date: Wed May 4 14:28:54 2005 from: Keith Briffa 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/