Response by the Authors of Taxing Air to comments made by Ian G. Enting

Response by the Authors of Taxing Air
to comments made by Ian G. Enting

Our attention has been drawn to Ian Enting’s extended review of our book Taxing Air, which was posted on the Skeptical Science website on August 23, 2013.

We thank Dr Enting for his commentary, from which we have drawn a number of useful minor corrections that will be incorporated in future reprintings of the book.

Before responding to some of Dr Enting’s other remarks in more detail, let us make it plain that we wish to discuss only matters of science.

Regrettably, the analysis of Taxing Air at Skeptical Science contains a long list of inappropriate, perjorative terms and phrases, which include the following: polemic; discredited; half-truths; slanted misrepresentation; systematic failure; poor writing; strawman arguments; re-writing of history; data fabrication; hypocrisy; bias; poor editorial co-ordination; emotional rubbish; denialist; contrarian; deceptive comment; cherry picking; and “similar to the tobacco lobby”.

Dr Enting also dismisses the influential TV documentary program The Great Global Warming Swindle, which is referred to in John Spooner’s Intoduction to Taxing Air, as “widely discredited”. This is wishful thinking, not least because Spooner cited many other sources for the facts that he deployed in his essay (some of which Durkin also used). Martin Durkin’s excellent and thought-provoking documentary in fact underscores most of the uncertainties regarding dangerous warming that IPCC advocates such as Dr Enting seek to ignore. And Swindle also pointed to a legitimate alternative to the DAGW hypothesis for explaining late 20th century warming climate, to whit natural climate variability.

Other matters raised – such as remarks about books by other authors, about the trade distribution of Taxing Air and about who might or might not have provided financial support for the book or its authors - are simply irrelevant.

Such provocative language and tendentious discussion of course has nothing to do with the scientific issue at hand, which is consideration of the evidence for and against possibly dangerous global warming of human causation.

We therefore simply ignore all such material. At the same time, we are moved to note the lack of scientific self-confidence that must exist within those who feel the need to deploy such ad hominem techniques against the many IPCC-independent scientists who provide critical analysis of the weak case for dangerous human-related warming (see, for example, NIPCC 2013).

Many of the most perjorative terms occur in the opening Overview, which therefore does not provide an auspicious start to Dr Enting’s article. Criticism from a person who exhibits such self-evident bias is difficult to take seriously. Nonetheless, amongst the unnecessary verbiage Dr Enting does make a number of useful scientific points, to which we respond below under headings that summarize the criticisms.

Ambiguous usage of the term climate sensitivity

Dr Enting points out that usage of the term climate sensitivity in its wider sense has become ambiguous, and that recent research literature maintains a distinction between what are now termed the transient climate response (TCR) and the equilibrium climate sensitivity (ECS).

These statements are true, but the historical reality is that early deterministic computer models were so primitive that an assumed “equilibrium sensitivity” was the only output that they offered. For example, for its Second Assessment Report in 1995 the IPCC used models that coupled only a primitive ocean to the atmosphere - with the result that the climate warmed independently of CO2 concentration; the climate sensitivity was therefore assumed to be the difference between the temperatures projected by one model run with increasing CO2 and another (default) run in which CO2 was set constant. In effect the models were essentially unstable, a quality that IPCC scientists subsequently sought to rectify. Accordingly, in the Third Assessment Report (2001) the models were made more stable by the use of an artificial ‘flux adjustment’. Such a technique immediately leads to the question as to whether the climate sensitivity determined by IPCC in 2001 was (and remains in later model projections) unduly influenced by the ‘flux adjustment’ methodology?

More sophisticated methods have since been adopted to constrain the most recent computer models. But the fact remains that without controls the models are inherently unstable, because the natural stabilising feedbacks of the climate system, like much of the physics, are represented in the models by parameterisation approximations and assumptions.

Currently, IPCC scientists define the TCR as the amount of warming that results from a doubling of atmospheric CO2 given a rate of increase of 1% per year, i.e. over about 70 years. This warming is significantly less than the long term (>1000 years) ECS, which is the amount of warming expected from a doubling of atmospheric CO2 as the climate system tends towards an assumed equilibrium.

