In climate, CO₂ plays an important role. The gas enhances the greenhouse effect, and therefore causes extra global warming. But what are the long-term consequences of an increasing CO₂ concentration for the rest of the carbon cycle? To find out, NESSC PhD candidate Robin van der Ploeg of Utrecht University investigates changes in the carbon cycle during periods of global warming in the past.

The idea of the long and complicated carbon cycle in a nutshell: the earth has its own feedback system to regulate CO₂ concentrations and thus temperature. Robin: ‘When temperature rises, chemical weathering occurs faster. More CO₂ is removed from the atmosphere, reducing the greenhouse and enabling the earth to cool. The CO₂ ends up in the ocean. The ocean pH becomes lower, but under normal circumstances,  buffer system controls the acidification. Ultimately, the extra CO₂ is stored in bedrock at the sea floor.’

The problem is: this negative feedback does not seem to work properly at all times, says Robin. An example is the Middle Eocene Climatic Optimum (MECO), a global warming period, 40 Mya. It took a good 500 thousand years before the climate system and carbon cycle were restored. Too long, based on what you would expect from theory, says Robin. ‘We hypothesize that too little weathering occurred at that time to neutralize the increasing CO₂ concentrations. In fact, it’s possible that the global warming during MECO was caused by a reduced weatherability of the continents.’

To confirm this hypothesis, Robin is collecting more data about the carbon cycle in that period. He uses osmium isotopes – osmium is found as a trace element in rocks on land as well as in the ocean, allowing scientists to trace parts of the carbon cycle using this element. For his research, Robin takes samples from drill cores from different locations. He then isolates osmium and measures the isotopes in a mass spectrometer.

Robin’s research is especially important to find out the exact interaction between the climate system and the carbon cycle. ‘What causes climate change? How does the system regulate itself? If we figure that out, we might not be able to predict climate change for the coming century – but we will understand it fundamentally,’ says Robin.