date: Wed, 08 Mar 2000 16:00:47 +0000 from: Phil Jones subject: Nature paper and beyond to: k.briffa@uea.ac.uk,t.osborn@uea.ac.uk [[[the original email appears deleted, only this forwarded copy survives]]] >Date: Tue, 7 Mar 2000 15:53:11 -0500 (EST) >From: "Henry N. Pollack" >To: p.jones@uea.ac.uk >cc: Shaopeng Huang >Subject: Nature paper and beyond > > >Dear Phil, > >Thanks for your note about the Nature paper -- we will be pleased to send >some reprints for you and colleagues as soon as they arrive. Thanks also >for your help and encouragement along the way with that paper. > >We (Shaopeng and I) welcome the opportunity to participate in a group >effort addressing the broad issues you outline in your e-mail. We would >all be well served by getting together for a free-for-all brainstorming >session to develop a strategy of getting at these questions. Is there any >possibility of this? > > >For now, let me offer some of my thoughts on the two topics you set forth. >(Shaopeng may or may not agree with all of my perspectives). > >1. We too are very interested in understanding what underlies the >differences between the reconstructions from the high resolution proxies >and the borehole temperatures. You suggest that we segregate the western >North American sites from the eastern sites. We have done that, and there >is a difference: the western sites have warmed only half as much (ca. 0.75 >K) as the eastern sites (ca. 1.5 K) over the past five centuries. But I >am not fully persuaded that the difference is due to different land-use >histories, although that might contribute. > >You might have a look also at the paper by Mareschal et al [JGR 103(B4), >7385-7397, 1998]. They remark on very careful site-selection criteria, >rejecting many boreholes for various reasons, including vegetation >changes. The boreholes they describe are not in the database we have >assembled because of the recent publication. Their reconstruction is >parameterized differently than ours, but it is similar to a reconstruction >we did for some forty boreholes in eastern Canada alone (not published). >Both show a warming of about 1.5 K in the 19th and 20th centuries. > >A related and puzzling observation is a comparison we did some years ago >between the Hansen and Lebedeff and your CRU instrumental SAT time series. >For most regions around the world the agreement was very good between the >two, but for eastern North America there was a considerable difference, >for reasons I have never been able to determine. The borehole >reconstructions, however, looked more like the H&L SATs than the CRU >series. Do you have any insights as to why the two SAT series might be >different? > >You also raise the question about whether snow cover renders the >subsurface temperatures a seasonal rather than annual archive. This has >always been an interesting question, and the answer (in part) lies in when >and how long the snow covers the land surface. Dave Chapman has done a >simple analysis that shows that the ground surface can be warmer if the >snow is on the ground during the coldest part of the winter, but if the >snow comes late in the season, it may shield the ground from some of the >early warming of spring. The thickness of the snow is not very critical >beyond some minimal thickness. It is possible to acquire snow-cover >information from climatological archives (NCDC, Environment Canada, >UEA-CRU?) to see just how much of the winter might be "lost" in various >locations. It is possible that the North American east-west difference is >in part due to earlier-longer-deeper snow cover in the mountainous west. >But at least a part of the American west is desert, with almost no snow >cover. > >Perhaps equally important is the freezing of soil moisture in the winter, >at least at latitudes where temperatures fall to or below the freezing >point. The latent heat release produces what is known as the zero curtain, >i.e. a zero temperature that exists until all the soil moisture is frozen; >depths below the curtain do not feel subzero winter temperatures that may >be occurring at the surface. This too might be amenable to a compilation >from climatological archives of the number of days when the SAT drops >below zero for some suitable period of time. I am not so familiar with the >archival data to know if this is an easily determined quantity. > >Related to these questions, I am getting quite interested in the details >of how the temperature signal gets imprinted at the ground surface, i.e >within the uppermost few meters of the subsurface. I am just getting a >project underway with a university colleague in engineering. He has >developed a Land Surface Processes Model to understand how the microwave >signal that satellites see is generated by surface processes. I have >persuaded him that a good test of the model is to see if it can generate >the temperature signal going down into the Earth as well as the >upward-directed microwave signal. The model is driven by time series of >meteorological observables. > >Now on to the second topic: > >2. About the past 1000 years. The very best boreholes are in the ice in >Greenland and Antarctica (low noise big signal environments). The >geothermal reconstructions for Greenland continue to show a temperature >maximum around 1000 AD (see the paper by Dahl-Jensen et al, Science 282, >268-271, 1998) that is about 1K above present day temperatures, and the >temperature around 1600 as the coldest (not by much) of the past >millennium. The 19th century cooling is also apparent. > >In another geothermal study (GRL 24, 1947-1950, 1997), using a very >different approach with much lower quality data but lots of it, Shaopeng >and I also showed a reconstruction very similar to that of Dahl-Jensen et >al, at least for the past 10 kyr. The dataset we used in that study was a >very heterogeneous global (predominantly northern hemisphere) ensemble >with a lot of noise that certainly made us cautious about the result. But >after seeing Dahl-Jensen's similar result, we have some additional >confidence in the broad picture that we developed. Our picture also shows >a peak of temperature around 1000 AD that is above present-day. > >In Antarctica things are different, with 1000 years ago cold and the LIA >warm, i.e. anti-correlated (see the 1999 IUGG abstract MC02/E/08-B1 by >Gary Clow). What this demonstrates is that it is possible that many of the >favorite climatic episodes, the MWP, the LIA, may not be global phenomena. > >We will review carefully the borehole database for deep high quality data >that may shed some additional light on the full millennium in different >regions. We might even think about different parameterizations for >some of the best data. But it will be very difficult to make the MWP go >away in Greenland. > >Let me also call to your attention another very recent paper (Werner et >al, GRL 27(5), 723-726, 2000) that makes an interesting case of explaining >why borehole and oxygen isotope temperature reconstructions for Greenland >are substantially different. The authors argue that there was a >deficiency in winter snowfall that biased the isotopes toward summer >snowfall temperatures, again a seasonality argument. I am on a steep >learning curve with respect to the subtleties of the various proxies, and >can only make guesses as to why proxies differ from each other and from >boreholes. I have some ideas, probably naive, to try out on you more >experienced proxy practitioners. > >Finally (this has turned out to be a very long reply!), we have just >received from Tom Crowley his current best estimates of the various >forcings, and will be looking at these in the context of our efforts to >integrate high resolution reconstructions with the borehole results, along >the lines of Shaopeng's paper at IUGG last summer. > >Well, the bottom line is that we are indeed interested in making headway >on these problems. What are the next steps? > >Cheers, >Henry > > Prof. Phil Jones Climatic Research Unit Telephone +44 (0) 1603 592090 School of Environmental Sciences Fax +44 (0) 1603 507784 University of East Anglia Norwich Email p.jones@uea.ac.uk NR4 7TJ UK ----------------------------------------------------------------------------