Unchecked global emissions set to trigger mass extinction of marine life

Researchers from Princeton University have reported in a new study that unless greenhouse gas emissions are reduced, marine biodiversity could plummet to levels not seen since the extinction of the dinosaurs. Scientists modeled future marine biodiversity under projected climate scenarios and found that species such as dolphinfish (shown) would be at risk as warming oceans decrease the supply of oxygen from the ocean while increasing the metabolic demand of marine life. Credit: Photo by Evan Davis

As greenhouse gas emissions continue to warm the world’s oceans, marine biodiversity could be on course to plunge over the next few centuries to levels not seen since the extinction of the dinosaurs. This dire prediction is according to a new research study to be published by[{” attribute=””>Princeton University researchers in the journal Science today (April 28, 2022).

The researchers modeled future marine biodiversity under different projected climate scenarios. They discovered that if emissions are not reduced, species losses due to warming and oxygen depletion alone could come to mirror the substantial impact humans already have on marine biodiversity by around 2100. Tropical waters would suffer the greatest loss of biodiversity, while polar species would face the highest risk of extinction, the authors reported.

“Aggressive and rapid reductions in greenhouse gas emissions are critical for avoiding a major mass extinction of ocean species,” said senior author Curtis Deutsch, professor of geosciences and the High Meadows Environmental Institute at Princeton.

Big Five Extinction Timeline

The researchers compared their model to the magnitude of Earth’s “Big Five” mass extinctions. The illustration above indicates the percentage of biodiversity lost during each event (left). At lower right, the loss of marine life from the present to 2300 is projected for high- and low-greenhouse gas emissions scenarios with the accompanying global temperature change. Credit: Data by Justin Penn and Curtis Deutsch with illustrations by Yesenia Román

The study found, however, that reversing greenhouse gas emissions could reduce the risk of extinction by more than 70%. “The silver lining is that the future isn’t written in stone,” said first author Justin Penn, a postdoctoral research associate in the Department of Geosciences. “The extinction magnitude that we found depends strongly on how much carbon dioxide [CO2] we emit while advancing. There is still enough time to change the trajectory of CO2 emissions and prevent the magnitude of warming that would cause this mass extinction.

Deutsch and Penn, who initiated the study when they were both in University of Washington, combined existing physiological data on marine species with models of climate change to predict how changes in habitat conditions will affect the survival of marine animals around the world over the next few centuries. The researchers compared their model to the scale of past mass extinctions captured in the fossil record, building on their previous work that linked the geographic pattern of the Late Permian extinction over 250 million years – Earth’s deadliest extinction event – to the underlying factors, namely global warming and oxygen loss from the oceans.

Biodiversity map

The researchers found that as emissions increased (in grey), biodiversity loss (in red) would be greater in tropical waters, while polar species are most at risk of extinction. Researchers have found that reversing greenhouse gas emissions could reduce the risk of extinction of marine life by more than 70%. Credit: Figure by Justin Penn

The researchers found that their model projecting future marine biodiversity, the fossil record of the late Permian extinction and, in fact, the distribution of species we see now follow a similar pattern – as sea temperature rises. ocean increases and as the availability of oxygen decreases, there is a pronounced decrease. in the abundance of marine life.

Water temperature and oxygen availability are two key factors that will change as the climate warms due to human activity. Warmer water is itself a risk factor for species adapted to cooler climates. Warm water also contains less oxygen than cooler water, resulting in slower ocean circulation that reduces oxygen supply at depth. Paradoxically, metabolic rates of species increase with water temperature, so oxygen demand increases as supply decreases. “Once the oxygen supply drops below what species need, we expect to see substantial species losses,” Penn said.

Marine animals have physiological mechanisms that allow them to cope with environmental changes, but only up to a point. Researchers have found that polar species are more likely to become globally extinct if global warming occurs because they won’t have suitable habitats to move to. Tropical marine species will likely fare better because they have characteristics that allow them to cope with the warm, low oxygen waters of the tropics. As the waters north and south of the tropics warm, these species may be able to migrate to newly adapted habitats. The equatorial ocean, however, is already so hot and oxygen-poor that further increases in temperature – and a concomitant decrease in oxygen – could render it locally uninhabitable for many species.

The researchers report that the pattern of extinction projected by their model – with greater overall extinction of species at the poles compared to the tropics – mirrors the pattern of past mass extinctions. A Deutsch and Penn study published in Science in 2018 showed that temperature-dependent increases in metabolic oxygen demand—coupled with decreased oxygen availability caused by volcanic eruptions—can explain geographic patterns of species loss during the extinction of the end of the Permian, which killed 81% of marine species.

The new paper used a similar model to show that anthropogenic warming could drive extinctions of the same physiological mechanism on a comparable scale if warming becomes large enough, Penn said. “The latitude pattern in the fossil record reveals the fingerprints of predicted extinction due to temperature and oxygen changes,” he said.

The model also helps solve a permanent puzzle in the geographical pattern of marine biodiversity. Marine biodiversity increases steadily from the poles to the tropics, but decreases at the equator. This equatorial plunge has long been a mystery – researchers aren’t sure what causes it, and some have even wondered if it’s real. Deutsch and Penn’s model provides a plausible explanation for the decline in equatorial marine biodiversity – the oxygen supply is too low in these warm waters for some species to tolerate.

The big concern is that climate change will make large swaths of the ocean similarly uninhabitable, Penn said. To quantify the relative importance of climate in extinctions, he and Deutsch compared future extinction risks due to global warming to data from the International Union for Conservation of Nature (IUCN) on current threats to various Marine animals. They found that climate change currently affects 45% of marine species threatened with extinction, but is only the fifth most important stressor after overfishing, transport, urban development and pollution.

However, Penn said, climate change may soon eclipse all of these stressors in importance: “Extreme warming would drive climate-related extinctions that by the end of the century will rival all current human stressors combined.

Reference “Avoiding mass extinction of the oceans due to global warming” April 29, 2022, Science.
DOI: 10.1126/science.abe9039

The work was supported by grants from the National Science Foundation (OCE-1737282), the National Oceanic and Atmospheric Administration (NA18NOS4780167), the California SeaGrant and Ocean Protection Council, and the UW Climate Change Program.

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