Study: Ocean Surface Tipping Point Could Accelerate Climate Change

By Lynnette Harris, lynnette.harris@usu.edu
and Constantino Panagopulos, costa@ig.utexas.edu

The oceans help to limit global warming by soaking up carbon dioxide emissions. But in a recently published study scientists have discovered that intense warming in the future could lessen that ability, leading to even more severe warming. The researchers developed the first model that demonstrates that changes in rainfall and ocean temperatures will change ocean chemistry at the surface.

The discovery comes from a study led by Megumi Chikamoto, who began the work at the University of Texas at Austin and is currently teaching in Utah State University's Department of Plants, Soils and Climate. Chikamoto and co-authors at UT Austin and the University of Colorado

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 a surface layer of low-alkalinity water that emerges during extreme warming and hinders the ability of the oceans to absorb carbon dioxide. Alkalinity is a chemical property that affects how much carbon dioxide 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 remove an important buffer of climate change severity, the authors said.

“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,” said Chikamoto, who led the study as a research fellow at the University of Texas Institute for Geophysics. “As we noted in the research summary, the oceans have been doing us a big favor by absorbing some carbon dioxide and lowering the amount of warming that occurs, but it will begin to lose its ability to do that by 2100.”

The study, funded by the National Science Foundation, was published in the journal Geophysical Research Letters with the title Long-Term Slowdown of Ocean Carbon Uptake by Alkalinity Dynamics.

Today, the oceans soak up about a third of the carbon dioxide emissions generated by humans. Climate simulations had previously shown that the oceans slow their absorption of carbon dioxide 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 carbon dioxide, its alkalinity falls and with it, its ability to absorb carbon dioxide. The end result is a surface layer that acts like a barrier for carbon dioxide absorption. That means less of the greenhouse gas goes into the ocean and more of it is left in the atmosphere. This in turn produces faster warming, which sustains and strengthens the low-alkalinity surface layer.

Co-author Pedro DiNezio, an affiliate researcher at the University of Texas Institute for Geophysics and associate professor at University of Colorado, said that the discovery was a powerful reminder that the world needs to reduce its carbon dioxide emissions to avoid crossing this and other tipping points.

“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 said. The next step, he said, is to figure out whether the alkalinity mechanism is triggered under more moderate emissions scenarios.

Co-author Nikki Lovenduski, a professor at the University of Colorado who contributed to the Intergovernmental Panel on Climate Change 2021 climate report, said that the study’s findings will help scientists make better projections about Earth’s climate future.

“This paper demonstrates that the climate change problem may be exacerbated by things that are as yet unknown,” she said. “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 and past climate change and perhaps come up with solutions for future problems.”