December 2007 Session ‘The “Divergence Problem’ In Northern Forests

This is a copy of the e-mail sent to a number of scientists about an important upcoming meeting. I made a suggestion to add a topic, which I have included at the end of this weblog. The topic of the ‘divergence problem” was discussed on Climate Science; see

A New Paper On The Differences Between Recent Proxy Temperature And In-Situ Near-Surface Air Temperatures

“Dear colleagues,

We would like to encourage you to submit an abstract to the following session for the fall AGU meeting to be held in San Francisco, CA from Dec 10-14, 2007. Please also pass on to any interested parties. This is for the Paleoclimatology and Paleoceanography session PP04 – The “Divergence Problem” in Northern Forests.

Go to for the most recent program listing and the abstract submission tool. The abstract deadline is September 6, 2007.

The session abstract is:

The “Divergence Problem” in Northern Forests

An anomalous reduction in forest growth indices and temperature sensitivity has been detected in tree-ring width and density records from many circumpolar northern latitude sites in recent decades. This phenomenon, also known as the “divergence problem”, is often expressed as an offset between warmer instrumental temperatures and their underestimation in reconstruction models based on tree rings. The divergence problem has potentially significant implications for large-scale patterns of forest growth, the development of paleoclimatic reconstructions based on tree-ring records from northern forests, and the global carbon cycle. The causes of this phenomenon, which appear to be several and sometimes regionally specific, are not well understood and are difficult to test due to the existence of a number of covarying environmental factors that may potentially impact recent tree growth. Although limited evidence suggests that the divergence may be anthropogenic in nature and restricted to the recent decades of the 20th century as well as higher latitudes, one current challenge is to confirm these observations. We welcome papers that address this issue using tree rings, remote sensing, vegetation models, and other methods.


Some issues to keep in mind:

1.. The “Divergence Problem” (DP) is not noted at all sites. Which regions in the Northern Hemisphere show this problem and which do not. Why are some regions affected worse than others? Are there species specific issues to consider as well?

2.. Has the DP been observed in the Tropics or Southern Hemisphere?

3.. Implications for palaeoclimate reconstruction

4.. Implications for forests as carbon sinks

5.. Is the DP restricted to only temperature sensitive tree-ring chronologies, or has anyone noted it in precipitation sensitive TR series as well.

6.. How about tree-ring isotopic data? Is the DP observed in any isotopic series? If not, could isotopic series aid the identification of the DP in the past?

7.. Can the use of forward modeling approaches aid the identification of the DP as well as explore reasons for it?

8.. In some regions the DP may be physically observed as a browning of the needles. Can remote sensing identify regions where DP is occurring?

9.. And of course, causes of the DP. Anthropogenic vs. natural reasons etc etc.

A critical mass of 20 abstracts is likely needed for this session to be oral in nature. There are, however, no guarantees though. The more the merrier!!

We look forward to your submissions and seeing you all in December.

Best regards,

Rob Wilson and Rosanne D’Arrigo

My reply is

“Dear Drs. Wilson and D’Arrigo

Thank you for your announcement and invitation for this very important
session. While I will not be able to attend the AGU Conference this
December, I did want to e-mail to encourage you to add another topic to
your list of questions. This is

How accurately does the in-situ (station data), when used to construct the
regional temperature trends, compare with the tree-ring data that are used
represent the actual temperature environment in which the trees grow?
Also, is the statistical relationship improved when the comparision with
the tree ring derived data is compared with maximum and minimum
temperatures, as well as different temperature measures of the growing
season, such as first and last date below selected threshold temperatures.

For the growing set of documentation of the USHCN sites, the siting of the
in-situ temperature measurement sites is a major problem (see and A
presentation of photographs for the surface temperature stations that are
used as part of the calculation of the temperature trends for each region
might be very insightful. Satellite derived surface temperatures (e.g. see
Comiso, 2006: Weather. pages 70-76) can be very helpful also in this
assessment, but the interpretation to the heights that the tree responds
to is also a challenge, as well as that the satellite is not sampling on
all days.

The testing of the robustness of the air temperature data trends would be
quite informative, and the availability of these photographs would be

Best Regards


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