Understanding the Last Millennium Ocean Carbon Cycle with a GFDL Earth System Model

The ocean plays a vital role in the Earth's carbon cycle. Total ocean carbon is about 50 times greater than the amount of carbon in the atmosphere, and is actively exchanged on a range of time scales. However, the exact mechanisms and pathways of natural and anthropogenic ocean carbon uptake and release are not fully understood. We used the Geophysical Fluid Dynamics Laboratory Earth System Model, ESM2G, forced with well-constrained climate data (solar, volcanic, greenhouse gases, and land use) for the last millennium (850 CE – 1850 CE) to better understand ocean dynamics and carbon flux variability prior to the industrial era under natural climate forcing. To better constrain last millennium climate and carbon variability, we executed ensembles with various configurations of time-varying forcing data; all climate forcing data varying in time over the past 1000 years, and isolated, single-forcing time varying climate data while holding other forcing constant at 1860 values. For example, the last millennium Southern Ocean carbon sink decreased - from 0.24 PgC/a over the first 100 years to 0.09 PgC/a over the last 100 years - evident when all climate forcing data and only greenhouse gas forcing data were time-varying. The magnitude of Southern Ocean carbon uptake and outgassing depends on the sector examined. Atlantic and Pacific Southern Ocean sectors switch from a CO₂ sink at the start of the last millennium, to an outgassing of CO₂ at the end of the millennium with a range of ~0.02-0.03 PgC/a. The Indian Southern Ocean sector remains a comparatively larger CO₂ sink, ~0.2 PgC/a, for the duration of the last millennium and weakens over the last millennium. Analysis on the physical mechanisms driving global and regional ocean carbon sinks and outgassing and their impact on the global carbon cycle will be presented.