Global warming starting to rob oceans of oxygen

New research detects climate change pattern

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New climate study pinpoints ocean deoxygenation. @bberwyn photo

Staff Report

Deoxygenation caused by global warming is already detectable in some of the world’s warmest ocean areas, climate scientists said last week, announcing the results of a new study that shows how increasing global temperatures will play out.

Based on their modeling, the researchers said they could detect the influence of human-caused climate change in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins. They expect to be able to detect similar signs in many other ocean areas between 2030 and 2040.

The long-term effects of deoxygenation caused by global warming won’t be pretty. A press release announcing the results of the study says that scientists are expecting a warming climate to sap oceans of oxygen, leaving fish, crabs, squid, sea stars, and other marine life struggling to breathe. But they had encountered difficulties in determining whether this anticipated oxygen drain was already having a noticeable effect.

“Loss of oxygen in the oceans is one of the serious side effects of a warming atmosphere, and a major threat to marine life,” said Matthew Long, scientists with the National Center for Atmospheric Research in Boulder, Colorado. The study is published in the American Geophysical Union journal Global Biogeochemical Cycles. The research was funded by the National Science Foundation (NSF).

“Since oxygen concentrations in the ocean naturally vary depending on variations in winds and temperature at the surface, it’s been challenging to attribute any deoxygenation to climate change. This new study tells us when we can expect the effect from climate change to overwhelm the natural variability,” Long said.

To explain their findings, the scientists described the basic mechanisms of ocean oxygenation, explaining that the entire ocean gets  its oxygen supply from the surface, either from the atmosphere or from phytoplankton, which release oxygen into the water through photosynthesis.

But warmer surface waters absorb less oxygen. And whatever oxygen is absorbed by warmer waters can’t penetrate deeper down as easily because warmer water forms a cap on the ocean that inhibits natural vertical mixing.

The modeling also enabled the scientists to create maps showing where ocean oxygen levels are affected by global warming. The climate change pattern became evident in the model runs around 2030, adding confidence to the conclusion that widespread deoxygenation due to climate change will become detectable around that time.

The maps could also be useful resources for deciding where to place instruments to monitor ocean oxygen levels in the future to get the best picture of climate change effects. Currently, ocean oxygen measurements are relatively sparse.

“We need comprehensive and sustained observations of what’s going on in the oceans to compare with what we’re learning from our models, and to understand the full effect of a changing climate,” Long said.

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