As greenhouse gas emissions continue to warm the world’s oceans, marine biodiversity could be on track to plummet within the next few centuries to levels not seen since the extinction of the dinosaurs, according to research from the 91̽�� and Princeton University.

Oceanographers modeled future marine biodiversity under different projected climate scenarios. They found that if emissions are not curbed, species losses from warming and oxygen depletion alone could come to mirror the substantial impact humans already have on marine biodiversity by around 2100. Tropical waters would experience the greatest loss of biodiversity, while polar species are at the highest risk of extinction, according to the April 28 in the journal Science.

“Aggressive and rapid reductions in greenhouse gas emissions are critical for avoiding a major mass extinction of ocean species,” said senior author , who began the research as a professor of oceanography at the 91̽��and is now at Princeton University.

The study found, however, that reversing greenhouse gas emissions now 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 , who began the study as a graduate student at the 91̽��and is now a postdoctoral researcher at Princeton. “The extinction magnitude that we found depends strongly on how much carbon dioxide we emit moving forward. There’s 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 combined existing physiological data on marine species with models of climate change to predict how changes in habitat conditions will affect the survival of sea animals around the globe over the next few centuries.

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 that are adapted for cooler climates. Warm water also holds less oxygen than cooler water, and leads to more sluggish ocean circulation that reduces the oxygen supply at depth. Paradoxically, species’ metabolic rates increase with water temperature, so the demand for oxygen rises as the supply decreases.

“Once oxygen supply falls short of 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. The researchers found that polar species are likely to go globally extinct if climate warming occurs because they will have no suitable habitats to move to.

Tropical marine species will likely fare better because they have traits that allow them to cope with the warm, low-oxygen waters of the tropics. As waters north and south of the tropics warm, these species may be able to migrate to newly suitable habitats. The equatorial ocean, however, is already so warm and low in oxygen that further increases in temperature — and accompanying decrease in oxygen — might make it locally uninhabitable for many species.

The researchers compared their model to past mass extinctions captured in the fossil record. They built on their earlier work done at the UW that linked the geographic pattern of Earth’s deadliest extinction event — the end-Permian extinction about 250 million years ago — to its underlying drivers: climate warming and oxygen loss from the oceans.

The new paper used a similar model to show that anthropogenic warming could drive extinctions from the same physiological mechanism and at a comparable scale if warming becomes great enough, Penn said.

“The latitude pattern in the fossil record reveals the fingerprints of the predicted extinction driven by changes in temperature and oxygen,” Penn said.

The model also helps resolve an ongoing puzzle in the geographic pattern of marine biodiversity. Marine biodiversity increases steadily from the poles towards the tropics, but drops off at the equator. This equatorial dip has long been a mystery — researchers have been unsure about what causes it and some have even wondered whether it is real. Deutsch and Penn’s model provides a plausible explanation for the drop 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 driving extinctions, the authors compared future extinction risks from climate warming to data from the International Union for Conservation of Nature on current threats to various marine animals. They found that climate change currently affects 45% of the marine species at risk of extinction, but is only the fifth-most important stressor overall, after overfishing, transportation, urban development and pollution.

However, Penn said, climate change could soon become the top stressor, eclipsing all the others.

“Extreme warming would lead to climate-driven extinctions that, near the end of the century, will rival all current human stressors combined,” Penn said.

The research was funded by the National Science Foundation, the National Oceanic and Atmospheric Administration, California Sea Grant, California Ocean Protection Council and the 91̽��Program on Climate Change.

This article is adapted from a Princeton . For more information contact Deutsch at cdeutsch@princeton.edu or Penn at jpenn@princeton.edu.

“Temperature alone does not explain where in the ocean an animal can live,” said Deutsch. “You must consider oxygen: how much is present in the water, how well an organism can take up and utilize it, and how temperature affects these processes.”

