From the conclusions of the Science paper:
"The geographic pattern of Holocene climate fluctuations remains murky, but several things are clear. The Little Ice Age and the subsequent warming were global in extent. Several Holocene fluctuations in snowline, comparable in magnitude to that of the post-Little Ice Age warming, occurred in the Swiss Alps. Borehole records both in polar ice and in wells from all continents suggest the existence of a Medieval Warm Period. Finally, two multidecade-duration droughts plagued the western United States during the latter part of the Medieval Warm Period. I consider this evidence sufficiently convincing to merit an intensification of studies aimed at elucidating Holocene climate fluctuations, upon which the warming due to greenhouse gases is superimposed."This paper also prompted an internet posting critical of Michael Mann's hockey stick paper, which concludes, "global warming is natural and the recent warming is probably no exception."
The Climategate emails include two email exchanges here and here in which Mann appears to be infuriated with the suggestion that the MWP was global and well as any suggestion that the current warm period was similar.
This article prompted the following entry at a now defunct site (climatechangedebate.org) which is quoted in full in the above climategate emails:Global warming is natural and the recent warming is probably no exception. This is the controversial argument made by prominent climatologist Wallace S. Broecker in today’s issue of Science. Broecker’s bombshell bears the seemingly innocent title “Was the Medieval Warm Period Global?” It may seem esoteric, but whether the apparent warmth reported in Europe about 1000 years ago was global or simply local is becoming a central issue in climate science. What makes it contentious is the recent claims by the Umted Nations Intergovernmental Panel on Climate Change that the earth is warmer now than it has been for millennia, and that therefore human carbon dioxide emissions are to blame. Broecker, a leading figure at Lamont-Doherty Earth Observatory, Columbia University, questions both IPCC claims.
The focus of the debate is a 1OOO-year temperature reconstruction known in climate circles as the “hockey stick”. Produced in 1999 by M. E. Mann, R. S. Bradley, M. K. Hughes, the long handle of the hockey stick shows global temperatures for the first 8 centuries as basically unchanging, followed by the sharply up-tilting blade of the last 150 years or so. The Mann et al hockey stick is the central feature of the recently released IPCC working group one Summary for Policy makers, which claims to embody the best of climate science.
Broecker does not like the hockey stick, nor the conclusions the IPCC draw from it. He says ” A recent, widely cited reconstruction (Mann’s) leaves the impression that the 20th century warming was unique during the last millennium. It shows no hint of the Medieval Warm Period (from around 800 to 1200 A.D.) during which the Vikings colonized Greenland, suggesting that this warm event was regional rather than global. It also remains unclear why just at the dawn of the Industrial Revolution and before the emission of substantial amounts of anthropogenic greenhouse gases, Earth’s temperature began to rise steeply. Was it a coincidence? I do not think so. Rather, I suspect that the post-1860 natural warming was the most recent in a series of similar warmings spaced at roughly 1500-year intervals throughout the present inter-glacial, the Holocene.”
Broecker presents the evidence for a global Medieval Warm Period, as well as for a Little Ice Age from around 1300 to 1860, when the present temperature rise begins. He also argues that the “proxy” evidence used by Mann et al, such as tree ring data, is ill suited to the time period and temperature variation -- less than one degree C -- in question.
As he puts it, “In my estimation, at least for time scales greater than a century or two, only two proxies can yield temperatures that are accurate to 0.5 C: the reconstruction of temperatures from the elevation of mountain snowlines and borehole thermometry. Tree ring records are useful for measuring temperature fluctuations over short time periods but cannot pick up long-term trends because there is no way to establish the long-term evolution in ring thickness were temperatures to have remained constant.”