In the context of rapidly warming Earth, a better understanding of the nature and long-term effects of positive climate feedback loops (the processes that accelerate the effects of warming) becomes critical.
One way to assess the role and impact of climate feedback processes is to use modelling studies that look to possible futures based on what we know now. For example, climate prediction models are the tools behind the 1.5°C global warming threshold adopted by the Intergovernmental Panel on Climate Change.
Alternatively, you can look back in time and see what happened when the Earth was 1-1.5°C warmer than it is today.This is what Syee Weldeab of the University of California, Santa Barbara, published in Proceedings of the National Academy of SciencesA professor of paleoclimatology has discovered a feedback process that has important implications for our modern-day continuing warming.
To understand global warming from a paleoclimate perspective, Weldeab and his colleagues went back to the peak of the Hermian Warm Period some 128,000 to 125,000 years ago. The ocean was 1-1.5°C warmer than the Holocene (our current geological age). The authors examined marine sediments in the tropical Atlantic and found that the intermediate water column warmed unusually strongly during brief intervals between the peaks of the Emiya Warm Period.
“Remarkably, the greatly reduced Greenland ice sheet was able to generate enough meltwater to disrupt the density-driven circulation in the Atlantic,” Weldeab said. “This contributes significantly to the substantial warming of the intermediate waters that we reconstruct.”
Typically, warm, salty water travels northward along the ocean surface from the tropics and cools as it reaches the northern mid-high latitudes. At this point, the now cooler, denser water drops into the deep ocean and back to the tropics. The interaction of this density difference resulted in the electrical currents we are familiar with today.
“What happens when you inject a lot of fresh water into the North Atlantic, basically it disrupts ocean circulation and reduces the mid-depth where cold water flows into the tropical Atlantic, thereby warming the waters at that depth,” he said.
While previous studies have discussed the disruption meltwater can cause to ocean currents and temperatures at intermediate depths, the new paper suggests that this warming is “larger than previously thought.”
“We demonstrate a hitherto unrecorded and very large water warming at intermediate depths, showing a temperature increase of 6.7°C above the average background value,” Weldeab said.
This unusually strong warming can have serious consequences, as the warm water hits ocean sediments that contain large amounts of methane hydrate, a mix of frozen water and methane. These deposits are located just below the seafloor surface.
At high pressure and low temperatures, the introduction of unusually warm water heated seafloor sediments, and the gas encased in ice began to dissolve, releasing methane, Weldeab explained. Weldeab and colleagues used carbon isotopes (13C/12C) in microbial shells to reveal the fingerprints of methane release and methane oxidation in the water column.
“This is one of several amplifying climate feedback processes in which warming causes accelerated melting of ice sheets,” he said. “Meltwater weakens ocean circulation and, as a result, waters at intermediate depths warm significantly, destabilizing shallow subsurface methane hydrates and releasing methane, a potent greenhouse gas.”
It is uncertain whether this feedback loop will play a role in this round of global warming, although human-induced warming is occurring at a faster rate than during the Emician period. According to the researchers, the findings “record and connect a series of climate events and climate feedback processes associated with and triggered by the penultimate climate warming peak and can serve as paleosimulations of modern, sustained warming.”
“The paleo perspective is a useful way to help us assess what might have happened,” Weldeab said. “It doesn’t have to happen like we found; every situation is different, but it will give you a direction.”
As climate changes, the deep ocean warms
Syee Weldeab et al., Evidence for massive methane hydrate instability during the penultimate interglacial warming, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2201871119
Courtesy of UC Santa Barbara
Citation: Paleoclimatologists discover ancient climate feedback loops that accelerated the effects of Earth’s last warming event (22 August 2022), accessed 23 August 2022 from https://phys.org/news /2022-08-paleoclimatologist-uncovers-ancient-climate-feedback Retrieved. html
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