A New Ice Age

Saturday Night Fever or Saturday Night Shiver? (case study)

One particular attack on the consensus requires its own section. If you have ever seen a debate about global warming on one of the political talk shows, you have probably heard someone proclaim, “Scientists were predicting a new ice age in the ‘70s. Why should we believe them now?”

Two concerns

There are two concerns when it comes to cooling, and both are still valid today.

The first is aerosols. These are small particles suspended in the air. There are both natural and manmade aerosols — “natural” like dust and the plumes from erupting volcanoes, and manmade pollution. Aerosols cause both cooling and warming depending on their type and location. In the case of both natural and manmade aerosols, the primary cooling aerosol is sulfur dioxide, which reflects sunlight back out into space, and it enhances clouds, which also reflect sunlight. (See the Introduction for more on sulfur dioxide from volcanoes.)

The second concern is the natural end to the current interglacial period. We are currently between ice ages, and this warm period will eventually come to an end if the climate is left to its own devices. The mechanism that controls this process are the Milankovitch cycles, named after one of the first scientists to describe them. These cycles involve changes to the earth’s orbit and the wobble and tilt of its axis. (See section 7 for a more complete explanation of Milankovitch cycles.)

“The Cooling World”

So, was there a global cooling scare? There were a few articles and books in the popular press that talked about global cooling. This one page Newsweek article from 1975 is the most famous, and is cited endlessly by the skeptics.1

There are similar articles in Time Magazine, The New York Times, and elsewhere.

Amidst other claims, it quotes a 1975 report from the National Academy of Sciences (NAS), titled Understanding Climatic Change which says, “A major climatic change would force economic and social adjustments on a worldwide scale . . . because the global patterns of food production and population that have evolved are implicitly dependent on the climate of the present century.” All of that is true. It works for a change in the climate for either direction. Notice that it doesn’t specifically mention cooling, and there’s a reason for that.

The problem with this article and others like it is that they don’t represent the state of the science in the ‘70s. By then, we knew that we were capable of both heating up and cooling down the planet. The scientific debate going on at that time was over which influence was going to dominate in the future. We also knew that the current interglacial period was going to end naturally, but the question was when and how rapidly.

More accurately . . .

A more accurate summation of our understanding of climate change was published in the November 1976 edition of National Geographic.

It correctly stated that we weren’t sure what direction the climate was heading due to all of the competing influences, both manmade and natural. It said:2

That earth’s climate changes, and even now may be changing quite rapidly is widely recognized. The questions facing worried experts are: Is the world as a whole cooling off, and perhaps heading into another onset of huge ice sheets? Or are we instead warming the atmosphere of our planet irreversibly with our industry, automobiles, and land-clearing practices? What sort of weather will our children and our grand children know? On the answers may rest the fate of nations and millions of people.

The state of the science

A survey of the published literature from 1965 to 1979 produces these statistics.3

The graph shows the cumulative articles predicting cooling, warming, and those remaining neutral. During that time, the survey counted a total of 7 papers expecting cooling, 19 neutral papers, and 42 for warming. So if warming outnumbered cooling 6 to 1, why were so many in the popular press talking about cooling?

Why “cooling”?

For the media, it was easier to talk about cooling, since the world had been cooling for about 30 years. Or more accurately, the world’s temperatures had been flat for about 30 years.4

Temperatures in the northern hemisphere, shown above in red, had been falling since the early ‘40s. Until more sophisticated temperature analyses came along, most of our temperature data came from the Northern Hemisphere, where most of civilization is. Since then, more effort has gone into the southern hemisphere data, shown in blue, which shows a more consistent and gradual rise in temperature. Taken together, we get the modern land-ocean temperature index shown below.5

Although there were unusually warm years centered around 1940, temperatures during the mid century were essentially flat.

However, in the ‘70s the northern hemisphere data was the most influential so claims of significant cooling were common, which lent credence to the Newsweek article and other similar reports.

The skeptics’ best shot

When skeptics are asked to provide a scientific paper that supports the assertions of these popular articles, they invariably point to a paper written by Rasool and Schneider and published in the Journal Science in 1971, which has among the most explicit predictions.6 Rasool and Schneider look at the opposing effects of increasing CO2 and aerosols and they conclude that “an increase by only a factor of 4 in global aerosol background concentration may be sufficient to reduce the surface temperature by as much as 3.5 °K(elvin),” and Kelvin is equivalent to Celsius in terms of the size of the units. They continue, “If sustained over a period of several years, such a temperature decrease over the whole globe is believed to be sufficient to trigger an ice age.”

Although Rasool was the lead author of this paper, he is almost never mentioned today. All of the attention is placed on Stephen Schneider who was a post-doc when he wrote this paper. Schneider later endorsed a popular book on global cooling, although by the end of the ‘70s he’d changed his position and has since become a vocal proponent for action on Global Warming. Because of this, Schneider more than anyone else, became the poster child for the skeptics’ claims that “the same scientists were predicting a new ice age.”

Returning to the ‘71 paper, they greatly underestimated “Climate Sensitivity,” which is short hand for the change in surface temperature resulting from a doubling of atmospheric CO2 concentration. By 1975, Schneider had recognized that their calculation for climate sensitivity was, at best, half of what it should have been and as little as one quarter the actual figure.7

In 1975, Schneider estimated that climate sensitivity was somewhere between 1.5 and 3 °C. That range is still low compared to the calculations of the 1979 Charney Report which put the value between 1.5 and 4.5 °, with a best estimate of 3 °.

