New research from Cornell University and Rensselaer Polytechnic Institute by Jane, and 6 co-authors, indicates that a continually warming world is leading to extended, late-summer weeks of water stratification, which prompts oxygen deprivation in the water -- provoking conditions called hypoxia (low oxygen) and anoxia (no oxygen) -- and negative consequences for fish and other species. The work published Dec. 6 in the journal Global Change Biology.
"Lakes with dissolved oxygen losses strongly outnumber those with gains," said lead author Stephen Jane, a postdoctoral fellow at the Cornell Atkinson Center for Sustainability. "At large scales, aerobic organisms are losing available habitat as warming of lakes continues. This is particularly the case for organisms that rely on well-oxygenated cool waters deep in lakes to survive warm periods."
Jane and his colleagues examined about 25 years of data available for more than 400 lakes -- mostly from the United States -- to identify dissolved oxygen loss. In addition to Rondaxe Lake, the group studied New York's Neversink (Sullivan County) and Cannonsville (Delaware County) reservoirs, and Jockeybush and Sagamore lakes (Hamilton County).
In temperate climate lakes, the researchers found that the amount of low oxygen water is increasing by 0.9% to 1.7% per decade on average and found that the volume of lake water lacking oxygen has increased by more than 50% from three decades ago. Reducing oxygen in lake water can have many effects. For example, anoxic portions of the water column might see a buildup of methane -- a powerful greenhouse gas. Nutrients such as phosphorus from agricultural fertilizer, released from unsettled lake sediment, may enter the water column, which increases the likelihood of harmful algal blooms.
Regan also reported in early 2023 that right now, life is thriving on our oxygen-rich planet, but Earth wasn’t always like this — and scientists have predicted the atmosphere will revert to being methane-rich and oxygen-poor in the future. This probably won’t happen for another billion years or so. But when the change comes, it will happen fairly quickly, according to a study published in 2021.This shift will return the planet to roughly the state it was in about 2.4 billion years ago, before what is known as the Great Oxidation Event (GOE). Additionally, the researchers behind the study say that atmospheric oxygen is unlikely to be a persistent feature of habitable worlds in general, which has implications for our efforts to detect signs of life further out in the universe.
“The model posits that atmospheric deoxidation, in which atmospheric O2 falls sharply to levels reminiscent of Archean Earth, most likely preceded the onset of humid greenhouse conditions in the Earth’s climate system and prior to the extensive loss of surface water from the Earth atmosphere is triggered”, explain the researchers in their work.
At that point it will be the end of the road for humans and most other life forms that depend on oxygen to get through the day. So let’s hope we figure out how to leave the planet sometime within the next billion years.
To reach their conclusions, the researchers ran detailed models of Earth’s biosphere, accounting for changes in the Sun’s brightness and the corresponding decrease in carbon dioxide levels as the gas is broken down by increasing heat. Less carbon dioxide means fewer photosynthesizing organisms like plants, which would result in less oxygen. Scientists have predicted that increased radiation from the sun would wipe ocean water off the face of our planet within about 2 billion years, but the model here – based on an average of just under 400,000 simulations – says the reduction in oxygen will be deadly at first from life.
“The oxygen drop is very, very extreme,” said Georgia Institute of Technology Earth scientist Chris Reinhard New scientist. “We’re talking about a million times less oxygen than today.”
What makes the study particularly relevant today is our search for habitable planets outside the solar system. As more powerful telescopes are coming online, scientists want to know what to look for in the vast amounts of data these instruments collect. It’s possible we need to look for biosignatures other than oxygen to have the best chance of spotting life, the researchers say. Their study is part of NASA’s NExSS (Nexus for Exoplanet System Science) project, which is investigating the habitability of planets other than our own.
According to calculations by Reinhard and environmental scientist Kazumi Ozaki of Toho University in Japan, Earth’s oxygen-rich habitable history could end up lasting only 20 to 30 percent of the planet’s entire lifespan — and microbial life will persist into existence long after we went.
“The atmosphere after the Great Deoxygenation is characterized by elevated methane levels, low CO2 levels, and no ozone layer,” Ozaki said.
“The Earth system will likely be a world of anaerobic life forms.”
The research was published in Nature Geosciences.
In a typical July or August, a lake surface may be about 70 degrees, while the lake bottom may be around 40 degrees. "Water temperature and density are related," Jane said. "So it becomes a situation where basically you have oil and vinegar, where strong water temperature differences between layers causes resistance to mixing -- which is stratification." The result is that oxygen from the atmosphere is prevented from replenishing dissolved oxygen in deep waters, Jane said. Because winter ends sooner than it did decades ago, seasonal stratification is starting earlier and ending later.
These stratification changes lead to more time for deoxygenation -- the interruption of the natural oxygenation process -- for deep-water habitats, he said.
"We show here that as warming is continuing, the amount of time that lakes exhibit stratification is increasing and this leads to increases in the amount of low-oxygen water in lakes," said co-author Kevin Rose, associate professor at Rensselaer Polytechnic Institute, Troy, New York. "The bad news is that given projected warming rates, we'll likely see even greater increases in the amount of oxygen-depleted water in lakes in the future."
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