from: Keith Briffa <k.briffaatXYZxyz.ac.uk>
subject: Re: Draft conclusions for report to Netherlands Environment
to: Martin Juckes <M.N.JuckesatXYZxyzac.uk>
been through this and please see my comments in square brackets.
Really am trying to get to the other stuff.
At 16:32 09/02/2006, you wrote:
I need to send in a draft report to RIVM soon. The summary should lay out
what we believe to be the state of knowledge on temperatures in the
I would be grateful for feedback on the text below.
IPCC (2001) concluded that ``The 1990s are likely to have been the warmest decade of the
the Northern Hemisphere, and 1998 is likely to have been the warmest
year," where ``likely'' implies a greater than 66\% probability [this implied a
confidence level of between 66 and 95%]
(this conclusion will be referred to below as ``C1'').
The Northern Hemisphere temperatures are believed to have shown a
gradual cooling trend from the start of the millenium until the
mid 19th century, and a warming trend since then. Substantial
interannual, decadal and centennial scale variability was superimposed
on these trends.
[In the Tar the focus was on Mannet al 1998,1999 and they did not show what I would call
"substantial centennial" variability]
The warming trend contains a signifcant natural component,
but an anthropogenic contribution was clearly detectable towards the end of the 20th
This conclusion was based on a wide range of results,
including that of Mann et al., (1999).
Since publication of the IPCC (2001) report there has been much criticism of
the techniques used to estimate temperatures, particularly those used by Mann et al.
The criticism of the latter work has drwan [drawn] attention to incomplete
documentation of the wide range of data sources used and to incomplete
description of some aspects of the analysis algorithm.
[The situation has not been helped by the dis-information spread by certain sceptics ,
however, that in my opinion act deliberately to confuse the issue]
The debate has attracted much public interest and generated considerable confusion.
(C1) is sometimes paraphrased as ``there was no hemispheric wide Medieval Warm Period'',
terminology leads to confusion: there is no agreed definition of what would constitute
a `` Medieval Warm Period''.
[Actually Martin I do not believe anyone says or believes that there was NO medieval warm
period - merely that it
is time transgressive , spatially poorly documented and , as you imply, not precisely
defined or quantified. There
was a period of relative warmth , but the question is how warm and when (actually that is
two questions!). ]
A second conclusion of the IPCC report, which is related to but distinct from (C1), is
that current temperature trends have a signifcant anthropogenic
component (referred to as ``C2'' below).
Conclusion (C2) is based mainly on GCM simulations and is not directly addressed in this
study. Conclusion (C1) is based mainly on
the interpretation of proxy climate records: this is the specific
issue addressed here. Reconstructions of past climates are also used to evaluate
GCM simulations of those climates and hence to evaluate the GCMs: this provides some
indirect input into conclusion (C2).
The following concpetual [conceptual] model can help us
to understand how studies of the past millenium can contribute to
discussion of future climate change:
Temperature anomaly- = [ ( climate sensitivity-) times ( sum of forcings-) ]
plus ( natural variability-)
This is a drastic simplification: the different ``forcings'' (solar variability,
volcanic and other natural changes to atmospheric composition, anthropogenic changes
to atmospheric composition) can not be wholly characterised by a single number:
their influcence on the climate system is extremely complex and the response
of the climate is neither instantaneous nor uniform. Nevertheless, scientists have found
this simple conceptual model to be a useful basis for discussion.
By testing the models
against observed climate variability it can be dtermiend [determined] whether they
have a climate sensitivity which is realistic. The problem is that the period of
global measurements is too short to carry out this exercise comprehensively.
[this begs the fascinating question of constitutes "realistic" climate sensitivity - given
in defining the concept to account for transience on different timescales - but your
summary is good]
In the last 5 years a number of studies using different techniques and different,
though overlapping [suggest say something like "using some common input data" rather
than use the word "overlapping"], data collections have re-inforced (C1), though they
disagree, both with Mann et al. and among themselves, on other issues. In
particular, there is a relatively wide range of estimates as to the magnitude
of the cold anomaly in the 18th century (during the ``Little Ice Age'').
[larger difference related to the cold of the 13th and 14th centuries]
It is clear that regional temperature anomalies can be much larger than
those on the hemispheric scale. IPCC (2001) did not suggest that
current temperattures are above the extremes experienced by
any region in the past thousand years. Recent modelling work has
led to greater understanding of climate variability on different
scales. A lot of discussion in the popular and electronic media,
and also, to a limited extent, in the peer reviewed literature,
neglects this crucial distinction between what is happening on the global
and regional scales. [agree wholeheartedly]
Data centres have improved the transparency with which data is [are] available and the
quality of the information accompanying the data, recording its provenance has
The use of a wide range of different data sources and different analysis techniques
makes evaluation of the differences among published results difficult.
Within this project we have subjected data collections from a variety of
authors to several analysis techniques.
It is found that the range of different results is still spanned by the
results when a single analysis technique is used.
This suggests that a priority for further work to reduce the uncertainty
will be to improve understanding of the data.
Professor Keith Briffa,
Climatic Research Unit
University of East Anglia
Norwich, NR4 7TJ, U.K.