New Insights Into The Reasons For Global Warming Slowdown

The warming trend of global averaged surface air temperature?SAT?has shown a slowdown or stagnation in the first decade of this century.Many researchers have investigated the mechanisms for this phenomenon,and most conculuded this decadal global SAT variation as a non-linear climate response to the combined effects of the anthropogenic forcing and natural variability on multi-timescales.Based on the Had CRU observation and model simulations of CMIP5 and CMIP6,this study presents a new method that quantifies the SAT changes induced by the increasing greenhouse gases?GHGs?,analyzes the characteristics of global and regional SAT response to GHG forcing,and derived two modes of internal variability from ocean.The study points out the decadal trend in SAT over the past 160 years could be explained by the combined effects of global GHGs changes,Atalantic Multi-decadal Variability?AMV?and Pacific Decadal Variability?PDV?,evaluates their influence and relative contribution to GMSAT variations,and explains the reasons of two global warming slowdown events since 20th century.The potential occurance and duration of global and regional warming slowdown in the 21st century under various future scenarios are also discussed.These analyses are helpful to deepen the understanding of multi-decadal variability of global/regional SAT.The main conclucsions are as follows:1.The analyses on the observation and realaysis datasets show that,there have been two global warming slowdown events since the 20th century,one occurs in the middle of 20th century?1941-1975?,and another in the early stage of the 21st century?1998-2013?.During the mid-20th century warming slowdown period,the decrease trend of SAT over the Nothern Hemisphere appeared in all the year round,but in the early 21st century,the evident decrease trend was mainly found in the boreal wintertime?Dec.-Jan.-Feb.?and springtime?Mar.-Apr.-May?over the mid-latitude region in Nothern Hemisphere.The CMIP5 and CMIP6 simulations?multi-model ensemble mean?can reasonably simulate the global warming slowdown in mid-20th century and the warming acceleration in late-20th century,but are not good enough for the muted warming trend post-2000.The discrepancy between the model simulations and observation is mainly due to the externally forced GMSAT response in climate models?climate sensitivity?differs,the transient climate response?TCR?and equilibrium climate sensitivity?ECS?of the CMIP6 are higher than those of the CMIP5 models,and the ECS of CMIP6 models is close to the upper limit of the ECS range given by IPCC AR5?4.5K?.2.This study reveals that,in both observations and CMIP simulations,a quasi-linear relationship exists between long-term global and regional SAT change and equivalent CO₂ concentration change?CO₂e,which characterizes the radiative effect of global well-mixed GHGs?.Based on this linear relationship,three key factors affecting the decadal SAT trends are derived by using observations:CO₂e,internal variability modes AMV and PDV.Atlantc Multi-decadal Variability,defined as the combination of SST variability over Atlantic and Pacific basins,resembles the AMO pattern in Atlantic;Pacific Decadal Variability resembles the IPO pattern in Pacific.Regression analysis of CO₂e,AMV and PDV suggests that for the observed global mean SAT changes from 1880 to 2017,the contribution from increased GHGs accounts for 70%,while AMV and PDV contributes 30%with a larger contribution from AMV.During the two warming slowdown periods,with the AMV and PDV indexes are of in-phase negative trends,the interdecadal variability suppressed the GHG-induced warming.On regional scale,the linear regression fitting of CO₂e,AMV and PDV reasonably recovered SAT change in most parts of the globe.3.Based on the analyses of CMIP model simulations,including historical,projection runs?RCP8.5 and SSP5-8.5?and the idealized 1pct CO₂ runs,it is found a quasi-logarithmic relationship holds between global mean SAT transient changes and CO₂e changes as the CO₂e concentration is above to 500ppmv or its overall increment is more than 1000ppmv,that is,when the CO₂e concentration increment is in a relative lower range,less than 500ppmv,the linear relationship is an approximation of the logarithmic relationship.By using Had CRUT and the equivalent CO₂ concentration of CMIP future scenario,the GHGs-induced global and regional SAT changes can be estimated logarithmically via the increment of CO₂e under different scenarios in future.We explored the changes in the duration of externally and natural generated variability features with special focus on warming slowdown,and discussed the occurrence and potential duration of the warming slowdown in next 80 years.The estimation shows that the duration of global warming slowdowns would become shortened as the GHG emissions continue to rise.However,the warming slowdown could still occur towards to the end of the 21st century even under the highest emission scenario SSP5-8.5/RCP8.5.Under the SSP5-8.5/RCP8.5 scenario,the potential duration of warming slowdown might be 8-10 years in the next 40 years?2021-2060?,and 7-9 years towards to the end of the 21st century.