Using regional warming levels to describe future climate change for services and adaptation: Application to the French reference trajectory for adaptation

To describe regional climate change, climate services typically rely on an ensemble of climate model simulations. The development and arrival of observational constraints at regional scales are questioning this approach, as some simulations may not align with warming trajectories estimated by these techniques. This study proposes a methodology for describing future regional changes that combines multiple sources of information: global and regional observational constraints applied to the CMIP6 ensemble, along with existing regional climate model simulations driven by CMIP5. This approach uses Regional Warming Levels (RWLs), mirroring the use of Global Warming Levels (GWLs) in the IPCC AR6. We apply it to mainland France, a region with discrepancies in warming projections between global models, regional models, and observational constraints. Results show that the standard GWL approach produces unrealistically low warming estimates due to overly low regional-to-global warming ratios in some models. Using RWLs allows separation of the annual mean warming estimation (based on observational constraints) from the detailed climate change characteristics (based on regional models). We explore ways to link RWLs and GWLs and assess associated uncertainties. This methodology has been selected to describe future climate change in mainland France, as part of the definition of a reference trajectory for adaptation set by the French government. It can be replicated in other regions and applied to existing or upcoming climate projections to express them in terms of regional warming levels at the national scale.

Practical implications

The French government has recently chosen to adopt a reference trajectory for adaptation to climate change in France, known as the TRACC (Trajectoire de Réchauffement de référence pour l’Adaptation au Changement Climatique). This trajectory defines 3 levels to which the country needs to prepare for, corresponding to +1.5 °C global warming in 2030, +2 °C in 2050 and +3 °C in 2100 compared to 1850–1900. The aim is to establish a single framework for climate change impact studies including climate services, the definition and analysis of adaptation actions, standardizing practices nationwide and facilitating a coherent response to climate challenges. This article describes the methodological choices associated with this trajectory, based on a description of future changes at a fixed regional warming level (RWL) consistent with the chosen global trajectory. For mainland France, the 3 TRACC levels are expressed as an average warming over the country of 2 °C in 2030, 2.7 °C in 2050 and 4 °C in 2100 compared to 1850–1900. These are derived from observational constraints, combining models and observations. The subsequent description of local scale climate change is based on existing regional climate model simulations. The article finally provides a description of some of the changes associated with these 3 regional warming levels.