1995 oceanic anthropogenic carbon content. Integrated with latitude for observational estimates of Khatiwala et al. (2009) in pink, Sabine et al. (2004) in green, and the CMIP5 models in blue, (a) unadjusted, and (b) adjusted relative to 1791 start date. The value at 70°N is the total carbon uptake in the year 1995.
Bronselaer, B., M. Winton, J. Russell, C.L. Sabine, and S. Khatiwala (2017): Agreement of CMIP5 simulated and observed ocean anthropogenic CO2 uptake. Geophys. Res. Lett., 44,doi:10.1002/2017GL074435.
Since the start of the Industrial Revolution, human activity has caused atmospheric CO2 levels to rise. During this time, the ocean has absorbed roughly one third of emitted anthropogenic (human-derived) carbon, so ocean carbon uptake therefore influences how much of this important greenhouse gas remains in the atmosphere. Accurately measuring and simulating ocean carbon storage is important for assessing the current environmental conditions and projecting future climates.
Uncertainties in ocean carbon storage observational estimates are of the order of 25% and generally disagree with global carbon models by as much as 25%. While the differences between observations and models are within the uncertainties, these differences are significant when evaluating the global carbon budget. This paper demonstrates that the largest biases in the various estimates are due to the difference in time period covered by each estimate. The Coupled Model Intercomparison Project phase 5 (CMIP5) models used for the latest IPCC assessment report collectively begin their simulations in the year 1850, while observational estimates evaluate anthropogenic carbon from 1791 or 1765. While fossil fuel emissions prior to 1850 are negligible, changes in land use resulted in increased atmospheric CO2 levels since 1765, leading to increased ocean carbon.
The authors present two methods for adjusting ocean anthropogenic carbon uptake due to prior rises in atmospheric CO2. They show that after adjusting the CMIP5 and two observational estimates to the 1791–1995 period, all three carbon uptake estimates agree to within 3 petagrams of carbon: about 4% of the total. Although there are still regional patterns in the biases, this “correction” greatly improves the consistency of the various approaches for estimating ocean anthropogenic carbon storage.