Recent research has found that under a business-as-usual emissions scenario, marine ecosystems are likely to experience “mass extinctions on par with past great extinctions.” The reason? Ocean warming and depleted levels of dissolved oxygen.
“Oxygen is a fundamental requirement, and there’s no substitute for it. The question then becomes how much oxygen is enough and what is the minimal amount that a given organism needs to survive,” said Curtis Deutsch, coauthor of an article published earlier this year in Science. Deutsch and his coauthor, Justin L. Penn, are researchers with both the Department of Geosciences at Princeton University and the School of Oceanography at the University of Washington.
Penn and Deutsch used existing data to better understand how much oxygen marine species need relative to how much is available at different activity and temperature levels. Using such data, the researchers built a mathematical model to predict thermal limits and their impacts on species. Deutsch said they sought to answer questions like “Would a given species be able to survive at minimal activity in a resting state?… Would it be able to be active enough to sustain and reproduce? And would it be able to survive in a state of maximum exertion?”
“As the temperature goes up, the ocean has less oxygen, but marine species need more oxygen,” explained Penn.
The warming data used by Penn and Deutsch were based on emissions scenarios laid out in the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Under a low-emissions scenario, in which the temperature stops rising at around 1.9°C of warming by the end of the century, their model predicted species losses consistent with levels we see today. Under a high-emissions scenario, in which warming could reach around 4.9°C, extinction losses are “markedly elevated.”
They noted that polar species are most at risk. “Polar species have the highest sensitivities to temperatures and oxygen relative to the tropics, because species in the tropics have already adapted to life in regions with higher temperatures and lower levels of oxygen,” Penn explained.
Comparisons to the Great Dying
In a previous paper, the authors compared the current period of global warming and loss of ocean oxygen to marine extinctions during the “Great Dying” event at the end of the Permian. That event, around 250 million years ago, was the most severe extinction event in Earth’s history.
“As someone who studies past extinctions, it is easy to think about the magnitude and intensity of extinction events, but imagining this in our current time and how potentially damaging it could be is difficult. This is why the paper and its projections are important and sobering,” said Pedro Manuel Monarrez, a postdoctoral fellow in the Department of Geological Sciences at Stanford University. Monarrez was not involved in the new research. “Policymakers really need to start looking at these comparisons [between oxygen levels, temperature, and species needs] because it’s not hyperbole to say that the modern crisis is going to be a big extinction event,” he added.
Extinction intensity has rarely been as severe on land as it has been in the ocean, but marine ecosystems are often overlooked in such studies, Monarrez said. “From what we’ve seen in extinction mechanisms in the fossil record in the oceans, the main factors are oxygen [depletion] and warming, which do not have the same effects on land,” he explained. “But most conservation efforts focus on terrestrial species because that’s easier to see.”
However, “we know from terrestrial studies that if you remove one species from an ecosystem, or introduce one, you can get all kinds of unpredictable changes,” Deutsch added.
Extinctions and even smaller reductions in ocean biomass have consequences far beyond marine ecosystems, the researchers noted. “Marine resources are an important part of the global human diet. So not only extinctions but losing abundance of marine species would also have huge impacts on peoples’ ability to feed themselves,” Deutsch said.
In May, for instance, the biggest producer of king salmon in New Zealand shut down three farms because of fish die-offs related to ocean warming. New Zealand King Salmon chief executive officer Grant Rosewarne told Radio New Zealand that “we’re a bit like the canary in the coal mine when it comes to global warming. We’ve got a cold-water species that’s very susceptible to half-a-degree change or 1-degree change…. We’re very much hoping to work with the government, for them to mitigate climate change as they have committed to do.”
And mitigation is still very possible, according to Penn and Deutsch. “Because extinction risk depends on greenhouse gas emissions, it means that there’s still time to change the trajectory of future emissions,” Penn explained. “So this is the silver lining.”
—Rishika Pardikar (@rishpardikar), Science Writer