The U.S. death toll from heat waves made more intense by climate change will rise by 150,000—just in the 40 largest cities—by the end of the century.

Unless strong measures are adopted to reduce heat-trapping carbon dioxide emissions and those cities ramp up procedures to deal with the coming "excessive heat events," the current annual average of 1,300 heat-related deaths will double by mid century and triple by 2100.

And all the estimates are probably on the conservative side.

Those are some of the major findings of a report published by the National Resources Defense Council based on two peer-reviewed studies led by Larry Kalkstein, senior professor of geography and regional studies at the University of Miami.

"Climate change is here now," said Daniel Lashof, director of the NRDC's Climate and Clean Air program at a recent news conference on the report. "It's already having an impact, and we can expect that to continue to get worse unless we take strong action to reduce these threats."

Kalkstein's team assumed emission trends would continue unless new policies are adopted to significantly abate carbon pollution. They projected the increase in the number of days per year that have dangerously hot conditions and then calculated the number of heat-related fatalities that can be expected.

The effects of the coming killer heat waves won't fall evenly across urban centers. Of the 40 cities Kalkstein examined, 37 will likely see increases in heat-related deaths, but some cities will have it much worse than others.

Louisville, Ky., is expected to see a jump of nearly 19,000 deaths, while Detroit's number rises by 18,000 and Cleveland's by 17,000. Only Seattle, Miami and Atlanta are not projected to see a rise in deaths.

The poor, infirm, elderly and solitary are most vulnerable during heat waves, but where they live will greatly affect their chances of survival. Some of the risk can be attributed to geography, but much of it also depends on the attitudes and approaches in urban centers.

"There's clearly a wide range of how much attention individual cities pay to the threat of heat-related deaths," Pete Altman, who wrote the NRCD report, told InsideClimate News. "There's a wide range of preparedness."

Philadelphia, which is expected to see only a small increase in deaths, is a model city in handling heat waves, Kalkstein said in the news conference. It has set up a heat task force and cooling centers where people can go to escape high temperatures; employed an effective early warning system; assigned block captains to check on vulnerable residents; and spent roughly $150,000 a year on the program. The city even has an agreement with its electricity provider to suspend disconnects for nonpayment during heat waves.

Several other factors figure into a city's death-risk equation, including the amount of green space it has, the prevalence of air conditioning, the age and makeup of the population, and the current frequency of heat waves.

Cities in the Southeast and Southwest are accustomed to dealing with heat, Altman said, while some cities in the Midwest and north are not. Places that usually have summer highs in the 80s, then suddenly see a multi-day spike into the 100s, will be at most risk.

Chicago was a prime example. In 1995, more than 800 people died in a long, intense heat wave. That event was a wake-up call and turning point for the city, which has since set up systems to deal with extreme heat events.

Global warming is projected to raise temperatures in North America by 4 to 11 degrees this century, but the expected jump in the death toll is largely attributable to an increase in the duration of heat waves.

"Most people can withstand two days in a row of that kind (100 degrees or more) of weather," Kalkstein said. "Once you get beyond two days, that's when it gets problematic."

Roughly two-thirds of the U.S. population lives in the 40 cities examined in the study. Kalkstein said rural areas should be less affected, because they don't feel the impact of the urban heat-island effect, where temperatures rise even higher because of the heat absorbed by asphalt and glass surfaces. But he said a study conducted by a colleague showed that rural areas will also see some increase in heat-related deaths in coming years.

The estimate of an additional 150,000 deaths nationwide by the end of the century is probably on the low side. The study didn't adjust for population growth or an increase in the percentage of elderly citizens, one of the most vulnerable groups.

Kalkstein said the impact of heat on human health has been historically understated, because it isn't accompanied by the visible damage that tornadoes and hurricanes leave behind. If you took a picture of a street before and after a heat wave, it would look the same.

"We consider heat to be a silent killer," he said. "It's difficult for medical examiners to detect heat-related deaths."

Heat waves bring verifiable spikes in heart attacks, strokes and respiratory failure, but medical examiners generally don't list heat as the cause of death in those cases, he said.