Reply to Emil Karlsson, “NIPCC and Climate Change Denialism,”, October 31, 2013, 

We thank Emil Karlsson for his commentary on the recent Nongovernmental International Panel on Climate Change (NIPCC) report, Climate Change Reconsidered II: Physical Science (henceforth, NIPCC-II).

Let us start with title of Karlsson’s critique, which refers to climate change “denialism”. Search as we may, we have been unable to find any statements made by NIPCC authors in either the full report or its Summary for Policymakers (SPM) that comprise an assertion, or contain the implication, that climate change does not occur. Precisely to the converse, the leitmotif of NIPCC’s evidence-based assessment is that climate is always changing for natural reasons. That human activities also have the potential to effect global climate measurably is certainly possible, but no such human effect has yet been isolated and measured.

Karlsson’s chosen title, then, is not an auspicious start, and the tone and accuracy of his article is downhill from that point. For instance, the very first paragraph does not discuss any scientific issue but instead comprises a weary and oft-rebutted ad hominem attack on the NIPCC’s scientific authors (“… purport to be independent ….”) and publisher (“… an anti-science organization …” that spends “… a lot of effort trying to spread pseudoscientific uncertainty …”).

Criticism from a person who starts off with such self-evident naked bias is difficult to take seriously, especially given the number of ‘straw-men’ arguments that Karlsson deploys. Nonetheless, for the record we provide below brief replies to some of the substantive scientific issues that are listed in the NIPCC-II SPM, to all of which Karlsson has taken exception.  We use Karlsson’s own headings as the framework for our replies.

Carbon dioxide is not saturated

We agree with Karlsson’s statement that carbon dioxide is not yet fully saturated in the atmosphere, and nowhere does NIPCC make a statement that it is. But anyway, that is not the point.

Instead, the point is that the relationship between increasing carbon dioxide and increasing temperature is quasi-logarithmic. Feedback effects aside, at 390 ppm the lower atmosphere is already saturated enough that additional increases in carbon dioxide are only capable of exerting very minor warming effect. Note that this argument applies to all levels of the atmosphere, including the upper levels where carbon dioxide is relatively less abundant, and that the line of reasoning also becomes more potent (in the sense that less additional warming is forced) the more carbon dioxide that either humans or nature emit.

Note too, and importantly, that our disagreement with Karlsson does not relate to disputed physics, but to the fact that empirical evidence is lacking for the proposition that dangerous (as opposed to minor) additional warming is resulting, or will result, from human emissions.

The key pieces of empirical data are that average global temperature has now failed to increase for 17 years despite an accompanying increase of about 8% in carbon dioxide which represents 34% of all the human emissions since the start of the industrial revolution (NIPCC SPM, Figure 6).


Feedback effects are crucial for understanding climate change and cannot be dismissed

Quite so, we fully agree.

But the water vapour positive feedback effect that IPCC modelling relies upon so heavily only concerns itself with one of many feedbacks, both positive and negative.

Empirical data show clearly that the IPCC’s deterministic models overestimate the amount of warming associated with increases in water vapour (see paper summaries in NIPCC-II, Chapter 1). In addition, cloud physics is notoriously complex and not fully understood, therefore being represented in the models by parameterization. This parameterization fails to account adequately for the cooling effects of an increase in low cloud cover that generally accompanies the increasing levels  of water vapour that IPCC asserts to cause (as yet unmeasured, and certainly not currently occurring) warming.

The profound uncertainty that accompanies estimates of the net outcome of all carbon dioxide-related feedbacks is encapsulated in Karlsson’s closing statement that “researchers consider positive feedback process to be stronger than negative feedbacks”. Which researchers? Why, the IPCC’s of course.

It is the weakest of arguments in science to assert that the opinion of my experts counts for more than the opinion of yours. For what counts is never opinion, however expert, but rather empirical evidence.

Global warming has not taken a break

The truth of this statement depends upon context.

Considering the long-term climate trend through the Holocene (last 10,000 years) (Davis & Bohling, 2001), it is indeed true that global warming “has not taken a break”, because over that time period the trend has been one of cooling, not warming.

Considering instead the short-term trend in average global temperature since the late 1990s, the statement becomes untrue. All major indices of average atmospheric temperature show a lack of warming since 1997 (NIPCC SPM, Figure 6), and several other datasets suggest that there may have been no warming since the 1950s (NIPCC SPM, Figure 4).

To counteract the undeniable fact that no significant warming has occurred since about 1997, Karlsson produces the breathtakingly self-interested assertion that “… statistical significance relates to how probable the observe[d] data, or more extreme data, are on the null hypothesis, not the practical significance of the observed trend. This alleged “lack of statistical significance” [arises] only because of not controlling for the short-term noise.

The concern is that the rate of [human-caused] climate change is faster than during natural variability

It is certainly a legitimate concern to raise that late 20th century warming (which has now ceased) might have occurred at a rate faster than normal natural variability.

However, it is wrong to assert, as Karlsson does, that late 20th century warming did indeed occur at such exceptional rates. Instead, the empirical evidence is completely clear that both short term and long term climate variability has in the recent geological past occurred at rates, and to magnitudes of warming and cooling, that far exceed even all the warming of perhaps 1 deg. C that may have occurred since the end of the Little Ice Age around 1860.

