Organization: Penn State University
Date: Thu, 30 Jul 1992 09:05:29 EDT
From: <RMG3@psuvm.psu.edu>
Newsgroups: sci.environment,sci.geo.meteorology,sci.skeptic
Subject: Productivity and Paleo-CO2 (Was Re: Lawyers ...
 <1992Jul25.210755.21103@techbook.com>
 <1992Jul27.163959.2414@daffy.cs.wisc.edu> <1992Jul29.101143.5817@techbook.com>

In article <1992Jul29.101143.5817@techbook.com>, szabo@techbook.com (Nick Szabo)
says:

>In article <1992Jul27.163959.2414@daffy.cs.wisc.edu> orf@scrap.ssec.wisc.edu
>(Leigh Orf) writes:

>>Increasing atmospheric CO2 will increase agricultural productivity?

>CO2 is indeed biolimiting for most plants.  Plants originally
>evolved at CO2 levels up to 100 times present levels, so it is
>no surprise that they are adapted to higher levels.  Higher CO2 levels
>increase the productivity of over 80% of agricultural plants, including
>most grains and legumes.

  A little knowledge is a dangerous thing, as shown here.  _Some_
plants evolved to deal with CO2 levels which are _believed_ to have
been up to 100 (or 1000 or 10, depending on who you read) times
higher than present.  Other plants evolved for approximately present
values.

  There are two classes of plants, based on their mechanisms for
photosynthesis.  They are called C3 and C4 plants.  I forget
(another example of little knowledge) which is which, but the
two type cases are trees (high CO2, or at least ancient lineage)
and grasses (wheat, rice, ... --> low CO2, recent lineage).
The experiments to which Nick has referred tested the effect of
trees to CO2 'fertilization'.  Actually a specific species of
tree.  If _anything_ were to be favorably affected by CO2, it
would be trees since they are indeed believed to have evolved under
conditions of elevated CO2 (relative to the last 30 Million years
or so).

  But, the grass-type plants evolved and extended their ranges
during the low CO2 portion of earth history.  Experiments have
yet to be done with such plants.  Since this class includes
major food crops (not replaceable by tree-derived products
yet), the question of whether more CO2 is good for plants is
still open.

  There is also a curious inconsistency on the one hand of
disbelieving GCM's and other tools of climate scientists, but
believing the reconstructions of higher CO2 in the atmosphere
through geological history.  One of the major clues that there
was more CO2 in the atmosphere earlier in geological history
has come from climate modelling.  The models were consistently
too cold (particularly at the poles) when run for simulations
of paleoclimates -- for example the late Cretaceous (when the
dinosaurs got nailed).  It was found that by adding more CO2
to the model atmosphere (to levels of 3-5 times pre-industrial)
that the model's agreement with climate reconstruction improved.
Geologists have since constructed models of the geological carbon
cycle which include an atmospheric component, and, using some of
the atmospheric modeller's estimates, constructed curves
for atmospheric CO2 through time.

  So, in a sense, the climate models have _already_ been tested
under increased CO2 conditions and found to work.  It is
inconsistent, at least, to believe the model results for
60 million years ago, but not for the present.  Within the
climate science community, the faith runs the other direction.
They believe the models much more for the present (when we
have lots of data to check against and have a good idea of
how it was derived) than for the paleoclimate (when the data
are sparser, less time-labelled, and harder to interpret).

Bob Grumbine
rmg3@grebyn.com