Climate studies have shown that areas of the world with historically low oxygen conditions are growing in size. At the same time, new low-oxygen areas are being formed. One study estimates that the total volume of deadly low-oxygen waters has quadrupled since 1960.
As climate change results in warmer waters containing less oxygen, Puget Sound could expect to see an increase in species that can live in lower-oxygen waters but a decrease in species sensitive to the warmer conditions. Some species are likely to disappear if trends continue on their current path, according to climate researchers, but much depends on the amount of greenhouse gas emissions in the coming years.
Figuring out how various species will fare under differing conditions throughout the world has been the focus of intense research by Justin Penn, who completed his doctoral degree in oceanography from the University of Washington in 2020. One of Penn’s major findings, published last year in the journal Science, is his prediction that sea life abundance and distribution will begin to change dramatically by the end of the century if greenhouse gas emissions are not curbed soon.
Species living near the poles might not be able to meet their oxygen demands in the warmer waters of the future. Since they are already in the coolest water on Earth, they would have nowhere to go in a search of cooler water.
Tropical waters would experience the greatest loss of diversity over time, according to his findings, while species living close to the poles would face potential extinction.
Underlying Penn’s work on climate change is an understanding that most marine animals increase their respiration rates and their demand for oxygen at higher temperatures. Since warmer waters hold less oxygen, climate change produces a double-whammy for creatures trying to survive under warming conditions.
Penn’s graduate school adviser, Curtis Deutsch, has spent years studying the interrelated effects of oxygen and temperature. He developed a “metabolic index” to describe the combined temperature and oxygen requirements of a species and to compare those needs with environmental conditions in various places. Deutsch, a former UW oceanography professor, is now affiliated with Princeton University in New Jersey, where Penn is completing his post-doctoral research.
A study published in 2020 by Deutsch, Penn and Brad Seibel of the University of South Florida suggests that a wide variety of marine creatures — from vertebrates to crustaceans to mollusks — already occupy, to varying extents, all the areas that meet their “breathability” needs, described by their metabolic threshold.
Penn combined the metabolic index for various species with predictions of temperature and oxygen from climate-change models. His maps show how the animals might fare in a warming climate. For example, species living in the warm tropics appear to have traits that allow them to better cope with the warming waters compared to those in more temperate areas. But some tropical species are likely to shift their populations toward the poles to find cooler waters with more oxygen.
At the equator, warm, low-oxygen water already constrains the number of species compared to the middle latitudes between the tropical and polar regions. Further warming in the equatorial region would tend to reduce species diversity even more.
At the same time, some species living near the poles might not be able to meet their oxygen demands in the warmer waters of the future. Since they are already in the coolest water on Earth, they would have nowhere to go in a search of cooler water. If unable to adapt, their risk of extinction would be high.
Ancient climate clues to future
Projections of ongoing climate effects on marine life — assuming an accelerating rate of warming — turn out to be not so different from events that took place some 252 million years ago during the “Great Dying,” according to a 2018 study by Penn and Deutsch along with fellow researchers at Stanford University.
At the end of the Earth’s Permian Period, up to 96 percent of all marine species and 70 percent of all terrestrial species went extinct. It was a time of massive volcanic activity that raised the temperature of the air and water. Researchers have long debated the cause of the mass die-off, speculating about ocean acidity, metal and sulfide poisoning, lack of oxygen and straight-on higher temperatures.