WHEN WE THINK about global warming, we might fret about rising sea levels, higher temperatures, and melting Mount Hood glaciers. Here's what we don't consider, but probably should: never-ending allergies, more cases of asthma, and even malaria.

And now local public health officials and environmental advocates—sounding the alarm and working on ways to deal with all that bad-health fallout—say there may be nothing we can do to stop it.

"Global warming is probably the biggest public health threat of the 21st century," says Maye Thompson, environmental health program director for Oregon's chapter of Physicians for Social Responsibility.

Thompson paints a bleak picture. Hotter summer months could be lethal for elderly Portlanders. Allergy seasons will last longer and intensify. More days with air pollution will cause, or worsen, heart disease and respiratory illnesses like asthma.

Weather will become more "severe." That means more heat waves in the summer, and heavier rain or snow in the winter. Lingering floodwaters would become a breeding ground for waterborne illnesses like cholera.

Then there's the small slice of the region's mosquito population known to carry nasty germs like West Nile virus, encephalitis, and malaria.

Malaria was once a fact of life in Portland, especially throughout the 19th and early 20th centuries. Cases were recorded as late as the 1930s, when insecticides and the dredging of marshes reduced the mosquito population.

"This idea of more diseases carried by mosquitoes," Thompson says, "that kind of thing could come back."

Two species of the Anopheles mosquito, the genus of mosquito that transmits malaria, remain in Multnomah County, making up about 6 percent of the total population, says Multnomah County vector control specialist Carl Pierce.

And a species native to Asia but first discovered here in 2006, Ochlerotatus japonicus, has also established itself, Pierce says. The newcomer bites during the day, not just at night, and is known to transmit West Nile virus and dengue fever. The Anopheles species, also present in Multnomah County, transmits malaria.

"Having another species that can transmit viruses, and is a day-biter, kind of increases the potential for disease transmission," Pierce says. For now, the weather is too cool for that to happen. But if temperatures heat up even slightly, he thinks the mosquito population would increase.

That global warming will cause health problems is beyond doubt, says Stacy Vynne, the adaptation and preparedness program director at the University of Oregon's Climate Leadership Initiative.

But no one really knows how severe the problems will be.

"Right now, we have generalizations like 'asthma might get worse' or 'pulmonary disease might get worse,'" says Kari Lyons-Eubanks, a policy analyst in Multnomah County's environmental health office.

But, Thompson, Vynne, and others say it may not take so long, either. "In terms of really vulnerable parts of the globe, like the Arctic, changes predicted for 2050 are already happening," Thompson says.

In 2010, the Centers for Disease Control split a $5.2 million grant between 10 states, including Oregon, to tackle the health effects of global warming.

Lyons-Eubanks is creating a detailed map of Multnomah County that will show, down to the street level, climate forecasts, air quality trends, pollen counts, tree canopies, and other elements that affect our climate. The map, expected to take a year, will also delineate health disparities and show where people of different socioeconomic classes, ethnicities, and health problems live.

Officials will then use the data to make what changes they can—seeking out heat pockets, for example, and then swapping pavement for trees when possible.

It doesn't sound like much. But even small changes can help, especially if they're all a community can do.

"It gets down to 'turn off your lights' and 'don't drive as much,'" Thompson says. "Totally unsexy."

This article has been corrected since it's original version. The diseases transmitted by a mosquito species were initially incorrect.