By Sabrina Tavernise / Source: nytimes

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Air pollution, like heavy smog in Beijing, is contributing to climate change, a phenomenon that is starting to affect people’s health around the world. Agence France-Presse— Getty Images

Is climate change a serious threat to human health?

Simple logic would suggest the answer is yes, a point that the Obama administration is using to build support for the president’s effort to make climate change a centerpiece of his final months in office.

A White House report listed deepening risks. Asthma will worsen, heat-related deaths will rise, and the number and traveling range of insects carrying diseases once confined to the tropics will increase.

But the bullet points convey a certainty that many scientists say does not yet exist. Scientists agree that evidence is growing that warmer weather is having an effect on health, but they say it is only one part of an immensely complex set of forces that are influencing health.

For example, scientists note that global travel and trade, not climate change, brought the first cases of chikungunya, a mosquito-borne tropical disease, to Florida.

Temperatures may be rising, but overall deaths from heat are not, in part because the march of progress has helped people adapt — air conditioning is more ubiquitous, for example, and the treatment of heart disease, a major risk for heat-related mortality, has improved.

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Ticks and mosquitoes carrying infectious diseases may be altering their ranges in response to temperature changes. Robert F. Bukaty/Associated Press

The resurgence of forests in the eastern United States and the subsequent increase in the deer population have helped drive a sharp growth in ticks and Lyme disease. But the increase in the prevalence of the illness in the United States has little to do with the climate, federal health experts say.

“There’s a lot of evidence showing that extreme weather can hurt people, but what we don’t know is whether those effects are getting worse,” said Patrick L. Kinney, director of the Columbia University Climate and Health Program, adding that scientists don’t have the long-term data needed to pinpoint how climate change is affecting health.

Still, climate change is a contributing factor. Ragweed now blooms about two to three weeks longer in the north central United States than it did a few decades ago, extending sneezing and watery eyes further into the fall, according to research led by Lewis H. Ziska, a plant scientist at the United States Department of Agriculture.

The Asian tiger mosquito, which came to the southern United States from Japan in the 1980s, likely in a shipment of used tires, has recently spread as far north as Connecticut, an encroachment scientists have connected to rising temperatures, said Dina Fonseca, an entomology professor at Rutgers University.

Mary H. Hayden, a scientist at the National Center for Atmospheric Research in Boulder, Colo., who studies climate and health, said of dengue fever, a tropical disease carried by mosquitoes: “I don’t think we can dismiss the role of climate. But can we say there is a direct causal link? No, we can’t. It’s more complex than that.”

The science is in its infancy. Data on insects, pollen counts and diseases in developing countries is often patchy. Many studies show associations, meaning two things are happening at the same time, but it is not clear that one is causing the other. Some experts compare it to the state of science in the early days of understanding smoking’s effect on lung cancer.

Evidence is accumulating, however. In 2000, the first National Climate Assessment, a government document weaving together the best evidence on climate change, had just 21 pages on health. The most recent assessment included a special section on health that filled more than 400 pages.

Two peer-reviewed British journals — Philosophical Transactions B and The Lancet — have dedicated many pages to the topic this year. Europeans, unburdened by the level of political controversy over climate change in the United States, often give more conclusive interpretations of the science.

“We are in a far more certain place now,” said Nick Watts of the University College London Institute for Global Health and a co-author of the Lancet analysis. “We feel very comfortable talking about direct effects of climate change on health.”

The climate’s effect on health is generally less pronounced in wealthier countries like the United States, where so many people are protected from the elements in their homes. A study comparing Laredo, Tex., and a city just across the border in Mexico found the incidence of dengue fever was far higher in Mexico, even though the mosquitoes that carry it were more abundant in Texas. Researchers attributed the Texan advantage to economics — air conditioning and windows that shut — not climate.

But climate change is affecting health in developed countries, too. In Canada, the tick population has exploded in recent years, with 13 areas where ticks were living and reproducing locally, up from just two in 1997. Researchers have found that some areas have become warmer, and thus more suitable for ticks. Warmer weather allows more immature ticks to survive into adulthood, expanding the population.

“The areas that are suitable for ticks to colonize are changing,” said Patrick Leighton, an assistant professor of veterinary epidemiology at the University of Montreal.

Insects like ticks and mosquitoes cannot regulate their own body temperatures, so their breeding, feeding and life cycles are extremely sensitive to temperatures. Canada now has about 700 locally acquired cases of Lyme disease a year, up from about 40 cases 15 years ago. Nearly all past infections used to happen outside Canada. Now most are acquired locally.

“This whole Lyme disease issue has gone from theory to reality for us,” said Dr. Nick Ogden, a senior scientist for the Public Health Agency of Canada. “Now the ticks are really moving in.”

But Lyme disease is also an example of just how difficult it is to draw broad conclusions about how climate change affects health. The disease is also moving south, with large sections of Virginia and parts of North Carolina now inundated with ticks that carry the disease. But that pattern appears to have little to do with climate.

Dr. C. Ben Beard, associate director for climate change at the Centers for Disease Control and Prevention, said reforestation in the eastern United States and the expanding populations of deer and people appear to be factors.

“Climate is probably not driving the southward expansion,” he said. In general, the temperature effects of climate change on ticks are more significant in northern climates, he said.

Heat has caused hundreds of deaths in Pakistan recently, with victims concentrated among older adults and people who spend a lot of time outside, like the homeless. Scientists say it is all but impossible to tie a specific weather event to climate change but say with increasing certainty that temperatures are rising.

But even heat is complicated. A recent review of heat mortality in the United States found that the rate of heat-related deaths declined by more than half from 1987 to 2005. The researchers concluded that the population had become more resilient to heat over time, which might have resulted in part from the increased use of heat warning systems by cities and improvements in medical treatment for conditions that are risk factors for heat mortality.

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