Tuesday, January 26, 2010

Mann upset about paper in Science suggesting Medieval Warm Period was global

A 2001 article in Science entitled "Was the Medieval Warm Period Global?" states ,"The Little Ice Age and the subsequent warming were global in extent... Borehole records both in polar ice and in wells from all continents suggest the existence of a Medieval Warm Period."

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:

Climate Guru Kicks The Hockey Stick   by David  Wqjick...

     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.”

1 comment:

  1. http://wattsupwiththat.com/2013/12/28/gavin-was-for-solar-forcing-of-climate-before-he-was-against-it/#comment-1515939