Temperature measurements over the past century show a slight warming trend on Earth, and most reputable climate scientists agree that the warming trend will increase dramatically in the coming century.
In 2001 the UN's authoritative Intergovernmental Panel on Climate Change reviewed published results of computer models and concluded that, by the end of this century, average temperatures could rise by as much as 5.8 degrees (Celsius), though most projections were in the range of a one to three degree increase.
"We know that severe heat waves kill people. So do extremes of cold," says Anthony McMichael, who explores the implications of a warmer planet on human health in the British journal The Lancet. "We know that a lot of infectious diseases are sensitive to temperature, to humidity, to rainfall, particularly the ones that are spread by mosquitoes, like malaria, dengue fever and yellow fever," he says. "We also know that as the world warms, the weather patterns are likely to become much more variable around the world and therefore populations are going to be exposed to an increased frequency and increased intensity of extreme weather events."
McMichael, who heads the National Center for Epidemiology and Population at the Australian National University at Canberra, says that could lead to more cyclones, storms, floods and droughts, all of which have consequences for human health.
He points out that the impact of extended heat waves has already been observed in numerous places. He cites examples in Alaska, Canada, and Australia, and the unusually high death rate in Western Europe brought on by a heat wave in 2003. "It was associated with an extraordinary excess of deaths during and immediately after the heat wave, a total of about 35,000 excess deaths, mostly in France, Italy and Spain, but throughout western Europe, and including England."
According to historical records, that deadly heat wave was a once-in-four-centuries event. But forecasters say this scenario could be repeating itself every 4 years by 2050, if current trends continue.
Climate scientists see a warming trend behind the reduction in Arctic sea ice, and the retreat of mountain and land glaciers. McMichael notes a worrisome increase in correlation with the rising temperatures, the emergence of infectious disease.
"That includes malaria in Africa, tick-born encephalitis in Sweden, patterns of cholera in Bangladesh, which come and go with changing temperatures of coastal waters, and a recent report of an increased annual frequency of food poisoning off the north coast of Alaska from bacterially-contaminated oyster beds known to be very temperature dependent," he says.
McMichael, who contributed to the report by the United Nations Intergovernmental Panel on Climate Change, says scientists from various disciplines have begun to collaborate on climate issues to predict, for example, the path of malaria in a warmer world.
"We want to obtain from them their best models of how patterns of temperature and rainfall will change by gridded location around the world," he says, adding that data will be connected "with our own biological models that express what we know about how temperature, rainfall, and humidity affect the biology of the mosquito and the biology of the malaria parasite. And from that, [we would] work out approximately the effects on future patterns of malaria."
McMichael says human pressure, especially the growing volume of human-produced greenhouse gas emissions in the atmosphere, is weakening the world's life support systems. "It is not just a matter of economic inconvenience, or loss of environmental amenities or risks to the physical built infrastructure in and around cities," he says. "It is, in the long term, a risk in our capacity to remain healthy and, in many parts of the world, [to] actually survive."
Anthony McMichael believes that increased public education, community support and advance policy planning, especially among the poorest and most vulnerable communities, will be essential to mitigating the health effects of climate change before it is too late.