Misconceptions of the marine biological carbon pump in a changing climate: Thinking outside the “export” box

Output of Earth system model simulations in support of the publicationI. Frenger, A. Landolfi, K. Kvale, C. J. Somes, A. Oschlies, W., Yao, and W. KoeveMisconceptions of the marine biological carbon pump in a changing climate: Thinking outside the “export” boxGlobal Change Biology (GCB-23-1265.R1) accepted for publication, 2023The marine biological carbon pump (BCP) stores carbon in the ocean interior, isolating it from exchange with the atmosphere and thereby coregulating atmospheric carbon dioxide (CO2). As the BCP commonly is equated with the flux of organic material to the ocean interior, termed “export flux”, a change in export flux is perceived to directly impact atmospheric CO2, and thus climate. Here, we recap how this perception contrasts with current understanding of the BCP, emphasizing the lack of a direct relationship between global export flux and atmospheric CO2. We argue for the use of the storage of carbon of biological origin in the ocean interior as a diagnostic that directly relates to atmospheric CO2, as a way forward to quantify the changes of the BCP in a changing climate. The diagnostic is conveniently applicable to both climate model data and increasingly available observational data. It can explain a seemingly paradoxical response under anthropogenic climate change: Despite a decrease in export flux, the BCP intensifies due to a longer reemergence time of biogenically-stored carbon back to the surface and thereby provides a negative feedback to increasing atmospheric CO2. This feedback is notably small compared to anthropogenic CO2 emissions and other carbon-climate feedbacks. In this Opinion paper we advocate for a comprehensive view of the BCP's impact on atmospheric CO2, providing a prerequisite for assessing the effectiveness of marine CO2 removal approaches that target marine biology.Methods are described in detail in METHODS.pdf.All model experiments have been carried out on the CAU NEC cluster.High resolution output and code to reproduce the experiments is available from:https://b2drop.eudat.eu/s/aAzKo6feZACjbNsFor questions arrising, please contact [e-mail]