I doubt geoengineering will ever be practical as a primary strategy for dealing with climate change (see here and here). That said, I don’t consider most of the efforts to pull CO2 out of the air geoengineering — that is ungeoengineering our self-inflicted climate wound. And those efforts are only plausible with super-aggressive mitigation that keeps concentrations close to 450 ppm.

It’s strategies like injecting sulfur into the atmosphere that should worry people the most. Those strategies have many flaws, but among the worst is that they do nothing to stop humanity from turning the oceans into one giant acidic deadzone.

A new study in Nature Geoscience, ($ub. req’d, abstract below) makes crystal clear why very serious mitigation must always be humanity’s primary strategy for averting climate catastrophe. As AFP reported on the study:

Global warming may create “dead zones” in the ocean that would be devoid of fish and seafood and endure for up to two millennia …

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Its authors say deep cuts in the world’s carbon emissions are needed to brake a trend capable of wrecking the marine ecosystem and depriving future generations of the harvest of the seas.

Precisely. This study makes a matching pair with NOAA stunner: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe.

Even worse, of course, is that while there are many plausible, albeit expensive and untried on large scale, strategies for removing CO2 from the atmosphere, it is far from clear how one does that from the ocean.

Here is more detail on this important study and on oceanic dead zones:

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In a study published online by the journal Nature Geoscience, scientists in Denmark built a computer model to simulate climate change over the next 100,000 years.

At the heart of their model are two well-used scenarios which use atmospheric levels of carbon dioxide (CO2), the main greenhouse gas, as an indicator of temperature rise.

Under the worst scenario, CO2 concentrations would rise to 1,168 parts per million (ppm) by 2100, or about triple today’s level.

[This worst-case scenario is, of course, just the business-as-usual emissions scenario, see here.]

Under the more optimistic model, CO2 would reach 549 ppm by 2100, or roughly 50 percent more than today.

The temperature rise that either would yield depends on several factors: when the peak in carbon emissions is reached and how quickly it falls, and whether the warming unleashes natural triggers, or tipping points, that enhance or prolong the warming in turn.

Taking such factors into account, the scientists predict a possible rise of around five to seven degrees Celsius over pre-industrial times under the worst scenario. Under the other scenario, there would be warming of roughly between two to four degrees Celsius.

Either scenario spells bad news for the ocean, said Jens Olaf Pepke Pedersen, a physicist at the Technical University of Denmark.

Under the worst scenario, warmer seas and a slowdown of ocean circulation would lower marine oxygen levels, creating “dead zones” that could not support fish, shellfish and other higher forms of marine life — and may not revive for 1,500 to 2,000 years.

“They would start slowly by the end of this century, it’s not something that would happen tomorrow or in the near future but over the next few generations,” Pedersen told AFP.

“But because of the inertia in the ocean, once you have the process going, it’s not feasible to reverse it again just like that, so it would continue for hundreds of years …

Wide oxygen depletion of the ocean, though, poses a far greater threat, touching at the heart of biodiversity, the paper warns.

Around 250 million years ago, a chemical change of the seas led to a massive wipeout of marine species.

Lead scientist Gary Shaffer of the Niels Bohr Institute at the University of Copenhagen said it was unclear, in the grim light of this study, whether future generations could look to the oceans as a major reserve of food.

“Reduced fossil-fuel emissions are needed over the next few generations to limit ongoing ocean oxygen depletion and acidification and their long-term adverse effects,” he said.

Here is the study abstract:

Ongoing global warming could persist far into the future, because natural processes require decades to hundreds of thousands of years to remove carbon dioxide from fossil-fuel burning from the atmosphere. Future warming may have large global impacts including ocean oxygen depletion and associated adverse effects on marine life, such as more frequent mortality events, but long, comprehensive simulations of these impacts are currently not available. Here we project global change over the next 100,000 years using a low-resolution Earth system model, and find severe, long-term ocean oxygen depletion, as well as a great expansion of ocean oxygen-minimum zones for scenarios with high emissions or high climate sensitivity. We find that climate feedbacks within the Earth system amplify the strength and duration of global warming, ocean heating and oxygen depletion. Decreased oxygen solubility from surface-layer warming accounts for most of the enhanced oxygen depletion in the upper 500 m of the ocean. Possible weakening of ocean overturning and convection lead to further oxygen depletion, also in the deep ocean. We conclude that substantial reductions in fossil-fuel use over the next few generations are needed if extensive ocean oxygen depletion for thousands of years is to be avoided.

Any geoengineering strategy that lets the oceans die is no cure at all.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.