Constrained estimates of externally forced past and future warming for Canada

Canada’s land mass, which extends well into the Arctic, has experienced some of the most rapid warming on Earth in recent decades. Here we apply a Bayesian observational constraint method to consistently assess the externally forced past, present and future warming in Canada, despite the presence of possible internal variability. Using observational constraints based on annual observational records of six Canadian sub-regions together with historical CMIP6 climate change simulations, we estimate that external forcing, which is almost entirely due to human activity, has warmed Canada by 2.1 [1.3, 2.8] °C between the 1850-1900 pre-industrial period and the recent 2015-2024 decade. Applying these same observational constraints to CMIP6 simulations of future climate conditions indicates that Canada will warm to 4.8 [3.5, 6.3] °C above pre-industrial levels by the end-of-century under an intermediate emissions scenario SSP 2-4.5, and to 6.2 [4.7, 7.9] °C under a high-emissions scenario SSP 3-7.0, with the largest warming projected for Northern Canada, followed by Quebec. The observational constraints, which were chosen and extensively evaluated using an imperfect model testing approach, reduce projected warming estimates for Canada relative to their unconstrainted counterparts, also materially reduce projection uncertainty.