Lesson one: scientists are people, people are jerks and that Mann guys seems to be an especially big person.
Lesson two: its way too fun reading other people’s email.
Lesson three (the real lesson): data and methods should be freely available. Reading all the nonsense that goes on trying to replicate policy-important studies (like the Mann et al 1999 paper) is infuriating. Luckily, in my experience, we don’t have this problem in economics.
Contrary to McArdle and Cowen, my prior on the joint claim “the last century was the hottest of the millennium” or “tree ring data is a good proxy for temperature” has decreased reading some of the replication studies. While thermometer readings show a pronounced increase in temperatures in recent decades, the tree ring data probably doesn’t. There’s even a good chance the tree ring data, if a good temperature proxy, show several periods in the past 1000 years that were warmer than the last century.
Here’s a paper in Science that uses a different data set from Mann paper and shows the so-called “Medieval Warm Period” probably had temperatures at least as high as the 20th century.
I haven’t seen a good explanation for why thermometer readings show different trends than tree rings (or other temperature proxies). Of course, I trust thermometers over proxies, but still.