I am sure you are correct, but at best it can still only produce calculated estimates, rather than authenticated information. As an aside, I have in my History records, copies of two letters written in the early 1800's which, amongst other things, talk about the weather, one written at the end of December 1818 in Street, Somerset, refers to the warm sunny weather with fruit trees in leaf and about to flower, which I am sure you will agree is early. The other written during the winter of the early 1820's on False Duck Island in Lake Ontario Canada, refers tho the lake not freezing over that year, thus preventing the occupants from taking their horses on the annual trip across the lake to the Quaker Monthly meeting on the mainland. It is possible to suggest that these two anecdotal incidents contribute to evidence of unusually warm weather across the globe during the early decades of the 19th century. Or maybe not!