Washington -- As the Earth warms, snow and ice melts and sea levels rise, the effects of climate change threaten more than the physical environment.

Unless warming trends are controlled, people throughout the world will face more injury, disease and death related to an increase in natural disasters and heat waves. People will experience higher rates of illnesses transmitted by food, water or vectors (insects or animals). Some will contract, and perhaps die from, diseases related to rising concentrations of air pollution.

Populations will be displaced by rising sea levels and affected by drought and famine. As glaciers melt, the hydrological cycle -- the continuous movement of water above, on and below Earth’s surface -- will shift and alter the productivity of farmable land.

This is not a science-fiction scenario. The World Health Organization (WHO), on a Web site dedicated to World Health Day (celebrated on April 7 each year) states, “We are beginning to be able to measure some of these effects on health even now.”

WHO estimates, for example, that by 2000 the global burden of disease from climate change was more than 150,000 excess deaths annually. The theme of World Health Day 2008 is “Protecting Health from Climate Change.”

CLIMATE AND HEALTH

Weather and climate have affected human health since the earliest people shivered with cold, baked in the sun or starved because vegetation was scarce.

“Injuries, displacement and death result from floods, hurricanes, tornadoes and forest fires,” Dr. Howard Frumkin and colleagues write in “Climate Change: The Public Health Response,” published in March in the American Journal of Public Health.

Frumkin directs the National Center for Environmental Health, part of the U.S. Agency for Toxic Substances and Disease Registry.

“An entire category of diseases -- the tropical diseases -- is named for a particular climate; climate and weather affect the distribution and risk of many vector-borne diseases such as malaria, Rift Valley fever, plague and dengue fever,” they write. “Weather also affects the risk of food-borne and water-borne diseases and of emerging infectious diseases such as hantavirus, Ebola hemorrhagic fever and West Nile virus.”

Recent examples of climate-related health effects include abnormally high temperatures in Europe in summer 2003 that were associated with 35,000 more deaths than the same period in previous years; a global toll from diarrhea, malaria and malnutrition of more than 3.3 million deaths in 2002; and 55,000 cases of infection by the mosquito-borne dengue virus over the past four months in Brazil, with nearly 70 deaths in Rio de Janeiro.

“I view climate change as one of the most serious health challenges,” Dr. Jonathan Patz, professor in the Department of Population Health Sciences at the Nelson Institute at the University of Wisconsin-Madison, told America.gov, “because it cuts across so many pathways that affect our health.”

Patz, a principal lead author for the U.N. Intergovernmental Panel on Climate Change Assessment reports since 1995, said changing climate affects several infectious diseases, many of which are carried by insects.

“Just a tiny change in temperature can affect the transmission cycle and development time of parasites inside these cold-blooded insects,” he said, “which is why one or two degrees of temperature rise, even half a degree, can have a tremendous influence in the transmission of malaria, for example. The parasite develops much faster inside the mosquito, temperature can change biting rates, and there are all sorts of amplifying factors when you deal with a biological system like a mosquito-borne disease.”

PUBLIC HEALTH RESPONSE

There is little doubt among experts that climate change is real and will affect lives around the world for years, perhaps centuries, to come.

“There is vast consensus in the scientific community that the planet is warming and that man has a demonstrable impact on that,” Antonio Busalacchi, director of the Earth System Science Interdisciplinary Center at the University of Maryland-College Park, told America.gov.

The only uncertainty, he added, is in taking the global-scale metrics of climate change down to the regional scale, at the scale of small countries or counties or states.

“The uncertainties essentially deal with on what spatial scales are our projections of temperature and precipitation valid and, with respect to warming, how warm and how fast?” Busalacchi said. “The field is going to reduce those uncertainties so we have improved and enhanced confidence at these regional scales.”

Health implications of climate change also will be local and regional in scale, Frumkin told America.gov, and each part of the world will have its own set of problems.

“Agricultural productivity is expected to be a problem in sub-Saharan Africa and south Asia but not so much in North America,” he said. “Heat waves are expected to be a problem in the northern tier of North America but not so much in the first two regions I mentioned. That means forecasting and predicting and preparedness all need to take place on a local-to-regional scale.”

The public health approach begins with data collection on a range of variables -- climate, ecosystem, severe weather, mosquito infectivity and others. Disease surveillance, research into modeling and forecasting methods, preparedness planning, outbreak investigation and training are all public health functions that are important in addressing climate change.

“These are all existing tools in the public health toolbox,” Frumkin added. “There’s nothing radically innovative here but we do need to be undertaking this work on a different scale than we have before and with a wide range of problems perhaps broader than we ever have before.