Predicting the magnitude, and therefore the impact, of climate change is a huge challenge facing scientists. The most recent climate report issued by the Intergovernmental Panel on Climate Change (IPCC) reports that the Earth’s climate is on course to ultimately warm up by 1.5 to 4.5 degrees Celsius with a doubling in the CO₂ concentration compared to the Industrial Revolution. That is the equivalent of the Greenland ice sheet existing or not existing and a variation in sea level rise of 6 m.

Influence of ice sheets on temperature

Examining past climate behaviour can help reduce the uncertainty about the relationship between CO₂ and temperature. Prof Appy Sluijs is reconstructing the climate of millions of years ago, in both a relatively warm period with no polar ice (the early Eocene climatic optimum, 53-51 million years ago) and a warm period in which there was an ice sheet in Antarctica (the middle Miocene climatic optimum, 17-14 million years ago). Sluijs not only aims to find out what influence temperature has on the ice, but also what impact the ice sheets have on the temperature.

Two climatic characteristics

“I plan to use this grant to focus my research on two important characteristics of climate change,” explains Sluijs. “Firstly, we want to have a more precise idea of the extent to which the worldwide climate heats up when the CO₂ concentration doubles. Secondly, we want to know the extent to which the warming is more intense in polar areas compared to the tropics. The latter is partly caused by the fact that the disappearance of ice makes the surface darker, as a result of which it reflects less sunlight and the Earth’s surface heats up more. This polar amplification will also determine how much ice there is on land in the future and therefore influence the extent of rises in sea level. I will be studying a wide variety of climate changes in the Eocene and Miocene periods in order to effectively assess these two climate characteristics.”

Microscopically-small fossils

For his climate reconstructions, Sluijs is using microscopically-small fossils of single-cell organisms that lived in those periods. This is because the chemical composition of these fossils is related to the amount of CO₂ in the atmosphere, enabling Sluijs to extrapolate how much CO₂ there was in the atmosphere at that time. He is using other methods to investigate how warm it was in tropical and polar regions.

More accurate predictions

Estimates of future climate changes not only depend on the amount of CO₂ that humans will produce, but also the sensitivity of the climate to the increase in CO₂. The calculations of this sensitivity and the polar amplification that Sluijs and his team plan to do will serve as a crucial test case for our understanding of the climate. “This understanding is effective for the current climate, because climate models can simulate the current climate very accurately but we are less certain for a very warm climate. We will be able to make future projections more accurate, if, based on information from the warm past, we can rule out sensitivity being no lower than 2 °C for each doubling of CO₂ and the poles warming up no less than twice as fast as the tropics, for example. It will enable more accurate estimations of both the temperature and rises in sea level. This will be essential in order to take measures to combat the consequences of these.”