from: Keith Briffa <k.briffaatXYZxyz.ac.uk>
subject: Re: A quick question if i may.
At 23:55 03/05/2005, you wrote:
Dear Dr Briffa
I know its marking season, but i wonder if you might answer me a few quick
will have to be brief , 'cause got to go to China at weekend and need to do loads of stuff
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
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
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
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.