Are We Seeing Global Warming?

Measurements over the last century show that the global mean temperature has increased by about 0.5 degrees C. But how much, if any, of that increase can be attributed to human activity is uncertain and highly controversial. In his Perspective, Hasselmann discusses recent efforts to untangle anthropogenic climate change from natural climate variability. Improvements in computer modeling have reduced the scatter in climate change simulations from 50 to 20%, but significant differences remain. To resolve the contentious issues of anthropogenic warming, more work will be needed on the role of aerosols, clouds, and ocean-atmosphere coupling in climate change.