During the Viking Medieval times, the Earth was warmer than it is now; how much warmer we do not know, but some working dairy farms, hundreds of years old, are emerging from Greenland Ice today. Growing seasons were longer (according to both European monastery and Chinese bureaucracy records). This started in about 850 AD and ended rather abruptly in the early part of the 1300’s.
The Earth is currently in a recession of the Ice Ages that covered much land with miles of Ice; this remission was thought to be temporary when I was in school, but the general notion that we are in a period of remission was not really questioned. There was no general agreement on what caused the Ice Ages. Ben Franklin, having witnessed some violent eruptions of Iceland volcanoes, hypothesized that it might be volcanic ash rising the reflectivity of the Earth to Solar radiation.
Whatever the cause, the Viking Warm period ended rather abruptly with a very wet period in the 1320’s, and a period of cooling began. This has become known as the “Little Ice Age.”
Several hundred years into this period, sunspots were discovered and serious study of them began. https://en.wikipedia.org/wiki/Maunder_Minimum
During much of this period very few sunspots were observed. About 1750 the number of sunspots increased, and – perhaps coincidentally, perhaps not – the Earth began to warm. It was still cold enough in 1776 for the cannon captured by Ethan Allen from the British fortress Ticonderoga to be carried across the frozen Hudson River to General George Washington in Harlem Heights, facilitating his retreat and the survival of the Continental Army. Sometime before 1850 the Hudson was no longer freezing hard enough to walk across, and little ice forms now; it is safe to say the Earth was colder in then, than it is now.
Dr. Osborne has been recording the sunspot counts for the past few years; this may or may not be an indication of future solar activity, and thus insulation of the Earth. That is independent of any estimate of Earth temperature, which is difficult; the operations taken to generate a number called the annual Earth temperature for any given year are very complex, and the same procedures are not always – sometimes cannot be – used each year. There are a number of ‘adjustment’ variables, and these are not generally discussed nor does everyone agree on the adjustments.