A critique on Veizer’s Celestial Climate Driver

Veizer suggests the recent global warming might be driven by changes in solar activity (his Fig. 14a – dashed line is temperature, solid line with diamonds is solar cycle length). But the figure only shows the temperature record from 1935-1990, even though of course more recent data are available. Veizer must be aware that in view of these more recent data, the scientist who originally proposed this correlation in 1991, Knud Lassen, has concluded in 2000 that solar cycle variations cannot explain the ongoing warming trend [1].

In the figure below, we have superimposed the standard CRU data set (blue curve) of global mean temperature on Veizers graph.

We see that the graph selectively shows a part of the time series dominated by the well-known temporary “dip” in the global warming trend, reaching a minimum around 1970. The correlation of temperature and solar cycle length applies mainly to this “dip” in the curves. Thus, solar cycle variations may or may not explain this “dip” (one similar-looking dip in two curves is not a significant correlation and could easily be coincidence; there is a better-founded explanation for this “dip” resulting from cooling by aerosol emissions – see the figure from Hansen et al. coming up below).

But there is no indication that solar cycle variations could explain the ongoing warming trend. There is simply no significant trend in this (or any other) solar indicator since 1940. Veizer fails to discuss the key issue (namely, whether the data support the idea that the warming trend, rather than just the temporary dip, could be explained by solar cycle variations). The (often faulty or misleading) use of these solar cycle data by “climate sceptics” has quite a pre-history, see e.g. Damon and Laut (2004). Note that the relative vertical scale of the curves is arbitrary. The article by Damon and Laut (2004) also shows that when solar cycle length is scaled up as much as it is in the above figure in order to match the 1940-1990 temperature record, that this implies a great mismatch for the earlier periods in time. Veizer should be aware of this problem of his proposed match, but he fails to mention it.

As an aside, one of the data points in the above graph is incorrect; the National Geophysical Data Center gives a solar cycle length of 11.8 years for 1963, where Veizer plots 11.0 years.

[1] P. Thejll and K. Lassen, 2000: Solar forcing of the Northern hemisphere land airtemperature: New data. Journal of Atmospheric and Solar-terrestrial Physics, Vol. 62 (13),1207-1213.

Greenland temperature variations

Veizer’s paper argues that the local preindustrial climate fluctuations over the past millennium in Greenland were not driven by CO2. As a proof, Veizer shows a temperature reconstruction for Greenland together with the CO2 concentration in his Fig. 12. Now, the preindustrial CO2 concentration over this period was constant, and therefore nobody has ever proposed that CO2 could have caused any climate variations during this time. It obviously can’t. Showing this in a plot is thus a completely moot point that is clearly not aimed at any fellow scientist; it is something meant to impress uninformed lay-people. Indeed, the graph is not from a scientific publication, but was reproduced from a popular climate-sceptics book that has been distributed in Germany by the coal-industry lobby.

Using a time period when CO2 did not change at all, to support a general conclusion that “CO2 is not the driver of climate change” and hence is not responsible for the current warming, is hardly logical. It merely highlights the fallacy of looking for one single “driver” that explains climate variations on all time scales. It is well-established in climatology that different causes and mechanisms have caused climate changes in the past (orbital variations, plate tectonics, solar variability, volcanic eruptions, etc.), so that a cause-effect relationship has to be determined for each individual case, rather than looking for one overall “driver”.

(As an inconsequential but telling aside, Veizer wrongly attributes the CO2 curve in Fig. 12 to the Greenland ice core GISP2. It is well-known amongst paleoclimatologists that there are no successful CO2 measurements from Greenland, the record shown is obviously from Antarctica. It is strange how this slipped by the journal’s review.)

A strange temperature graph

Veizer, Fig. 14d

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