Global Warming

Global warming increasing spread of dead zones in oceans, rivers

Global warming is likely playing a bigger role than previously thought in dead zones in oceans, lakes and rivers around the world and it's only going to get worse, according to a new study.

Dead zones occur when fertilizer runoff clogs waterways with nutrients, such as nitrogen and phosphorous. That leads to an explosion of microbes that consumes oxygen and leaves the water depleted of oxygen, harming marine life.

Scientists have long known that warmer water increases this problem, but a new study Monday in the journal Global Change Biology by Smithsonian Institution researchers found about two dozen different ways — biologically, chemically and physically — that climate change worsens the oxygen depletion.

Warmer water holds less oxygen, adding to the problem from runoff, but warmer water also affects dead zones by keeping the water more separate, so that oxygen-poor deep water mixes less.

When the water gets warmer, marine life's metabolism increases, making them require more oxygen just as the oxygen levels are already dropping. Other ways that climate change affects dead zones includes longer summers, ocean acidification and changing wind and current patterns, the study said.

A duck swims in Lake Michigan's Green Bay near an accumulation of algae. The bay is one of the many bodies of water that have developed dead zones, areas where fertilizer and wastewater runoff has created excessive levels of nutrients that build up microbes but deprive marine life of oxygen.