Abstract
The export flux of organic carbon from the upper ocean is the starting
point of the transfer and long term storage of photosynthetically-fixed
carbon in the deep ocean. This “biological carbon pump” is a
significant component of the global carbon cycle, reducing atmospheric
CO2 levels by ~ 200 ppm. Carbon exported out of the
upper ocean also fuels the productivity of the mesopelagic zone,
including significant fisheries. Here we show that, despite its
importance, export flux is poorly constrained in Earth System Models,
with the modelled range in projected future global-mean changes due to
climate change spanning +1.8 to -41%. 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 prioritises 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 remineralisation)
of export. We also identify the observations required to achieve more
robust characterisation, and hence improved model parameterization, of
export flux, and thus decrease uncertainties in current and future
estimates of this important planetary carbon flux.