Environmental Research Letters (Jan 2021)
On the impossibility of extreme event thresholds in the absence of global warming
Abstract
The exceptional severity of recent climate extremes has raised the question of whether some events would have been impossible in the absence of global warming. This question is critical for climate adaptation, but is challenging to answer given the length and non-stationarity of the observational record. The large single-model ensemble climate simulations archived in the Coupled Model Intercomparison Project (CMIP6) offer a unique opportunity to explore whether the hottest temperatures of the current climate are more extreme than any that could have occurred in the absence of human forcings. We first analyze the one CMIP6 model that has daily data archived for large ensembles in both the historical all-forcings and historical natural forcings experiments. We find that, for large areas of the world, the maximum daily-, seasonal- and annual-scale thresholds of the large single-model ensemble with natural and human forcings (‘all-forcings’) are never reached in the large single-model ensemble with only natural forcings. However, we also identify widespread areas—notably in the northern hemisphere mid-latitudes—where the hottest thresholds of the all-forcings ensemble are frequently exceeded in the absence of human forcings. Further analysis suggests that human forcings other than greenhouse gases (GHGs) are a primary cause of this discrepancy. For example, when comparing the late and early periods of the CMIP6 historical all-forcings experiment, other large single-model ensembles exhibit similar muting of extremely warm years over northern-hemisphere mid-latitude regions. However, under GHG-only forcing, all years in the recent period are hotter than the hottest early-period year over most of the globe. These results suggest that, although the hottest possible events in the current climate may have been virtually impossible in the absence of historical GHG emissions, other non-GHG anthropogenic forcings have muted the emergence of previously impossible events.
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