Relative importance of internal climate variability versus anthropogenic climate change in global climate change

Authors: CHEN, JIE - Wuhan University; BRISSETTE, FRANCOIS - University Of Quebec; MARTEL, JEAN-LUE - University Of Quebec; Zhang, Xunchang; FREI, ALLAN - City University Of New York

Submitted to: Journal of Climate
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2020
Publication Date: 1/15/2021
Citation: Chen, J., Brissette, F.P., Martel, J., Zhang, X.J., Frei, A. 2021. Relative importance of internal climate variability versus anthropogenic climate change in global climate change. Journal of Climate. 34(2):465-478. https://doi.org/10.1175/JCLI-D-20-0424.1.
DOI: https://doi.org/10.1175/JCLI-D-20-0424.1

Interpretive Summary: To better understand the role of internal climate variability (ICV) in climate change impact studies, this study separates and quantifies the relative importance of ICV in relation to the anthropogenic climate change trend (ACC) in global and regional climate change using a criterion of Time of Emergence (ToE). The ToE is defined as the 30-year period when the ACC emerges from ICV. The ACC is defined as the multi-model ensemble mean and the ICV is defined as the inter-member variability of multi-member ensembles of climate models for mean precipitation and temperature. The results show that the annual mean precipitation ACC are expected to emerge within this century over extra-tropical regions as well as along the equatorial band. However, ToEs are unlikely to occur, even by the end of this century, over intra-tropical regions outside of the equatorial band. In contrast, annual mean temperature ACC have already emerged from the temperature ICV for most of the globe. Similar spatial patterns are observed at the annual and seasonal scales, while a weaker ACC for June-July-August precipitation and additional ICV for December-January-February temperature translate to later ToEs for some regions. In addition, similar ToE estimates are observed when using 5-, 10-, and 40-member ensembles, suggesting that a 5-member ensemble may be sufficient to estimate ICV for mean precipitation and temperature at the multi-decadal scale. Overall, this study implies that in many cases, medium term adaptation to changing mean precipitation should also focus on ICV. For temperature, mitigation strategies should therefore focus on ACC.

Technical Abstract: To better understand the role of internal climate variability (ICV) in climate change impact studies, this study separates and quantifies the importance of ICV in relation to the anthropogenic climate change (ACC) in global and regional climate change using a criterion of Time of Emergence (ToE). The ToE is defined as the 30-year period when the ACC emerges from ICV. The ACC is defined as the multi-model ensemble mean and the ICV is defined as the inter-member variability of multi-member ensembles of climate models for mean precipitation and temperature. The results show that the annual mean precipitation ACC are expected to emerge within this century over extra-tropical regions as well as along the equatorial band. However, ToEs are unlikely to occur, even by the end of this century, over intra-tropical regions outside of the equatorial band. In contrast, annual mean temperature ACC have already emerged from the temperature ICV for most of the globe. Similar spatial patterns are observed at the annual and seasonal scales, while a weaker ACC for June-July-August precipitation and additional ICV for December-January-February temperature translate to later ToEs for some regions. In addition, similar ToE estimates are observed when using 5-, 10-, and 40-member ensembles, suggesting that a 5-member ensemble may be sufficient to estimate ICV for mean precipitation and temperature at the multi-decadal scale. Overall, this study implies that in many cases, medium term adaptation to changing mean precipitation should also focus on ICV. For temperature, ICV does not play a dominant role and mitigation strategies should therefore focus on ACC.