Here’s how the ice age trapped carbon dioxide in oceans

The ocean: not just scenic, but a good carbon dioxide sink. Image: Getty.

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. Image: Josef Reischig/Wikipedia Commons.

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.

The ConversationIt 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.

Karen Kohfeld is professor of climate, oceans, and paleo-environments at Simon Fraser University. Zanna Chase is associate professor at the University of Tasmania.

This article was originally published on The Conversation. Read the original article.


Covid-19 is highlighting cities' unequal access to green space

In the UK, Londoners are most likely to rely on their local park for green space, and have the best access to parks. (Leon Neal/Getty Images)

As coronavirus lockdowns ease, people are flooding back to parks – but not everyone has easy access to green space in their city.

Statistics from Google show that park attendance in countries across the globe has shot up as people have been allowed to move around their cities again.

This is especially true in urban areas, where densely populated neighbourhoods limit the size of private green space – meaning residents have to go to the park to get in touch with nature. Readers from England can use our interactive tool below to find out how much green space people have access to in their area, and how it compares to the rest of the country.


Prime Minister Boris Johnson’s announcement Monday that people are allowed to mingle in parks and gardens with groups of up to six people was partially following what people were doing already.

Data from mobile phones show people have been returning to parks across the UK, and also across Europe, as weather improves and lockdown eases.

People have been returning to parks across the world

Stay-at-home requirements were eased in Italy on 4 May, which led to a flood of people returning to parks.

France eased restrictions on 1 May, and the UK eased up slightly on 13 May, allowing people to sit down in public places so long as they remain socially distanced.

Other countries have seen park attendance rise without major easing of lockdown – including Canada, Spain, and the US (although states there have individual rules and some have eased restrictions).

In some countries, people never really stopped going to parks.

Authorities in the Netherlands and Germany were not as strict as other countries about their citizens visiting local parks during lockdown, while Sweden has famously been avoiding placing many restrictions on people’s daily lives.

There is a growing body of evidence to suggest that access to green space has major benefits for public health.

A recent study by researchers at the University of Exeter found that spending time in the garden is linked to similar benefits for health and wellbeing as living in wealthy areas.

People with access to a private garden also had higher psychological wellbeing, and those with an outdoor space such as a yard were more likely to meet physical activity guidelines than those without access to outdoor space. 

Separate UK research has found that living with a regular view of a green space provides health benefits worth £300 per person per year.

Access is not shared equally, however, which has important implications for equality under lockdown, and the spread of disease.

Statistics from the UK show that one in eight households has no garden, making access to parks more important.

There is a geographic inequality here. Londoners, who have the least access to private gardens, are most likely to rely on their local park for green space, and have the best access to parks. 

However the high population in the capital means that on the whole, green space per person is lower – an issue for people living in densely populated cities everywhere.

There is also an occupational inequality.

Those on low pay – including in what are statistically classed as “semi-skilled” and “unskilled” manual occupations, casual workers and those who are unemployed – are almost three times as likely as those in managerial, administrative, professional occupations to be without a garden, meaning they rely more heavily on their local park.

Britain’s parks and fields are also at significant risk of development, according to new research by the Fields in Trust charity, which shows the number of people living further than a 10-minute walk from a public park rising by 5% over the next five years. That loss of green spaces is likely to impact disadvantaged communities the most, the researchers say.

This is borne out by looking at the parts of the country that have private gardens.

The least deprived areas have the largest gardens

Though the relationship is not crystal clear, it shows at the top end: Those living in the least deprived areas have the largest private green space.

Although the risk of catching coronavirus is lower outdoors, spending time in parks among other people is undoubtedly more risky when it comes to transmitting or catching the virus than spending time in your own outdoor space. 

Access to green space is therefore another example – along with the ability to work from home and death rates – of how the burden of the pandemic has not been equally shouldered by all.

Michael Goodier is a data reporter at New Statesman Media Group, and Josh Rayman is a graphics and data visualisation developer at New Statesman Media Group.