cc: "Keith Briffa" <k.briffa@uea.ac.uk>
date: Fri, 08 Apr 2005 14:29:52 +0100
from: Tim Osborn <t.osborn@uea.ac.uk>
subject: Re: Emailing: ECHO-G and solar weighting
to: "Rob Wilson" <rjwilson_dendro@blueyonder.co.uk>

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At 10:36 08/04/2005, Rob Wilson wrote:
>1. I have to admit that I still find it strange that the natural forcing 
>ECHO-G run does not show higher values in the recent period when compared 
>to the 1100-1225 period. The higher modelled temperatures through this 
>earlier period are obviously forced by solar input. The fact that the 
>natural run does not show equivalently high (or higher) values in the 
>recent period means that the relative weighting of the forcings must have 
>changed over time. This seems completely illogical to me, but in my 
>ignorance, I think I must go with what the model shows. However, I think 
>some sort of mention of this is warranted in the paper - or am I being too 
>simplistic about all this.

I believe the early warmth in the ECHO-G run is due, at least in part, to 
the initial conditions being too warm (nearer to present-day than 
pre-industrial) and hence would have been cooler if the model had started 
with more realistic conditions.  At least partly an artefact of the 
experimental design rather than the real world.  I tried to show that at 
the Reading meeting with comparisons with the MAGICC simulations.

>2. As you and Keith will be included on the coral paper (will not hear 
>back from Sandy until the 20th) we can discuss this issue further later as 
>this is relevant for that paper also. I am troubled about the variance of 
>the modelled data being higher than the actual instrumental data. For 
>example, the temperature amplitude (defined as the difference between the 
>warmest and coldest decade) in the ECHO-G model of NH temperatures is 2.08 
>(see table below). However, the variance of the model is much higher than 
>the instrumental data over their common period. If I scale the modelled 
>data to the same mean/variance of the instrumental data (in this case Land 
>only 20-90N), then the amplitude reduces to 1.27. This value sits nicely 
>between my RCS reconstruction and Jan's curve when they have also been 
>scaled to the same instrumental data-set. This analysis suggests that 
>amplitude of the Moberg series is possibly too high.

Also, ECHO-G has no cooling effect in 20th century due to tropospheric 
sulphate aerosols and hence warms too much.  Hence amplitude too great.

