cc: mitrie -- Anders Moberg <andersatXYZxyzu.su.se>, Eduardo Zorita <Eduardo.ZoritaatXYZxyzs.de>, Jan Esper <esperatXYZxyz.ch>, Keith Briffa <k.briffaatXYZxyz.ac.uk>, Myles Allen <m.allen1atXYZxyzsics.ox.ac.uk>, Nanne Weber <weberatXYZxyzi.nl>, t.osbornatXYZxyz.ac.uk
date: Thu, 9 Feb 2006 22:57:39 -0500
subject: Re: Draft conclusions for report to Netherlands Environment
to: Martin Juckes <M.N.JuckesatXYZxyzac.uk>
This is fine by me, and the conclusion is interesting.
My only comment would be on the section that says that different
reconstructions are inconsistent with each other - are they really, given
the large uncertainties? the ones I looked at (excluding Mann) did
not seem inconsistent with each other for example in terms of their
little ice age coolign given the large uncertainties...but maybe I am not
up to date on what exactly you did there - sorry if this is the case!
Quoting Martin Juckes <M.N.Juckes@rl.ac.uk>:
> 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
> last millenium.
> 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 millennium in
> the Northern Hemisphere, and 1998 is likely to have been the warmest
> year," where ``likely'' implies a greater than 66\% probability
> (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. The warming trend contains a signifcant natural component,
> but an anthropogenic contribution was clearly detectable towards the
> end of the 20th century.
> 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 attention to incomplete
> documentation of the wide range of data sources used and to incomplete
> description of some aspects of the analysis algorithm.
> The debate has attracted much public interest and generated
> considerable confusion.
> (C1) is sometimes paraphrased as ``there was no hemispheric wide
> Medieval Warm Period'', but this
> terminology leads to confusion: there is no agreed definition of what
> would constitute
> a `` Medieval Warm Period''.
> 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 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
> 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 whether they
> have a climate sensitivity which is realistic. The problem is that
> the period of reliable,
> global measurements is too short to carry out this exercise comprehensively.
> In the last 5 years a number of studies using different techniques
> and different,
> though 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'').
> 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.
> Data centres have improved the transparency with which data is
> available and the
> quality of the information accompanying the data, recording its
> provenance has
> also improved.
> 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.