In the context of a rapidly warming planet, the need to better understand the nature and long-term impact of positive climate feedback loops – processes that accelerate the effects of warming – becomes of crucial importance.
One way to assess the role and impact of climate feedback processes is to use modeling studies to examine the likely future based on what we know now. Climate projection models, for example, are the tools behind the 1.5°C global warming threshold adopted by the Intergovernmental Panel on Climate Change.
Alternatively, you can look into the past to see what happened when the Earth was up to 1-1.5°C warmer than today. This is what Syee Weldeab of UC Santa Barbara did in an article published in the Proceedings of the National Academy of Sciences. The paleoclimatology professor has discovered feedback processes that have concerning implications for our modern, ongoing warming.
To get a paleoclimate perspective on global warming, Weldeab and his colleagues dated back to around 128,000 to 125,000 years ago at the peak of the Eemian Warm Episode. The oceans were up to 1-1.5°C warmer than during the Holocene (our current geological epoch). The authors examined tropical Atlantic marine sediments and found unusually strong warming of the intermediate water column during a brief interval during the peak Eemian warm episode.
“Remarkably, a significantly reduced Greenland ice sheet was able to produce enough meltwater to disrupt the density-driven Atlantic Ocean circulation,” Weldeab said. “This has contributed significantly to the significant warming of the intermediate waters that we have reconstructed.”
Typically, warm, salty water moves north from the tropics along the surface of the ocean and cools as it reaches the middle and high northern latitudes. At this point, the now cooler and denser water falls into the deep sea and back down to the tropics. This interaction of density differences results in the currents we know today.
“What happens when you put a lot of fresh water into the North Atlantic is that it disrupts ocean circulation and reduces cold water advection in the middle depth of the tropical Atlantic, and therefore warms the waters at this depth,” he added. he said.
While previous studies have discussed the disruption that meltwater caused to currents and temperatures at intermediate depths, the new paper finds that this warming was “greater than previously thought”.
“We show a hitherto undocumented and remarkably large warming of the water at intermediate depths, exhibiting a temperature increase of 6.7°C over the mean background value,” Weldeab said.
This exceptionally strong warming has serious consequences, because the warm water encroaches on the marine sediments which contain abundant methane hydrates, a mixture of frozen water and methane. These deposits are not far below the surface of the seabed.
Weldeab explained that at high pressure and low temperature, the introduction of unusually warm water heats up the seafloor sediments and the gases encapsulated in the ice begin to dissolve, releasing methane. Weldeab and his colleagues used carbon isotopes (13C/12C) in the shells of microorganisms to uncover the fingerprint of methane release and methane oxidation in the water column.
“This is one of many amplified climate feedback processes where global warming has caused accelerated ice sheet melting,” he said. “The meltwater weakened ocean circulation and as a result waters at intermediate depth warmed considerably, leading to destabilization of shallow subterranean methane hydrates and release of methane, a potent greenhouse gas .”
It is not known for certain whether this feedback cycle will play out in the current cycle of global warming, although anthropogenic activity has created a higher rate of warming than that which occurred during the Eemian period. These findings, the researchers say, “document and connect a sequence of climate events and climate feedback processes associated with and triggered by the penultimate peak of global warming that may serve as a paleo-analogue to ongoing modern warming.” .
“The paleo perspective is a useful approach to help us assess what might happen,” Weldeab said. “It doesn’t have to be exactly the way we found it; every situation is different, but it gives you a direction to look in.”
Deep ocean warming as climate changes
Syee Weldeab et al, Evidence for massive methane hydrate destabilization during penultimate interglacial warming, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2201871119
Provided by University of California – Santa Barbara
Quote: Paleoclimatologist discovers ancient climate feedback loop that accelerated effects of Earth’s last warming episode (2022, August 22) Retrieved August 23, 2022 from https://phys.org/news/2022-08-paleoclimatologist -uncovers-ancient-climate-feedback. html
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