Water Thinking: Climate Change isn’t just a land and air problem.

I’m at the Ocean Sciences meeting in New Orleans this week, tweeting (@explomary #OSM16) and getting caught up on all things ocean. The conference opened on Monday night with a talk by Lisa Levin of Scripps Institute. Lisa spends a lot of time at sea studying the impacts of ocean change on deep-water marine species and ecosystems. You may already know that excess carbon dioxide in the atmosphere is absorbed in the ocean, changing seawater chemistry and making life more difficult for shell-building creatures. Dr. Levin didn’t talk much about that--there are multiple sessions here on coral reefs and ocean acidification. Instead she wanted to raise awareness about the decline of oxygen in the world’s oceans, a response to climate change that is less well known.

For billions of years, oxygen has been the key to life on earth (and in the oceans). Every second breath we take comes thanks to oxygen produced by marine plankton, which also support marine ecosystems. But the oxygen is not evenly distributed in the ocean, there are naturally-occurring oxygen-minimum zones (OMZ) typically occurring at depths between 100 and 1000 meters, where oxygen saturation is 30% or less. Organisms that live in the OMZ are adapted to low oxygen but most species that live near the surface avoid these regions of the ocean.

On coastal margins, like the west coast of North and South America wind-driven spring upwelling brings these low-oxygen waters to the surface. That can be a good thing, as the deeper waters also contain nutrients that phytoplankton need to grow and reproduce which in turn feed zooplankton and up the food web to fish, birds and even whales. But if the low oxygen water sticks around too long, the ocean can become hypoxic and drive life away from the zone. Scientists have been seeing an expansion of these hypoxic zones from such far-flung regions as Chesapeake Bay, the Gulf of Mexico, and the bottoms of San Francisco Bay and the East China sea.

Photo of Lisa Levin’s slide showing the decline of oxygen in multiple world oceans, seas and bays.

Data about the oxygen levels of the ocean is hard to come by but there are places where long-term records are available and they show a decline in dissolved oxygen saturation along the west coast from Alaska to Southern California and these low oxygen areas are expanding globally.

Now, Lisa tells us this ocean “deoxygenation” (sometimes referred more graphically as “ocean suffocation”) is a problem not just in hypoxic and upwelling zones but is spreading thanks to the warming of the ocean caused by climate change. This affects the metabolism of fish, reducing their body size, reproduction and survival rates and constraining where they can live. It’s not just a problem for fish; low oxygen can be lethal to less mobile marine creatures such crustaceans, snails and bivalves (oysters, clams and scallops) that have a harder time escaping the hypoxic waters.

Why is this happening? It’s largely because the ocean is warming, a response to climate change. A warming ocean holds less dissolved gas, something well known to people who have opened a warm can of soda. Over the past century, sea surface temperatures have increased about 1 degree F. That may sound small, but Lisa put it in perspective: that’s the equivalent heat caused by the detonation of five atomic weapons a second absorbed by the global ocean.

Runoff from land also contributes to oxygen depletion and it's occurring in an ocean that is being impacted by ocean acidification, pollution and overfishing. One thing we can do to help ocean ecosystems is reduce carbon emissions and other ecosystem stressors. But Lisa says we also need to increase observations and work together scientifically to understand and acknowledge the problem.