We know so little about solar cycles and the sun in general. It seems pointlessly arrogant to assume that we are the cause of the climate change (if there is any). It reminds me of the rooster who thinks his crowing is what brings on the dawn.
I think it is exactly that --(the rooster that brings on the dawn)-- to assume that such a huge (earth)system could be negatively impacted by something so insignificant to geological events (cow farts, humans, bbq's). Do the data (global warming/ cars) correlate? Yes, perhaps. Is car exhaust causative? It is so highly unlikely. This is an interesting study, in the Alps. Always good when existing theories are challenged.
From Wikipedia:
The Medieval Warm Period (MWP) or Medieval Climate Optimum was an unusually warm period during the European Medieval period, lasting from about the 10th century to about the 14th century. It has been argued a better name would be the Medieval Climatic Anomaly.
Initial research on the MWP and LIA was largely done in Europe, where the phenomenon was most obvious and clearly documented.
It was initially believed that the temperature changes were global. However, this view has been questioned by some scientists, amongst them Bradley and Jones, 1993; Hughes and Diaz, 1994; Crowley and Lowery, 2000. The 2001 IPCC report summarises this research, saying: "…current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of 'Little Ice Age' and 'Medieval Warm Period' appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries"
During this time wine grapes were grown in Europe and southern Britain (however, factors other than climate strongly influence the commercial success of vineyards; and the time of greatest extent of mediaeval vineyards falls outside the MWP). The Vikings took advantage of ice-free seas to colonize Greenland and other outlying lands of the far north. The period was followed by the Little Ice Age (LIA), a period of cooling that lasted until the 19th century when the current period of global warming began.
An interesting question, since Ariana Huffington blames SUV's for global warming--and there weren't a lot of those around in the thirteenth and fourteenth centuries.