Data going back further than a century or two is derived from "proxy" indicators such as tree-rings and stalactites which, again, are subject to large errors.
To find out how warm the earth was before that, climatologists have had to resort to indirect methods, such as measuring tree-rings, or counting the different isotopes of oxygen in layers of polar ice.
Fusa Miyake discovered Carbon-14 and Beryllium-10 traces in tree rings from the era, which point to a gamma ray burst from a celestial body other than the Sun.