date: Mon Jul 2 09:47:40 2007 from: Keith Briffa subject: Re: crowley forcing to: Tim Osborn , 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/