In the interests of simplicity, this complexity of history and definition of climate sensitivity was not addressed in the original text of Taxing Air. However, on reflection we agree with Dr Enting that the distinction is important and we will therefore incorporate appropriate editorial adjustments to future versions of the book.

The role of aerosols has been ignored

Taxing Air deals with more than 100 commonly asked questions about global warming and climate change. Inevitably the selection of topics to be covered required the exercise of judgement. Because of the paucity of high quality data, and the complexity of the physical relationships involved, a question about aerosols was not included in the first edition of Taxing Air, but will be considered for future reprints.

The matter is, however, complex. For instance in its 5th Assessment Report the IPCC recently concluded that “Greenhouse gases contributed a global mean surface warming likely to be in the range of 0.5-1.3 deg. C over the period 1951-2010 with the contribution from other anthropogenic forcing, including the cooling effect of aerosols, likely to be in the range of -0.6 to +0.1 deg. C”.

The IPCC’s hypothesized reduction in theoretical warming by 0.6 deg. C is an arbitrary outcome based upon a subjective choice of estimates of aerosol counter-forcing. But the effects of aerosols should be considered in both a temporal and spatial framework, which current generation deterministic computer models do not provide. Overall, the effects of aerosols are highly complex and uncertain, and IPCC only considers a few out of a total of more than 50 different types of aerosol that are known to exist.

The role of other greenhouse gases has been ignored, especially methane

Water vapour (the most important greenhouse gas) is discussed on pp. 92, 94-97 and 104 of Taxing Air; methane is discussed on pp. 92, 94, 95 and 114-116; and ozone on pp. 92, 94 and 116.

The usage of both Gt C and Gt CO2 in discussions of CO2-forced warming is confusing

We agree entirely with Dr Enting that this ambivalence of terminology is confusing, especially for lay persons. Indeed, we expressly noted this point in a footnote on p. 196 of Taxing Air.

Unfortunately, the dual usage is deeply embedded in the research literature, where Gt C is favoured in most discussions of the global carbon cycle but Gt CO2 is preferred in many discussions of the expected forcing from CO2 or in socio-economic analysis.

Given this state of affairs, we judged it best to maintain the discussion in terms of the relevant literature on any particular topic, whilst at the same time providing a clear explanation of the difference between the two measures and advice on how to convert between them.

Minor errors in the plotted data from the GISP-2 ice-core data (Fig. 5)

Dr Enting points out a typographic error in the x-axis text annotation of Fig. 5, and the lack of any explanatory key for the red-portion of the graph that depicts 20th century instrumentally-measured warming.

Needless to say, neither of these slips were intended as “misrepresentation”, as alleged, and both have already been corrected.

Inappropriate choice of units for expressing rates of warming or cooling

Dr Enting takes exception to the use of the informal units of deg. C/century in description of historical and Holocene temperature change, and prefers the equally informal units of deg. C/year or deg. C/decade.

In the absence of any rational justification for Dr Enting’s preferred choice of units (which in itself reveals a focus on weather variability rather than climatic change), we stand by our adoption of the unit of measurement that is conventional in palaeo-climatic research.

No evidence exists for warming rates up to 2.5 deg./century being sustained for a century

As Dr Enting himself notes, the evidence is contained in the GRIP-2 ice core (Davis and Bohling, 2004), and also in many other palaeoclimatic records. For example, Figure 5 in Taxing Air shows that a period of sustained Greenland warming occurred from 8,230 BP to 7,820 BP; the warming was from -32.4 OC to -28.7 OC, its rate was comparable with that of the late 20th century warming and it was sustained for four centuries.

That neither the Greenland nor other high quality palaeoclimatic records represent a measured global temperature is to a large degree beside the point.

Carbon dioxide should have been acknowledged to be a “pollutant”

Dr Enting cites a decision by the Supreme Court of the United States in support of his belief that CO2 is a pollutant.

Given that court decisions are based upon law and precedent, this does not strike us as a particularly convincing scientific argument. For example, the same reasoning advanced by the Supreme Court in its demonization of CO2 could also be used to claim that oxygen is a pollutant, because anthropogenic emissions have altered its natural balance in the atmosphere over recent centuries.

