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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"):


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

   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      
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