“Gravity exists. The world is round. Climate change is happening.”

So tweeted Barack Obama's advocacy group Organizing for Action on Monday, adding the hashtag #ScienceSaysSo. Had the hashtag read #ThePresidentSaysSo, no one would have bought the bogus appeal to authority. But many will buy the appeal to scientific authority.

Few nowadays defer to the traditional authority figures of old – parents, priests or politicians. But many are inclined to take scientists' word for things. If scientists say that anthropogenic climate change is happening, well, then anthropogenic climate change is happening. (Mr. Obama's tweeters must mean anthropogenic climate change, since no one denies that the climate is changing, as it always does.)