The inescapable scientific fact remains that CO2 is the natural fertiliser that sustains plant life and is therefore essential for most forms of life on Earth. It has also been widely demonstrated that plants grow better, and use less water, under CO2 enriched atmospheres, which points to beneficial rather than deleterious outcomes from enhanced human-related CO2 contributions.

We repeat our view that to term CO2 a pollutant is an abuse of logic, an abuse of language and an abuse of science.

Carbon dioxide residence time in the atmosphere is a complex matter

We agree with Dr Enting, and indeed describe in Taxing Air (p. 95), that different “residence” times can be relevant to injections of CO2 into the atmosphere, depending upon the context of the discussion. Nonetheless, it is largely IPCC’s failure to follow a terminology consistent with that of other scientists that has caused the confusion that exists - as is well manifest in the breakout box in Section 2.1 of the Second Assessment Report that Dr Enting refers us to.

In strict use, no ambiguity is attached to the term “residence time”, which simply refers to the average length of time that a molecule of CO2 remains in the atmosphere before it is recycled into an alternative reservoir or sink. Given an atmospheric reservoir of about 780 Gt of carbon and an annual flux of about 210 Gt (mostly CO2 the oceans and plants), the theoretical residence time for an individual molecule is just 4 years. This calculation of a short residence time is supported by the empirical result that it took about 7 years for the spike of radiocarbon to decay after atomic testing in the 1960s.

In contrast, the IPCC’s concept of residence time being related to the time of decay after a “spike” injection of CO2 has occurred is theoretical and not supported by empirical evidence. Though the IPCC has often quoted 50-200 years as the “residence time” that they use for such calculations (e.g., in their First Assessment Report), Princeton Professor Harvey Lam has shown that the number actually averages 400 years for all published IPCC models up to the those of the Third Assessment Report (Lam, 200).

Perhaps Dr Enting should review p.21 of the IPCC’s Second Assessment Report (WG1 Section), and in particular the discussion of Global Warming Potential (GWP) there. GWP is defined as “an index defined as the cumulative radiation forcing between the present and some time horizon caused by a unit mass of gas emitted now , expressed relative to that for some reference gas (here CO2 is used)”. The text goes on to say that although the GWP is quoted as single values the typical uncertainty is 35%, not including the uncertainty of the CO2 reference itself. These uncertainties are then magnified again for some molecules for which IPCC scientists have miscalculated the GWP by basing their estimates on atomic mass rather than on a molecule to molecule basis - as they did in assigning an inflated GWP of 20 or more to CH4 when the correct figure is actually 7 (Flood, 2011).

These comments notwithstanding, Dr Enting’s remarks suggest that our explanation of the matter of CO2 residence time is not as clear as it might have been, a matter that we will try to improve in our next text revision.

Fig. 20 fails to include IPCC’s estimated error boundaries for their model projections

The shaded grey field that indicates IPCC’s estimated error boundaries for their projected future temperatures was omitted from Fig. 20 because it has no validity. At the same time, the conventionally calculated (and probably meaningful) error bars associated with the actual temperature measurements are plotted.

Whilst weather forecasting methods make successful use of probabilistic ensemble averaging to provide a numerical range of uncertainties for individual forecasts, IPCC’s climate models are not run in this mode. Ensemble averages are mostly based upon a statistically inadequate and inconsistent number of runs, generally less than five. As discussed by Singer (2013), the chaoticity of modeling can only be overcome by using a large number of runs.

Furthermore, it is obvious also that different climate models in CMIP5 use different parameterizations and adopted forcings in calculating both the present and future climate. No meaningful statistical probability or error field can be derived by averaging such an inhomogeneous set of model outputs, not only in a statistical sampling sense but also from the structural and methodological point of view.

Fig. 27, which summarizes the relationship between corrected global temperature and solar effects after Svensmark and Friis-Christensen, needs to be treated with caution

We agree with Dr Enting’s comments about this matter, and, indeed, included appropriate cautionary remarks in our original description of this graph.

The relationship between solar radiation intensity, magnetic disturbances, cosmic rays and climate is complex, and yet remains to be understood in detail. Nonetheless, strong evidence exists for the importance of a combination of solar factors as controls on terrestrial climate (e.g., Neff et al., 2004; Soon and Legates, 2013).

