Forecasters are calling for another hot summer, and the longer-term outlooks for summers to come are chilling.
Worldwide climate change is expected to intensify summer heat, particularly in urban areas.
Heat remains the nation's biggest weather killer. In the last 30 years, the government blames it for more than 5,000 deaths. Flooding is a distant second, with fewer than 3,000.
It is the hot nights that drive up death tolls in heat waves, and it already is well-documented that city nights are getting warmer. That's partly because buildings and paving retain heat.
In Philadelphia, for example, National Weather Service records show that the increase in nighttime warming is outpacing that of daytime. For the last six summers, daily low temperatures have averaged 2.3 degrees above the long-term mean while daily highs averaged 1.8 degrees above. That half a degree equals a 30 percent difference.
Regardless of any climate trends, however, summers almost certainly will become deadlier, especially in temperate cities such as Philadelphia, Washington and New York, where extreme heat comes in spurts and the population isn't acclimated to it.
Sheer demographics are likely to overpower any global-warming or human adaptations to it, including air-conditioning and early warning systems for heat waves.
Elderly in single-person households are the most likely to die in heat waves. And people are living longer, and more people are living alone. The nation's live-alone population age 85 and over is projected to hit 2.3 million in 2020, up from 1.1 million in 1990, according to the U.S. Administration on Aging.
But if the victims are so old, how can we know if the heat is what's killing them?
Health experts look at mortality data. If deaths are way above average during a heat wave and then return to near normal levels when the hot spell ends, it is reasonable to assume that heat was a factor.
While that may seem simple enough, estimating heat-related mortality is a tricky business precisely because so many of the victims are isolated. Deaths may go unreported for several days, observed Haresh Mirchandani, retired Philadelphia medical examiner.
When Mirchandani came to Philadelphia from Detroit in the late 1980s, he began counting cases in which heat was a contributing factor. He ordered investigators to look for clues such as closed windows or the absence of fans and air conditioners.
The method gained national attention when the city reported 118 heat-related deaths from a 1993 heat wave that other cities said killed very few people. An investigation by the U.S. Centers for Disease Control and Prevention determined that Mirchandani's method should be used everywhere.
In short, Philadelphia has changed the way the nation looks at heat impacts.
The city now has one of the world's most sophisticated heat-response systems, says Laurence Kalkstein, a retired University of Delaware professor who is an expert on heat stress.
When heat is imminent, the city alerts its 7,000 block captains and urges relatives, neighbors and volunteer groups to check on the elderly.
It is the key component of a low-cost, low-tech program that appears to be working. In a study, Kalkstein and four other researchers estimated that it saved more than 115 lives from 1995 through 1998.
If the weather forecasts are accurate, this could be another busy season in Philadelphia. Both the federal Climate Prediction Center and WSI Corp., a Massachusetts-based energy-forecasting company, are calling for a hotter than normal summer in the region.
And if the climate models are right, expect more hot summers. No matter what, the probability of a swelling vulnerable population is near 100 percent.
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