New research from the University of St Andrews has uncovered a thus unknown process which may have helped drive the Earth in to past ice ages.
Published today in Journal of Climate , the paper links ice sheets in the Northern Hemisphere to changes in ocean circulation and sea ice in the Southern Hemisphere understood to drive down atmospheric CO2. Linking these two key features of the ice ages has long been a challenge, given how far apart the two regions are.
Earth was mainly driven into ice ages by more CO2 being stored in the ocean, which is a process largely driven by changes in the Southern Ocean around Antarctica. Yet how the initial growth of large ice sheets in the Northern Hemisphere (brought about by natural cycles in the Earth's orbit) then lead to changes in the far-flung Southern Ocean which led to CO2 drawdown, was unknown.
This new paper, proposes that, once northern ice sheets start growing, they cool nearby North Atlantic waters, which eventually circulate all the way down to the Southern Ocean. If the cooling of the northern waters is strong enough, it could induce significant circulation changes in the south, of the sort is known lead to more CO2 being stored in the ocean.
Researchers tested this hypothesis in ocean model simulations by only cooling the far northern latitudes (mimicking the effect of a large ice sheet) and measuring whether it affected the Southern Ocean. The results dramatically demonstrated that cooling the northern latitudes alone caused nearly the same amount of CO2 drawdown (74%) as a simulation where researchers cooled the entire planet.
This result firmly backs up the idea that ice sheets can cool northern waters which eventually make their way south and cause changes in the Southern Ocean – having been an important part of driving Earth into its past ice ages.
Historically, scientists studying ice ages have long focused on the Southern Ocean, and it's only recently that other remote parts of the ocean have started to be more appreciated.
Lead author Dr Madison Shankle from the School of Earth and Environmental Science said; "Essentially, we've produced evidence of a plausible mechanism connecting these two far-flung puzzles pieces – Northern Hemisphere ice sheets and Southern Hemisphere ocean circulation changes leading to CO2 drawdown – which has been a tricky thing to explain in our theory of Earth's past ice ages. Our study takes the step of actually demonstrating them working in model simulations, which is a big step forward for this field."
The ice ages are natural case studies of how carbon moves in and out of the atmosphere. The more known about them, the more researchers can understand how Earth naturally moves carbon around in the carbon cycle. This knowledge is critical in considering how the planet will respond to the added CO2 within the Earth system today.
