective heat wave mortality rates, which in the 1999 event were almost exactly the same in both metropolitan areas. This represents a great improvement for the city of Chicago compared to the 1995 heat wave. Suggestions are made for further improving municipal heat wave response efforts based on the 1999 experience.