Challenges in comparing observed and model-simulated climate trends on regional scales

Disentangling the effects of internal variability and anthropogenic forcing on regional climate trends remains a key challenge with far-reaching implications. Due to its largely unpredictable nature on timescales longer than a decade, internal climate variability confounds interpretation of historical trends and complicates climate model evaluation. In this overview talk, I shall synthesize new findings from coupled model initial-condition Large Ensembles and related single-forcing Large Ensembles, with those based on statistical methods applied to the observational record, to address the following questions: How well do we know the characteristics of internal variability from the short observational record? How well do we know the forced response in observations? What is the range of historical climate trends that nature could have produced? Do climate models simulate realistic spatial patterns and amplitudes of internal variability and forced response? Are models structurally different in their forced responses and internal variability, or are their differences attributable to sampling fluctuations? I will conclude with some open questions and prospects for future progress.