Species-specific characteristics, overall oxygen levels and water temperature combine to determine which parts of the ocean are “breathable” for different ocean-dwelling creatures. New research led by Deutsch shows that a wide variety of marine animals — from vertebrates to crustaceans to mollusks — already inhabit the maximum range of breathable ocean that their physiology will allow.

The findings, Sept. 16 in Nature, also provide a warning about climate change: Since warmer waters will harbor less oxygen, some stretches of ocean that are breathable today for a given species may not be in the future.

“Organisms today are basically living right up to the warmest temperatures possible that will supply them with adequate oxygen for their activity level — so higher temperatures are going to immediately affect their ability to get enough oxygen,” said Deutsch. “In response to warming, their activity level is going to be restricted or their habitat is going to start shrinking. It’s not like they’re going to be fine and just carry on.”

Oxygen levels and temperatures vary throughout ocean waters. Generally, water near the equator is warmer and contains less oxygen than the cooler waters near the poles. But moving from the surface ocean to deeper waters, both oxygen and temperature decrease together. These principles create complex 3-D patterns of oxygen and temperature levels across depths and latitudes. An organism’s anatomy, physiology and activity level determine its oxygen needs, how effectively it takes up and uses the available oxygen in its environment, and how temperature affects its oxygen demand.

Deutsch and his co-authors — , a 91̽��doctoral student in oceanography, and , a professor at the University of South Florida — wanted to understand if breathability was a limiting factor in determining the ranges of marine animals today. They combined data on temperature and oxygen content across the oceans with published studies of the physiology, oxygen demand and metabolism of 72 species from five different groups of marine animals: cold-blooded vertebrates, like fish, and their relatives; crustaceans; mollusks; segmented worms; and jellyfish and their relatives.

The team modeled which parts of the ocean are and aren’t habitable for each species. Researchers show that a species’ current range generally overlapped with the parts of the oceans predicted to be habitable for it. Their model predicts that the northern shrimp, a crustacean, should be able to get enough oxygen in cool waters north of about 50 degrees north latitude — and that is generally the shrimp’s range today. The small-spotted catshark can inhabit temperate and cool waters at a variety of depths, but near the tropics only near-surface waters — above about 300 feet — are breathable, which is also reflected in its current range.

In many cases, species ranges are right up to the edge of breathability, which indicates that for marine animals the ability to get enough oxygen may be a major limiting factor in determining where they can live, Deutsch added. Outside of that range, organisms run the risk of hypoxia, or not getting enough oxygen.

Temperature affects both how much oxygen that seawater can hold, and how much oxygen an animal needs to maintain the same level of activity. The already-tight overlap the researchers saw between breathability and current ranges indicate that long-term rises in temperature, as expected under climate change, will likely restrict the ranges of many marine animals.

This new study follows a 2015 study of four Atlantic Ocean species by Deutsch’s team, and builds on its findings by showing that diverse species in all ocean basins are generally inhabiting the maximum range they currently can.

In the future, Deutsch wants to include additional species, and further explore the relationships among temperature, oxygen and physiology.

The researchers would also like to find historical examples of marine species shifting their range in response to water breathability, as the team showed earlier this year with the northern anchovy.

“What we really want to find are more observations of marine species moving around in accordance with what we’d expect with temperature conditions and oxygen availability,” said Deutsch. “That will give us firm examples of what to expect as temperature and oxygen conditions fluctuate, and shift permanently with climate change.”

The research was funded by the Gordon and Betty Moore Foundation, the National Oceanic and Atmospheric Administration and the National Science Foundation.

For more information, contact Deutsch at 206-543-2189 or cdeutsch@uw.edu.

A new study led by the 91̽�� finds that the animals’ ability to breathe in that water may be key to where and when they thrive. The , published May 15 in Science Advances, uses recent understanding of water breathability and historical data to explain population cycles of the northern anchovy. The results for this key species could apply to other species in the current.

“If you’re worried about marine life off the west coast of North America, you’re worried about anchovies and other forage fish in the California Current. Ultimately it’s what underpins the food web,” said lead author , a 91̽��postdoctoral researcher in oceanography.

