What Caused the Last Glacial Maximum? Ice Age Climate Puzzle Solved

About 21,000 years ago, the Last Glacial Maximum (LGM) occurred. At the time, our planet's northern continents were covered in vast ice sheets, and carbon dioxide levels were well below levels seen before the Industrial Revolution. Now, though, scientists are uncovering exactly why the LGM occurred and what causes our planet to undergo these fluctuations in climate.

"We have all of these scattered pieces of information about changes in the ocean, atmosphere, and ice cover, and what we really want to see is how they all fit together," said Raffaele Ferrari, one of the researchers, in a news release.

In fact, it's the ocean itself that may be to blame for changing climatic conditions. That's why scientists took a closer look at the Southern Ocean in particular, which encircles Antarctica. This body of water is a critical part of the carbon cycle since it provides a connection between the atmosphere and the deep ocean abyss. Deep, carbon-rich waters rise to the surface in this area to "breathe" CO2 in and out.

While the modern Southern Ocean has lots of room for waters to mix and churn out CO2, that wasn't the case during the LGM. Permanents sea ice covered far more of the Southern Ocean's surface, which prevented gas exchange. In order to see what effect this might have on the atmosphere and climate, the researchers turned to equations.

The scientists calculated the amount of water that was trapped under the sea ice by currents in the LGM. In the end, they found that the ice cover created a huge impact on the Earth's processes. It turns out that, in fact, the Southern Ocean's CO2 was never exhaled into the atmosphere during the LGM. This, in particular, explains the big drop in atmospheric CO2 during this time period.

The findings reveal that there's a dynamic link between sea-ice expansion and the increase of ocean water insulated from the atmosphere. This explains not only the low levels of atmospheric CO2 during the LGM, but also during other glacial periods.

"To me, that means that all the events that co-occurred must be incredibly tightly linked, without much freedom to drift beyond a narrow margin," said Ferrari in a news release. "If there is a causality effect among the events at the start of an ice age, then they could happen in the same ratio."

The findings are published in the journal Proceedings of the National Academy of Sciences.