There and back again: The journey of sinking marine microplankton and its implication for past, present and future climate

Anthropogenic climate change is one of the greatest challenges for humanity today. How does the Earth system react to the atmospheric greenhouse gas increase that has never happened before with such speed in Earth's history? So far, climate models project an increase of the global average temperature, sea level and the number of `extremely' warm days in the coming decades and centuries. Although it is clear that the Earth is warming, uncertainty remains in the progression of those developments. These climate models are inherently wrong, but are they useful? Climate models give a proper representation of present-day climate, which we know from validations with observations. These models, which contain fundamental physical processes and have been developed over the past decades, cannot be validated in the increased greenhouse-climate of the future. We can however, compare the models with observations of warmer climates in the past, which are similar to the future climate, to get an understanding of these type of `extreme' climates. Although (unfortunately) no direct observations are available for the past hundred millions of years, we do find indirect evidence about the past climate (for example fossils or ice cores). These so-called proxies provide an archive of past climate and can be used to compare with climate model simulations. As a result, the combination of models and proxies of past climate can be used to get a better understanding of how a future climate, which is warmer than today, may look like. A primary part of the Earth's archive to reconstruct past climates is provided by marine sediments, consisting of (fossil remains from) microplankton. The microplankton species in the bottom sediments originated from a location close to the ocean surface before they started sinking to the bottom. Hence, microplankton at the ocean bottom is representative of the ocean surface environment. It is often assumed that these planktonic species sunk vertically downwards. H