The study shows that species respond to how breathable the water is — a combination of the oxygen levels in the water and the species’ oxygen needs, which are affected by water temperature. The anchovy historical data matches this pattern, and it suggests that the southern part of their range could be uninhabitable by 2100.

“Climate change isn’t just warming the oceans — it is causing oxygen to decrease, which could force fish and other ocean animals to move away from their normal range to find higher-oxygen waters,” Howard said.

Anchovy populations are known to cycle through time, but the reasons have been mysterious. Other explanations — that drew on food supplies, predator-prey interactions, competition with other species, and temperature preferences — failed to fully explain the anchovy populations cycles from the1950s to today, which have been carefully recorded.

Since the late 1940s, the , or CalCOFI, a partnership between California state and federal agencies, has monitored marine life and conditions offshore. It was established after the economically devastating crash of the sardine fishery in the 1940s with the goal of avoiding another fisheries collapse and better understanding marine populations.

“They weren’t just measuring anchovies, they were measuring everything they could get their hands on,” Howard said. Because the anchovies are numerous and their populations soared after the sardine collapse, these fish provide a good record over time and space for the past half-century.

Previous research by the 91̽��group showed that water “breathability,” the combined effects of temperature and oxygen levels, are key for marine animals’ survival. The 2015 research used models to combine the effects of warmer seawater that can hold less oxygen with marine animals’ increased metabolic needs in a warmer environment.

The new study also drew on a 2018 paper that analyzed the oxygen needs for various types of marine animals at different water temperatures. The two previous studies focused on the future, under climate change, and the distant past, for a major extinction event.

Researchers combined observations with ocean models to fill gaps in the data and showed that the breathability index changes over time and corresponds with when anchovy populations rise and fall, and when they move deeper or closer to shore.

“This study is the first one that demonstrates on a timescale of decades that a species is responding in really close alignment with this metabolic index — how breathable the ocean in its habitat has become,” said senior author , a 91̽��associate professor of oceanography. “It adds a new, independent line of verification that species in the ocean are arranged in accordance with how breathable their habitats are.”

The authors then looked at the extent of anchovy habitat in the future under climate change. Projected changes in the water conditions will likely make the southern part of the anchovies’ range, off the coasts of Mexico and Southern California, uninhabitable by 2100.

“We expect habitats to shift for all species that depend on oxygen for survival,” Howard said. “If we understand how these animals are responding to their environment, we can better predict how these populations will be affected as the conditions change.”

Co-authors include graduate student Justin Penn and research scientist Hartmut Frenzel in the 91̽��School of Oceanography; Daniele Bianchi, Lionel Renault and James McWilliams at the University of California, Los Angeles; Brad Seibel at the University of South Florida; and Fayçal Kessouri and Martha Sutula at the Southern California Coastal Water Research Project. This research was funded by the National Science Foundation; the National Oceanic and Atmospheric Administration; California Sea Grant and the California Ocean Protection Council; and the Gordon and Betty Moore Foundation.

For more information, contact Howard at ehoward2@uw.edu and Deutsch at cdeutsch@uw.edu or 206-543-5189.

The largest extinction in Earth’s history marked the end of the Permian period, some 252 million years ago. Long before dinosaurs, our planet was populated with plants and animals that were mostly obliterated after a series of massive volcanic eruptions in Siberia.

Fossils in ancient seafloor rocks display a thriving and diverse marine ecosystem, then a swath of corpses. Some 96 percent of marine species were wiped out during the “Great Dying,” followed by millions of years when life had to multiply and diversify once more.

What has been debated until now is exactly what made the oceans inhospitable to life – the high acidity of the water, metal and sulfide poisoning, a complete lack of oxygen, or simply higher temperatures.

New research from the 91̽�� and Stanford University combines models of ocean conditions and animal metabolism with published lab data and paleoceanographic records to show that the Permian mass extinction in the oceans was caused by global warming that left animals unable to breathe. As temperatures rose and the metabolism of marine animals sped up, the warmer waters could not hold enough oxygen for them to survive.

The was published Dec. 7 in .

“This is the first time that we have made a mechanistic prediction about what caused the extinction that can be directly tested with the fossil record, which then allows us to make predictions about the causes of extinction in the future,” said first author , a 91̽��doctoral student in oceanography.

Researchers ran a climate model with Earth’s configuration during the Permian, when the land masses were combined in the supercontinent of Pangaea. Before ongoing volcanic eruptions in Siberia created a greenhouse-gas planet, oceans had temperatures and oxygen levels similar to today’s. The researchers then raised greenhouse gases in the model to the level required to make tropical ocean temperatures at the surface some 10 degrees Celsius (20 degrees Fahrenheit) higher, matching conditions at that time.

The model reproduces the resulting dramatic changes in the oceans. Oceans lost about 80 percent of their oxygen. About half the oceans’ seafloor, mostly at deeper depths, became completely oxygen-free.

To analyze the effects on marine species, the researchers considered the varying oxygen and temperature sensitivities of 61 modern marine species — including crustaceans, fish, shellfish, corals and sharks — using published lab measurements. The tolerance of modern animals to high temperature and low oxygen is expected to be similar to Permian animals because they had evolved under similar environmental conditions. The researchers then combined the species’ traits with the paleoclimate simulations to predict the geography of the extinction.

“Very few marine organisms stayed in the same habitats they were living in — it was either flee or perish,” said second author , a 91̽��associate professor of oceanography.

The model shows the hardest hit were organisms most sensitive to oxygen found far from the tropics. Many species that lived in the tropics also went extinct in the model, but it predicts that high-latitude species, especially those with high oxygen demands, were nearly completely wiped out.

To test this prediction, co-authors and at Stanford analyzed late-Permian fossil distributions from the , a virtual archive of published fossil collections. The fossil record shows where species were before the extinction, and which were wiped out completely or restricted to a fraction of their former habitat.

The fossil record confirms that species far from the equator suffered most during the event.

“The signature of that kill mechanism, climate warming and oxygen loss, is this geographic pattern that’s predicted by the model and then discovered in the fossils,” Penn said. “The agreement between the two indicates this mechanism of climate warming and oxygen loss was a primary cause of the extinction.”

The study builds on led by Deutsch showing that as oceans warm, marine animals’ metabolism speeds up, meaning they require more oxygen, while warmer water holds less. That earlier study shows how warmer oceans push animals away from the tropics.

The new study combines the changing ocean conditions with various animals’ metabolic needs at different temperatures. Results show that the most severe effects of oxygen deprivation are for species living near the poles.

“Since tropical organisms’ metabolisms were already adapted to fairly warm, lower-oxygen conditions, they could move away from the tropics and find the same conditions somewhere else,” Deutsch said. “But if an organism was adapted for a cold, oxygen-rich environment, then those conditions ceased to exist in the shallow oceans.”

The so-called “dead zones” that are completely devoid of oxygen were mostly below depths where species were living, and played a smaller role in the survival rates.

“At the end of the day, it turned out that the size of the dead zones really doesn’t seem to be the key thing for the extinction,” Deutsch said. “We often think about anoxia, the complete lack of oxygen, as the condition you need to get widespread uninhabitability. But when you look at the tolerance for low oxygen, most organisms can be excluded from seawater at oxygen levels that aren’t anywhere close to anoxic.”

Warming leading to insufficient oxygen explains more than half of the marine diversity losses. The authors say that other changes, such as acidification or shifts in the productivity of photosynthetic organisms, likely acted as additional causes.

The situation in the late Permian — increasing greenhouse gases in the atmosphere that create warmer temperatures on Earth — is similar to today.

“Under a business-as-usual emissions scenarios, by 2100 warming in the upper ocean will have approached 20 percent of warming in the late Permian, and by the year 2300 it will reach between 35 and 50 percent,” Penn said. “This study highlights the potential for a mass extinction arising from a similar mechanism under anthropogenic climate change.”

