Ancient ice holds clues to climate change

The cylinders, known as ice cores, are drilled from ice sheets in the regions. They are usually 10 centimetres in diameter and can be taken from up to 4 kilometres deep in the ice.

As the ice froze many thousands of years ago, artefacts such as air bubbles and dust particles were gradually trapped as the snow was compacted into ice. By extracting the cores, scientists can analyse the ice, and the air and dust trapped in it can be used to reconstruct the climate of the past.

Scientists can obtain a wealth of information from a single ice core – data about sea ice extent, past surface temperature, the chemical composition of air and more. Ice cores contain dust particles from South America or Australia, providing an indication of how air moved around the globe.

‘Air bubbles trapped in ice are like little time capsules that record the past atmospheric composition,’ said Dr Emilie Capron of the British Antarctic Survey. ‘So we measure loads of different gases and essentially we can measure greenhouse gases such as carbon dioxide and methane.’

Dr Capron was one of the scientists involved in the EU-funded PAST4FUTURE project, which extracted and analysed ice cores from Antarctica and Greenland and combined the results with other historical climate data to better understand past climate change.

The idea was to use this information to more accurately predict future climate change.

Interglacial periods

The project, which involved more than 170 scientists, gathered information from natural sources such as ice cores, marine sediment cores and land-based records. Researchers focused on collecting data for two time periods: the current interglacial period, which began around 11 000 years ago, and the last interglacial period, which occurred 129 000 to 116 000 years ago.

Climate information from the last interglacial period is particularly relevant because during that time temperatures on earth were significantly higher at the poles than they are now. The sea level was also between five and nine metres higher than current levels because of the melting of ice in Greenland and Antarctica.

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‘We can use past climates as a natural experiment on the earth’s system to unveil the way it reacts and the way it works.’

‘Under this warmer climate, Antarctica and Greenland were melting,’ said Dr Capron. ‘So we’d like to know under that former climate that was the last interglacial, how much contribution to sea level there was from Greenland, how much there was from Antarctica. It can give us a clue about how Greenland and Antarctica are going to react in the future to the fact that it’s getting warmer.’

The warming during the last interglacial period was due to natural causes, such as changes in the solar radiation hitting the earth due to the tilt of the earth on its axis. However, because no previous climate has been affected by human infuence, this warm period is probably the closest we can get to an estimation of what the future has in store.

Refining climate models

PAST4FUTURE researchers also tested the accuracy of existing climate change models by comparing the historic data collected from ice cores against what the models predicted for climate conditions during that period. New information revealed by the ice cores data was fed back into the climate change models to increase their accuracy.

‘For instance, colleagues produce climate model simulations at the same sites where we have information from ice cores, and we can then assess whether they are in agreement or not,’ said Dr Capron.

‘If they are, it suggests that the climate models are likely representing correctly the climate processes that were happening at that time. It gives us confidence in those models that are then used to make future climate predictions.’

As an example, researchers were able to demonstrate the importance of including the freshwater produced by melting ice sheets into existing climate models. By doing so, the models were able to correctly represent the difference in temperature changes between the northern and southern hemispheres that were observed at the beginning of the last interglacial period.

Refining the accuracy of climate change models in this way will help us more confidently predict what is likely to happen to the current climate under different scenarios, and help decide what action to take.

Dr Valérie Masson-Delmotte, a senior climate scientist at Institut Pierre Simon Laplace, France, who also worked on the PAST4FUTURE project, says that understanding the past is vital for increasing our confidence in the models used to anticipate the future. ‘The problem with climate change of the future is that we cannot measure it, we can only model it,’ she said.

‘We can use the last interglacial period as a benchmark to test the same models of climate and ice sheets that are used to explore possible future risks. We can use past climates as a natural experiment on the earth’s system to unveil the way it reacts and the way it works. And that’s really how we can make best use of past climates with respect to future risks.’