date: Mon Jul  2 09:47:40 2007
from: Keith Briffa <k.briffa@uea.ac.uk>
subject: Re: crowley forcing
to: Tim Osborn <t.osborn@uea.ac.uk>, t.kleinen@uea.ac.uk

   Tim/Thomas
   I agree that this is he only logical way to justify the periods - the very issueof the
   period definition itself , is a source for useful discussion on the concept/definition of a
   Little Ice Age (in the context of previous papers that all address this from the
   observational/proxy side in isolation. In other words the specificaton of the periods and
   the sensitivity to specific forcings should constitute a fair slice of such a paper on its
   own.
   Keith
   At 17:04 29/06/2007, Tim Osborn wrote:

     Hi Thomas,
     thanks for the figs.  I'd already made a plot too, using the forcings used in the ECHO-G
     simulation which came from Crowley (2000) -- please see attached.  Very similar to yours
     except I've used a 30-yr Gaussian weighted filter.  The zero level is arbitrary.
     The red line at -2.5 W/m**2 helps highlight the minima in the last 500 years (1450 AD is
     lower, but less good proxies back then!).  Late 1600s and 1810s are similar with this
     filter.  I've put vertical markers around 1670-1700 and 1810-1820... they are slightly
     delayed from the peak negative forcing, but given the lag expected in the climate
     response to these forcings, they seem like reasonable periods to use to search for
     strong LIA conditions.  1670-1700 is indeed not very much out of the ordinary compared
     with the preceding century, but in the context of the whole 500 yr it is unusual.
     Keith -- the background to this is that Thomas and I were briefly discussing the proxy
     results that Thomas showed earlier (for 1690s and 1810s) and I said that if they can be
     made into a paper there needs to be a justification for choosing the periods.  A good
     justification would be that in the last 500 years they are periods with lowest forcing
     and therefore an expectation of coldest forced climate change.  This will of course
     depend on whose forcing you use (principally ratio of solar to volcanic magnitudes) and
     filter etc.  But the point isn't to try to prove that these periods definitely had the
     strongest negative forcing of the last 500 yr, but instead to use the analysis of
     Crowley (2000) forcings to say that this may be true and therefore these periods are
     worth looking at in terms of proxy evidence.  So, 1810s still, but change 1690s to
     1670-1700???
     Can we talk about this next Friday?
     Cheers
     Tim
     At 15:18 29/06/2007, you wrote:

     Hi Tim.
     Since I am preparing figures anyway, I thought I'd have a quick look at your
     forcing timeseries suggestion.
     The blue line are the annual values, red is a 10 (well, 11, actually) year
     running mean, black is a 25 year running mean.
     In the 25yr mean, the early 19th century is somewhat remarkable, since it's a
     noticeable dip in the otherwise increasing trend. The late 17th century
     forcing is certainly a local minimum, but appears rather unremarkable (I
     guess mainly since the early 17th century also has rather low forcing).
     In the decadal mean, the 1810s (or thereabouts) certainly look rather
     interesting, but in the 17th century one should possibly go to some other
     period, e.g. the 1670s, or the 1640s.
     The idea for using the 1690s came from the CET timeseries, where it is the
     coldest period (see HadCET figure).
     The natural forcings run agrees nicely in that respect (see gm_temp.jpg).
     Natural forcings is black, our control run is blue, the thick lines are 10yr
     running means. There, quite obviously the 1690s is the coldest decade as
     well, with possibly similarly low temperatures at the beginning of the run.
     Cheers,
     Thomas

     Dr Timothy J Osborn, Academic Fellow
     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:      [1]http://www.cru.uea.ac.uk/~timo/
     sunclock: [2]http://www.cru.uea.ac.uk/~timo/sunclock.htm

   --
   Professor Keith Briffa,
   Climatic Research Unit
   University of East Anglia
   Norwich, NR4 7TJ, U.K.

   Phone: +44-1603-593909
   Fax: +44-1603-507784
   [3]http://www.cru.uea.ac.uk/cru/people/briffa/

