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Weather-Related Morbidity and Mortality

Impacts on Risk

A changing climate coupled with changing demographics is expected to magnify the already significant adverse effects of extreme weather on public health. For example, the intensity and frequency of precipitation events in the United States have increased over the past 100 years in many locations. In the Midwest and Northeastern United States, heavy rainfall events (defined as those in excess of 1 inch of rainfall) have increased by as much as 100%, and recent flooding events, such as the June 2008 flooding in the Midwest, have caused billions of dollars of damage and significant loss of life. In line with this observed trend, there is a projected increase in intensity of precipitation events in some areas of the country, particularly in the Northeast, which has experienced a 67% increase in the amount of heavy precipitation events in the past 50 years. Precipitation extremes also are expected to increase more than the mean. Regional variability appears to be increasing so that even though extreme precipitation events will become more common, some areas will concurrently experience drought, especially in the northeast and southwest. The intensity of extreme precipitation events is projected to increase with future warming. This could limit the ability to capture and store water in reservoirs, leading to flash flooding events. Climate variability resulting from naturally occurring climate phenomena such as El Niño, La Niña, and global monsoons, are associated with extreme weather events around the globe. El Niño and La Niña conditions lead to changes in the patterns of tropical rainfall and in the weather patterns in mid-latitudes, including changes in the frequency and intensity of weather extremes. El Niño is also projected to increase in both frequency and intensity as the climate warms, though there is uncertainty about the relative frequency of El Niño and La Niña in the future. Increased precipitation associated with stronger El Niño events would affect the Western United States, particularly California, the Pacific Northwest, and the Gulf Coast more than other regions of the country. Heavy precipitation events will be highly variable in magnitude, duration, and geographic location. Increased variability in weather and climate extremes is difficult to predict, and will impact the ability of human systems to manage for and adapt to heavy precipitation and flooding events. An observed divergence of precipitation patterns has lead to increased variability in the amount of precipitation per event, resulting in both extreme amounts of precipitation, as well as abnormally small precipitation events. These effects have already been observed globally. A study in Germany found that winter storms from 1901 to 2000 showed an increasing trend of precipitation events both exceeding the 95th percentile and falling below the 5th percentile; while from 1956 to 2004 the Dongxiang River in China became increasingly likely to be at either extreme flood flow or abnormally low-flow. The current evidence is insufficient to determine if the frequency of tropical cyclones in the Atlantic Basin will change. The observed frequency of tropical cyclones in the Atlantic Basin has increased since the mid-1990s, though the numbers are not unprecedented and must be reconciled with active multi-decadal periods of the past, such as the 1950s and 1960s. Increases in sea surface temperature and decreases in wind shear may lead to more intense Atlantic hurricanes, though some models also show a decrease in the number of intense hurricanes in the Atlantic Basin. The spatial distribution of hurricanes also is likely to change, with storm surges becoming more damaging in areas unaccustomed to facing large hurricanes. The combination of sea-level rise with increasing storm intensity could lead to significant destruction of coastal infrastructure and more costly hurricanes. In addition, flooding and coastal changes could lead to ecosystem changes such as loss of wetlands that could indirectly impact human health. Hurricane track forecasting and modeling methods have improved, and mortality rates for major storms have declined over time, but the combination of increased coastal population density, increased intensity of tropical storms, and sea-level rise will result in significantly increased risk going forward.

Some models show that what were 20-year floods in 1860 in the United Kingdom are now 5-year floods; even greater impacts are expected in tropical regions. In the United States, large floods are more frequent now than at the beginning of the 20th century. Monsoon-related flooding results in damaged infrastructure, increased disease, and loss of life. During El Niño, areas including Indonesia, southern Africa, northeastern Australia, and northeastern Brazil usually experience extensive periods of dry weather and warmer-than-average temperatures. These conditions have historically resulted in a variety of adverse effects, such as mudslides, forest fires and resulting increased air pollution, mass migrations, and famines.

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