Details

Autor(en) / Beteiligte

• Palevsky, Hilary I.
• Doney, Scott C.

Titel

Sensitivity of 21st Century Ocean Carbon Export Flux Projections to the Choice of Export Depth Horizon

Ist Teil von

• Global biogeochemical cycles, 2021-02, Vol.35 (2), p.n/a

Ort / Verlag

Washington: Blackwell Publishing Ltd

Erscheinungsjahr

2021

Quelle

Wiley Online Library

Beschreibungen/Notizen

• Global Earth system model simulations of ocean carbon export flux are commonly interpreted only at a fixed depth horizon of 100 m, despite the fact that the maximum annual mixed layer depth (MLDmax) is a more appropriate depth horizon to evaluate export‐driven carbon sequestration. We compare particulate organic carbon (POC) flux and export efficiency (e‐ratio) evaluated at both the MLDmax and 100‐m depth horizons, simulated for the 21st century (2005–2100) under the RCP8.5 climate change scenario with the Biogeochemical Elemental Cycle model embedded in the Community Earth System Model (CESM1‐BEC). These two depth horizon choices produce differing baseline global rates and spatial patterns of POC flux and e‐ratio, with the greatest discrepancies found in regions with deep winter mixing. Over the 21st century, enhanced stratification reduces the depth of MLDmax, with the most pronounced reductions in regions that currently experience the deepest winter mixing. Simulated global mean decreases in POC flux and in e‐ratio over the 21st century are similar for both depth horizons (8%–9% for POC flux and 4%–6% for e‐ratio), yet the spatial patterns of change are quite different. The model simulates less pronounced decreases and even increases in POC flux and e‐ratio in deep winter mixing regions when evaluated at MLDmax, since enhanced stratification over the 21st century shoals the depth of this horizon. The differing spatial patterns of change across these two depth horizons demonstrate the importance of including multiple export depth horizons in observational and modeling efforts to monitor and predict potential future changes to export. Plain Language Summary The ocean's biological pump plays an important role in the global carbon cycle by transferring carbon from the surface to the deep ocean, where it can be stored away from contact with the atmosphere. A question that has therefore garnered significant interest is how the biological pump will change over the 21st century under climate change scenarios. Past analyses of global climate model simulations, however, have focused on the amount of carbon that sinks out of the surface‐most 100 m rather than considering the depth that sinking carbon particles must reach to be stored away from contact with the atmosphere long term. Here we re‐analyze one of these model simulations, introducing the consideration that, to be considered as stored long term, organic carbon particles must sink deep enough to escape the deepest layer mixed into contact with the atmosphere each winter. Using this definition, we find different spatial patterns for where we expect the biological pump to weaken or strengthen under 21st century climate change. These differences are largely driven by decreases in the depth of deep winter mixing, reducing how deep sinking particles must penetrate to be stored long term. Key points Projected changes to biological carbon export are evaluated for the first time using the maximum annual (winter) mixed layer depth horizon Spatial patterns of projected changes in flux below the winter mixed layer depth differ from those assessed using the 100 m depth horizon Differing patterns of projected change are largely driven by strengthened stratification in high‐latitude regions with deep winter mixing