Roles of climate feedback and ocean vertical mixing in modulating global warming rate

Despite the rapid increase of greenhouse gases (GHGs) in the atmosphere during the past 50 years, observed global mean surface temperature (GMST) showed a pause in the warming trend during the first decade of the twenty-first century. This is referred to as the global warming "hiatus". A dominant hypothesis emphasizes that the superimposition of the cold phase of the Pacific decadal variability and the global warming trend can lead to the hiatus. Using simply energy balance models, we explore two potential mechanisms that may supress the GMST warming trend: enhanced negative climate feedback and downward heat mixing. Forced by linearly increasing heating, a stronger negative climate feedback can reduce the GMST warming rate, but cannot result in a warming hiatus. Downward mixing of heat can cause a short-lived hiatus of surface warming rate due to enhanced nonlinear ocean heat uptake by the lower ocean, but the surface warming would be accelerated in the long run due to the decline of downward heat mixing rate. This study provides further evidence, both theoretically and numerically, that in the long run, the only route to contain the global warming effectively is to reduce GHG emissions.