Posted on 07/14/2006 6:27:00 AM PDT by mathprof
It is routine these days to read in newspapers or hear -- almost anywhere the subject of climate change comes up -- that the 1990s were the "warmest decade in a millennium" and that 1998 was the warmest year in the last 1,000.
This assertion has become so accepted that it is often recited without qualification, and even without giving a source for the "fact." [snip]
The claim originates from a 1999 paper by paleoclimatologist Michael Mann. Prior to Mr. Mann's work, the accepted view, as embodied in the U.N.'s 1990 report from the Intergovernmental Panel on Climate Change (IPCC), was that the world had undergone a warming period in the Middle Ages, followed by a mid-millennium cold spell and a subsequent warming period -- the current one. That consensus... held that the Medieval warm period was considerably warmer than the present day.
Mr. Mann's 1999 paper eliminated the Medieval warm period from the history books, with the result being the bottom graph you see here. It's a man-made global-warming evangelist's dream, with a nice, steady temperature oscillation that persists for centuries followed by a dramatic climb over the past century. [snip] Al Gore uses it in his movie.
The trouble is that there's no reason to believe that Mr. Mann, or his "hockey stick" graph of global temperature changes, is right.[snip] The three researchers -- Edward J. Wegman of George Mason University, David W. Scott of Rice University and Yasmin H. Said of Johns Hopkins University -- are not climatologists; they're statisticians. Their task was to look at Mr. Mann's methods from a statistical perspective and assess their validity. Their conclusion is that Mr. Mann's papers are plagued by basic statistical errors that call his conclusions into doubt...Mr. Mann's methodology is biased toward producing "hockey stick" shaped graphs.
(Excerpt) Read more at online.wsj.com ...
Must I register with the WSJ?
I'm afraid you must register to read the article. I did my best to excerpt the main points it made.
This is from the offical report commissioned by the House Energy Committee, due to be released today on Mann's statistical methods in the hockey stick.
It looks to be the "stick a fork in it" death knell for the hockey stick.
McIntyre has more about this at http://www.climateaudit.org/?p=749
Not only must you register, but you must buy a paid subscription also.
These two figures show former temperatures with major periods of glaciation labeled. The dashed lines are the present global average temperature of about 15° C (59° F). Thus the solid curves show small changes from this average; note that the temperature drops only about 5° C during a glaciation. This has occurred about every 100,000 years, with smaller wiggles in between. That is, there has been a 100,000 year glaciation cycle for the past million years or so, and there may be shorter cycles as well.
The most recent glaciation, 20,000 years ago, is called the Laurentide, and Earth is still recovering from it. This map from the The Illinois State Museum exhibit on ice ages shows the extent of that ice.
The most recent small drop in average temperature caused the Little Ice Age of 1500-1700 AD, which history describes. Mountain glaciers advanced in Europe and rivers like the Thames in England froze solid, which doesn't happen now.
Click image for animation
The growth of the ice sheets began about 120,000 years ago as ice built up on the continents in the Northern Hemisphere, especially in Canada and Europe. The largest extent of these ice sheets occurred 18,000 years ago. At that time the largest ice sheets were between 3.5 and 4 km thick. In North America the largest ice sheet was the Laurentide Ice Sheet centered on Hudson Bay with other sheets centered on Greenland and in the Canadian Rocky Mountains. As these ice sheets expanded they grew together, covering Baffin Bay and eventually the Great Lakes and New England. In northwestern Europe the Fennoscandian Ice Sheet began to grow and expand south to cover what is now Norway and Sweden and north to cover the exposed continental shelf. Over time the ice sheet grew to cover Finland and the United Kingdom. This ice sheet extended east to the Ural Mountains where it met the Siberian Ice Sheet. Before the last ice age ice sheets already existed on Antarctica and on Greenland.
Most people seem surprised when we say current levels are relatively low, at least from a long-term perspective - understandable considering the constant media/activist bleat about current levels being allegedly "catastrophically high." Even more express surprise that Earth is currently suffering one of its chilliest episodes in about six hundred million (600,000,000) years. Given that the late Ordovician suffered an ice age (with associated mass extinction) while atmospheric CO2 levels were more than 4,000ppm higher than those of today (yes, that's a full order of magnitude higher), levels at which current 'guesstimations' of climate sensitivity to atmospheric CO2 suggest every last skerrick of ice should have been melted off the planet, we admit significant scepticism over simplistic claims of small increment in atmospheric CO2 equating to toasted planet. Granted, continental configuration now is nothing like it was then, Sol's irradiance differs, as do orbits, obliquity, etc., etc. but there is no obvious correlation between atmospheric CO2 and planetary temperature over the last 600 million years, so why would such relatively tiny amounts suddenly become a critical factor now? 
Which is normal the Medieval Maximum ca 1200 AD or the Little Ice Age ca 1620 AD?
The most interesting part of this report is found in Figure 4.1. I thought that the pro-hockey stick folks had discredited the McIntyre/McKitrick analysis that produced a hockey-stick effect by pumping random data into Mann's model. However, this report claims to have reproduce it:
We have been able to reproduce the results of McIntyre and McKitrick (2005b). While at first the McIntyre code was specific to the file structure of his computer, with his assistance we were able to run the code on our own machines and reproduce and extend some of his results. In Figure 4.1, the top panel displays PC1 simulated using the MBH98 methodology from stationary trendless red noise. The bottom panel displays the MBH98 Northern Hemisphere temperature index reconstruction.
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