date: Wed May  4 14:28:54 2005
from: Keith Briffa <k.briffa@uea.ac.uk>
subject: Re: A quick question if i may.
to: R.Platt@geo.hull.ac.uk

   At 23:55 03/05/2005, you wrote:

     Dear Dr Briffa

   Hi Rob

     I know its marking season, but i wonder if you might answer me a few quick
     questions.

   will have to be brief , 'cause got to go to China at weekend and need to do loads of stuff
   before

     Having conducted some reading into the climate change debate, i became rather
     unstuck as i found myself reading in energy and environment of the rejection of
     mann's climate curve by mckintyre and mckittrick last year.  This led me to
     look into more of the proxy data records, yours being one them.  As i read the
     various discussion i suddenly had a thought, and i'm not sure where to get an
     answer so i hope you don't mind me asking you.

   Of course not

     I may be rather over simplifying dendrochronolgy, but am i correct to believe
     that the signal for temperature is based simply on the size of the tree ring,
     or is it more complex than this.

   It is often mean width of rings from many trees at a site , averaged for each year AFTER
   measurements have been processed to remove geometric bias due to rings getting thinner as
   they
   are laid down round an increasing circumference ie young (inner trunk) rings are thicker
   and older (outer trunk) rings are progressively thinner - even in constant climate. Maximum
   ring density (hardness of wood  - related to how densely packed the cells are and how wide
   their cells walls are) is also used , and also has a geometric bias that needs to be
   accounted for.

      If its not, surely the size of the tree ring,
     which represents growth during a certain season, can be affected by many, if
     not all environmental parameters.

   This is a much discussed , and potentially true , issue. In fact, many theoretical models
   of tree growth (such as the vegetation schemes used in some large climate models) assume
   that tree productivity (and hence carbon sequestration) increases as CO2 increases. There
   is conflicting literature arguing that we can , and can not, observe such changes (over and
   above the influence of climate) on the growth rates of some trees in the late 20th century.
   Any "fertilizing" effect , such as the increased transport of nitrogen compounds to higher
   latitudes (that might be expected to be nitrogen poor) from increasing industrialisation
   might be expected to result in increased tree growth , possibly exaggerating (or obscuring)
   the apparent role of warming in causing modern ring widths in these areas to increase.
   However , while direct fertilization in trees (by N,P,K ) undoubtedly causes increased ring
   widths (in the absence of other limitation such as by water shortage) , it is still hotly
   debated as to whether the controlled greenhouse experiments , or open top chamber
   experiments using increased CO2 levels, actually indicate any real evidence of
   fertilization  (except perhaps for very brief periods). It is interesting to note , that
   stomatal density changes have been used to infer past atmospheric CO2 levels , during the
   last 10000 years, suggesting that trees adapt to the ambient CO2 , and so may not simply
   increase in growth proportionately.

     Could an increase in carbon in the atmosphere therefore give the same result as
     an increase in temperature? How can one distinguish the two? and what would
     this mean for our understanding of proxy based climate change?

   We can not give a definitive answer as of yet , but the general idea is to attempt to
   separate them using statistical techniques . The short answer is that we should not rule
   out the possibility that the apparent increase in 20thcentury tree growth around the world
   , might be partly due to higher CO2 levels.

     Any thoughts would be gratefully received. Cheers.

   In fact the issues are much more complex , due to the confounding effects of the need to
   manipulate tree-ring  measurements before environmental interpretation , and because
   various aspects of the environment have shown (partly parallel ) trends over the 20th
   century  but I have to do other stuff now

     Rob

   cheers
   Keith

   --
   Professor Keith Briffa,
   Climatic Research Unit
   University of East Anglia
   Norwich, NR4 7TJ, U.K.

   Phone: +44-1603-593909
   Fax: +44-1603-507784
   [1]http://www.cru.uea.ac.uk/cru/people/briffa/

