I don’t know how to tell for sure, but I’ll hazard a guess that this is the first time Donald Rumsfeld has been quoted in the climate literature.

It’s a discussion of the latest version of the Hadley global temperature data set, and the team has gone to significant effort to quantify the uncertainties:

A definitive assessment of uncertainties is impossible, because it is always possible that some unknown error has contaminated the data, and no quantitative allowance can be made for such unknowns. There are, however, several known limitations in the data, and estimates of the likely effects of these limitations can be made.

And then the authors cite Rumsfeld’s famous disquisition on the topic:

There are things we know that we know. There are known unknowns. That is to say there are things that we now know we don’t know. But there are also unknown unknowns. There are things we don’t know we don’t know.