Oceans’ ability to soak up emissions has a tipping point

"We need to think about these worst-case scenarios to understand how our CO2 emissions might affect the oceans not just this century, but next century and the following century," says Megumi Chikamoto. (Credit: Giga Khurtsilava/Unsplash)

The oceans soak up carbon dioxide emissions, helping to limit global warming—but intense warming in the future could lessen that ability, leading to even more intense warming, scientists warn.

For a new study, the researchers analyzed a climate simulation configured to a worst-case emissions scenario and found that the oceans’ ability to soak up carbon dioxide would peak by 2100, becoming only half as efficient at absorbing the greenhouse gas by 2300.

The decline happens because of the emergence of a surface layer of low-alkalinity water that hinders the ability of the oceans to absorb CO2. Alkalinity is a chemical property that affects how much CO2 can dissolve in seawater.

Although the emissions scenario used in the study is unlikely because of global efforts to limit greenhouse gas emissions, the findings reveal a previously unknown tipping point that if activated would release an important brake on global warming, the authors say.

“We need to think about these worst-case scenarios to understand how our CO2 emissions might affect the oceans not just this century, but next century and the following century,” says Megumi Chikamoto, who led the research as a research fellow at the University of Texas Institute for Geophysics.

Today, the oceans soak up about a third of the CO2 emissions generated by humans. Climate simulations had previously shown that the oceans slow their absorption of CO2 over time, but none had considered alkalinity as explanation. To reach their conclusion, the researchers recalculated pieces of a 450-year simulation until they hit on alkalinity as a key cause of the slowing.

According to the findings, the effect begins with extreme climate change, which supercharges rainfall and slows ocean currents. This leaves the surface of the oceans covered in a warm layer of fresh water that won’t mix easily with the cooler, more alkaline waters below it. As this surface layer becomes more saturated with CO2, its alkalinity falls and with it, its ability to absorb CO2. The end result is a surface layer that acts like a barrier for CO2 absorption.

That means less of the greenhouse gas goes into the ocean and more of it is left behind in the atmosphere. This in turn produces faster warming, which sustains and strengthens the low-alkalinity surface layer.

The discovery is a powerful reminder that the world needs to reduce its CO2 emissions to avoid crossing this and other tipping points, says coauthor Pedro DiNezio, an affiliate researcher at the University of Texas Institute for Geophysics and associate professor at University of Colorado.

“Whether it’s this or the collapse of the ice sheets, there’s potentially a series of connected crises lurking in our future that we need to avoid at all costs,” he says. The next step, he says, is to figure out whether the alkalinity mechanism is triggered under more moderate emissions scenarios.

The study’s findings would help scientists make better projections about future climate change, says coauthor Nikki Lovenduski, a professor at the University of Colorado.

“This paper demonstrates that the climate change problem may be exacerbated by things that are as yet unknown,” she says. “But the ocean climate feedback mechanism this particular study revealed will open up new avenues of research that will help us better understand the carbon cycle, past climate change, and perhaps come up with solutions for future problems.”

The study appears in Geophysical Research Letters and received funding from the National Science Foundation.

Source: UT Austin