29 July 2019

New Insights into Natural Variability and Anthropogenic Forcing of Global/Regional Climate Evolution

Science

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%.

Impact

By assuming a constant rate of GHG increase in next few decades, our analysis indicates that the GHG-induced warming will dominate the internal decadal-timescale variability in most land regions except southeastern China and North America where a less than global mean warming is likely to be experienced due to natural decadal variability. Further, in most ocean regions, AMV and PDV can still insert significant influence, resulting in large uncertainty on projecting the SAT changes there.

Summary

Here, by combining both observations and model simulation, we study how the interplay of anthropogenic climate change and internally generated decadal climate variability would determine the transient global and regional surface temperature changes. It is found that the GHG-induced SAT changes can be linearly scaled by the GHG change. Defined as the combined variability in the Atlantic and Pacific basins after the removal of GHG-induced SAT changes, both AMV and PDV play a significant role in modulating global mean and regional SAT changes in the past a century-and-a-half. Regression analysis indicates that the observed global mean SAT changes from 1880 to 2017 come 70% from contributions from GHGs, with AMV and PDV contributing a combined 30% with a possible higher contribution from AMV. Overall, the AMV contributes significantly to the global mean SAT transient changes on multidecadal timescale, however, the contributions of PDV and AMV to global mean SAT have similar magnitudes on decadal timescales with the PDV leading AMV in most parts of the 20th century. Moreover, when PDV and AMV are in-phase, the contribution of natural decadal variability to global and regional climate can be significantly larger than that from GHGs, but an out-of-phase change of PDV and AMV could minimize their contribution to global and regional climate. As the

GHGs increase further, our study shows a declined influence of the PDV and AMV to global and regional climate relative to that of GHGs.

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)
Publications
Wu, T, A Hu, F Gao, J Zhang, and G Meehl.  2019.  "New Insights into Natural Variability and Anthropogenic Forcing of Global/Regional Climate Evolution."  npj Climate and Atmospheric Science 2(1).  https://doi.org/10.1038/s41612-019-0075-7.