One of the most easily imagined impacts of global warming is an increase in the number and severity of heat waves. Heat stress is a well-known danger during prolonged bouts of hot weather, especially in cities, which tend to trap heat. In both New York and Shanghai, for instance, records show that daily mortality rates increase sharply once temperatures exceed a certain threshold . During intense heat waves, the death toll attributed to heat stress can be surprisingly high, as occurred in Chicago in July 1995, when heat stress killed 726 people during a 4-day heat wave  .
Midlatitude cities including Washington, D.C., Athens, and Shanghai seem to be at greatest risk for deadly heat waves. In these cities, residents (especially the elderly, the very young, and the poor) are not acclimatized to extremely hot weather and are thus more vulnerable to heat stress. Among these vulnerable groups, the existence of previous health problems, greater heat exposure due to substandard housing, and lack of access to air conditioning are all factors leading to higher heat-related mortality. By the middle of the next century, climate change could increase the frequency of very hot days severalfold in a city similar to Washington, D.C., according to one estimate . The normally hotter average temperatures in tropical and subtropical cities seem to help residents accommodate heat waves better, so they suffer fewer heat-stress problems, although heat-related deaths during a 1995 heat wave in New Delhi indicate that even residents in the tropics can be susceptible to extreme temperatures .
Conversely, a potential health benefit of warmer global temperatures could be fewer cold-related deaths as winters become milder. A recent British study estimated that by 2050, an increase in the average wintertime temperature by 2.0