evidence: This finding strongly suggests that over the vast majority of the global area analyzed, the observed twentieth-century warming trends on a regional scale are unlikely to be due to internal climate variability alone. [...] The climate model simulations provide additional evidence for the hypothesis that the observed surface temperature warming during the twentieth century is too large to have been caused by internal climate variability or natural climate forcings alone. The observed trends (1901–2000 and 1949–2000) exceed model-generated internal variability and are significantly inconsistent with trends from natural-only forcing experiments over a wide majority of the global regions tested. There is little evidence that the model simulates too little internal climate variability; in fact, the model appears to simulate excessive internal climate variability on interannual time scales in many regions and at the global scale. [...] In summary, anthropogenic forcing agents, as opposed to natural forcing agents or internal climate variability, are simulated to be the dominant cause of the pronounced warming in the late twentieth century. This agrees with a number of previous global modeling studies of natural versus anthropogenic forcing (e.g., Tett et al. 1999; Stott et al. 2000; Broccoli et al. 2003; Meehl et al. 2004). Establishing the relative contributions of individual forcing agents in CM2 will require further specialized experiments and will be the topic of a future study.