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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.

Plain Language Summary What will be the consequence of global warming on regional soil moisture at the end of the 21st century? The response found in the fifth Assessment Report (AR5) of Intergovernmental Panel on Climate Change is blurred by many uncertainties, even when the focus is on a single business-as-usual scenario for the projected concentrations of greenhouse gases. Such a confusion is dominated by climate model uncertainties on the long term but might be also due to internal climate variability on the near term. Here we use a detection-attribution methodology to demonstrate that recent trends in soil moisture and in near-surface relative humidity averaged over the boreal midlatitude continents in summer have been mainly driven by human activities. Then we show that there is a fairly strong relationship between the near-term versus long-term aridity response among a set of 20 climate models, thereby supporting the limited influence of internal climate variability on near-term variability. Finally, we use this emergent relationship to constrain the long-term model response with the recent trends estimated from the observations and find that the projected long-term drying was probably underestimated by most global climate models explored in the AR5.