The biological carbon pump in CMIP6 models: 21st century trends and uncertainties

Abstract

The biological carbon pump (BCP) stores ∼1,700 Pg C from the atmosphere in the ocean interior, but the magnitude and direction of future changes in carbon sequestration by the BCP are uncertain. We quantify global trends in export production, sinking organic carbon fluxes, and sequestered carbon in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) future projections, finding a consistent 19 to 48 Pg C increase in carbon sequestration over the 21st century for the SSP3-7.0 scenario, equivalent to 5 to 17% of the total increase of carbon in the ocean by 2100. This is in contrast to a global decrease in export production of –0.15 to –1.44 Pg C y –1 . However, there is significant uncertainty in the modeled future fluxes of organic carbon to the deep ocean associated with a range of different processes resolved across models. We demonstrate that organic carbon fluxes at 1,000 m are a good predictor of long-term carbon sequestration and suggest this is an important metric of the BCP that should be prioritized in future model studies.

Regionally disparate ecological responses to microplastic slowing of faecal pellets yields coherent carbon cycle response

“…Over the nearer term, the net air-sea gas exchange is hardly impacted, even when using the larger-sized microplastic particle assumption. This is an anticipated result because neither adjustment of particle fluxes nor the biological pump itself have been demonstrated to influence ocean carbon uptake over human timescales (Koeve et al, 2020;Wilson et al, 2022). Regionally, however, changes to ecosystem function can be significant and of the same order as climate warming impacts, with alteration of phytoplankton detrital fluxes exceeding 50% in the North Pacific and disproportionate impacts on low-productivity gyres.…”

An Assessment of Vertical Carbon Flux Parameterizations Using Backscatter Data From BGC Argo

“…Despite the significance of the BCP, current estimates of its magnitude vary greatly (Bisson et al., 2020; DeVries & Weber, 2017; Henson et al., 2011, 2012, 2015; Siegel et al., 2014) and its future projections are highly uncertain (Henson et al., 2022; Laufkötter et al., 2016; Wilson et al., 2022). While biogeochemical models are considered as the best tools for predicting the future response of the BCP to climate change, their utility critically depends on their ability to make skillful simulations and predictions.…”

Seasonality in carbon flux attenuation explains spatial variability in transfer efficiency

“…Despite the evidence for, and the importance of temporal and spatial variability in the flux attenuation and its potential influence on carbon sequestration, both sinking and remineralisation -as well as the flux attenuation parameter -are often assumed to be constant both in space and time. This is also the case in higher complexity models such as the CMIP6 generation [33,34], which have mechanistic representations of remineralisation but often model detritus as sinking at a constant speed.…”

“…In this case, we note that most CMIP6 models adopt constant (in time, space and depth) sinking speed [33,34], with only two models using a variable formulation: one has a sinking speed that is constant in time but increases with depth [48], and another has a sinking speed that varies according to the nutrient stress [49].…”

“…Hence, assuming a 473 fixed spatial and temporal pattern in flux attenuation 474 limits the model assessment of the BCP and seques-475 tration under climate change in the IPCC scenarios. 476 Currently, all CMIP models have invariant in time and 477 space (and all but one have a constant in depth) sink-478 ing speed [33,34], and incorporating mechanistic mod-479 els for sinking particles is a challenge for the CMIP7 480 generation and beyond.…”