Climate Model Projections of 21st Century Global Warming Constrained Using the Observed Warming Trend

The Coupled Model Intercomparison Project Phase 6 (CMIP6) archive includes larger ensembles, longer historical simulations, and models with a broader range of climate sensitivity than CMIP5. These features favor the application of observationally constrained climate projections. The 1970-2014 trend in global mean temperature is well-correlated with projected future warming across the CMIP6 multimodel ensemble. We first evaluate an approach that weights simulations based on the realism and degree of independence of their 1970-2014 trends, by treating each historical simulation in turn as pseudo-observations, and using the other models and weighting method to predict 21st century warming in the model concerned. The method performs well based on correlation and probabilistic measures. Applying the method using the observed 1970-2014 warming trend results in only small changes in the mean and lower bound of CMIP6 projected warming but substantially reduces the upper bound of projected early-, mid- and late-21st century warming under all SSP scenarios. Plain Language Summary Different climate models predict different amounts of future warming over the 21st century. Most previous studies have weighted models equally to derive a range of projected future warming. Here, we test and apply an approach that gives more weight to models, which are better able to match the observed 1970-2014 warming trend. This approach substantially reduces the upper bound of projected warming over the 21st century.