The research was funded by the Gordon and Betty Moore Foundation and the National Science Foundation.

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For more information, contact Penn at jpenn@uw.edu and Deutsch at cdeutsch@uw.edu or 206-218-7112.

NSF grant: OCE-1419323, OCE-1458967; GBMF grant: 3775

But new research is showing that climate change is expected to accelerate rates of crop loss due to the activity of another group of hungry creatures — insects. In a published Aug. 31 in the journal , a team led by scientists at the 91̽�� reports that insect activity in today’s temperate, crop-growing regions will rise along with temperatures. Researchers project that this activity, in turn, will boost worldwide losses of rice, corn and wheat by 10-25 percent for each degree Celsius that global mean surface temperatures rise. Just a 2-degree Celsius rise in surface temperatures will push the total losses of these three crops each year to approximately 213 million tons.

“We expect to see increasing crop losses due to insect activity for two basic reasons,” said co-lead and corresponding author , a 91̽��associate professor of oceanography. “First, warmer temperatures increase insect metabolic rates exponentially. Second, with the exception of the tropics, warmer temperatures will increase the reproductive rates of insects. You have more insects, and they’re eating more.”

In 2016, the United Nations estimated that at least 815 million people worldwide don’t get enough to eat. Corn, rice and wheat are staple crops for about 4 billion people, and account for about two-thirds of the food energy intake, the UN Food and Agriculture Organization.

“Global warming impacts on pest infestations will aggravate the problems of food insecurity and environmental damages from agriculture worldwide,” said co-author , a professor in the Department of Earth System Science at Stanford University and founding director of the Center on Food Security and the Environment. “Increased pesticide applications, the use of GMOs, and agronomic practices such as crop rotations will help control losses from insects. But it still appears that under virtually all climate change scenarios, pest populations will be the winners, particularly in highly productive temperate regions, causing real food prices to rise and food-insecure families to suffer.”

To investigate how insect herbivory on crops might affect our future, the team looked at decades of laboratory experiments of insect metabolic and reproductive rates, as well as ecological studies of insects in the wild. Unlike mammals, insects are ectothermic, which means that their body temperature tracks the temperature of their environment. Thus, the air temperature affects oxygen consumption, caloric requirements and other metabolic rates.

The past experiments that the team studied show conclusively that increases in temperature will accelerate insect metabolism, which boosts their appetites, at a predictable rate. In addition, increasing temperatures boost reproductive rates up to a point, and then those rates level off at temperature levels akin to what exist today in the tropics.

Deutsch and his colleagues found that the effects of temperature on insect metabolism and demographics were fairly consistent across insect species, including pest species such as aphids and corn borers. They folded these metabolic and reproductive effects into a model of insect population dynamics, and looked at how that model changed based on different climate change scenarios. Those scenarios incorporated information based on where corn, rice and wheat — the three largest staple crops in the world — are currently grown.

“Temperate regions are currently cooler than what’s optimal for most insects. But if temperatures rise, these insect populations will grow faster,” said co-author , a researcher at the University of Vermont’s College of Agriculture and Life Sciences and the Gund Institute for Environment. “They will also need to eat more, because rising temperatures increase insect metabolism. Together, that’s not good for crops.”

For a 2-degree Celsius rise in global mean surface temperatures, their model predicts that median losses in yield due to insect activity would be 31 percent for corn, 19 percent for rice and 46 percent for wheat. Under those conditions, total annual crop losses would reach 62, 92 and 59 million tons, respectively.

The researchers observed different loss rates due to the crops’ different growing regions, Deutsch said. For example, much of the world’s rice is grown in the tropics. Temperatures there are already at optimal conditions to maximize insect reproductive and metabolic rates. So, additional increases in temperature in the tropics would not boost insect activity to the same extent that they would in temperate regions – such as the United States’ “.”

