Bradley, R. S., M. K. Hughes, H. F. Diaz, 2003: Climate in Medieval time. Science, 302, 404-405.
Climate in Medieval time is often said to have been as warm as, or warmer than, it is "today." Such a statement might seem innocuous. But for those opposed to action on global warming, it has become a cause célèbre: If it was warmer in Medieval time than it is today, it could not have been due to fossil fuel consumption. This (so the argument goes) would demonstrate that warming in the 20th century may have been just another natural fluctuation that does not warrant political action to curb fossil fuel use.
Careful examination of this argument must focus on three issues: the timing of the purported temperature anomaly, its geographical extent, and its magnitude relative to temperatures in the 20th century. The latter issue is especially important, because advocates of a warm Medieval episode commonly argue that solar irradiance was as high in Medieval time as in the 20th century. They maintain that 20th-century global warming was largely driven by this solar forcing, not by increasing greenhouse gas concentrations.
The balance of evidence does not point to a High Medieval period that was as warm as or warmer than the late 20th century. However, more climate records are required to explain the likely causes for climate variations over the last millennium and to fully understand natural climate variability, which will certainly accompany future anthropogenic effects on climate.
[Abstract courtesy of H. F. Diaz.]