Recent heat waves, droughts, and increasingly longer summers would likely not be happening without global warming caused by human-induced release of greenhouse gases, according to scientists at NASA’s Goddard Institute for Space Studies (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1205276109). Climate scientists have been hesitant to link extreme weather events, such as the heat wave and drought in Texas and Oklahoma in 2011, to human-induced climate change rather than chalking them up to natural climate variability, in particular when the analysis is based on theory and climate-change computational models. NASA’s James E. Hansen and colleagues have now used only temperature data to compare weather anomalies during a base period of 1951–80 with anomalies since 1980. They found that extremely hot summers—those with temperatures three standard deviations greater than the mean temperature during the base period—affected less than 1% of Earth’s land surface during the base period. But these extreme events now affect 10% of the land. The researchers say their results are important to help form public policy, because actions to sufficiently curb greenhouse gas emissions are unlikely until the public, which typically takes its cues from local variability in weather, perceives that human-caused global climate change is under way.