Observations reveal a poleward expansion of the tropics in recent decades, implying a potential role of human activity. However, although theory and modelling suggest increasing GHG concentrations should widen the tropics, previous observational-based studies depict disparate rates of expansion, including many that are far higher than those simulated by climate models. Here, we review the rates and possible causes of observed and projected tropical widening. By accounting for methodological differences, the tropics are found to have widened about 0.5° of latitude per decade since 1979. However, it is too early to detect robust anthropogenically induced widening imprints due to large internal variability. Future work should target the seasonal and regional signatures of forced widening, as well as the associated dynamical mechanisms.
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P.W.S., K.M.G., T.B. and S.M.D. are members of working groups related to the topic of this review—the International Space Science Institute (ISSI) Tropical Width Diagnostics Intercomparison Project and the US Climate Variability and Predictability Program (US CLIVAR) Changing Width of the Tropical Belt Working Group. We thank the the ISSI and US CLIVAR offices, and sponsoring agencies (the ESA, Swiss Confederation, Swiss Academy of Sciences, University of Bern, NASA, NOAA, NSF and DOE) for supporting these groups and activities. J.L. is supported by the US Department of Energy Office of Science Biological and Environmental Research (BER) as part of the Regional and Global Climate Modeling Program. We acknowledge F. Liu for his assistance in drawing Fig. 4.
The authors declare no competing interests.
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Staten, P.W., Lu, J., Grise, K.M. et al. Re-examining tropical expansion. Nature Clim Change 8, 768–775 (2018). https://doi.org/10.1038/s41558-018-0246-2
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