source (note that I have flipped the above graph horizontally so that it matches the first graph)
Another common theme of the skeptics: How come it was so warm in the years 800-1300 AD?
On the top graph down we see that the temperatures nearly touched modern values, yet there was no industrial carbon dioxide release then.
The second graph shows that there was an increase in sunspot activity at that time, and unusually long period of increased solar activity. That is one possible candidate for the causation of the Mediaeval Warm Period.
The third graph shows that there is no correlation between modern temperatures and solar activity. The fit is much better for CO2 levels.
The lesson here is that the earth's temperature is a result of six variables:
a. the distance from earth to sun, which varies very slowly
b. sunspot activity, which has a cycle of about 11 years
c. Albedo - the reflectivity of the earth's surface - mainly clouds and snow
d. aerosols and soot, mainly from volcanoes, which can cool the atmosphere by throwing dust into the air. Industrial soot plays a part here
e. greenhouse gases, which trap the sun's heat
f. Ocean currents, notably El Nino and La Nina
We are affecting d and e, but they are enough, in a finely balanced system, to create serious problems.
Note that there is some doubt about whether the MWP was local or global.