Posted on 26 June 2012 by Bart Verheggen

Also published at My view on climate change.

In their reply to our criticisms, Prof Fritz Vahrenholt and Dr Sebastian Lüning exhibit a misunderstanding of various key aspects of climate science. Their claim, that mainstream science is radically wrong, is unfounded and not backed up by sufficiently strong evidence. In our response, also published at EER, we only focused on a few selected issues of contention. Here we reply to all of Vahrenholt’s and Lüning’s arguments and allegations.

• A long reference list in their book is by itself not persuasive for their argument. In their reply they already show a tendency to misinterpret published research (e.g. Solanki 2004; Mann 2008; Berger 2011).

• They present their view as some kind of superior alternative to the IPCC. That is a very strong claim. The least they should do is to submit such a claim to scientific scrutiny, rather than hiding behind a long -partly misunderstood and partly cherrypicked- reference list.

• Vahrenholt and Lüning are correct in pointing out that the climate has a delayed response to changes in its radiation budget (whether from the sun, from CO2 or from other causes) due to the ocean’s heat capacity. This may cause the planet to continue to warm after the radiative forcing has stabilized, but the rate of warming will decrease and level off as the climate equilibrates to the new situation.

However, the observations show that both surface temperatures as well as ocean heat content started to increase (during the 1970s and 80s) long after solar activity had reached its plateau (during the 1950s). This is inconsistent with a lagged response to the sun, as suggested by Vahrenholt and Lüning. The relatively steady rate of warming of both ocean and atmosphere over the past four decades indicates that this must be caused by another process. The sun cannot be responsible for the warming of the past four decades, irrespective of how strongly one wishes to amplify its effect.

Updated graphic of total heat content from Church et al 2011

Vahrenholt and Lüning cite the work of Solanki and co-authors in support of their claim. However, Solanki et al made the same point as we do: "This comparison shows without requiring any recourse to modeling that since roughly 1970 the solar influence on climate (through the channels considered here) cannot have been dominant" (Solanki et al., 2003), and: "Although the rarity of the current episode of high average sunspot numbers may indicate that the Sun has contributed to the unusual climate change during the twentieth century, we point out that solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades." (Solanki et al., 2004, referring to their 2003 paper). This is just one of many examples where Vahrenholt and Lüning misinterpret what is written in the literature in order to bolster their point of view.

• Besides the sun, purported cyclical behavior is put forward by Vahrenholt and Lüning as a dominant driver of climate. There is much that is unclear about these cycles, but they manifest themselves mainly regionally. A mechanistic understanding of these cycles –instead of fitting observations to periodic signals, which by itself can be a mathematical artifact- is lacking. Moreover, currently all climate indices (air temperatures, sea temperatures, amount of ice, ecological aspects) are moving in the warmer direction, so a re-distribution of heat within the climate system is not the cause of the current warmth. They mention some scientists (e.g. Bond, Latif) who study these cycles and who concluded that they are very influential in our climate. This view is not widely shared because of the sketchy and conflicting information available. But Bond and Latif did not draw the same conclusions from their work as Vahrenholt and Lüning did; they did not use it to downplay the important role that greenhouse gases play in our climate.

• Vahrenholt and Lüning proclaim as fact that global temperatures in Medieval times were similar as now, although most (but not all) published research to date concluded that the warmth in Medieval times was not as globally widespread and as pronounced as the current warmth. E.g. Mann et al, in their 2008 PNAS paper (which they cite in their response) wrote “The reconstructed amplitude of change over past centuries is greater than hitherto reported, with somewhat greater Medieval warmth in the Northern Hemisphere, albeit still not reaching recent levels."). Their insinuating language like “the MWP and LIA suddenly re-appeared”, in reference to an oft discussed 14 year old paper is nothing more than a cheap dig, taken straight from the skeptics’ playbook.

• Vahrenholt and Lüning seem to have missed our reference to the work of Pearce and Adams, who modeled the amplifying effect of cosmic rays as proposed by Svensmark and found it falling short by orders of magnitude. Besides this evidence from a physics based climate model, it is also possible to do a regression analysis of global average temperatures to the different potential climate forcings, and including time lags in the fitting procedure. This comes down to letting the data speak for themselves, without any a-priori knowledge of working mechanism (including amplification) or physics. Lean and Rind did exactly that in their 2008 paper. The result: “None of the natural processes can account for the overall warming trend in global surface temperatures.” and “According to this analysis, solar forcing contributed negligible long-term warming in the past 25 years and 10% of the warming in the past 100 years”. Note that these regression results implicitly include any amplification effects.

• Vahrenholt and Lüning correctly conclude that fluctuations in short term trends “should not be misunderstood that warming on a 30 year scale might have stopped”. In contrast, in the original EER interview Vahrenholt said “Our critics say fifteen years is not enough to make judgments about the climate.” Apparently, he now agrees with his critics.

• Vahrenholt and Lüning confuse what are assumptions versus what are emergent properties or outcomes of climate models. For example, the amplifying feedback by water vapor is a result of the interplay of various elements of atmospheric physics as incorporated by General Circulation Models. They also state that the small effect of solar variability is an assumption of climate models, whereas it is an outcome. Rather than handwaving inconvenient results away, the proper response would be to demonstrate where the underlying physics is wrong, e.g. by building a better model.

• Vahrenholt and Lüning claim that stratospheric temperatures have remained stable, even though they are expected to decrease in response to a stronger greenhouse effect. Again they conclude something about a long term trend based on short term fluctuations, against their own advice. Moreover, the paper by Berger and Lübken which they cite is irrelevant because it presents temperature trends in the mesosphere, an even higher altitude. The CO2 fingerprint is mainly confined to the stratosphere and is distinct from the effect of ozone. The latter also induces cooling, but mainly above the poles, while increasing CO2 cools the stratosphere everywhere. The point remains that if the sun were the dominant cause of surface warming, this warming would also have occurred at higher stratospheric layers. This is not what has been observed.

In short, Vahrenholt’s and Lüning’s reply does not change our conclusion: Their claim that “the contribution of CO2 to global warming is being exaggerated” does not stand up to scientific scrutiny. The sun is not responsible for the warming seen during the past 4 decades. Greenhouse gases in all likelihood are.

Climate changes in the deep past (going back hundreds of thousands or even millions of years) can not be explained, let alone quantitatively modeled, without a substantial warming effect from CO2. We invite readers to view this excellent talk by the American geologist Richard Alley. It's telling that no physics-based climate model has been developed that can simulate past and recent climate changes without a substantial warming effect from an increase in greenhouse gas concentrations. Wishing away the effects of CO2 is not enough in science; it needs to be quantitatively demonstrated.