CoAuthors

Elodie Charles (CLS, France); Aurelien Ribes (CNRM, Météo-France, France)

Observations and climate models are combined to identify an anthropogenic warming signature in the upper ocean heat content changes (OHC) since 1971. We use a new detection and attribution analysis developed by Ribes et al. (2015) which uses a symmetric treatment of the magnitude and the pattern of the climate response to each radiative forcing. A first estimate of the OHC response to natural, anthropogenic, greenhouse gas and other forcing is derived from a large ensemble of CMIP5 simulations. Observational datasets from historical reconstructions are then used to constrain this estimate. A spatio-temporal observational mask is applied to compare simulations with actual observations and to overcome reconstruction biases. Results on the 0-700m layer show that the global OHC has increased significantly since 1971 in response to GHG emissions by 2.37±0.34 107J/m2/y. 33% of this increase has been compensated by other anthropogenic influences (mainly aerosols), which induced an OHC decrease. The natural forcing has also induced a slight global OHC decrease since 1971,compensating an extra 7% of the OHC increase due to GHG. Compared to previous studies we have separated the effect of the anthropogenic forcing into the effect of the GHG forcing and the effect of the other anthropogenic forcing. This has been possible by using a new D&A method and by analysing at the same time the global OHC trends over 1957-1980 and over 1971-2005.This bivariate method takes advantage of the different time variation of the GHG forcing and the aerosol forcing since 1957 to separate both effects. The next step is to develop a similar approch to provide an observational constraint on greenhouse gas and aerosol contributions to global mean sea level rise