Abstract
Because of natural decadal climate variability—Atlantic multi-decadal variability (AMV) and Pacific decadal variability (PDV) —the increase of global mean surface air temperature (GMSAT) has not been monotonic although atmospheric greenhouse-gas (GHG) concentrations have been increasing continuously. It has always been a challenge regarding how to separate the effects of these two factors on GMSAT. Here, we find a physically based quasi-linear relationship between transient GMSAT and well-mixed GHG changes for both observations and model simulations. With AMV and PDV defined as the combination of variability over both the Atlantic and Pacific basins after the GHG-related trend is removed, we show that the observed GMSAT changes from 1880 to 2017 on multi-decadal or longer timescales receive contributions of about 70% from GHGs, while AMV and PDV together account for roughly 30%. Moreover, AMV contributes more to time-evolving GMSAT on multi-decadal and longer timescales, but PDV leads AMV on decadal timescales with comparable contributions to GMSAT trends.
Highlights
The influence of decadal climate variability on regional and global climate has been studied previously,[1] additional attention has been brought to bear on this problem due to the 2000–2014 global warming slowdown[2,3,4,5,6,7,8,9,10] which occurred during a time of continuously increasing greenhouse gas (GHGs) concentrations
global mean surface air temperature (GMSAT) rises more than 1 °C in the last one and half centuries, half of which has been attributed to the increased GHG concentrations by many previous studies.[18,48]
Defined as the combined variability in the Atlantic and Pacific basins after the removal of GHG-induced surface air temperature (SAT) changes, both AMV and Pacific decadal variability (PDV) play a significant role in modulating global mean and regional SAT changes in the past a century-anda-half
Summary
The influence of decadal climate variability on regional and global climate has been studied previously,[1] additional attention has been brought to bear on this problem due to the 2000–2014 global warming slowdown[2,3,4,5,6,7,8,9,10] which occurred during a time of continuously increasing greenhouse gas (GHGs) concentrations Both observational and modeling studies have explored potential mechanisms to explain this warming slowdown and concluded that internal decadal variability played a major role. More energy can be transported into the subsurface and deep oceans instead of warming the upper few tens meters of ocean, contributing to a slowdown in global mean surface air temperature (GMSAT) increase.[13,34,35,36,37,38,39,40] A linkage between the time-evolution of observed GMSAT and PDV has been established whereby there is a weaker GMSAT increase for PDV negative phase (with the tropical Pacific somewhat cooler than average on decadal timescales) and accelerated warming for PDV positive phase since the early 20th century.[41,42]
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