Predicting future warming from our past

Southampton-led research on prehistoric levels of carbon dioxide in the oceans gives an insight into the Earth’s future warming. Dr Gavin Foster, from Ocean and Earth Science talked to the University of Southampton's New Boundaries magazine to elaborate on this high impact research.

During the Pliocene, the Earth was around 2ºC warmer than it is today and atmospheric CO2 levels were around 350 to 400 parts per million (ppm), similar to the levels reached in recent years. By studying the relationship between CO2 levels and climate change during a warmer period in Earth’s history, the scientists have been able to estimate how the climate will respond to increasing levels of CO2 in the future, a parameter known as ‘climate sensitivity’. The findings, which have been published in Nature, also show how climate sensitivity can vary over the long term.

“Today the Earth is still adjusting to the recent rapid rise of CO2 caused by human activities, whereas the longer-term Pliocene records document the full response of CO2-related warming,” says Dr Gavin Foster, Associate Professor of Isotope Geochemistry at the University, who co-led the study.

Innovative method

The international team, which also included academics from the Autonomous University of Barcelona and the Australian National University in Canberra, studied the composition of shells of ancient marine organisms that inhabited the surface waters of the ocean millions of years ago to measure its carbon content.

The established way of studying the atmosphere’s prehistoric composition is to analyse ice cores taken from Antarctica. “These ice cores give us the well-documented climate cycles every 100,000 years: warm ‘interglacials’, where CO2 levels are high, followed by cold ‘glacials’, where CO2 levels are low,” explains Gavin. “However, the ice cores only go back around 800,000 years and none of the interglacials during this period have been as warm as our climate today.” So in order to get an insight from the past about our current climate, a different approach was needed.

Gavin and his team developed a new way of measuring the CO2 content of ancient oceans: by analysing the ratio of two isotopes of the element boron – boron 10 and boron 11 – in the calcium carbonate of shells and skeletons of fossilised plankton-like organisms called foraminifera. The ratio of the boron isotopes indicates the pH that the shells originally grew at.

“By measuring the boron isotopic composition of these ancient fossils we can get an idea of what the ocean pH, or acidity, was like millions of years ago,” says Gavin. “That is important because the acidity of the ocean is determined by the amount of carbon that is dissolved in it, and, on long timescales, the amount of carbon in the ocean will determine the amount of carbon in the atmosphere. So we get a measure of the CO2 variability in the oceans and atmosphere through time.”

The advantage of looking at marine carbonates is that sediment cores from the sea bed give us at least 65 million years’ worth of data, Gavin explains. Drill ships take samples in the deep ocean as part of the Integrated Ocean Drilling Program, an international consortium of which the UK is a member, and this provides a valuable archive that researchers can study. The challenge is analysing these samples to reveal atmospheric CO2 in the past. “We used the boron isotopes to look at the relationship between CO2 change, sea level change and global temperature to give us an idea of how the Earth’s system behaved in the past, which helps us hone our predictions of how the warm climate of the future might play out,” says Gavin.

Warming world

By analysing the boron content in the foraminifera, Gavin and his team have reconstructed the atmospheric CO2 during the Pliocene. They found that the climate sensitivity was the same during the warmer Pliocene as during colder glacial periods that had previously been measured from the ice cores, once they had allowed for other factors such as reflective ice sheets covering large parts of the world during glacial periods.

The finding is exciting because it helps us understand how today’s climate is responding to rising CO2 levels. “As the climate warms, you might expect more temperature change for a given CO2 change because of amplifying factors such higher atmospheric water vapour content as the world gets warmer,” says Gavin. “Lack of understanding of these feedbacks leads to all the uncertainty around what the temperature of the future is going to be because we don’t yet know how all the feedbacks are going to play out. What we showed was that as the Earth got warmer or colder in the past, the feedbacks were operating at the same level.”

Getting an accurate measure of climate sensitivity is crucial for predicting the future climate. Climate sensitivity is thought to be around three degrees every time CO2 is doubled. “If we continue to burn fossil fuel at the current rate, by 2100 we are looking at about four degrees warming – perhaps a little more. If we are way off on our climate sensitivity estimate, by the time we get to 2100 it might well be more like five, six or seven degrees,” says Gavin.

“Our findings use the geological record to support the climate sensitivity quoted by the Intergovernmental Panel on Climate Change (IPCC) based on different observations, for example the modern climate system and the more recent geological record,” says Gavin, who was a contributing author on the last IPCC report. “The temperatures we see in the Pliocene are probably going to be reached on earth by around 2080 if we carry on producing CO2 at the current rate,” he adds.

World-leading expertise

This study is a particularly important contribution to the knowledge on past climates because of its high resolution: 100 data points were collected in a painstaking process, each of which took about two weeks to complete.  “Here at the University of Southampton we have the largest boron isotope group in the UK – we are one of the few labs in the world that can do this type of analysis,” says Gavin. Having the capability to achieve such high resolution data is crucial because the more data available, the better the reconstructions of past climates can be. “It’s not just the change in CO2 between the modern and the Pliocene that we are interested in,” says Gavin. “52 million years ago there was the Eocene climatic optimum – where global  temperatures were 12 to 14 degrees warmer than today – so by looking further into the past we are looking further into the future of our climate.”

For more information visit www.decentintotheicehouse.org or visit Gavin's research pages on our website.