Introduction
Anthropogenic fossil fuel emissions have significantly disrupted the global carbon cycle, leading to accelerated climate change since the onset of the industrial era and posing substantial risks to global livelihoods. The ocean and terrestrial biosphere partially mitigate the full effects of these emissions by sequestering and storing carbon over timescales that span decades to millennia. Yet, large uncertainties persists in the future uptake of carbon by these reservoirs due to limited understanding of the complex and interdependent processes and their response to the rapid climate change we experience today.
One of the most promising yet underutilized approaches to critically assess these uncertainties is investigating the large array of natural experiments from Earth’s geological past, during which the climate underwent profound changes. This approach enables the deciphering of triggers and feedback mechanisms driving these changes. In its recent geological past, Earth’s climate was marked by numerous rapid events as well as long-term climate change both thought to be facilitated by feedbacks between physical processes, such as ocean circulation, and biogeochemical ones that ultimately modulate atmospheric CO2 concentrations and thus Earth’s radiative balance.
To quantitatively interpret reconstructions of past climate, Earth system modelling has become indispensable. To overcome the challenges associated with the relatively long time scales of past climate change, computationally efficient models are required. Such Earth system models of intermediate complexity have a long history of being developed in Bern. We continue this with the current version of the Bern3D model that is extensively employed in the Global Biogeochemical Modelling group. Over the past years, the model has been equipped with a wide range of isotope tracers (see below) that allows for direct model-data comparisons, vastly improving the robustness of interpretations of proxy reconstructions. Further development of new isotope tracers will continue in the future. Our recent studies employing this approach provided new constraints, for instance, for the mean ocean temperature during the peak of the last ice age and the ocean circulation during the last deglaciation. With this paleo-informed Earth system model, we ultimately also simulate future climate change to better project changes of greenhouse gas concentrations, ocean acidification, and other climate change impacts on centennial to millennial time scales.
Methods
We are primarily employing and further developing the Bern3D (https://doi.org/10.1175/JCLI-D-23-0104.s1) Earth system model of intermediate complexity, which currently consists of geostrophic-frictional 3D ocean, a single-layer energy-moisture balance atmosphere, a thermodynamic sea-ice module, and the CISM ice-sheet model. The biogeochemistry comprises a dynamical global vegetation model (LPX-Bern), marine biogeochemistry with two size-classes, dynamic particle remineralization, and three nutrient tracers (PO4, SiO2, DFe), and a dynamic marine sediment module.
The model is further equipped with a large number of diagnostic and prognostic isotope tracers. The latter include stable and radio-carbon isotopes, Be isotopes, Nd isotopes, Pa and Th isotopes, Cr(III) and Cr(VI). Further, the Bern3D model includes noble gas tracers (N2, Ar, Xe, Kr) and 39Ar employed to explore the relationship between mean ocean temperature and noble gas ratios archived in polar ice cores.
The Bern3D model is currently adapted to become a more modular design that allows for the coupling of different models of Earth system components. The development of new diagnostic and prognostic tracers also continues.
Projects
We are involved in the following projects:
