Sun Feb 12th, 2006 at 07:04:44 PM EST
As promised, with some delay, the diary about climate reconstructions, solar changes and climate models, "projecting" the future.
To introduce myself: retired engineer process automation of a large chemical factory. Because of that, I have some knowledge of (and defined and used) models, be it for multivariate physico-chemical processes, not for climate.
Interested in (and activist for) the environment since 1968 and specifically interested in climate from some 30 years ago, after reading a book about sun-climate connections (like rain and drought, lake levels, ocean/land temperatures, even number of wars...).
I do believe that anthropogenic greenhouse gas emissions have an influence on climate, be it far less than the alarming stories that reach the general public. In my opinion, after reading a lot of literature and following the discussions of both sides of the debate, the influence of anthropogenic greenhouse gases and especially aerosols are overestimated, and solar influences are by far underestimated by current climate models.
That doesn't imply that we shouldn't do anything to reduce the use of fossil fuels. To the contrary, as we need to reduce pollution and our oil dependence of not so stable countries...
The hockeystick graph
I was rather shocked by the acception of the MBH98 "hockeystick" millennium temperature reconstruction, without much debate, which overturned the scientific understanding of that time with a warm NWP (medieval warm period) and a much cooler LIA (little ice age). That triggered for me a deeper search for what is known in climate...
Other reconstructions, Huang ea.
After MBH98, the first publication which differs in amplitude vs. the other temperature reconstructions was for borehole temperatures by Huang ea., 2000. This revealed a much cooler LIA of app. -1 degr.C, vs. the -0.2 degr.C in the shaft of the hockeystick of the MBH98 reconstruction. This is important, as that has consequences for the impact of natural changes (especially solar) on current and future temperature changes. Mann ea., 2003 optimised the Huang data (but corrected this partly in 2004), but Pollack and Smerdon protested against this optimisation.
The second reconstruction with a larger variability (and a higher MWP) was from Esper ea., 2002. He used a different method for (regional) calibration of tree rings (see Cook ea., 2004), which retains long-term trends better than the method used by MBH98, but also a different calibration period. The effect of this can be read in Esper, Wilson and Briffa, 2005. Mann ea. reacted on the Esper publication, and Cook and Esper responded to this critique.
While Esper's reconstruction is only based on extratropical NH tree ring sites, Moberg ea., 2005 used different non-tree ring proxies for the long-term temperature trend and tree rings only for superposed short term temperature variations, effectively downplaying the influence of tree rings on the total reconstruction.
The reconstructions of Esper, Moberg and Oerlemans, 2005 (the latter based on glacier length records), all show a difference of app. 0.8 degr.C between the LIA and the mid-20th century temperatures (and a MWP app. at the same height of around 1950) vs. a difference of only 0.2 degr. C in MBH98 and other reconstructions. This is discussed by Esper, Moberg, Luterbacher and others, 2005 for its importance towards future climate expectations. The variability in the pre-industrial period was mainly from two sources: volcanic and solar. Volcanic gives a long-term average variation of maximum 0.1 degr.C over the past 600 years (see Fig. 6 from Briffa ea., 1998), that means that solar changes had an influence between 0.1-0.9 degr.C (the latter for the borehole reconstruction). Further, several of the reconstructions don't show a hockeystick shape, but more sinusoidal variations.
The amplitude of the temperature variation is a very important difference. If there was a small solar influence in the past (with a 0.1 degr.C solar induced variation in climate), then the recent upswing in temperatures since 1850 is in large part GHG driven. If there was a large solar influence in the past (with a 0.7-0.9 degr.C solar induced change in climate), then the upswing was largely solar driven. In both cases, the instrumental temperature variation must be explained with a different ratio between GHG/aerosols at one side and (enhanced) solar at the other side (of course besides the accuracy of the instrumental temperature record). That means that climate models must be adjusted, depending of what is known of the pre-industrial (and pre-instrumental) climate era.
Thus we still are in need of (far) more accurate reconstructions of the past millennium to have a better view on what can be expected in the future...
The latest reports
Last week two new reports were published on millennium reconstructions. The first was by Osborne and Briffa (OB) (one need subscription for the full text), the other is from D'Arrigo, Wilson and Jacoby (DWJ), using a partly different dataset. This is discussed by both opponents in the reconstruction arena: ClimateAudit (CA) and RealClimate (RC).
RC sees OB as a confirmation of the MBH98 hockeystick graph, while the main critique by CA is that OB used the same disputable bristlecone pine data (where the 1850-1980 upswing probably has nothing to do with temperature) and the same disputable PC method, which mines for "hockeystick" forms, as was used by MBH98.
Interesting is that DWJ didn't use the bristlecone pine data (and other questionable series), as these have a bad correlation with local/regional temperatures, while OB use these series, as they show a good correlation with local temperature trends... Quite contradictory!