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Re: C-14 time scale and way of implementation



Dear Fortunat,

Your suggestion that we may still wish to run C-14 as an inorganic tracer
is interesting.  As you know, there has been some discussion amongst some
of us about this issue ever since the OCMIP-2 planning meeting in San
Diego, back in February.

A key issue is that results from inorganic and biotic runs should be the
same, to within a few permil, if atmospheric CO2 is held to the same value
(Bacastow and Maier-Reimer, Climate Dynamics, 1990).  When you and I
discussed this recently in Bern, I think we agreed that an inorganic model
with C-14 and C-12 as separate tracers would do a good job. Such a
2-tracer approach is slightly more complicated that that which you suggest
now, (1-tracer "fractionation-corrected ratio" from Toggweiler et al.,
1989).  However, since we are going to make the solubility pump run
anyway, adding C-14 to that would be about the same amount of work, and
would not require more CPU time.  Actually the 2-tracer run (for C-14 and
C-12) would be more efficient (in CPU time) than the alternative (one run
for the solubility pump plus another run for the C-14/C-12 ratio). 

Another concern is how one handles changes in oceanic and atmospheric CO2
with time (Suess + Bomb effects).  These changes are easier to handle when
using a 2-tracer approach.  Technically one should account for CO2 changes
in both the atmopshere and ocean, otherwise, differences of more than 7
permil seem likely.

There was a brief discussion of this issue at the last U.S. OCMIP meeting
(Boulder, 19 May 1998), which is recounted briefly in the minutes (see
the section entitled "Radiocarbon"):

http://www.ipsl.jussieu.fr/OCMIP/phase2/meetings/minutes.boulder_19may1998.html

Therefore, I believe the two-tracer inorganic approach is the way to go. 
However, Olivier Marchal's sensitivity tests are important to quantify by
how much the 1-tracer ratio method differs from more complicated
approaches.  I would be particularly interested in seeing your results for
the industrial and postnuclear era.  How will you model changes in surface
ocean CO2?

Best wishes,  Jim

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   James Orr                       Laboratoire des Sciences du Climat 
   LSCE, CEA Saclay                             et de l'Environnement    
   Bat. 709 - Orme                 Unite Mixte de Recherche CEA-CNRS    
   F-91191 Gif-sur-Yvette Cedex    Phone:   (33) (0)1 69 08 77 23      
   FRANCE                          Fax:     (33) (0)1 69 08 77 16
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   mailto:  orr@cea.fr            (country code)  (when in France)      
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