As our climate changes, extreme weather events such as droughts, floods, heat waves, heavy rainfall, tropical storms and hurricanes are expected to increase, according to a team of scientists, led by David R. Easterling of NOAA's National Climatic Data Center in Asheville, N.C. Scientists reached this conclusion after reviewing hundreds of studies that used data and climate models to examine past and future changes in climate extremes. Their work, which includes reviews of studies using observations, modeling, and impacts, is reported in the September 22 edition of Science magazine.

"Our review shows consistency between our climate models and what we have observed in the 20th century. Models of 21st century climate suggest that many of these changes in climate extremes are likely to continue. We also found that extreme weather events have had increasing impact on human health, welfare, and financial losses." Easterling continued, "this trend is likely to become more intense in the years to come both as the climate continues to change, and society continues to become more vulnerable to weather and climate extremes."

Recent years have seen a number of weather events cause large losses of life as seen from Hurricane Mitch, as well as tremendous increase in economic hardship. Losses caused by catastrophes, defined as greater than $5 million, have grown steadily in the United States, from about $100 million annually in the 1950s to $6 billion per year in the 1990s. The annual number of catastrophes grew from 10 per year in the 1950s to 35 per year in the 1990s.

Overall in the United States, the researchers found a slight downward trend in the number of extreme temperature events, despite an overall warming in the mean temperature. For the 1910-1998 period, there has been a slight decrease in the number of days below freezing over the entire country. Regionally, the start of the frost-free season for the Northeast occurred 11 days earlier in the mid-1990s than in the 1950s. The Northeast also saw a trend to fewer extreme cold days, but also a trend to fewer warm maximum temperatures as well.

Trends in one-day and multi-day heavy precipitation events show a tendency toward more days with heavy precipitation totals over the 20th century. The researchers also found that increases in extreme precipitation events are responsible for a disproportionate share of the observed 5 to 10 percent increase in total annual precipitation since the early 20th century.

Losses created by various weather types have grown. Annual hurricane losses have grown from $5 billion in the 1940s to more than $40 billion in the 1990s, adjusted for inflation. Flood damages also continue to increase with annual losses of $1 billion in the 1940s, growing to $6 billion per year during the 1980s and 1990s. The scientists report that most of the increase has been due to societal shifts. The growth of population, demographic shifts to more storm-prone locations, the growth of wealth have collectively made the nation more vulnerable to climate extremes.

Easterling also co-authored an opinion-piece titled "U.S. Policies Pertaining to Weather and Climate Extremes" in the same issue of Science magazine.