Computer simulations of temperature change due to natural factors alone (blue line) do not match the actual temperature change (black, dotted line) that has occurred over the past century. Only a combination of human activities and natural factors (red line) can explain recent temperature changes. The graph also shows how volcanic eruptions cause a brief cooling in both the long-term trends of measured temperatures and computer simulations. (Graph: NASA, adapted from Hegerl and Zwiers et al., 2007)