Biogeochemistry Paleoceanography Physical oceanography Sea Ice

Key Role of Sea Ice in Glacial Cycles

Source: Marzocchi, A., and M. F. Jansen (2019), Global cooling linked to increased glacial carbon storage via changes in Antarctic sea ice. Nature Geoscience. doi:10.1038/s41561-019-0466-8

Water is transported around the world’s oceans via a current system called the meridional overturning circulation (MOC), often referred to as the great ocean conveyer belt. Figure 1 below shows the approximate path of this conveyer belt–red indicates transport near the surface and blue represents deep transport. Along with water, these currents carry and redistribute heat, salt, and gasses. Therefore, the overturning circulation plays an important role in regulating the global climate system.

Figure 1: Schematic of the global overturning circulation (Robert Simmon via Wikimedia Commons)

Over the past 2.5 million years, Earth’s climate has varied between cold glacial and warm interglacial periods. This variability is linked to slight changes in Earth’s orbit, which determine the amount of warming from the sun. Rise and fall of atmospheric carbon dioxide (CO2) levels is also important in driving these glacial-interglacial cycles. Given the role of the ocean in regulating atmospheric CO2 concentrations, it follows that changes in the large-scale ocean circulation over time can also play a role in the glacial cycles. However, the relative importance of the ocean circulation in affecting these trends is not well understood.

Studying changes in the ocean circulation between glacial and interglacial periods is difficult since there is no observational data from thousands of years ago. Therefore, scientists have to infer past circulation using other methods such as analysis of sediment cores from the seafloor and ice cores from glaciers. These reconstructions are based on the properties of the atoms within the sediment or ice. For example, isotopes are different forms of the same element with varying numbers of neutrons. By measuring the ratio of different isotopes for a particular element in a sediment or ice sample, scientists can infer past temperature or carbon dioxide levels. This type of analysis suggests that the ocean circulation has experienced major glacial-interglacial modifications over the past 2.5 million years.

Figure 2: Aerial image of Antarctic sea ice (NASA/Nathan Kurtz via Wikimedia Commons)

In addition to reconstructions of oceanographic conditions from isotope ratios, scientists can also use numerical models to try to understand glacial cycles. A recent study in Nature Geoscience led by Alice Marzocchi at the National Oceanography Centre in the UK uses an idealized model of the Atlantic Ocean to investigate the role of Antarctic sea ice in forcing glacial cycles.

In the model simulations, Marzocchi lowered the atmospheric temperature to imitate the transition into a glacial period. As a result of this, Antarctic sea ice expanded and the deep ocean became more stratified. These changes are consistent with reconstructions of the circulation at the Last Glacial Maximum (LGM), about 21,000 years ago. Furthermore, the reorganization of waters in the deep ocean led to enhanced oceanic carbon uptake, which could account for up to half the glacial-interglacial variation in atmospheric CO2.

These results suggest that changes in Antarctic sea ice and circulation, triggered by atmospheric cooling, stimulate carbon drawdown and thus play a large role in glacial-interglacial transitions. By describing the mechanisms driving changes in oceanic carbon storage over long timescales, studies such as this are key to understanding glacial cycles.

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