Ocean carbon export flux projections in CMIP6 Earth System Models across
multiple export depth horizons
Abstract
The ocean’s biological carbon pump (BCP) plays a key role in global
carbon cycling by transporting biologically-fixed carbon from the
surface to the deep ocean. Prior analyses of the BCP in Earth System
Model (ESM) simulations have typically evaluated particulate organic
carbon (POC) flux at a fixed export depth horizon of 100 m. However,
this overlooks spatial and temporal variations in the depth that sinking
POC must penetrate to reach the mesopelagic, or to sequester carbon from
the atmosphere on climate-relevant timescales. We use depth-resolved POC
flux output from eight Coupled Model Intercomparison Project Phase 6
(CMIP6) ESMs to compare global and regional changes in POC flux at five
export depth horizons ‒100 m, the base of the euphotic zone (EZ depth),
the particle compensation depth (PCD), the maximum annual mixed layer
depth (MLDmax), and 1000 m ‒ under the high-emissions scenario SSP5-8.5.
We also examine the relationship among net primary production, export
efficiency from the surface ocean, and transfer efficiency to depth in
key regions of the ocean, identifying model- and region-specific
variations in the mechanistic drivers of POC flux changes in the deep
ocean. Globally and spatially, trends in POC flux magnitude and decline
are similar at the four surface export depth horizons, and multimodel
variability in POC flux change by 2100 is greatest at the 1000 m export
depth horizon (+4% to -55%). This indicates the importance of
improving model parameterizations of transfer efficiency and of POC flux
to the deep ocean.