The team notes that farmers and governments could try to lessen the impact of increased insect metabolism, such as shifting where crops are grown or trying to breed insect-resistant crops. But these alterations will take time and come with their own costs.

“I hope our results demonstrate the importance of collecting more data on how pests will impact crop losses in a warming world — because collectively, our choice now is not whether or not we will allow warming to occur, but how much warming we’re willing to tolerate,” said Deutsch.

Co-lead author is , director of Future Earth at the University of Colorado, Boulder. Additional co-authors are , a 91̽��research scientist in the Department of Atmospheric Sciences; , a 91̽��professor of atmospheric sciences; and , a 91̽��professor emeritus of biology. The research was funded by the National Science Foundation and the Gordon and Betty Moore Foundation.

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For more information, contact Deutsch at cdeutsch@uw.edu or +1 206-543-5189 and the 91̽�� News Office at +1 206-543-2580.

DOI: 10.1126/science.aat3466

Grant numbers: OCE-1419323, OCE-1458967, OCE-1542240, GBMF#3775

How much of this happens in different regions of the ocean would seem like an academic question, except during an era when humanity is spewing carbon dioxide into the air at record-high levels and wondering where all that carbon will go in the future.

A 91̽�� published this week (July 25) in the uses a new approach to get a global picture of the fate of marine carbon. It finds that the polar seas export organic carbon to the deep sea, where it can no longer trap heat from the sun, about five times as efficiently as in other parts of the ocean.

“The high latitudes are much more efficient at transferring carbon into the deep ocean,” said first author , who did the work as a postdoctoral researcher at the 91̽��and is now an assistant professor at the University of Rochester in New York. “Understanding how this happens will certainly allow a more complete prediction of ocean responses to climate change.”

The planet has many carbon sinks, or routes that transfer heat-trapping carbon from the atmosphere into other parts of the Earth system. This sink is a literal one. Carbon-rich plankton detritus clumps together to form that drifts down through the water and provides food for deeper-dwelling organisms. The continual supply of organic carbon in particles from the surface to the deep sea is known as the “biological pump.”

This pump had been thought to operate at similar strength throughout the oceans, but the new study finds a strong regional pattern. The authors find that about 25 percent of organic particles sinking from the surface in the polar oceans reach at least 1 kilometer (0.6 miles) — the depth required for long-term storage in deep waters or the seafloor. Just 5 percent of sinking carbon in the subtropics makes it that far, while the rest is released into shallower water where it can soon rejoin the atmosphere. The tropics have an intermediate value of about 15 percent.

“This highlights the importance of the polar ocean — the cold, high-latitude parts of the ocean — for their ability to store carbon over long time periods,” said co-author , a 91̽��associate professor of oceanography.

The growth of marine plants at the ocean’s sunlit surface is well-studied, but what happens a mile down is more mysterious. For many years, scientists have put floating at different depths to try to learn how deep the particles reach, but the results have been inconclusive.

“It’s obviously quite expensive to deploy these traps on a scale that you would need to make global estimates,” Weber said.

The new study takes a different approach. Researchers looked at phosphate, a nutrient taken in by plankton in the surface and released with carbon when particles decompose. They then used a computer model of ocean currents to determine the depth at which this nutrient is released.

“By looking at the products of the decomposition we could look at it in the opposite way but come to the same information, which is how deep stuff gets before it decomposes,” Deutsch said.

They found that, overall, about 15 percent of the carbon in ocean plankton makes it to long-term storage in the deep ocean, which agrees with previous estimates. But the regional pattern came as a surprise.

The authors tried to understand why. Temperature could be a factor, since cold water, like refrigerators, will slow decomposition on the way down. But the temperature difference could not fully explain the results.

What did explain a range of observations was the size of the organisms that form marine snow. Warm, nutrient-poor subtropical seas are so-called “marine deserts” where the life that survives is made up of tiny picoplankton. Nutrient-rich polar oceans, and to a lesser degree the equator, can support larger lifeforms, such as diatoms, that sink more like a proverbial stone.

