Ocean-dwelling species are slowly being strangled.
The amount of oxygen dissolved in the oceans is decreasing due to climate change, and unfortunately, this is already evident in some parts of the world. Now, according to a new study led by the National Center for Atmospheric Research (NCAR), this deoxygenation should be noticeable across large regions of the oceans between 2030 and 2040.
The entire ocean — from the deepest point to the shallowest — gets its oxygen supply at the surface from either the atmosphere or phytoplankton, which release oxygen into the water through photosynthesis.
However, warmer surface waters absorb less oxygen. And the oxygen that is absorbed has a more difficult time traveling deeper into the ocean. Why? Because as water heats up, it expands, becoming lighter than the water below it, which makes it less likely to sink. This can gradually drain oceans of oxygen, leaving fish, crabs, squid, sea stars, and other marine life struggling to breathe.
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Thanks to natural warming and cooling, oxygen concentrations at the sea surface are constantly changing, but deep in the ocean those changes can last for years or even decades.
"Loss of oxygen in the ocean is one of the serious side effects of a warming atmosphere, and a major threat to marine life," NCAR scientist Matthew Long and lead author of the study, said in a press release. "Since oxygen concentrations in the ocean naturally vary depending on variations in winds and temperature at the surface, it's been challenging to attribute any deoxygenation to climate change. This new study tells us when we can expect the impact from climate change to overwhelm the natural variability."
The study, published in the journal Global Biogeochemical Cycles, investigated the difference between this natural variation and the impact of climate change. The scientists used 12 different simulations that ran for the years 1920 to 2100 to study dissolved oxygen and how concentrations have varied in the past. Using this information, they could determine when ocean deoxygenation due to climate change is likely to become more severe.
What the team found was that deoxygenation caused by climate change can already be detected in the southern Indian Ocean and parts of the eastern tropical Pacific and Atlantic basins, and stated that more widespread deoxygenation would be detectable between the years 2030 and 2040. However, in some parts of the ocean, including areas off the east coasts of Africa, Australia, and Southeast Asia, deoxygenation caused by climate change was not evident even by 2100.
Using these same data, the researchers created a map of when deoxygenation due to climate change will become detectable (seen in the image below).
Oxygen loss in the oceans. Grey regions of the ocean will not have detectable loss of oxygen even by 2100. Photo credit: Matthew Long/NCAR
According to the researchers, these types of maps could be useful resources for deciding where to place instruments to monitor oxygen levels in the future. Currently, ocean oxygen measurements are few and far between.
Long concluded, "We need comprehensive and sustained observations of what's going on in the ocean to compare with what we're learning from our models and to understand the full impact of a changing climate."
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