[update, reference sheet is now linked at the bottom of post]

By Angus McFarlane,

There was an overwhelming scientific consensus in the 1970s that the Earth was headinginto a period of significant cooling. The possibility of anthropogenic warming was relegated to a minority of the papers in the peer-reviewed literature.

Introduction

Whether or not there was a global cooling consensus in the 1970s is important in climate science because, if there were a cooling consensus (which subsequently proved to be wrong) then it would question the legitimacy of consensus in science. In particular, the validity of the 93% consensus on global warming alleged by Cook et al (2103) would be implausible. That is, if consensus climate scientists were wrong in the 1970s then they could be wrong now.

Purpose of Review

It is not the purpose of this review to question the rights or wrongs of the methodology of the 93% consensus. For-and-against arguments are presented in several peer-reviewed papers and non-peer-reviewed weblogs. The purpose of this review is to establish if there were a consensus in the 1970s and, if so, was this consensus cooling or warming?

In their 2008 paper, The Myth of the 1970s Global Cooling Scientific Consensus, Peterson, Connolley and Fleck (hereinafter PCF-08) state that, “There was no scientific consensus in the 1970s that the Earth was headed into an imminent ice age. Indeed, the possibility of anthropogenic warming dominated the peer-reviewed literature even then.” This conclusion intrigued me because, when I was growing up in the early 1970s, it was my perception that global cooling dominated the climate narrative. My interest was further piqued by allegations of “cover-up” and “skulduggery” in 2016 in NoTricksZone and Breitbart.

Therefore, I present a review that examines the accuracy of the PCF-08 claim that 1970s global cooling consensus was a myth. This review concentrates on the results from the data in the peer-reviewed climate science literature published in the 1970s, i.e., using similar sources to those used by PCF-08.

Review of PCF-08 Cooling Myth Paper

The case for the 1970s cooling consensus being a myth relies solely on PCF-08. They state that,”…the following pervasive myth arose: there was a consensus among climate scientists of the 1970s that either global cooling or a full-fledged ice age was imminent…A review of the climate science literature from 1965 to 1979 shows this myth to be false. The myth’s basis lies in a selective misreading of the texts both by some members of the media at the time and by some observers today. In fact, emphasis on greenhouse warming dominated the scientific literature even then.” [Emphasis added].

PCF-08 reached their conclusion by conducting a literature review of the electronic archives of the American Meteorological Society, Nature and the scholarly journal archive Journal Storage (JSTOR). The search period was from 1965 to 1979 and the search terms used were “global warming”, “global cooling” and a variety of “other less directly relevant” search terms. Additionally, PCF-08 evaluated references mentioned in the searched papers and references mentioned in various history-of-science documents.

In total, PCF-08 reviewed 71 papers and their survey found 7 cooling papers, 20 neutral papers and 44 warming papers. Their results are shown in their Figure 1.

A cursory examination of Figure 1 indicates that there is a 62% warming consensus if we use all the data and this consensus increases to 86% pro-warming, if we were to ignore the neutral papers (as was done in the 93% consensus). Therefore, the Figure 1 data seems to prove the contention in PCF-08 that 1970s global cooling was a myth.

However, I find it difficult to believe that the 1970s media “selectively misread” the scientific consensus of the day and promoted a non-existent cooling scare. Therefore, I present an alternative to the PCF-08 analysis below.

Methodology of this Review

In this review, I use an identical methodology to PCF-08, i.e., I examine peer-reviewed scientific journals. Non-peer-reviewed newspaper and magazine articles are not used. A significantly larger number of papers are presented in the current review than were used in PCF-08.

The PCF-08 database of articles is used but this is extended to examine more literature. Note that examining all of the scientific literature would have been beyond my resources. However, my literature survey was facilitated by the work of Kenneth Richard in 2016 (hereinafter, KR-16) at NoTricksZone, in which he has assembled a large database of sceptical peer-reviewed literature.

Some people may wish to ignore the KR-16 database as being from a so-called “climate denier” blog. However, almost all of the papers in KR-16 are from peer-reviewed literature and consequently it is a valid database. It is also worth noting that 16 of the papers used in the KR-16 database are also contained in the PCF-08 database.

The combined PCF-08 and KR-16 databases form the benchmark database for the current review. It was intended to significantly extend the benchmark database but, on searching the relevant journals, only 2 additional papers were found and these were added to form the database for this review.

It should be noted that KR-16 states that there were over 285 cooling papers. However, many of these papers were deleted from the current review as not being relevant. For example, several papers were either outside the 1965-1979 reference period or they emphasise the minor role of CO2 but do not consider climate trends.

I agree with PCF-08 that no literature search can be 100% complete. I also agree that a literature search offers a reasonable test of the hypothesis that there was a scientific consensus in the 1970s. I reiterate that the resulting database used in this review is significantly larger than that used by PCF-08 and consequently it should offer a more accurate test of the scientific consensus in the 1970s.

