Findings show delicate balance of ice sheets, winds and ocean currents
FRISCO — The superstorm depicted in “The Day After Tomorrow” may be completely implausible, but that doesn’t mean the Earth’s climate system is always as stable as it seems now.
New research by a team of scientists at the Alfred Wegener Institute shows how there may have been significant shifts in ocean circulation and wind patterns that happened in the span of just a few decades — not even the blink of an eye by geological time standards.
Specifically, the researchers looked a series of rapid climate shifts known in the scientific world as the Dansgaard-Oeschger events, about 110,000 to 23,000 years ago.
“The rapid climate changes … did not take place at the extreme low sea levels, corresponding to the time of maximum glaciation 20,000 years ago, nor at high sea levels such as those prevailing today. They occurred during periods of intermediate ice volume and intermediate sea levels,” said researcher Xu Zhang, who used climate models and data from seafloor sediments to trace the climate shifts.
During the cold periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong westerly winds drove the Arctic sea ice southward, preventing the exchange of heat between the atmosphere and the ocean. Without that exchange, ocean circulation weakened, transporting less heat to the north,
During longer phases of the cold glacial period, ice sheets thickened over North America, lowering sea level. At the same time, the prevailing westerly winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north.
Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean.
According to the study, a thicker North American ice sheet strengthened ocean circulation, dramatically warming the northern hemisphere for a few decades until the glaciers once again retreated.
“Using the simulations performed with our climate model, we were able to demonstrate that the climate system can respond to small changes with abrupt climate swings,” said Professor Gerrit Lohmann, leader of the Paleoclimate Dynamics group at the Alfred Wegener Institute.
“At medium sea levels, powerful forces, such as the dramatic acceleration of polar ice cap melting, are not necessary to result in abrupt climate shifts and associated drastic temperature changes,” Lohmann said.
At present, the extent of Arctic sea ice is far less than during the last glacial period. The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Climate changes following the pattern of the last ice age are therefore not to be anticipated under today’s conditions.
“There are apparently some situations in which the climate system is more resistant to change while in others the system tends toward strong fluctuations,” Lohmann said. “In terms of the Earth’s history, we are currently in one of the climate system’s more stable phases. The preconditions which gave rise to rapid temperature changes during the last ice age do not exist today. But this does not mean that sudden climate changes can be excluded in the future.”