Uncertain response of ocean biological carbon export in a changing world

The transfer of organic carbon from the upper to the deep ocean by particulate export flux is the starting point for the long-term storage of photosynthetically fixed carbon. This 'biological carbon pump' is a critical component of the global carbon cycle, reducing atmospheric CO2 levels by similar to 200 ppm relative to a world without export flux. This carbon flux also fuels the productivity of the mesopelagic zone, including important fisheries. Here we show that, despite its importance for understanding future ocean carbon cycling, Earth system models disagree on the projected response of the global export flux to climate change, with estimates ranging from -41% to +1.8%. Fundamental constraints to understanding export flux arise because a myriad of interconnected processes make the biological carbon pump challenging to both observe and model. Our synthesis prioritizes the processes likely to be most important to include in modern-day estimates (particle fragmentation and zooplankton vertical migration) and future projections (phytoplankton and particle size spectra and temperature-dependent remineralization) of export. We also identify the observations required to achieve more robust characterization, and hence improved model parameterization, of export flux and thus reduce uncertainties in current and future estimates in the overall cycling of carbon in the ocean.

Automated summary

This article discusses the transfer of organic carbon from the upper to the deep ocean, known as the "biological carbon pump," and emphasizes its importance for the global carbon cycle and marine ecosystems. However, there is disagreement among Earth system models regarding the projected response of global export flux to climate change, mainly due to the complexity of the biological carbon pump, making it challenging to observe and model. The article suggests prioritizing key processes and identifying necessary observations to improve the accuracy of models and reduce uncertainties in current and future estimates of carbon cycling in the ocean.