Paleoclimatologist Discovers Ancient Climate Feedback Loop That Accelerated Effects of Earth's Last Warming Episode

In the context of rapid global warming, the need to better understand the character and long-term impact of positive climate feedback loops – processes that accelerate the effects of warming – becomes of crucial relevance.

One way to assess the role and impact of climate feedback processes is to use modeling studies to examine the possible future based on what we know now. Climate projection models, for example, are the tools behind the world wide warming threshold of 1.5°C adopted by the Intergovernmental Panel on Climate Change.

Alternatively, you can look into the past to see what happened when Earth was up to 1-1.5°C warmer than it is today. That’s 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.

For a paleoclimate perspective on global warming, Weldeab and his colleagues colleagues reassembled about 128 000 to 128 ago 000 years at the peak of the hot Eemian 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 midwaters we have reconstructed.”

Typically, warm, salty water moves north from the tropics the extensive of the ocean floor and cools as it reaches the middle and high northern latitudes. At this point, the now colder and denser water falls into the deep sea and back down towards 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 Atlantic North is that it disrupts ocean circulation and reduces cold water advection in the intermediate depth of the tropical Atlantic, and therefore warms the waters at this depth,” he added. he said.

While previous studies have discussed the disturbance that water of melting caused to currents and temperatures at intermediate depths, the new doc reveals that this warming was “furthermore vital than previously thought”.

“We show a hitherto undocumented and remarkably crucial 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. As 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 seafloor.

Weldeab explained that at high pressure and low temperature, the introduction of unusually warm water heats the sediments of the sea floor and the gases encapsulated in the ice begin to dissolve, releasing methane. Weldeab and his colleagues used carbon isotopes (125C/000C) in the shells of microorganisms to discover the fingerprint of methane release and methane oxidation in the water column.

“This is one of the many amplified climate feedback processes where a warming climate has caused an accelerated melting of the ice sheet,” he said. “The melt water weakened the ocean circulation and as a result the waters at intermediate depth warmed considerably, leading to the destabilization of shallow subterranean methane hydrates and the release of methane, a potent greenhouse gas .”

It is unclear 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 happen exactly as we found each condition is different, but it gives you a direction to look in.”

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