SEA surface temperatures in the tropical Pacific Ocean increased on average by serveral tenths of a degree during the 1980s and early 1990s1–4, contributing to the observed global warming during this period5. Here we investigate the possible causes of this Pacific warming, using a global coupled ocean–atmosphere general circulation model incorporating increasing concentrations of atmospheric carbon dioxide. In the model, cloud cover and cloud albedo feedbacks contribute to tropical Pacific sea surface temperature increases that are greater east of 180° longitude, with attendant shifts in large-scale precipitation patterns and mid-latitude circulation anomalies in the north Pacific. These anomalies resemble some aspects of El Nino events, as well as features associated with recent observed Pacific-region climate anomalies. The resemblance to El Nino complicates the problem of detection and attribution of climate change, and suggests that depletion of freshwater resources6 may be an additional hazard of greenhouse warming for populations in the western Pacific region.