Narrowing the spread in CMIP5 model projections of air-sea CO2 fluxes

Abstract

Large spread appears in the projection of air-sea CO2 fluxes using the latest simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Here, two methods are applied to narrow this spread in 13 CMIP5 models. One method involves model selection based on the ability of models to reproduce the observed air-sea CO2 fluxes from 1980 to 2005. The other method involves constrained estimation based on the strong relationship between the historical and future air-sea CO2 fluxes. The estimated spread of the projected air-sea CO2 fluxes is effectively reduced by using these two approaches. These two approaches also show great agreement in the global ocean and three regional oceans of the equatorial Pacific Ocean, the North Atlantic Ocean and the Southern Ocean, including the average state and evolution characteristics. Based on the projections of the two approaches, the global ocean carbon uptake will increase in the first half of the 21st century then remain relatively stable and is projected to be 3.68–4.57 PgC/yr at the end of 21st century. The projections indicate that the increase in the CO2 uptake by the oceans will cease at the year of approximately 2070.

Current and Future Decadal Trends in the Oceanic Carbon Uptake Are Dominated by Internal Variability

“…Thus, the contribution of internal variability to the ocean carbon uptake uncertainty, and the future evolution of this internal variability remain poorly quantified. Moreover, as of now, CMIP5 historical simulations are not expected to reproduce the decreasing decadal trends in the ocean carbon uptake during 1992–2001 (Wang et al, ) as found in observational estimates. Hindcast models forced with atmospheric reanalysis may better capture decadal variations of the ocean carbon uptake driven by climate variability.…”

“…Thus, the contribution of internal variability to the ocean carbon uptake uncertainty, and the future evolution of this internal variability remain poorly quantified. Moreover, as of now, CMIP5 historical simulations are not expected to reproduce the decreasing decadal trends in the ocean carbon uptake during 1992-2001 (Wang et al, 2016) ESM-based perturbed initial conditions ensembles allow for a consistent evaluation of both forced signals and internal variability within the same model. The important role of internal climate variability for projections of ocean biogeochemistry has been already confirmed, using such ensembles (containing 30 members) McKinley et al, 2016;Rodgers et al, 2015).…”

“…Data‐based estimates of oceanic carbon uptake show large uncertainties arising from the gaps in observations and different mapping methods that are used to derive observational products (Rödenbeck et al, ). Earth system models (ESMs), driven by future climate change scenarios, that is, those used in the fifth Phase of the Coupled Model Intercomparison Project (CMIP5), project an increase in the ocean carbon uptake in the recent decades and into the future, as long as CO 2 emissions increase (Frölicher et al, ; Nevison et al, ; Wang et al, ). Yet large discrepancies appear among these model simulations due to uncertainties related to internal variability, model response, and scenarios (Deser et al, ; Hawkins & Sutton, ; Lovenduski et al, ; Tebaldi & Knutti, ).…”

The [simple carbon project] model v1.0

“…The SCP-M simulations commence in 1751 and therefore incorporate an extra 100 years of fossil fuel and land use change emissions beyond the CMIP5 model results presented in Jones et al (2013). Wang et al (2016) quote a range of 412-649 PgC Figure 5. SCP-M modelling results compared with modern atmospheric and ocean GLODAPv2 data for (a) atmospheric CO 2 and δ 13 C, (b) atmospheric 14 C, (c) ocean δ 13 C, (d) 14 C, (e) DIC, and (f) the carbonate ion proxy.…”

“…Broecker, 1982;Sarmiento and Toggweiler, 1984;Kohfeld and Ridgewell, 2009;Sigman et al, 2010), alongside changes in the terrestrial biosphere stock of carbon (Francois et al, 1999;Ciais et al, 2012;Peterson et al, 2014;Hoogakker et al, 2016). Active theories within the ocean realm include changes in ocean biology (Martin, 1990;Watson et al, 2000;Martinez-Garcia et al, 2014), ocean circulation and mixing (Sarmiento and Togg- Figure 7. Panel (a) shows the annual uptake of CO 2 by the ocean in each of the RCPs over the period 1751-2100, modelled with SCP-M. By 2100, SCP-M estimates a range of 0-6 PgC yr −1 across the RCPs as estimated by CMIP5 models, reproduced from Jones et al (2013).…”