Global warming could deplete ocean oxygen, with severe consequences – ‘This inexorable force of human-induced warming will clearly result in widespread ocean deoxygenation in the future’

Desdemona Despair

[cf. Graph of the Day: Simulated catastrophic decline of plankton in warming oceans]

By Chris Mooney
28 April 2016

(Washington Post) – In the long list of troubling climate change scenarios, there’s one that gets relatively little attention, but definitely has enormous potential consequences.

It goes like this:

The oceans are getting warmer — they are, after all, where 90 percent of global warming actually ends up. And when they warm up they expand, because that’s what warm water does. This raises our sea levels, but it also has another effect — it reduces the amount of oxygen dissolved in the water. That’s simply physics: Warmer water contains less oxygen.

But it’s worse: If surface water is warmer, it doesn’t mix down as much into the ocean depths any longer. It’s less dense, and so less capable of doing that. That means that oxygen that enters the ocean in its upper layers — either through exchange with the atmosphere, or because it is generated by tiny photosynthesizing microorganisms, called phytoplankton, that hang out up there — won’t mix down into the deep as often.

“What’s happening is, there’s a physical mechanism that impedes the delivery of surface waters into the interior,” said Matthew Long, an oceanographer with the National Center for Atmospheric Research who is lead author of a troubling new study on what scientists call the “deoxygenation” of the oceans. The work appeared in Global Biogeochemical Cycles, co-authored with Curtis Deutsch of the University of Washington and Taka Ito of the Georgia Institute of Technology.

The problem is that marine life needs oxygen. If there’s less of it, that could expand the number of areas sometimes called “oxygen minimum zones” where plants, fish, and other organisms would struggle to survive.

Now, in the new study, Long and his colleagues have found that some parts of the ocean are already likely showing an oxygen deficiency, due to the effects of global warming. And by around the year 2030, their model suggests, the human role in driving widespread ocean oxygen loss will be even more apparent if greenhouse gas emissions continue unchecked.

“Its fairly widespread detection….is basically evident in the 2030s to 2040s decade,” Long said. […]

“There is a lot of variability in ocean oxygen, and it is a detection problem,” Long said, “but this inexorable force of human-induced warming will clearly result in widespread ocean deoxygenation in the future.” [more]

Global warming could deplete the oceans’ oxygen – with severe consequences

ABSTRACT: Anthropogenically forced trends in oceanic dissolved oxygen are evaluated in Earth system models in the context of natural variability. A large ensemble of a single Earth system model is used to clearly identify the forced component of change in interior oxygen distributions and to evaluate the magnitude of this signal relative to noise generated by internal climate variability. The time of emergence of forced trends is quantified on the basis of anomalies in oxygen concentrations and trends. We find that the forced signal should already be evident in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins; widespread detection of forced deoxygenation is possible by 2030–2040. In addition to considering spatially discrete metrics of detection, we evaluate the similarity of the spatial structures associated with natural variability and the forced trend. Outside of the subtropics, these patterns are not wholly distinct on the isopycnal surfaces considered, and therefore, this approach does not provide significantly advanced detection. Our results clearly demonstrate the strong impact of natural climate variability on interior oxygen distributions, providing an important context for interpreting observations.

Finding forced trends in oceanic oxygen