past temperatures are calculated from several sources:

--actual records (naval ship logs, weather stations, etc). Obviously, actual records are sparse.

--current temperatures and trends can be extrapolated backwards with some confidence

--temperature proxies like ice cores, tree rings, etc. can give a reasonable estimate of what the temperature may have been

Combining all of these, or what is available, allows scientists to make a statement, with some uncertainty, about temperature conditions in the past.

One of the complaints of the skeptics is that the calculators are very reluctant to share their raw data so that their results might be verified.

WW