“Simply because they sink faster, these large phytoplankton are more likely to reach the deep ocean before being consumed,” Weber said.

Under climate change, oceans are predicted to support fewer plankton overall. What’s more, it’s thought that water temperatures will rise, currents will slow and the tropics will expand.

“Even though this study is not directly about climate change, it provides us with a new way of thinking to apply to climate-change scenarios,” Weber said. “As those regions dominated by smaller plankton tend to expand, it’s likely that the ocean will become less efficient at locking carbon away from the atmosphere.”

The research was funded by the Gordon and Betty Moore Foundation. Other co-authors are 91̽��oceanography postdoctoral researcher and graduate student , and at the University of California, Santa Barbara.

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For more information, contact Weber at t.weber@rochester.edu or 585-275-2103 and Deutsch at cdeutsch@uw.edu or 206-543-5189.

91̽�� researchers and collaborators have found that the same principle will apply to marine species under global warming. The warmer water temperatures will speed up the animals’ metabolic need for oxygen, as also happens during exercise, but the warmer water will hold less of the oxygen needed to fuel their bodies, similar to what happens at high altitudes.

The , published June 5 in , finds that these changes will act together to push marine animals away from the equator. About two thirds of the respiratory stress due to climate change is caused by warmer temperatures, while the rest is because warmer water holds less dissolved gases.

“If your metabolism goes up, you need more food and you need more oxygen,” said lead author , a 91̽��associate professor of oceanography. “This means that aquatic animals could become oxygen-starved in the warmer future, even if oxygen doesn’t change. We know that oxygen levels in the ocean are going down now and will decrease more with climate warming.”

The study centered on four Atlantic Ocean species whose temperature and oxygen requirements are well known from lab tests: that live in the open ocean; Atlantic that live in coastal waters; sharp snout that live in the subtropical Atlantic and Mediterranean; and common , a bottom-dwelling fish that lives in shallow waters in high northern latitudes.

Deutsch used climate models to see how the projected temperature and oxygen levels by 2100 due to climate change would affect these four species’ ability to meet their future energy needs. If current emissions continue, the near-surface ocean is projected to warm by several degrees Celsius by the end of this century. Seawater at that temperature would hold 5-10 percent less oxygen than it does now.

Results show future rock crab habitat would be restricted to shallower water, hugging the more oxygenated surface. For all four species, the equator-ward part of the range would become uninhabitable because peak oxygen demand would become greater than the supply. Viable habitats would shift away from the equator, displacing from 14 percent to 26 percent of the current ranges.

The four animals were chosen because the effects of oxygen and temperature on their metabolism are well known, and because they live in diverse habitats. The authors believe the results are relevant for all marine species that rely on aquatic oxygen for an energy source.

“The Atlantic Ocean is relatively well oxygenated,” Deutsch said. “If there’s oxygen restriction in the Atlantic Ocean marine habitat, then it should be everywhere.”

Climate models predict that the northern Pacific Ocean’s relatively low oxygen levels will decline even further, making it the most vulnerable part of the ocean to habitat loss.

“For aquatic animals that are breathing water, warming temperatures create a real problem of limited oxygen supply versus elevated demand,” said co-author , a 91̽��professor of biology who has studied metabolism in land animals and in human mountain climbers.

“This simple metabolic index seems to correlate with the current distributions of marine organisms,” he said, “and that means that it gives you the power to predict how range limits are going to shift with warming.”

Previously, marine scientists thought about oxygen more in terms of extreme events that could cause regional die-offs of marine animals, also known as dead zones.

“We found that oxygen is also a day-to-day restriction on where species will live, outside of those extreme events,” Deutsch said. “Ranges will shift for other reasons, too, but I think the effect we’re describing will be part of the mix of what’s pushing species around in the future.”

Other co-authors are of the Alfred Wegener Institute in Germany; , a former graduate student at the University of California, Los Angeles; and at the University of Rhode Island. The research was funded by the Gordon and Betty Moore Foundation, the National Science Foundation and the Alfred Wegener Institute’s PACES program.

