50. Heidelberger Physik Graduiertentage

The Current State of the Global Carbon Cycle: Insights from Observations, Isotope Studies and Modelling

Samuel Hammer

Earth's global mean surface temperature has risen by 1.2°C since the 1850s. Man-made emissions of greenhouse gases (GHGs) like CO₂, CH₄ and N₂O are responsible for a large share of this temperature increase. Human-related emissions have led to GHG concentrations unprecedented in the last 800.000 years of Earth's history and are altering the natural cycles of those gases significantly.

Among the anthropogenically influenced GHGs CO₂ is the one with the single largest effect on the Earth’s radiation balance. CO₂ acts as an exchange molecule in the global carbon cycle, thereby connecting the fast-reacting carbon reservoirs atmosphere, biosphere and oceans. The interplay between these reservoirs is highly complex and depends on a multitude of variables, which are, last but not least, temperature dependent. Each exchange process between reservoirs leads to small changes in the isotopic composition of the carbon in these reservoirs. Studying the ratios of ¹³C/¹²C allows for quantifying the different exchange processes. With the help of the radioactive isotope ¹⁴C, it is possible to separate and quantify the CO₂ originating from the combustion of fossil fuels.

This lecture covers the current state of the global carbon cycle based on the 2022 report of the Global Carbon Project (www.globalcarbonproject.org). The lecture then delves into the individual aspects of the carbon cycle and the CO₂ exchange between the reservoirs. In particular, the course will address how to exploit the information contained in the isotopic signals of CO₂ and the processes leading to the respective isotopic changes. The lecture concludes with projected changes in the global carbon cycle and will introduce state-of-the-art carbon cycle observation systems.