Pathogens that have been restricted by seasonal temperatures can invade new areas and find new victims as the climate warms and winters grow milder, researchers say in a study in the journal Science.
"Climate change is disrupting natural ecosystems in a way that is making life better for infectious diseases," said Andrew Dobson, a Princeton University researchers and another co-author of the study in Science. "The accumulation of evidence has us extremely worried. We share diseases with some of these species. The risk for humans is going up."
Climate changes already are thought to have contributed to an epidemic of avian malaria that wiped out thousands of birds in Hawaii, the spread of an insect-borne pathogen that causes distemper in African lions, and the bleaching of coral reefs attacked by diseases that thrive in warming seas.
Humans are also at direct and dramatic risk from such insect-born diseases as malaria, dengue and yellow fever, the researchers said.
"In all the discussions about climate change, this has really been kind of left out," said Drew Harvell, a Cornell University marine ecologist and lead author of the study. "Just a one- or two-degree change in temperature can lead to disease outbreaks."
Richard S. Ostfeld, a co-author of the study, said, "We're alarmed because in reviewing the research on a variety of different organisms we are seeing strikingly similar patterns of increases in disease spread or incidence with climate warming." Ostfeld is an environmental researcher at the Institute of Ecosystem Studies.
In the study, the authors analyzed how warming temperatures already are letting insects and microbes invade areas where they once were barred by severe seasonal chills. They said mosquitoes are moving up mountainsides, spreading disease among animals formerly protected by temperature. They also found some pathogens reproduce more often in warmer temperatures, so there are more germs around to cause infection.
Among the possible effects they found:
Epidemics of Rift Valley fever, a deadly mosquito-borne disease, rage through northeastern Africa during years of unusual warmth. If the climate becomes permanently warmer and wetter, as some predict, Rift Valley fever epidemics will become frequent.
Malaria and yellow fever may become more common as milder winters permit the seasonal survival of more mosquitoes, which carry these diseases. A warmer climate also could enable them to move into areas where the cold once kept them out.
In Hawaii, a warming climate has chased the chill from some mountains, letting mosquitos thrive at higher and higher elevations. The bugs have carried with them a type of avian malaria, and the disease has attacked native birds that had no immunity to the disease.
"Today there are almost no native birds (in Hawaii) below 4,500 feet," (1,350 meters)(Dobson said in an interview.
Coral reefs in many parts of the world are becoming bleached and dying, killed by pathogens that thrive in the warming seas.
"Previously many of the waters were slightly below the optimal temperatures for these pathogens," said Ostfeld. "Now the temperatures are right on target. There is a strong link between the warming climate and diseases of corals."
Germs that attack oysters also are thriving in the warming waters. Ostfeld said oyster beds as far north as Maine are now being affected by pathogens once barred by a colder sea.
An outbreak of distemper killed many lions in Tanzania last year, and the scientists linked that to a climate change that enables flies that carry distemper to invade parts of East Africa.
A parasite that kills Monarch butterflies can survive only at warm temperatures, which protected the colorful insect in its northernmost habitats. A warming climate has allowed the parasite to spread. Ostfeld said where the Monarch is rare "it may disappear, and where it is common, it may become less abundant."