Global warming may cause human health problems due to microbes, bacteria, and toxic algae blooms in coming decades, new research suggests.



Scientists had already predicted more deaths and illnesses due to heat waves, natural disasters, and the expansion of tropical diseases such as malaria.

But other, less obvious health threats will likely arise as warming changes the ecology of ocean and freshwater environments in coming decades, experts said in February at an American Association for the Advancement of Science meeting in Washington, D.C.

Several of the scientists had been funded by the Oceans and Human Health Initiative of the National Oceanic and Atmospheric Administration (NOAA), which aims to identify and manage hidden health risks of global warming.

1. Toxic Algae May Contaminate More Seafood



Blooms of one kind of "red tide" algae—which create ocean dead zones—may be more frequent as the world warms, especially in North America's Puget Sound region (map), according to Stephanie Moore of NOAA's West Coast Center for Oceans and Human Health.

The Alexandrium catenella algae species produces a poison that can accumulate in seafood and subject humans to everything from vomiting to muscle paralysis to, in rare cases, death.

Moore and colleagues at the University of Washington created localized computer models that depicted future ocean and weather patterns. The group identified environmental conditions that are ideal for algae, particularly water temperature. By forecasting how oceans might warm, for example, the models predicted how climate change would change the algae's growth patterns down the road.

"We found that, not only will the risk for toxic blooms increase within the present-day bloom season, which is typically between July and October, but the bloom season itself will also expand," Moore said.

"We could see blooms beginning up to two months earlier in the year, compared to what we've seen historically, and they may also last up to one month later in the year."

As temperatures rise, some warmer-water algae species may also expand into traditionally cooler waters. Other spots currently suitable for blooms could simply become too warm for the tiny plants.

Likewise certain harmful algal species might no longer grow at all in the peak of summer, she added. For these species, "instead of having just one bloom season in later summer and early fall, we might start seeing two smaller seasons—one in spring and one in late fall."

Such shifts could temporarily close down shellfish fisheries due to safety concerns, and overall create more problems overall for those who try to keep the human food supply safe.

(Read about how we're running out of seafood in National Geographic magazine.)

"If they aren't testing for a certain toxin because it's never occurred in a place [or time], there is a potential for contaminated seafood to get to the public," Moore said.

"So it's really going to call for increased vigilance," she added. "We hope a benefit from studies like this might be to better prepare managers for the different blooms that might occur."

2. Harmful Ocean Bacteria Could Explode



Many climate change models suggest wet regions will get wetter and dry ones will become even drier.

This would increase desertification and the amount of dust kicked into the atmosphere from places like western Africa. (Related: "Africa-wide 'Great Green Wall' to Halt Sahara's Spread?")

Much of that dust ends up in the ocean, where researchers have found it supercharges the growth of harmful bacteria that can end up in seafood.

For instance, the University of Georgia's Erin Lipp and graduate student Jason Westrich have found that dust from Moroccan deserts can spark rapid growth of Vibrio, a genus of ocean bacteria. If ingested via contaminated seafood or water, the microbes can cause gastrointestinal infections and infectious diseases such as cholera.

For their experiment, the scientists took dust collected from Morocco, added it to seawater taken from the Florida Keys. Within 24 hours Vibrio growth jumped up to a thousandfold.

The scientists discovered that iron in the dust was stimulating most of the growth, Westrich noted.

Iron, a necessary ingredient for many life-forms, is in relatively short supply in seawater. That's why dumping iron into oceans to promote the growth of carbon dioxide-absorbing plankton, considered a global warming fix—though a controversial one. (See related pictures: "Seven Emergency Climate Fixes.")

Seafood-illness statistics also suggest that Vibrio poisoning in the U.S. has jumped 85 percent since 1996.

No one knows if iron in desert dust has played a role, but Lipp and Westrich hope to find out.

"In the near term, we're really trying to focus in on what's happening in the ocean water during one of these big dust events," Lipp said.

3. More Sewage May Taint Drinking Water

At the University of Wisconsin-Milwaukee, Sandra McLellan has used modeling to explore how increased rainfall from climate change is projected to impact her state and some already stressed sewer systems around the Great Lakes (see map).

In Wisconsin, urban growth has exceeded the capacity of many sewer systems. During severe storms, sewer systems can overflow into the lakes—potentially introducing disease-inducing bacteria and viruses into drinking-water supplies.

Only 1.7 inches (4.3 centimeters) of rain in 24 hours can cause such overflows, which occur in Milwaukee about three times a year and more often in some Great Lakes cities.

Climate models predict that spring rains in Wisconsin are likely to increase over the next half century, and, in a worst-case scenario, raise the volume of stormwater overflowing into the lakes by as much as 20 percent, McLellan said. While her overflow analysis was specific to Milwaukee, the research provides a model that may be used by other cities around the Great Lakes and beyond, she added.

"It's not that climate change is creating a new problem," she explained. "We already have a big problem of combined sewage overflows. We're making investments to try to reduce them, but we may be outpaced by climate change and by development."