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For more information, contact Deutsch at cdeutsch@uw.edu or 206-543-5189 or Huey at hueyrb@uw.edu or 206-543-1505.

“The tropics should actually get better oxygenated as the climate warms up,” said , a 91̽��associate professor of oceanography. He is lead author of the study published Aug. 8 in .

Warmer water contains less gas, so climate change is expected to reduce oxygen levels. Observations show this is already taking place in many places around the world. Declines during the past 20 years in the tropical low-oxygen zones, the lowest-oxygen waters on the planet, had led to proposing that these zones would also get worse over time.

Tropical regions are usually associated with an abundance of life, but they have some of the most inhospitable places for ocean dwellers. The oxygen minimum zones off Mexico and Peru have oxygen levels already too low to support most animals (so, unlike in other low-oxygen zones, here there’s no risk of killing fish).

But when those levels drop even further, a particular group of bacteria, which can use nitrogen instead of oxygen as a source of energy, thrive. Nitrogen is an essential and very scarce nutrient for marine plants. When oxygen levels get low enough for that particular group of bacteria to take over, significant amounts of the ocean’s fertilizer get deep-sixed to the bottom of the tropical ocean.

The shows that water flowing into the tropics is indeed likely to get lower in oxygen, decreasing the initial oxygen supply. But demand will also shift under climate change. Specifically, as the trade winds weaken, the whole sequence of events that feeds this bacterial food chain will slow down, and the low-oxygen zone will shrink.

“If we want to understand how biological and chemical aspects of the ocean will change in the future, we really have to pay a lot of attention to what happens with the winds,” Deutsch said. “The winds can lead to conclusions that are exactly the opposite of what you’d expect.”

Trade winds from the west cause deep water to percolate up along western coasts, bringing nutrients up from the deep sea. These nutrients feed marine plants, which feed marine animals, which decompose to feed bacteria that use up the remaining oxygen. As trade winds weaken, less nutrient-rich water percolates up from the deep. Fewer plants grow at the surface. Finally, fewer oxygen-gobbling bacteria can survive.

Deutsch is a climate modeler who studies tropical ocean circulation. He learned of sediment cores, collected off Mexico by co-authors at the University of Southern California and at Columbia University, that showed a puzzling longer-term trend. The authors worked together to interpret the samples. Results show that for most of the time since 1850 the population of these nitrogen-eating bacteria has been going down, coincident with warming oceans and weakening trade winds. This implies that the local oxygen levels, for which few direct measurements exist, have been rising.

“I find it an interesting question for understanding the way the ocean functions on climatic or geologic timescales,” Deutsch said.

Most climate models predict that trade winds will continue to weaken in the future, shrinking the oxygen-minimum zones in the Pacific Ocean off the coasts of Mexico, Chile and Peru, and in the Indian Ocean off western Australia.

Decreasing oxygen in the wider ocean is still a major concern, Deutsch said, as are overfishing, ocean acidification and warming water temperatures.

“This study shows that what happens to the winds, which is sometimes overlooked, is really important for predicting how the oceans will respond to climate change,” Deutsch said.

The research was funded by the National Science Foundation, the Gordon and Betty Moore Foundation, the U.S. Geological Survey and the Lamont-Doherty Earth Observatory. Other co-authors are Tom Weber, a research scientist at the UW; Robert Thunell at the University of South Carolina; Caitlin Terms at the University of Southern California; James McManus at the University of Akron; John Crusius at the U.S. Geological Survey; Taka Ito at the Georgia Institute of Technology; Timothy Baumgartner and Vicente Ferreira at Mexico’s Autonomous University of Baja California; and Jacob Mey at the City University of New York.

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For more information, contact Deutsch at 206-543-5189 or cdeutsch@uw.edu.

NSF grant numbers: OCE-0851483, OCE-0727123, OCE-0624777