As the average temperature rises, heat waves likely will become more frequent and more intense, which could mean tens of thousands of heat-related deaths each year in the United States at a multibillion-dollar cost.
The 1995 heat wave in Chicago claimed around 600 lives in five days, and the mammoth heat wave that overtook Europe in 2003 caused an estimated 50,000 or more deaths. At least 140 people died from heat-related complications during the heat wave that struck California in July, and that number is likely significantly underreported.
Matters could be made worse by indirect health impacts such as a rise in mosquito-borne diseases such as West Nile virus because the pests flourish in warmer weather.
For regions with higher average summer temperatures and higher humidity, more heat is particularly worrisome.
But California's enviably mild, Mediterranean-style climate could, ironically, make its population more vulnerable as well. Far fewer California homes, schools and other buildings have air conditioning compared with hotter parts of the country, leaving many residents, particularly in poor communities, without an easy way to cope when unusually high temperatures hit.
And it doesn't take prolonged, intense heat to cause problems.
"We don't need heat waves or extreme ambient temperatures or high humidity levels to find a temperature-mortality association," said epidemiologist Rupa Basu of the California Office of Environmental Health Hazard Assessment.
Basu and Bart Ostro, head of air pollution and epidemiology at COEHHA, compared data for temperatures and deaths from nine California counties, including Contra Costa, for the months of May through September from 1999 to 2003.
They found that for every 10-degree increase in apparent temperature, which includes the effect of humidity, there was a corresponding 3 percent rise in deaths on any given day.
The number is slightly higher for seniors and children younger than five, and the highest rate was among African Americans, at nearly 5 percent. The main causes of death were heart related, including congestive heart failure and heart attacks.
This is because the biggest impact of heat is on blood circulation. In cooler temperatures, circulation is concentrated around the vital organs in the body's core, including the heart. When temperatures rise, more blood is circulated under the skin to attempt to cool the body down, which can put more stress on the heart.
Next, Basu's team projected results into the future using a middle-of-the-road climate change scenario for California, based on a 2004 study led by Daniel Cayan of the Scripps Institution of Oceanography in La Jolla, published in the Proceedings of the National Academy of Sciences.
In this scenario, for the year 2034, with a three to four degree increase in average temperature, there would be 477 excess heat-related deaths in the nine counties they studied, and 783 for the whole state that year.
And these estimates don't account for deaths related to extreme heat, Ostro said.
The time period Basu and Ostro studied from 1999 to 2003 had no significant heat waves, and the average monthly temperatures didn't rise above 80 degrees.
They are currently studying the heat wave in California in July and the preliminary results show that the effects are much larger for extreme heat.
"It's conceivable that these numbers could be three to five times higher," Ostro said.
This would far outstrip the current rate for the entire United States of around 400 heat-related deaths reported annually.
The number of annual deaths from heat nationwide could rise to 35,000 by the end of the century if greenhouse gas emissions continue unabated, according to economist Olivier Deschênes of UC Santa Barbara.
In a preliminary study that has yet to be peer-reviewed and published, Deschênes and Michael Greenstone of the Massachusetts Institute of Technology analyzed the effect of random year-to-year variation in temperatures on death rates across the United States.
They estimate that a 6.6-degree rise in the average temperature could boost the country's death rate by 1.5 percent. With the life expectancy of men and women taken into account, this translates into 440,000 lost years of life annually.
This amounts to a cost of $44 billion every year using a standard economic calculation to value years of life lost. Their study also predicts a sizeable increase in energy consumption to compensate for the heat.
And this doesn't take nonfatal illnesses into account.
"We may never be able to get the full cost of climate change in terms of health. Here we only look at mortality," Deschênes said. "Let's say that more people get asthma in the future as a result of climate change. Quality of life would go down, which has an economic cost, but we currently don't have comprehensive enough data to say anything about that cost."
Basu and Ostro are trying to address this the same way they looked at mortality, by analyzing how the number of hospitalizations is affected by temperature. They expect that with rising temperatures, there may be an increase in cardiovascular problems, respiratory issues and brain circulation disorders such as stroke.
Warmer weather will have indirect impacts on health as well.
Mosquitoes, which can carry and transmit diseases such as West Nile virus and western equine encephalitis, thrive in warm weather. And if climate models are right, these pests should be quite pleased with the forecast for California.
Simply lengthening the warm season will give mosquitoes more time to reproduce, and consequently, more time to potentially infect people.
"If you increase the temperature in the spring and fall, it will elongate the transmission season," said entomologist William Reisen at the University of California, Davis.
And warmer temperatures also speed up the insect's life cycle, which means more generations of mosquitoes will come and go each year. And when their reproduction rate is faster, mosquitoes feed more frequently, which means more bites.
Mosquitoes may also be able to expand their range as areas that were too cold for them before become hospitable, particularly in the higher elevations.
To make matters worse, the heat could also speed up the reproduction of mosquito-borne pathogens such as West Nile virus.
"For the mosquito as well as the virus, hotter is usually better," Reisen said.
Betsy Mason covers science and the national laboratories. Reach her at 925-847-2158 or email@example.com.