From: SCHLOERER@rzmain.rz.uni-ulm.de (Jan Schloerer)
Subject: Re: Lawyer's science vs. unbiased presentation of the facts
Organization: University of Ulm, Germany
Date: Fri, 24 Jul 1992 16:25:41 GMT
X-News-Reader: VMS NEWS 1.23


In  <1992Jul22.202258.14769@midway.uchicago.edu>
    rtp1@quads.uchicago.edu (raymond thomas pierrehumbert)   writes :

> Does anybody know if there is any historical precedent for a 4-5 degree
> WARMING in a period of less than 1000 years?  Especially for a warming
> of global mean temperatures  (Y[ounger] D[ryas] was a cooling of
> (I think) about this magnitude, but it is supposed to have been
> pretty localized).


It appears that for the past 150,000 years or so the answer is "no",
and that for earlier periods the answer is not (yet?) precisely known.
Sources at the bottom, corrections and amendments welcome.

The warming preceding the Eem interglacial, roughly 130 000 years before
present, may have been a bit swifter than the warming starting roughly
15 000 years before present, after the most recent ice age  (Breuer fig.
2b, Crowley fig. 2 & 3).  The Antarctic Vostok core indicates a Pre-Eem
warming rate of about 2 oC / 1000 years for about 5000 years, a Southern
Ocean sea surface temperature record (Crowley fig.2) indicates a Pre-Eem
warming rate of roughly 1.5 oC / 1000 years for about the same time span.
(The global warming rate may have been a bit different.)

Breuer reports from the Dahlem Workshop that the absolute amounts of
earlier temperature changes, in particular those before the pleistocene
ice ages (roughly the last 1.6 million years) are not precisely known.

Breuer also reports that after the Younger Dryas cold spell, about
10 000 years before present, average temperatures, at least for the
Northern Hemisphere, increased by roughly 2 oC.  Greenland ice cores
seem to indicate a warming of  ~7 oC within a 50-year period after the
Younger Dryas (Lehman & Keigwin); this extreme 'jump' was most likely
a regional and not a global phenomenon.

Both Breuer and Crowley stress that paleoclimate studies, while helpful
in understanding the climate system, are not likely to provide valid
analogies to the present situation.  No fully analogous situation is
known in the geologic record.  Continents have drifted, mountain ranges
like Himalaya or Rocky Mountains have formed, the Isthmus of Panama
has closed roughly 3 million years ago, ocean circulation has changed,
and the warmest interglacial, the Eem, seems to have been only 1 to 2 oC
warmer than the modern climate.  "If the climate does change in the
future, it is likely to result in a climate state for which there may be
no geologic analog, that is, it may be a unique climate realization
in earth history."  (Crowley, p. 43).


   ... whether anthropogenic global warming is the fastest change
   the world has ever seen doesn't matter;  only whether it is
   a change that is too fast for *us* to adjust to
   without significant disruption ...

                         men@bjerknes.colorado.edu  (Matthew Newman)
                         in   <1992Jul21.170734.28363@colorado.edu>


Sources :

   Breuer, Georg,  Weltweite Erwaermung - Einst und Jetzt,
      Wissenschaft und Fortschritt 42, 3 (March 1992), 116-120.
      Conference report to appear, according to Breuer:  Eddy, J.A. &
      H. Oeschger (eds): Global Changes in the Perspective of the Past
      (66th Dahlem Workshop), Wiley & Sons, Chichester, UK
   Crowley, Thomas J.,  Utilization of paleoclimate results to validate
      projections of a future greenhouse warming.  In  M.E.Schlesinger
      (ed.),  Greenhouse-gas-induced climatic change: a critical
      appraisal of simulations and observations,
      Elsevier, Amsterdam 1991, pp. 35-45.
   Lehman, Scott J.  &  Lloyd D. Keigwin,  Sudden changes in
      North Atlantic circulation during the last deglaciation.
      Nature 356 (30 April 1992), 757-762.


Jan Schloerer              Internet: schloerer@rzmain.rz.uni-ulm.de
Klinische Dokumentation    Univ. Ulm       Schwabstr. 13
Postfach 3880              D-W-7900 Ulm    Germany
Newsgroups: sci.environment