Karlsson also refers tonatural variability during the Cambrian”, but fails to inform his readers that at that time atmospheric carbon dioxide levels exceeded the pre-industrial level of 280 ppm by 15-times (yes, fifteen times) without any known parallel dangerous global warming.

[Human-caused] Climate change will have harmful consequences for human health, the economy and environment

This is a totally unsubstantiated assertion for which there is no convincing evidence. The Human Health chapter of Climate Change Reconsidered II: Impacts, Adaptation, and Vulnerabilities, expected to be released next March, will provide a thorough summary of the research and discussion of relevant issues.

Natural forcings alone cannot account for the observed warming

This argument, which refers to an infamous graph in the IPCC’s 4AR (2007), has been debunked so many times that it is astonishing that it is still being repeated.

The graph in question purports to show that climate models that include only selected natural forcings fail to accurately simulate the measured temperature curve for the 20th century, whereas inclusion of a human forcing effect in the models results in an output temperature curve that matches the 20th century curve. After describing this, Karlsson asserts thatThus, the claim that it (observed modern climate change) is just “all natural” (i.e. no substantial human influence) is falsified by the data.”

First, we cannot fully quantify many climate processes, which as already explained are thus represented in the models by parameterization rather than fundamental physics. Second, we do not know all of the processes that affect climate, but only a subset of processes that we currently think are important. Third, Karlsson’s claimed test is based upon unvalidated computer models and not on data. Fourth, the null hypothesis (that observed modern climate variation is due to natural causes) is NOT tested by those computer models. And fifth, the IPCC exercise that Karlsson describes is simply one of sophisticated curve fitting of no fundamental scientific merit.

The carbon dioxide fertilization effect is more complex and variable than NIPCC thinks

The discussion is not about what the NIPCC “thinks”, but what the published empirical data indicate.

Of courseplants with different forms of metabolism (C3, C4 and CAM) will react differently” to enhanced levels of atmospheric carbon dioxide, but as thousands of peer-reviewed studies have shown, the response is nonetheless near universal and overwhelmingly positive in all three major plant types.

Karlsson appears not to comprehend this well-substantiated fact. And as to his claim that there may be “places around the world where global warming will lead to less crop success and yield, even when taking into account the carbon dioxide fertilization effect,” he appears to be equally ignorant that rising levels of atmospheric CO2 tend to raise the temperature of optimum plant photosynthesis beyond the predicted temperature values associated with global warming, effectively nullifying this worn out claim (Idso & Idso, 2011).

These facts help explain why, in spite of the Earth’s air temperature increasing to a level that the IPCC claims is unprecedented in the the past millennium or more, a recent study by Randall et al. (2013) found that the 14% extra carbon dioxide fertilization caused by human emissions between 1982 and 2010 caused an average worldwide increase in vegetation foliage by 11% after adjusting the data for precipitation effects.

Despite the unrelenting propaganda mounted against it by environmental activists, human carbon dioxide emissions are indubitably a net environmental benefit.

Bidirectional causation between higher temperatures and more atmospheric carbon dioxide

Karlsson claims that human emissions of carbon dioxide and other anthropogenic greenhouse gases is [sic] a substantial influence on the current warming trend.”

First, there is no significant current warming trend. Over both the short term (last 17 years) and long term (last 10,000 years) global average temperature has decreased or at best failed to warm.

Second, by “bidirectional causation” Karlsson seems to be referring to the idea that “… after the last ice age initial warming cause[d] additional carbon dioxide to enter the atmosphere from the ocean, which led to even more warming.” In other words, Karlsson is asserting that a positive feedback loop applies.

But no evidence exists for the proposition that such a feedback loop will cause dangerous or runaway warming. Rather, the ice core record shows clearly that changes in temperature precede changes in carbon dioxide throughout the glacial-interglacial cycle (Mudelsee, 2001), and that for the last half million years the climate system has oscillated in a self-limiting way between glacials and interglacials by about 6 deg. C.

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By Craig Idso, Fred Singer and Bob Carter

NIPCC advisory scientists and lead authors, Climate Change Reconsidered II: Physical Science.



Davis, J.C. and Bohling, G.C. 2001. The search for pattern in ice-core temperature curves. American Association of Petroleum Geologists, Studies in Geology 47: 213–229.

Idso, C.D. and Idso, S.B. 2011. The Many Benefits of Atmospheric CO2 Enrichment. Vales Lake Publishing, 366 pp.

Lindzen, R.S and Choi, Y.-S. 2011. On the observational determination of climate sensitivity and its implications. AsiaPacific Journal of Atmospheric Science 47: 377–390. doi:10.1007/s13143-011-0023-x

Mudelsee, M. 2001. The phase relations among atmospheric CO2 content, temperature and global ice volume over the past 420 ka. Quaternary Science Reviews 20: 583–589.

NIPCC Summary for Policymakers 2013. Climate Change Reconsidered II: Physical Science 22 pp.

Randall J. Donohue, Michael L. Roderick, Tim R. McVicar, Graham D. Farquhar. CO2 fertilisation has increased maximum foliage cover across the globe’s warm, arid environments. Geophysical Research Letters, 2013; DOI: 10.1002/grl.50563