Health Effects of Climate Change

Late in the summer of 2007, residents of Castiglione di Cervia, Italy, began coming down with symptoms unusual for inhabitants of a small village in the plains of northern Italy. In the town of 2,000, more than 100 people were struck with a high fever, rashes, and crushing pain in their bones and joints - caused, doctors eventually discovered, by a tropical mosquito-borne infection called chikungunya.

It was the first time the dengue-like virus - a disease previously known only in the tropics - had broken out on the European continent.

Brought from India by an Italian tourist, in a normal year it would have most likely have spread no further. But in 2007, a mild winter in northern Italy had allowed Asian tiger mosquitoes to start breeding early and in large numbers. When the virus arrived, the insects provided the perfect vector.

In Forecast, my 2008 book on the impacts of climate change, the chapter on health may have been the hardest to write. I held it for last, looking for as clear a connection as possible between the warming of the planet and outbreaks of disease. In the end, I managed to track down a few examples, such as the one in Italy, where the links were strong.

But by then I realized that specific diseases were only part of a bigger picture. When it comes to climate change, the most important impacts of the emissions from our cars, power plants and factories are likely to be broad and indirect. To put it in the language of health care, global warming needs to be examined not just from the perspective of medicine, but from public health.

As the international conversation on climate change intensifies in the next few years, journalists are likely to be called upon to find local examples of the effects on health. They will be wise to be skeptical of most claims, and demand hard evidence. At the same time, they should remain alert to the unintended consequences of fossil fuel burning and aware that cause and effect do not always become clear until much later.

Likely: More tropical diseases in Northern climes

To be sure, a lot of evidence points towards a direct link between the warming of the earth and outbreaks of disease, particularly those carried by vectors. As the world warms, ecosystems are shifting northward and uphill, and travelling with them will be all sorts of pathogens.

Take malaria, for instance. Mosquito larvae mature more rapidly when the water in which they grow is warm. Female mosquitoes digest blood faster and bite more frequently as the mercury rises. Even more important is the impact of temperature on the reproductive ability of the parasite that causes the disease.

A mosquito infected with malaria will only live a few weeks. If it dies before the parasite reaches maturity, the disease will die with it. And here's the important bit: At 68 degrees Fahrenheit (20 degrees Celsius), the Plasmodium falciparum parasite, which causes one of the disease's more deadly strains, takes 26 days to complete its reproductive cycle. At 77 degrees Fahrenheit (25 degrees Celsius), it's ready to re-infect after just 13 days.

Dengue, another mosquito-borne tropical disease, has reemerged in Brazil and is climbing towards the United States through Mexico.Dengue cases in Mexico have jumped 600 percent since 2001, reaching as far north as the formerly dengue-free state of Chihuahua, which borders Texas.

In addition, the Canadian government has attributed the increase in West Nile virus cases to climate change and warned residents in 2005 that dengue fever, yellow fever and malaria might not be far behind.

The big picture is clear: As the world warms, we're likely to see more outbreaks of tropical disease in places where they once were absent.

The problem comes when you try to narrow down. One of the biggest challenges in reporting on climate change is that it's currently nearly impossible to attribute any given phenomenon to climate change. Mild winters, such as the one that led to the outbreak in Castiglione di Cervia, may be what we would expect from global warming. In fact, officials at the World Health Organization have speculated that the epidemic may have marked the first climate-change-triggered outbreak of a tropical disease in Europe.

How to differentiate "the signal from the noise"

But it's impossible to say what the temperature that year would have been if we had never burned a single barrel of oil. There's considerable variation in the natural climate, and since we're in the early stages of climate change, it's hard to differentiate the signal from the noise. Complicating things further, the computer models of the earth from which we draw a lot of our conclusions are still imprecise and often ambiguous. Dramatic changes are on the horizon - and may be even happening now-but it's difficult to draw a direct causal link.

My solution was to try to find analogues-not necessarily linked to our emissions-where changing local climates have had health impacts.

For instance, in the summer of 2003, temperatures in Europe spiked higher than ever recorded previously. The brutal heat wave killed as many as 45,000 people in two weeks. As climate change continues to develop, California is expected to see an increasing number of hot summers. When trying to understand what Californians might be headed for, it doesn't matter if the European heat wave was caused by climate change. The consequences of those high temperatures hold lessons either way.

One of the cases I examine closely in my book is Four Corners disease, which illustrates one of the most important points when thinking of climate change and health. The fatal flu-like disease takes its name from the region in northwest New Mexico in which it was discovered. When the epidemic was first noticed in 1993, the way it killed caused widespread panic.Victims in the prime of life suffocated when their lungs suddenly filled with fluid.

The link between a warmer ocean and a rat-borne disease

Scientists were baffled as to its cause until an ecologist studying mice at the University of Mexico made the connection between unusually wet weather and the rodent population. The year of the outbreak and the previous one had been El Niño years, when the Pacific Ocean off the coast of South America warms up, disrupting weather patterns and showering the southwestern United States with rain. The result had been more food for rodents, leading to a higher population of mice, and the spread of a hitherto unnoticed virus.

For rich countries like the United States, it's these types of unexpected impacts that will pose the greatest challenge. After all, areas like Florida already provide conditions favorable to malaria, but the disease is kept in check through mosquito control, health care, and preventive devices like screen doors and air conditioners.

While rising temperatures may boost mosquito populations, the parasite will likely be contained through the redoubling of these efforts. What will be harder to plan for is what's unexpected-sudden sparks like the one in New Mexico. Outbreaks find opportunity in times of uncertainty. Diseases are easier to fight when you know when and where they'll hit.

Thus, reporters writing on climate change should keep their eyes on the unexpected.

One of my favorite examples is the story of the spruce bark beetle and its effects on human health. In North America, a series of mild winters in the 1990s facilitated a boom in the population of the insect-normally killed off during hard frosts-at a time when droughts were weakening the trees on which they fed. The result was a massive die off of trees, followed by waves of forest fires, and-in communities downwind-a reported rise in cases of asthma, chest pain, and bronchitis.

It's important not to forget that the biggest health impacts from climate change may have little to do with disease. Global warming will knock the world off balance. It has the potential to cause stronger storms, food shortages, even wars.

If climate change leads to longer droughts, will that lead to malnutrition? How will the sick reach hospitals in Northern communities if melting permafrost ruins the roads? Assuming for a moment that Hurricane Katrina was to some extent fueled by climate change, how do you measure its effects on human health?

While clear-cut connections between warmer weather and disease, such as the one in Castiglione di Cervia, are important to report, it's imperative to remember that the impacts from global warming on health will come from many directions.

Stephan Faris is the author of Forecast: The Consequences of Climate Change, from the Amazon to the Arctic, from Darfur to Napa Valley, published by Henry Holt & Co. in 2008.

Photo credit: Takver, 2009