Normal conditions (top), strengthening due to natural variability (middle) and weakening due to greenhouse warming (bottom). Black arrows represent horizontal and vertical winds with the shading on the background map illustrating ocean temperatures. Over the past few decades, natural variability has strengthened the Pacific Walker circulation leading to enhanced cooling in the equatorial central-to-eastern Pacific (middle). Climate models forced by increasing greenhouse gas concentrations simulate weakening of the Walker circulation (bottom). (Right) Temporal evolution of model-simulated Walker circulation trends, with the dark blue line and orange shading denoting anthropogenically-induced changes and the impact of natural processes, respectively.