Most of the papers in the review database acknowledge the global cooling from the 1940s to the 1970s (typically 0.3 °C global cooling). Therefore, deciding between cooling, neutral or warming was relatively straightforward in most cases; namely did the paper expect the climate regime during the 1940s-1960s period to either to continue from the date that the paper was published, or did it expect a different climate regime in the medium-to-long-term?

Notwithstanding the straightforward test described above, some of the papers make contradictory statements and are thus more difficult to classify. Consequently, their classification can include an element of subjectivity. Fortunately, there are very few papers in this category and consequently an inappropriate classification does not materially affect the overall results.

The test criteria are summarised in Table 1.

Classification Test of Classification of Papers Typical Examples from Papers Cooling Cooling expected to either continue or initiate Kukla & Kukla (1972) “…the prognosis is for a long-lasting global cooling more severe than any experienced hitherto by civilized mankind.” Neutral Either non-committal on future climate change or expects warming or cooling to be equally possible Sellers (1969) “The major conclusions that removing the arctic ice cap would have less effect on climate than previously suggested, that a decrease of the solar constant by 2-5% would be sufficient, to initiate another ice age, and that man’s increasing industrial activities may eventually lead to the elimination of the ice caps and to a climate about 14C warmer than today…” Warming Warming expected to either continue or initiate Manabe & Weatherald (1967) “According to our estimate, a doubling of the CO, content in the atmosphere has the effect of raising the temperature of the atmosphere (whose relative humidity is fixed) by about 2C.”

Table 1: Summary of Classification System for Papers

The search terms “global cooling” and “global warming” used by PCF-08 are used in this review but they have been expanded to include “cool”, “warm”, “aerosol” and “ice-age” because these, more general terms, return a larger number of relevant papers. Additional search terms such as “deterioration”, “detrimental” and “severe” have also been included. These would fit into the PCF-08 category of “other less directly relevant” search terms.

Several of the papers in the database are concerned about the effects of aerosol cooling and they state that this effect dominates the effect of the newly emerging CO2-warming science. Indeed, a few papers warn of CO2 cooling.

However, PCF-08 do not include any papers that refer to aerosol cooling by a future fleet of supersonic aircraft (SST’s) but several papers in the 1970s assumed an SST fleet of 500 aircraft. This seems incongruous now but, to show that this number of aircraft is not unrealistic; Emirates Airlines currently have a fleet of 244 (non-supersonic) aircraft and 262 more on order. Therefore, I have included papers that refer to the effects of aerosols from supersonic aircraft and other human activities. Of course, supersonic travel was killed-off by the mid-1970s oil crisis.

Furthermore, a number of PCF-08 and KR-16 papers were re-classified (from cooling, neutral or warming) as summarised Table 2.

Reference Original Amended Sellers (1969) Warming Neutral Benton (1970) Warming Neutral Rasool and Schneider (1972) Neutral Cooling Machta (1972) Warming Neutral FCSTICAS (1974) Warming Cooling National Academy of Sciences (1975) Neutral Cooling Thompson, 1975 Warming Neutral Shaw (1976) Neutral Cooling Bryson and Dittberner (1977) Neutral Cooling Barrett, 1978 Neutral Cooling Ohring and Adler (1978) Warming Neutral Stuiver (1978) Warming Neutral Sagan et al. (1979) Neutral Cooling Choudhury and Kukla, 1979 Neutral Cooling a. Amended Classifications to PCF-08 Reference Original Amended Budyko, 1969 Cooling Warming Benton (1970) Cooling Neutral Mitchell, 1970 Cooling Neutral Mitchell (1971) Cooling Warming Richmond, 1972 Cooling Neutral Denton and Karlén, 1973 Cooling Warming Schneider and Dickinson, 1974 Cooling Neutral Moran, 1974 Cooling Neutral Ellsaesser, 1975 Cooling Neutral Thompson, 1975 Cooling Neutral Gates, 1976 Cooling Neutral Zirin et al., 1976 Cooling Neutral Bach, 1976 Cooling Warming Norwine, 1977 Cooling Warming Paterson, 1977 Cooling Neutral Schneider, 1978 Cooling Warming b. Amended Classifications to KR-16

Table 2: Amendments to Classification of Papers in Database

Two examples of the amendments to the classification of the papers in the database are explained below:

1. The Benton (1970) paper is classified as “Cooling” in KR-16 but the paper states that, “In the period from 1880 to 1940, the mean temperature of the earth increased about 0.60C; from 1940 to 1970, it decreased by 0.3-0.4°C…The present rate of increase of 0.7 ppm per year [of CO 2 ] would therefore (if extrapolated to 2000 A.D.) result in a warming of about 0.60C – a very substantial change…The drop in the earth’s temperature since 1940 has been paralleled by a substantial increase in natural volcanism. The effect of such volcanic activity is probably greater than the effect of manmade pollutants… it is essential that scientists understand thoroughly the dynamics of climate.” [Emphasis added]. Consequently, this paper is re-classified as neutral in this review. Not the “Cooling” classification in KR-16 and not the “Warming” the classification in PCF-08).

