Depending on where you are, this is going to be a hotter, wetter, drier, windier, calmer, dirtier, buggier or hungrier century than mankind has seen in a while. In some places, it may be deadlier, too.
The effects of climate change are diverse and sometimes contradictory. In general, they favor instability and extreme events. On balance, they will tend to harm health rather than promote it.
That is the majority view of scientists trying to solve an equation whose variables range from greenhouse gas concentrations and the El Niño weather pattern to mosquito ecology and human cells' ability to withstand heat.
"We are not dealing with a single toxic agent or a single microbe where we can put our finger with certainty on an exposure and the response," said Jonathan A. Patz, a physician and epidemiologist at the University of Wisconsin at Madison. "Climate change affects everything."
Predictions of how global warming could affect people's health are crude. They are based on the experience of the past several decades, when there has been a small, well-documented rise in the temperatures of the planet's atmosphere and oceans. What that says about the future -- a time when warming is expected to accelerate, but people may be able to prepare for it -- is quite uncertain.
In the last quarter of the 20th century, the average atmospheric temperature rose by about 1 degree Fahrenheit. By 2000, that increase was responsible for the annual loss of about 160,000 lives and the loss of 5.5 million years of healthy life, according to estimates by the World Health Organization. The toll is expected to double to about 300,000 lives and 11 million years of healthy life by 2020.
The biggest tolls were in Africa, on the Indian subcontinent and in Southeast Asia. Most of that increased burden of death and disease was from malnutrition, diarrhea, malaria, heat waves and floods. But those diseases will play a minor role, at best, in many regions that nevertheless will feel the effects of global warming.
To organize their thinking -- and to focus the attention of policymakers -- researchers tend to put the health effects of climate change into five groups.
The most obvious effect of global warming is hotter weather.
Scientists predict that heat waves will be longer and more frequent in the future. Their worst-case effects may have been glimpsed in Europe's summer of 2003, the hottest spell there since the 1500s. About 30,000 people died of heat-related illness, including 14,800 in France in three weeks in August.
People who were old, very young, ill, immobile or poor were at highest risk. Although the human body can adapt somewhat to chronically higher temperatures, those groups will remain vulnerable -- and they are likely to make up a bigger slice of the population in the future.About 20 percent of people in industrialized countries are over age 60 today. That figure will rise to 32 percent by 2050. More people will also live in cities -- 61 percent of the world's population by 2030, compared with 45 percent now. Cities are "heat islands," 9 degrees Fahrenheit warmer on average than surrounding rural areas and resistant to the cooling effects of night.
Aging and urbanization -- and possibly more obesity -- will put people at greater risk for heat-related illness. Nevertheless, that consequence of global warming may be easier to avoid than others, as a study published three years ago suggests.
It examined mortality on hot days in 28 cities in the last third of the 20th century. Death rates were lower in the 1980s and 1990s than in the 1960s and 1970s in most places, with the least reduction in cities of the Northeast and the Midwest. (A heat wave in Chicago in 1995 caused more than 500 deaths, the biggest U.S. toll in years.)
This steady decline in heat-stress death was almost certainly the consequence of air conditioning, better awareness of the problem and improved medical care.
"If there is a very effective response system, then even in hotter temperatures you may not see more deaths," said Kristie L. Ebi, an epidemiologist and consultant in Alexandria. She helped write the health chapter of the most recent report of the United Nations' Intergovernmental Panel on Climate Change, which was this year's Nobel Peace Prize winner along with Al Gore.
Climate change is expected to increase the severity of storms, especially ones associated with cyclical events such as the El Ni¿o Southern Oscillation.
Flooding is the most common weather disaster, responsible for the deaths of about 100,000 people and the displacement of 1.2 billion from 1992 to 2001. The worsening of this hazard will vary by region. It is expected to change little in Southeast Asia by 2030, but it may increase 50 percent in West Africa and quadruple in Central and South America.
In addition to storms, rising oceans threaten coastal populations. Of the world's 20 megacities, 13 are at sea level. Storm surges, while short-lived, can cause permanent damage, eroding land and damaging water supplies and cropland with saltwater.
When it comes to food production, climate change will have varying effects. Overall, it will tend to slow the long historical decline in the number of hungry people.
In 1990, there were 520 million people at risk of hunger, according to a study by British and American scientists published in 2005. In the absence of global warming, that number was predicted to fall to 300 million by 2080. With global warming, it is expected to fall to 380 million, although under various scenarios of greenhouse gas reductions it could drop to 320 to 340 million, according to recent mathematical modeling.
Climate change affects air pollution in two ways.Heat speeds chemical reactions and consequently may worsen pollution from ozone and airborne particulates, or soot. It may also spur pollen production by some plants, which could in turn worsen asthma and allergies in some people.
One model of global warming's effects on air pollution in 15 eastern U.S. cities predicts that the number of days exceeding ozone standards will rise from the current average of 12 to 20 per summer by 2050. Deaths linked to that pollutant -- nearly all in people who have lung or heart ailments -- could go up 5 percent under that scenario.
Waterborne and Food-Borne Disease
Higher temperatures and torrential rains are likely to cause outbreaks of some diarrheal diseases.
The incidence of cholera -- a bacterial infection whose home is South Asia but that circles the world in slow epidemics -- depends in part on water temperatures in the Bay of Bengal and on monsoon rains. A recent study of waterborne-disease outbreaks in the United States in the past 50 years found that 67 percent were preceded by heavy rainfall.
Researchers in Australia have shown that the number of food-borne infections from salmonella bacteria goes up in hot weather.
Overall, climate change is expected to increase the burden of diarrhea, mostly in developing countries, by 2 to 5 percent by 2020.
Scientists suspect that many diseases transmitted by insects and animals will become more common, although there is more uncertainty about this than other consequences of global warming.
Dengue and malaria, carried by mosquitoes, are most likely to increase. Under some projections, Africans will be exposed to malaria 25 percent more of the time in 2100 than they are now.
That risk, however, could be offset by controlling mosquitoes with pesticides, the use of bed nets by children and pregnant women, and better medical care.
Other diseases that may become more prevalent are yellow fever (also carried by mosquitoes), schistosomiasis (by snails), leishmaniasis (sand flies) and Lyme disease (ticks).
The Role of Planning
In the United States, most public discussion of global warming has been about ways to slow the phenomenon, and not about ways to dampen or prevent effects that are already inevitable.
"We are a good decade behind Europe in designing and developing adaptations that will decrease our vulnerability and increase our resilience," said Ebi, the epidemiologist.
Such planning is wise not only for the federal government and states, but for cities and towns as well, Ebi believes."The impacts of climate change really do depend on your local context," she said