New research could explain why the Arctic was much warmer during a period millions of years ago that scientists say most closely resembles Earth's climate today.
The climate during the mid-Pliocene Epoch, roughly 3 million years ago, is a good overall match for conditions today with regard to average temperature and the level of carbon dioxide in the atmosphere. But there are some differences that scientists have had trouble explaining.
Analyses of fossils, pollen and chemicals contained in core samples of seafloor sediments suggest the North Atlantic Ocean and the Arctic were much warmer during the mid-Pliocene than they are today. But climate models haven't been able to accurately reproduce that past Arctic warming.
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Now, researchers in the United States and United Kingdom say the difference between the Arctic of the Pliocene and that of the present may boil down to shifts in the topography of the ocean floor -- in particular, of a group of underwater ridges and channels that extend from Greenland to Scotland.
Known as the Greenland-Scotland Ridge, the undersea mountain range divides the basin of the North Atlantic Ocean from those of the Nordic Seas. Researchers believe it was about 300 meters (roughly 984 feet) deeper during the mid-Pliocene.
"Geophysicists have studied this ridge in the past. It's a hot spot, with mantle coming up from underneath" the ocean floor, said lead author Marci Robinson, a micropaleontologist with the U.S. Geological Survey. "The Earth's crust will swell when it's hot and condense when it's cold, and that changes the height of the ridge."
When Robinson and her colleagues tweaked their climate model to account for the ridge's depth during the mid-Pliocene, "almost like magic, the data and the model results matched up really well," she said.
A help to 'hindcasting'
Today, the ridge traps North Atlantic Ocean water as it sinks to lower depths, redirecting it back toward the equator. But the new study suggests that during the mid-Pliocene, when the ridge was lower, it allowed deep ocean currents to flow freely. That ultimately increased the flow of warm surface water, leading to the unusual warming in the North Atlantic and Arctic.
Robinson said the study results could help improve the accuracy of climate models, leading to better predictions of the future climate.
That's because scientists who work with the models test them by seeing how well they can replicate the climate of the past, a process called "hindcasting." If a model can accurately reproduce past conditions, the thinking goes, researchers can be more confident about the model's ability to project the future climate.
But Robinson said there is still work to be done to test the new theory about the influence of the Greenland-Scotland Ridge on the Pliocene-era Arctic and North Atlantic Ocean.
She and her colleagues used a simplified model of the ocean ridge for their initial research, for example.
"The [paleoclimate] data say the ridges only dropped by about 300 meters," Robinson said. "In this study, we just removed them. It's the equivalent of dropping by about 800 meters, more than what's realistic. But this was a pilot study."
The research team plans to conduct new analyses that incorporate a more realistic picture of the drops and ridges, she said.
The study was published online last month in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. Its authors include researchers from the U.S. Geological Survey, the British Geological Survey and several universities in the United Kingdom.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500
