Midlatitude Summer Drying: An Underestimated Threat in CMIP5 Models?

Abstract

Early assessments of the hydrological impacts of global warming suggested both an intensification of the global water cycle and an expansion of dry areas. Yet these alarming conclusions were challenged by a number of latter studies emphasizing the lack of evidence in observations and historical simulations, as well as the large uncertainties in climate projections from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Here several aridity indices and a two‐tier attribution strategy are used to demonstrate that a summer midlatitude drying has recently emerged over the northern continents, which is mainly attributable to anthropogenic climate change. This emerging signal is shown to be the harbinger of a long‐term drying in the CMIP5 projections. Linear trends in the observed aridity indices can therefore be used as observational constraints and suggest that the projected midlatitude summer drying was underestimated by most CMIP5 models. Mitigating global warming therefore remains a priority to avoid dangerous impacts on global water and food security.

Holocene fire activity during low-natural flammability periods reveals scale-dependent cultural human-fire relationships in Europe

“…Although during the last century only minor fire events have occurred in the CEL compared to other areas of the world, forest cover is expected to increase in the future and fuel will accumulate in widespread human-planted pine and spruce monocultures (Caudullo et al, 2016;Houston Durrant et al, 2016). Future climate change scenarios predict drier and warmer summers, and the frequency of natural ignition by lightning might increase (Douville and Plazzotta, 2017;Lhotka et al, 2018;Romps et al, 2014). These factors increase fire hazard and fire risk (Hardy, 2005), leading to a much more flammable landscape-a situation comparable to the early Holocene.…”

Advances in understanding large‐scale responses of the water cycle to climate change

“…24 Multimodel coupled CMIP5 simulations underestimate declining relative humidity observed over global land. 25,26 This discrepancy also applies to atmosphere-only experiments applying observed sea surface temperature (SST): a single model simulated a −0.05 to −0.25%/decade trend (1996-2015) compared with an observed estimate of −0.4 to −0.8%/decade. 25 It is not clear if this discrepancy is explained by potential deficiencies in representing ocean to land moisture transport, 27 land-atmosphere coupling, 24 or inhomogeneity of the observational records.…”

On the Connection Between Global Hydrologic Sensitivity and Regional Wet Extremes

“…It has been shown that most of the intermodel spread in wet extremes arises from tropical and subtropical regions (O'Gorman, 2012;O'Gorman & Schneider, 2009;Pendergrass & Hartmann, 2014b;Pfahl et al, 2017). Previous attempts to directly constrain projections in precipitation extremes are rather limited but have suggested that the increase in both wet and dry extremes, especially over land, may be underestimated by GCMs (Borodina et al, 2017;Douville & Plazzotta, 2017;O'Gorman, 2012).…”