And regarding Dr Enting’s scepticism that ENSO could play a controlling role on global temperature variation, it should be noted that recent research by de Freitas and McLean (2013) has amplified and reinforced the earlier conclusion of McLean, de Freitas and Carter (2009a, b) that it does. In addition to which , the criticism of McLean et al. (2009) that Dr Enting refers to has been shown to be completely misplaced (McLean, de Freitas and Carter, 2010).

Improvement to Fig. 34, which summarizes the flood history of the Brisbane River since 1840

Dr Enting makes the good point that the later part of the downstream flood record depicted in this figure has been affected (mitigated) by the construction of the Somerset (1935) and Wivenhoe (1985) Dams.

We have added an appropriate annotation to the figure caption.

p. 203 footnote, climate sensitivity equation after the IPCC

Dr Enting points out a proofing error in the IPCC equation that we reproduce here. The equation should indeed read DT = X * log2(C/C0), where ∆T represents the incremental increase in atmospheric carbon dioxide level from C0 to C, assuming an equilibrium climate sensitivity of X.

Note, however, that Dr Enting’s imputation that the value of 3.3O C for climate sensitivity that is used in Taxing Air must therefore be faulty, as must be inferences based upon it, is incorrect. In fact, the numeric estimate that we quote for climate sensitivity comes directly from the IPCC rather than having been calculated by independent application of the (typographically flawed) equation.


In conclusion, we thank Dr Enting again for his thorough review, which has enabled us to correct a number of typographic or other minor errors, and to clarify our writing in a number of places.

Bob Carter, John Spooner, Bill Kininmonth, Stewart Franks and Bryan Leyland

December 18, 2013



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.

de Freitas, C.R. and McLean, J.D. 2013. Update of the chronology of natural signals in the near-surface mean global temperature record and the Southern Oscillation Index. International Journal of Geosciences, 4(1), 234-239. Open access at:

Flood, W. 2011. The methane misconception. Energy & Environment, 22: 233-239.

Lam, H.S.H., 2003. Residence Time of Atmospheric CO2. 4 pp.

McLean, J. D., C. R. de Freitas, and R. M. Carter 2009b. Correction to ''Influence of the Southern Oscillation on tropospheric temperature''. Journal of Geophysical Research, 114, D20101, doi:10.1029/2009JD013006. ISSN 0148-0227.

McLean, J.D., de Freitas, C.R. and Carter, R.M. 2009a. Influence of the Southern Oscillation on tropospheric temperature. Journal of Geophysical Research, 114:D14104, doi:10.1029/2008JD011637, 2009.

McLean, J. D., C. R. de Freitas, and R. M. Carter 2009b. Correction to ''Influence of the Southern Oscillation on tropospheric temperature''. Journal of Geophysical Research, 114, D20101, doi:10.1029/2009JD013006. ISSN 0148-0227.

J. D. McLean, C. R. de Freitas and R. M. Carter. 2010. Censorship at AGU: scientists denied the right of reply. SPPI Original Paper, March 30, 2010.

Neff, U. et al. 2001. Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago. Nature 411, 290.

NIPCC 2013. Climate Change Reconsidered II: Physical Science.

Singer, S. F. 2013. Overcoming chaotic behaviour of general circulation models (GCMS). Energy and Environment 24: 397-403.

Soon, W. and Legates, D.R. 2013. Solar irradiance modulation of equator-to-pole (Arctic) temperature gradients: Empirical evidence for climate variation on multi-decadal timescales. Journal of Atmospheric and Solar-Terrestrial Physics, 93: 45–56.S

Original Article here

Taxing Air by Bob Carter and John Spooner

by Bob Carter & John Spooner
with Bill Kininmonth, Martin Feil,
Stewart Franks, Bryan Leyland

ISBN: 9780646902180
Full colour paperback, 288 pp
Distributed by Dennis Jones
Pub date: 1 July 2013

$30.00 + p&p

Order for Delivery to Australia

Note: For quantities of 3 or more books, please email:


Order for International Delivery

We recommend online bookseller Fishpond for best-value price and postage rates. Click here

Bookshop release, July 1st

All inquiries to Kelpie Press at

Kelpie Press