(Re)Confirmed: Earth’s Wobble, Plus CO2, Ended Last Ice Age
April 5, 2012
As the Earth’s orbital and axial characteristics changed over thousands of years, increases in corresponding solar incidence at key northern latitudes gradually brought the planet out of the last ice age. This is the picture that has long been the best fit for the observations, and has now been further confirmed by a team from Oregon State.
The prevailing view over the last decade has been that the change in insolation gradually brought about a rise in greenhouse gases, which took a long time to begin percolating out of ocean and soils, and were a critical component of the forcing that brought the planet into the current interglacial. This lag has often been deviously used by denialists in arguing that greenhouse gases have no effect on global temps. The video above tells that story.
One of the difficulties in telling this story, is that climate scientists have been putting together a puzzle using only one piece – the ice core data from a single location in Antarctica. The “lag” issue emerged as an artifact of the paucity of data.
The new research, drawing on 80 proxy records from diverse locations around the planet to show that “temperature is correlated with and generally lags CO2 during the last (that is, the most recent) deglaciation”.
Rising levels of carbon dioxide have for the first time been definitively linked to the global warming that led to the last Ice Age.
A team at Oregon State University reconstructed globally averaged temperature changes during the end of the last Ice Age, in contrast to previous studies, which only compared CO2 levels with local temperatures.
They found that average temperature around the Earth correlated with – and generally lagged behind – rising levels of CO2.
“Carbon dioxide has been suspected as an important factor in ending the last Ice Age, but its exact role has always been unclear because rising temperatures reflected in Antarctic ice cores came before rising levels of CO2, says Jeremy Shakun, a former doctoral student at OSU and now a post-doctoral fellow at Harvard and Columbia.
“But if you reconstruct temperatures on a global scale – and not just examine Antarctic temperatures – it becomes apparent that the CO2 change slightly preceded much of the global warming, and this means the global greenhouse effect had an important role in driving up global temperatures and bringing the planet out of the last Ice Age.”
The theory is that the Earth’s natural wobble affected the amount of sunlight striking the northern hemisphere, melting ice sheets that covered Canada and Europe. Fresh water flowed into the Atlantic Ocean, where it formed a lid over the sinking end of the Atlantic Meridional Overturning Circulation.
This put an end to the movement of warm water up from the tropics which had delivered heat to the northern latitudes.
The discovery has implications, of course, for how human-generated carbon dioxide will affect the planet in future.
“CO2 was a big part of bringing the world out of the last Ice Age, and it took about 10,000 years to do it. Now CO2 levels are rising again, but this time an equivalent increase in CO2 has occurred in only about 200 years, and there are clear signs that the planet is already beginning to respond,” says Shakun.
The team believes that small changes in the Earth’s orbit around the sun affected the amount of sunlight striking the northern hemisphere, which caused the ice to melt in Canada and Europe.
This ice turned into fresh water, which flowed off of the continent and into the Atlantic Ocean, where it formed a lid cover the sinking end of the Atlantic Meridional Overturning Circulation. This area is part of a global network of currents that brings warm water up from the tropics and keeps Europe temperate despite its high latitudes.
The researchers said when the fresh water draining off the continent at the end of the Ice Age entered the North Atlantic, it essentially put the brakes on the current and disrupted the delivery of heat to the northern latitudes.
“When the heat transport stops, it cools the north and heat builds up in the Southern Hemisphere,” Shakun said in the press release. “The Antarctic would have warmed rapidly, much faster than the time it takes to get CO2 out of the deep sea, where it was likely stored.
“The warming of the Southern Ocean may have shifted the winds as well as melted sea ice, and eventually drawn the CO2 out of the deep water, and released it into the atmosphere,” Shakun said. “That, in turn, would have amplified warming on a global scale.”
The team found that the average temperature around the Earth at the end of the Ice Age correlated with rising levels of carbon dioxide.
Below, Richard Alley tries to explain the concept to an intellectually challenged congress critter.