Oysterman Jim Aguiar had never had to deal with the bacterium Vibrio parahaemolyticus in his 25 years working the frigid waters of Prince William Sound. The dangerous microbe infected seafood in warmer waters, like the Gulf of Mexico. Alaska was way too cold. But the sound was gradually warming. By summer 2004, the temperature had risen just enough to poke above the crucial 59-degree mark. Cruise ship passengers who had eaten local oysters were soon coming down with diarrhea, cramping and vomiting — the first cases of Vibrio food poisoning in Alaska that anyone could remember. "We were slapped from left field," said Aguiar, who shut down his oyster farm that year along with a few others.

As scientists later determined, the culprit was not just the bacterium, but the warming that allowed it to proliferate. "This was probably the best example to date of how global climate change is changing the importation of infectious diseases," said Dr. Joe McLaughlin, acting chief of epidemiology at the Alaska Division of Public Health, who published a study on the outbreak. The spread of human disease has become one of the most worrisome subplots in the story of global warming. Incremental temperature changes have begun to redraw the distribution of bacteria, insects and plants, exposing new populations to diseases that they have never seen before.

A report from the World Health Organization estimated that in 2000 about 154,000 deaths around the world could be attributed to disease outbreaks and other conditions sparked by climate change. The temperature change has been small, about 1.4 degrees Fahrenheit over the last 150 years, but it has been enough to alter disease patterns across the globe.

In Sweden, fewer winter days below 10 degrees and more summer days above 50 degrees have encouraged the northward movement of ticks, which has coincided with an increase in cases of tick-borne encephalitis since the 1980s. Researchers have found that poison ivy has grown more potent and lush because of increased carbon dioxide in the atmosphere.

In Africa, mosquitoes have been slowly inching up the slopes around Mt. Kenya, bringing malaria to high villages that had never been exposed before. "It's going to get very warm," said Andrew Githeko, a vector biologist who heads the Climate and Human Health Research Unit at the Kenya Medical Research Institute in Kisumu. "That's going to mean a huge difference to malaria."

Githeko, 49, grew up in the central highlands in a tiny village near the town of Karatina, about 5,700 feet above sea level. His home was different from most of Africa. The air was damp and chilly. On clear days, he could see the glaciers on Mt. Kenya, the second-highest peak in Africa at 17,058 feet. When he was a child, lowland diseases like malaria were unknown in Karatina. But perhaps 10 years ago, a smattering of cases began to appear.

He had long ago left his home to study the great plagues of Africa — Rift Valley fever, malaria, cholera and others. The appearance of malaria in the highlands, however, was a mystery worth returning home for. Githeko dispatched a colleague to collect mosquito larvae in puddles and streams around Mt. Kenya, some as high as 6,300 feet. Tests later identified some of the mosquitoes as Anopheles arabiensis, one of the species that carry malaria.

Githeko's findings, published in 2006, marked the highest A. arabiensis breeding site ever recorded in Kenya and was the first published report of malaria infections in the central highlands, he said. He knew by watching Mt. Kenya's gradually disappearing glaciers that his world was warming, and that lowland diseases would eventually work their way higher. "But we did not expect this to happen so soon," he said.

Githeko's work has been echoed in a small number of studies around the world. In 1996, health authorities reported a human case of tick-borne encephalitis in the Czech village of Borova Lada, elevation 3,000 feet. Until then, the Ixodes rinicus tick, which carries the disease, had never been seen above 2,600 feet.

The case caught the attention of Milan Daniel, a parasitologist the Institute for Postgraduate Medical Education in Prague who has been studying the movement of ticks in the Czech Republic for half a century. He scoured the Sumava and Krkonose mountains and found that the ticks had migrated as high as 4,100 feet largely because of milder autumns over the last two decades, according to a series of studies published over the last four years.

From 1961 to 2005, the mean temperature in the Krkonose Mountains had increased about 2 1/2 degrees. "This shift of the ticks," Daniel said, "is clearly connected with climate changes." According to a landmark United Nations report released this month, global warming has reached a point where even if greenhouse gas emissions could be held stable, the trend would continue for centuries.

The report painted a grim picture of the future — rising sea levels, more intense storms, widespread drought. Predicting the future of disease, however, has proven difficult because of myriad factors — many of which have little to do with global warming. Diseases move with people, they follow trade routes, they thrive in places with poor sanitation, they develop resistance to medicines, they can blossom during war or economic breakdowns.

"No one's saying global warming is the whole picture here," said Dr. Paul R. Epstein, associate director of the Center for Health and the Global Environment at Harvard University. "But it is playing a role. As climate changes, it's projected to play an even greater role." In a Beltsville, Md., laboratory filled with bathroom-sized aluminum chambers, U.S. Department of Agriculture weed physiologist Lewis Ziska is peering into the future of one of the key components of global warming — rising carbon dioxide levels.

CO2 levels have been on the rise since the dawn of the Industrial Revolution more than 200 years ago. Today, they are at their highest point in more than 650,000 years. In the tightly sealed chambers, Ziska re-created pre-industrial conditions by turning down the concentration of carbon dioxide to 280 parts per million. In another box, he simulated the present with 370 parts per million. In a third box, he pumped up the carbon dioxide to 600 parts per million, the estimate for 2050.

Much of Ziska's work has centered on ragweed, a noxious plant that sets off allergy sufferers, such as Ziska himself. The weeds inside the tanks suck up carbon dioxide. "It's like feeding a hungry teenager," he said. Collecting yellow pollen in plastic bags fitted around the plants, Ziska found that current conditions produced 131% more pollen than pre-industrial conditions. Future conditions produced 320% more.

"For us weed biologists, this is the worst of times and the best of times," he said. The impact of global warming has not been all bad. Researchers recently found that rising temperatures have helped reduce some diseases related to cold weather. One British study found that the number of children infected with a cold-like virus known as respiratory syncytial virus has been declining with warming temperatures.

Combining meteorological data and emergency room admission rates from 1981 to 2004, physiologist Gavin Donaldson at University College London found each increase of 1.8 degrees clipped three weeks off the end the virus' winter season. "A small amount of warming can go a long way, as far as changing disease transmission dynamics," said Dr. Jonathan Patz, director of Global Environmental Health at the University of Wisconsin in Madison.

Given the gradual pace of warming, there are also some chances to adapt. After Prince William Sound's Vibrio outbreak in 2004, the state required more oyster testing in some areas. In the last two years, there have been only four cases of Vibrio food poisoning. Life in Aguiar's remote inlet has largely returned to the way it was before. This winter has been cold. Aguiar, a bear of a man with a riotous beard, huddled inside the houseboat for warmth recently as the temperature outside hovered around 20 degrees. The pale Northern Lights pulsed over the snow-laced Chugach Mountains, and skins of ice grew on the still water.

Come summer, Aguiar will start sending oyster samples to the state. When the temperature hits about 55 degrees, he'll drop his oyster baskets 60 or 100 feet in the water for about 10 days to clear out the bacteria. It's a solution he can live with in a warming world. "It's not all evil," he said. "I just don't like to see rapid change."
(Por Jia-Rui Chong, L.A.Times, 25/02/2007)