2. The Sagan et al. (1979) paper is classified as “Neutral” in PCF-08 but the paper states that, “Observations show that since 1940 the global mean temperature has declined by -0.2 K…Extrapolation of present rates of change of land use suggests a further decline of -1 K in the global temperature by the end of the next century, at least partially compensating for the increase in global temperature through the carbon dioxide greenhouse effect, anticipated from the continued burning of fossil fuels.” [Emphasis added]. Therefore, this paper is re-classified as cooling in this review (conforming to the KR-16 classification).

Results from Review & Discussion

The review database contains a total 190 relevant papers, which is 2.7 times the size of the PCF-08 database. Of the 190 papers in the review database, 162 full papers/books and 25 abstracts were reviewed (abstracts were used when the full papers were either pay-walled or could not be sourced). Furthermore, 4 warming papers from PCF-08 were not reviewed because they could not be sourced. Therefore, the PCF-08 classification was used for these papers in this review.

The results from the review are summarised in Figure 2.

It is evident from Figure 2 that, for the 1965-1979 reference period used by PCF-08, the number of cooling papers significantly outnumbers the number of warming papers. It is also apparent that there are two distinct sub-periods contained within the reference period, namely:

1. The 1968-1976 period when cooling papers greatly outnumber the warming papers (85% to 15%), if we ignore the neutral papers (as was done in the Cook et al (2103). The 85% to 15% majority is an overwhelming cooling consensus. Additionally, this is probably the period when the 1970s “global cooling consensus” originated because cooling was clearly an established scientific consensus – not the myth that PCF-08 contend.

2. The 1977-1979 period when warming papers slightly outnumber the cooling papers (52% to 48%) – a warming majority but not a consensus.

The following observations are also worth noting from Figure 2 for the 1965-1979 reference period:

1. Of the 190 papers in the database, the respective number of papers are 86 cooling, 58 neutral and 46 warming. In percentage terms, this equates to 45% cooling papers, 31% neutral papers and 24% warming papers, if we use all of the data.

2. The cooling consensus increases to 65% compared with 35% warming – a considerable cooling consensus, if we ignore the neutral papers (as was done in the Cook et al (2103).

3. The total number of cooling papers is always greater than or equal to the number of warming papers throughout the entire reference period.

Although not presented in Figure 2, it is worth noting that 30 papers refer to the possibility of a New Ice-Age or the return to the “Little Ace-Age” (although they sometimes they used the term “Climate Catastrophic Cooling”). Timescales for the New Ice Age vary from a few decades, through a century or two, to several millennia. The 30 “New Ice Age” papers are not insignificant when compared with the 46 warming papers.

Conclusions

A review of the climate science literature of the 1965-1979 period is presented and it is shown that there was an overwhelming scientific consensus for climate cooling (typically, 65% for the whole period) but greatly outnumbering the warming papers by more than 5-to-1 during the 1968-1976 period, when there were 85% cooling papers compared with 15% warming.

It is evident that the conclusion of the PCF-08 paper, The Myth of the 1970s Global Cooling Scientific Consensus, is incorrect. The current review shows the opposite conclusion to be more accurate. Namely, the 1970s global cooling consensus was not a myth – the overwhelming scientific consensus was for climate cooling.

It appears that the PCF-08 authors have committed the transgression of which they accuse others; namely, “selectively misreading the texts” of the climate science literature from 1965 to 1979. The PCF-08 authors appear to have done this by neglecting the large number of peer-reviewed papers that were pro-cooling.

I find it very surprising that PCF-08 only uncovered 7 cooling papers and did not uncover the 86 cooling papers in major scientific journals, such as, Journal of American Meteorological Society, Nature, Science, Quaternary Research and similar scientific papers that they reviewed. For example, PCF-08 only found 1 paper in Quaternary Research, namely the warming paper by Mitchell (1976), however, this review found 19 additional papers in that journal, comprising 15 cooling, 3 neutral and 1 warming.

I can only suggest that the authors of PCF-08 concentrated on finding warming papers instead of conducting the impartial “rigorous literature review” that they profess.

If the current climate science debate were more neutral, the PCF-08 paper would either be withdrawn or subjected to a detailed corrigendum to correct its obvious inaccuracies.

Afterword

I reiterate that no literature survey can be 100% complete. Therefore, if you uncover additional references then please send them to me in the comments. It would make this review much better if we could significantly increase the number of relevant references.

Additionally, if you disagree with the classification of some of the references then please let me know why you disagree and I will consider appropriate amendments. Your comments on classification would certainly increase the veracity of the review by providing an independent assessment of my classifications.

References

The references used in this review and their classification are included in the spreadsheet here:

References-Global Cooling Consensus.xlsx

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