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Sea ice trapped atmospheric carbon dioxide in the last ice age. Pearse Buchanan, Author provided

Carbon dioxide trapped by ice-age oceans raises questions for future

Over the past few decades, scientists have monitored the atmosphere and oceans using instruments, gauges and satellites. But modern climate variability remains small compared to what we can expect in the future due to human emission of carbon dioxide.

How can we be sure we understand how climate and carbon dioxide are linked? Our research examines the climate of past ice ages, revealing what we know – and still need to learn – about climate and carbon dioxide.

For the past million years or so, Earth’s climate has gone through regular ice age cycles. Ice ages were fundamentally different from warm periods. Global temperatures were about three to five degrees Celsius cooler. Large ice sheets blanketed North America and Eurasia, displacing the expansive forests found there today. Deserts expanded and moved more dust over land and seas. Sea ice covered the ocean around the poles, and ocean currents changed.

Ancient air bubbles trapped in the ice from Antarctica have also provided a glimpse of how the ice age atmosphere was different. Carbon dioxide levels were about one-third lower during ice ages compared to warm periods, and less than half of what our atmosphere holds today due to carbon emissions.

Ocean key to low carbon dioxide

Ever since scientists first discovered carbon dioxide levels were low during ice ages, they have been proposing theories to understand why. It’s widely accepted that the ocean lies at the centre of the carbon dioxide puzzle because the ocean holds about 60 times more carbon than the atmosphere. Many studies have focused on how carbon dioxide escaped from the ocean when the last ice age ended. But how did the carbon dioxide get there when the Earth cooled?

As climate scientists, we have long shared a fascination with the ice age carbon dioxide puzzle, and so decided to examine the fate of the ocean as the Earth moved into the last ice age, and how the ocean pulled carbon dioxide out of the atmosphere and into the deep sea.

The last warm period ended around 125,000 years ago, when our human ancestors were still in Africa. The planet cooled for more than 100,000 years until it reached the coldest part of the last ice age, 20,000 years ago. While the planet was cooling, carbon dioxide levels also dropped over three key periods between 125,000 and 20,000 years ago. With our research study, we hoped to understand how the ocean’s cooling behaviour was linked to drops in carbon dioxide.

Fossils yield clues

Direct measurements only give us information about ocean temperatures for about the last 100 years, so we turned to chemical and biological clues left by tiny fossils from the sea floor.

Animals called foraminifera live at the ocean surface and form shells the size of sand grains. When they die, their shells fall to the sea floor and leave behind an important record of the temperatures in which they lived. By counting the numbers of cold versus warm species, scientists can estimate past ocean temperatures.

We trawled the scientific literature for studies of past sea surface temperatures. In total, we found data from 136 locations around the globe, amounting to more than 40,000 estimates of temperature.

Foraminifera are tiny animals that live at the ocean surface. Josef Reischig, CC BY

Our findings show that carbon took different paths into the deep sea during each step. The first drop in carbon dioxide occurred 115,000 to 100,000 years ago. During this time, both polar oceans cooled significantly and sea ice expanded dramatically around Antarctica. But we found no evidence that the large-scale circulation of the deep-sea changed during this time.

This is significant, because many theories explaining the carbon dioxide puzzle involve changes in deep ocean circulation. So the most likely cause for the first major drop in carbon dioxide levels was an expansion of sea ice around Antarctica. Sea ice acts as a lid on the waters of the Southern Ocean, also known as the Antarctic Ocean, and prevents them from releasing their carbon to the atmosphere.

The second drop in carbon dioxide levels happened around 70,000 years ago. Ocean temperatures cooled even further, this time accompanied by changes in deep ocean circulation. There is also evidence of an uptick in the ocean’s biological production at this time. Fuelled by nutrients added to the ocean from desert dust, tiny ocean plants helped to pump carbon from the surface ocean into the deep sea.

How did carbon dioxide enter the oceans?

We conclude that a reorganization of deep ocean circulation, triggered by ocean cooling and even more sea ice in the Southern Ocean, was responsible for trapping most of the carbon dioxide in the ocean during this time.

The final slide into the last ice age saw the last additional uptake of carbon dioxide by the ocean. During this time, it was “all systems go” to trap carbon in the ocean, including strong cooling of the ocean surface (which allows seawater to hold more carbon dioxide), sea ice acting as a lid, an amped-up biological cycle and sluggish deep ocean circulation acting to retain carbon.

We combined the individual efforts of hundreds of scientists to show, for the first time, how global ocean temperatures were linked with carbon dioxide changes as the Earth entered the last ice age. This new understanding hints at some important thresholds in the climate system.

It’s clear that some parts of the system - such as sea ice around Antarctica - respond rapidly when the ocean cools. Other parts, like deep ocean circulation, change very little at first, until a nudge from extra cooling pushes the system into a new state.

But far from solving the carbon dioxide puzzle, this work raises new questions. For example, we still don’t have a good handle on how much sea ice was changing when the ice age began.

It also points to gaps in existing theories. Our work shows the deep ocean circulation changed dramatically around 70,000 years ago, but we still need to understand how. Our next step is to combine our new temperature database with ice age models to put some of these theories to the test.

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