>It ceases to amaze me that, on one hand, AGW proponents will voice their current understanding of the limited impact of solar irradiance on climate, but, then, on the other, they resurrect a graphic prepared using obsolete solar irradiance data to emphasize their belief that Anthropogenic Greenhouse Gases have dominated climate over the last 40 years.
Figure 1 is an illustration of Climate Change Attribution from the Global Warming Art website. It was recently used in a discussion on climate change at another blog.
Global Warming Art describes the illustration as, “This figure, based on Meehl et al. (2004), shows the ability with which a global climate model (the DOE PCM) is able to reconstruct the historical temperature record and the degree to which the associated temperature changes can be decomposed into various forcing factors.” Source:
Does it really show the ability with which GCMs can reconstruct the historical temperature record? Let’s check.
The reference listed by Global Warming Art is: Meehl, G.A., W.M. Washington, C.A. Ammann, J.M. Arblaster, T.M.L. Wigleym and C. Tebaldi (2004). “Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate”. Journal of Climate 17: 3721-3727.
A quick Google search brings the following link to the paper:
The abstract of Meehl et al states, “The late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing (mainly GHGs), while the early twentieth-century warming is mainly caused by natural forcing in the model (mainly solar).”
The solar study referenced by Meehl et al is Hoyt and Schatten (1993) “A discussion of plausible solar irradiance variations”. Yes, that’s right, 1993. Refer to Table 1 in Hoyt et al and to the discussion on page 3723 to confirm the source of solar data.
Figure 2 is a comparative graph created by Leif Svalgaard of several TSI reconstructions and composites. Note that the current understanding of TSI variability is represented by the Svalgaard (red) curve and that the Hoyt data is represented by the light gray one. That’s a significant difference. The Hoyt data is obsolete.
(Note: The gray curve is difficult to see; the Svalgaard and Hoyt TSI data are used again in Figure 3, with a shorter time span.)
So the graph that Global Warming Art uses is actually a confirmation of a GCM’s INABILITY to match the historical temperature record, because it relies on obsolete TSI data to make the curves fit in early years.
-THE EFFECT OF THE OBSOLETE DATA-
Figure 3 is a comparison of the Hoyt and Svalgaard TSI data from 1880 to present without the noise of the additional datasets.
To scale it to Deg C, I assumed that from peak to trough global temperature has varied 0.1 deg C for the past three Solar Cycles and that the average variation in TSI over those Solar Cycles is 1 Watt/Meter^2. That agrees with the current understanding of the impact of solar irradiance on global temperature. The scaling factor is therefore 0.1 Deg C/(Watt/Meter^2). To shift it into the range of global temperature anomalies, I subtracted 1366.5 Watts/Meter^2 before scaling it. Figure 4 is the result. The problem is becoming obvious.
Now let’s add GISTEMP Global Temperature anomaly data to the graph. Refer to Figure 5. The “match” between global temperature anomaly and scaled Hoyt TSI curve isn’t bad. Unfortunately, the Hoyt TSI data is obsolete. The scaled Svalgaard TSI curve represents the current understanding.
The Meehl et al study had to use a solar forcing that was extremely unrealistic in order to reproduce the warming in the early part of the 20th century. If the forcings they employed are so erroneous in the early years, is there any reason to believe the anthropogenic forcings in recent years are realistic?