In order to predict the human-induced climate change of the next century or so, climate scientists are desperate to understand past natural shifts. One of the most important, and most puzzling, occurred around a million years ago, and new research suggests it had a lot to do with the closing of the Bering Strait.
For the past 600,000 years, the Earth has experienced 100,000-year-long Ice Ages punctuated by shorter interglacials such as the one we are in now. The causes of these are well understood, being driven by cycles in our planet's orbit. Previously, however, cold periods were milder and lasted 40,000 years. The shift is known as the Mid-Pleistocene Transition (MPT), starting 1.2 million years ago and accelerating between 900,000 and 600,000 years before now. Many explanations have been proposed for its causes, but none have proven convincing.
A paper in Nature Communications presents an argument that the crucial factor was blocking of flow between the Arctic and Pacific Oceans.
"The subarctic North Pacific is composed of some of the oldest water on Earth, which has been separated from the atmosphere for such a long time that a high concentration of dissolved CO2 has built up at depth,” said Dr Sev Kender of the University of Exeter in a statement. “When this water upwells to the surface, some of the CO2 is released.” However, this deep water rises much less frequently in the North Pacific than similar waters in the Southern Ocean, helping keep atmospheric carbon levels down during the last Ice Age.
Using sediment cores from the Bering Strait region Kender and colleagues found that during the MPT the waters of the North Pacific became more stratified. The upwelling of waters from the bottom of the oceans was reduced to a minimum, preventing the release of deep-water carbon dioxide.
Kender argues this stratification reflected changes to the Bering Strait that prevented cold water reaching the Arctic Ocean, where it would have risen to the surface. The closure could have been a result of sediment build-up, or because the Earth at the time had cooled just enough for glaciers to block the Strait. Either way a feedback loop began, with colder temperatures meaning enough ice to keep the Strait closed for the majority of the last 900,000 years and intensifying cold spells.
The flow of water through the gap in the brief warm periods since then has been insufficient to restore the Earth's previous carbon balance, until industrialization began. We've already released enough carbon dioxide to restore the world to pre-MPT conditions, and current emission rates will soon induce atmospheric concentrations not seen for 15 million years.