Tim



>Coldest
>
>Warmest
>
>Amplitude
>
>Wilson STD
>
>1813-1822 (-0.74)
>
>1938-1947 (0.20)
>
>0.94
>
>Wilson RCS
>
>1600-1609 (-0.97)
>
>1937-1946 (0.17)
>
>1.14
>
>Esper02
>
>1345-1354 (-1.18)
>
>1950-1959 (0.15)
>
>1.34
>
>Briffa00
>
>1813-1822 (-0.80)
>
>1951-1960 (0.10)
>
>0.90
>
>Mann99
>
>1458-1467 (-0.68)
>
>1957-1966 (0.10)
>
>0.79
>
>Jones98
>
>1693-1702 (-0.77)
>
>1929-1938 (0.06)
>
>0.83
>
>Moberg05
>
>1576-1585 (-1.33)
>
>1104-1113 (0.23)
>
>1.56
>
>ECHO-G
>
>1689-1698 (-1.97)
>
>1953-1962 (0.11)
>
>2.08
>
>ECHO-G [scl]
>
>1689-1698 (-1.09)
>
>1953-1962 (0.18)
>
>1.27
>
>Does the higher variance in the ECHO-G model imply that it is TOO sensitive?
>
>comments are much appreciated
>
>regards
>Rob
>
>
>
>----- Original Message -----
>From: <mailto:t.osborn@uea.ac.uk>Tim Osborn
>To: <mailto:rjwilson_dendro@blueyonder.co.uk>Rob Wilson
>Cc: <mailto:k.briffa@uea.ac.uk>Keith Briffa
>Sent: Wednesday, April 06, 2005 10:22 AM
>Subject: Re: Emailing: ECHO-G and solar weighting
>
>At 13:07 05/04/2005, you wrote:
> >I was always under the impression that, in general, solar changes
> >controlled long term changes in climate and volcanic events caused short
> >term cooling.
> >...
> >I guess 'clusters' of volcanic events could cause a longer term response
> >of the climate system.
>
>Clustering of volcanic events does indeed enable them to have longer term
>influences.  But there probably is a changeover in which is dominant as you
>go to longer time scales.  But where that changeover happens depends on how
>strong the forcings are, which is particularly uncertain for solar (and
>fairly uncertain for volcanic).  The strength of the forcings is determined
>by how the modellers designed their experiments - they specify the forcing
>time series in advance.  For the ones they chose to use in ECHO-G (based on
>Crowley's earlier work, though not identical) the volcanic still looks to
>be dominant on time scales up to 30 years and probably of similar
>importance to solar on time scales 30-100 years (these are my guesses from
>eyeballing some figures of smoothed forcings, but could be quantified by
>looking at spectra etc.).  Above 100 years, then solar is probably bigger
>than volcanoes in ECHO-G, but not overwhelmingly so.
>
> >How are the relative weightings of the external forcing estimated in the
> >models?
> >Although I agree that GHGs are important in the 19th/20th century
> >(especially since the 1970s), if the weighting of solar forcing was
> >stronger in the models, surely this would diminish the significance of GHGs.
>
>The forcings for ECHO-G are selected in advance by (1) choosing the
>strength and time series of solar irradiance variability; (2) choosing the
>strength and time series of volcanic aerosol variability and converting
>this to a surrogate time series of solar irradiance reductions, which are
>then added to (1); and (3) choosing the time series of greenhouse gas
>concentrations.
>
>Thus (1) and (2) prescribe the forcings to the model - there is no role for
>the model itself to determine the strength of those forcings.  The model
>does however determine the strength of the forcing induced by the GHGs, but
>this is fairly accurately known as are the histories of the GHG
>concentrations (the combined uncertainty in history of GHGs and their
>conversion to radiative forcing is given by the IPCC TAR as just
>+-10%).  Thus the ECHO-G model's only role in determining the relative
>weightings of the forcings is in how it converts (3) to a forcing and this
>is very well known.  Hence the climate model doesn't really determine the
>relative weightings - they are determined by whoever designed the
>experiment and selected the inputs.
>
>If (1) had been chosen with stronger changes, ECHO-G would have responded
>by making last 150 years warming stronger - the GHG-induced warming would
>still be just as great, but would have extra solar-induced warming on
>top.  Thus GHG-warming would not be reduced.  This might give a poorer fit
>to the observed record, implying either that the solar forcing should be
>reduced (since the GHG can't be as it's so well known) or that the climate
>model is too sensitive (quite possible).  So, in the latter respect, your
>point could be correct because a lower sensitivity would then downplay the
>response (including the GHG-induced warming) until it fitted the
>observations once more.
>
>However, that recent paper by Foukal et al. suggests the evidence for
>long-term fluctuations of solar irradiance is biased and perhaps there
>isn't any!  Also, the people doing detection & attribution of 20th century
>climate changes reckon that the particular timing of the solar and GHG
>forcings during the 20th century (i.e. the interdecadal time series
>structure, not just the increasing trend) and the slightly different
>patterns of climate response to the two forcings can be used to distinguish
>between the two and they find the observed warming time series and patterns
>match better to that expected from the GHG forcing than the solar forcing -
>and if I understand it correctly, this result doesn't depend on the
>magnitude of the forcings, just the time series structure.
>
>Sorry to go on.  Hope some of this is useful/interesting.
>
>Cheers
>
>Tim
>
>Dr Timothy J Osborn
>Climatic Research Unit
>School of Environmental Sciences, University of East Anglia
>Norwich  NR4 7TJ, UK
>
>e-mail:   <mailto:t.osborn@uea.ac.uk>t.osborn@uea.ac.uk
>phone:    +44 1603 592089
>fax:      +44 1603 507784
>web:      <http://www.cru.uea.ac.uk/~timo/>http://www.cru.uea.ac.uk/~timo/
>sunclock: 
><http://www.cru.uea.ac.uk/~timo/sunclock.htm>http://www.cru.uea.ac.uk/~timo/sunclock.htm

Dr Timothy J Osborn
Climatic Research Unit
School of Environmental Sciences, University of East Anglia
Norwich  NR4 7TJ, UK

e-mail:   t.osborn@uea.ac.uk
phone:    +44 1603 592089
fax:      +44 1603 507784
web:      http://www.cru.uea.ac.uk/~timo/
sunclock: http://www.cru.uea.ac.uk/~timo/sunclock.htm

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