Do Responses to Different Anthropogenic Forcings Add Linearly in Climate Models?


Many detection and attribution and pattern scaling studies assume that the global climate response to multiple forcings is additive: the response over the historical period is statistically indistinguishable from the sum of responses to individual forcings. In a recent study, researchers used the National Aeronautics and Space Administration’s Goddard Institute for Space Studies (GISS) and National Center for Atmospheric Research Community Climate System Model (CCSM) simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive to test this assumption for multiyear trends in global-average, annual-average temperature and precipitation at multiple timescales. Findings show that responses in models forced by pre-computed aerosol and ozone concentrations are generally additive across forcings; however, the study demonstrates that there are significant nonlinearities in precipitation responses to different forcings in a configuration of the GISS model that interactively computes these concentrations from precursor emissions. These nonlinearities are attributed to differences in ozone forcing arising from interactions between forcing agents. Study results suggest that attribution to specific forcings may be complicated in a model with fully interactive chemistry and may provide motivation for other modeling groups to conduct further single-forcing experiments.


Marvel, K., G. Schmidt, D. Shindell, C. Bonfils, A. Legrande, L. Nazarenko, and K. Tsigaridis. 2015. “Do Responses to Different Anthropogenic Forcings Add Linearly in Climate Models?” Environmental Research Letters 10(10), 104010. DOI: 10.1088/1748-9326/10/10/104010.