Croker Sack

"Democracy is the theory that the common people know what they want, and deserve to get it good and hard." — Henry Louis Mencken (1880-1956)

Friday, June 23, 2006

Unprecedentedly popular word

"Unprecedented" keeps showing up in the news reports, but so far I haven't found for certain where the news reporters are getting their quotes.

I've only been able to read through the summary of the National Academy of Sciences report on global warming so far, but this is what it says:

The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on icecaps and the retreat of glaciers around the world, which in many cases appear to be unprecedented during at least the last 2,000 years. Not all individual proxy records indicate that the recent warmth is unprecedented, although a larger fraction of geographically diverse sites experienced exceptional warmth during the late 20th century than during any other extended period from A.D. 900 onward.

The report is saying, in other words:
  1. that Mann and his "hockey stick" claimed there was an unprecedented period of warmth in the latter part of the 20th century compared to the previous 1000 years;
  2. that Mann's conclusion is supported by "an array of evidence" -- some of which (melting icecaps and glacier retreat) involves "proxy" indicators of events which "appear to be unprecedented during at least the last 2,000 years;" and
  3. that some of the evidence indicates that the more recent warmth is not unprecedented.

The news reporters appear to be taking that paragraph as the basis for their statements that the National Academy of Sciences has found that the surface temperatures in the last few decades of the 20th century were unprecedentedly warm -- and even stretching it a little farther by implying that the rate of increase in temperatures is itself unprecedented:

After a comprehensive review of climate-change data, the pre-eminent U.S. scientific body found average temperatures on Earth have risen about 1 degree over the past century, a development that "is unprecedented for the last 400 years and potentially the last several millennia."

The NAS report doesn't appear to say what the news reporters say it says.

The next paragraph in the NAS report's summary is interesting -- both for what it says and for the omission of its words from news media reports:

Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium. The substantial uncertainties currently present in the quantitative assessment of large-scale surface temperature changes prior to about A.D. 1600 lower our confidence in this conclusion compared to the high level of confidence we place in the Little Ice Age cooling and 20th century warming. Even less confidence can be placed in the original conclusions by Mann et al. (1999) that “the 1990s are likely the warmest decade, and 1998 the warmest year, in at least a millennium” because the uncertainties inherent in temperature reconstructions for individual years and decades are larger than those for longer time periods, and because not all of the available proxies record temperature information on such short timescales.

That is much the same as the press release, so why have the reporters overlooked or omitted it?

It says, in short: The NAS has very little confidence in the claim made by Mann that the last decade was the warmest in a thousand years, much less his claim that 1998 was the warmest year.

The reporters appear to want to end the debate by simply twisting what the NAS said into a complete endorsement of Mann's "hockey stick."

Look at the graph on page 2 of the NAS report's summary (page 3 of the pdf file). It shows the reconstructions of surface temperatures in the years since approximately 900 A.D., and it isn't in the shape of a hockey stick.

Consider the obvious fact that anyone could have a "high level of confidence" that temperatures are warmer now than at any time since 1600 -- because the years from approximately 1250 to 1850 were the cooling period known as the "Little Ice Age." Imagine that! The world is warmer now than it was during the Little Ice Age.


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