Even with the low figures for CO2, Rasool and Schneider’s 1971 conclusion of cooling still depended on a fourfold increase in the aerosol levels of the atmosphere. Is that assumption valid?

J. Murray Mitchell vs. Rasool & Schneider

During this time period, the pre-eminent climate scientist was J. Murray Mitchell of the National Oceanic and Atmospheric Administration (NOAA). Among many accomplishments, he pioneered the effort to create geographically diverse temperature series. Mitchell’s papers provided much of the basis for the discussion of aerosols in the the 1975 NAS report quoted by Newsweek.

In a paper from 1970 (predating Rasool and Schneider), Mitchell looked at what it would take to cause cooling based on his calculations at that time.8 Recall that Rasool and Schneider were talking about a fourfold increase in particulate emissions, and a greatly underestimated sensitivity to CO2.

Aerosols are often lumped together, and for simplicity’s sake, we will only look at anthropogenic sulfur dioxide emissions. As we’ve discussed, sulfur dioxide is the primary cooling aerosol, both manmade and natural.

Mitchell looked at three scenarios. A doubling of particulates every 10, 15, and 20 years. This chart is an example of what that would look like in the case of sulfur dioxide, based on a recent study of anthropogenic emissions.

The chart stops at only one doubling, but emissions would continue to increase exponentially, effectively creating a vertical wall of sulfur dioxide with the next doubling.

In the most extreme case, a doubling every 10 years implied that significant cooling was imminent and going to occur in the mid ‘80s. A doubling time of 15 years implied cooling around the year 2040, but not before 1 degree of additional warming. On the other extreme, a doubling time of 20 years meant that warming would continue well past 2100, and by the time the effect of particulates caught up to CO2, the climate would have warmed 10 ° which would be a disaster for mankind and most life on earth. Civilization would collapse well before that happened.

So, given our 3 scenarios, which one most accurately described what really happened? None of them.9

Sulfur dioxide emissions were already flattening out and by the late ‘80s had peaked. If you recall, sulfur dioxide is the cause of acid rain. There was a large push to clean up coal power plants and by the ‘90s the emissions of sulfur dioxide had plummeted. Also contributing to slowing the emissions was mandated improvements in energy efficiency after the ‘70s which trickled down to the rest of the world. Another factor was the collapse of the Soviet Union in the ‘90s. This chart ends in 2000, and more recent data is spotty, but even taking into account the increased coal burning in China, global levels are far lower than they were in the ‘80s.

J. Murray Mitchell vs. Rasool & Schneider, part 2

A year after Rasool and Schneider, Mitchell published a paper examining their work. Based on CO2 sensitivity calculated by Manabe (see section 1) he concluded that the maximum cooling impact of particulates matched what was expected from CO2 warming, so the best that aerosols could do was cancel out the warming. 10

He writes:

In balance it is more likely than not that the net impact of human activities on the climate of future centuries will be in the direction of warming. Such being the case, it appears that man’s intervention in the natural climatic trends of the future-whatever their direction-would if anything tend to prolong the present interglacial, rather than to cause its premature breakdown.

In other words, Mitchell believed that if humans are doing anything, they are delaying the onset of the next ice age.

Even with the low figures for CO2, Rasool and Schneider’s 1971 conclusion of cooling still depended on a fourfold increase in the aerosol levels of the atmosphere. Is that assumption valid?

Wither aerosols?

We know now that the dramatic rise in post World War II aerosol emissions did play a part in the mid century “cooling”. For scientists in the ‘60s and ‘70s, it was at least plausible that aerosol emissions would continue rising at an exponential rate, but pollution controls made this impossible. In the mean time, the increase in CO2 continued unchecked.

Cleaning up the air has an almost immediate effect on climate — aerosols only last a few days before they are washed out of the atmosphere. In contrast, most of the emitted CO2 lasts for decades, and some lasts for centuries or longer. Present day warming is therefore caused by the accumulated emissions of CO2, not present day emissions. (See section 7)

Since the ’70s, the net result of aerosol emissions has been to reduce the warming of the earth’s surface that we would otherwise expect if the only issue was increasing greenhouse gas concentrations. There is an obvious catch-22: reducing pollution results in an immediate improvement of air quality but also makes global warming immediately worse. (See section 13 for a more complete description of aerosol “paradox” and section 14 for what we might do about it).

S. Fred Singer and the “hysterical fears” of the NAS

Skeptics have resorted to creative ways of inventing a consensus on cooling in the ‘70s. In a Washington Times commentary, the prominent skeptic S. Fred Singer uses the occasion to take swipes at the National Academy of Sciences.

He says that their “exaggerated concern about global warming contrasts sharply with an earlier NAS/NRC report . . . There, in 1975, the NAS ‘experts’ exhibited the same hysterical fears-this time, however, asserting a ‘finite possibility that a serious worldwide cooling could befall the Earth within the next 100 years.’”11

The 1975 report that Singer refers to is the same one quoted by the 1975 Newsweek article. He takes this from the appendix. Judge for yourself if this is a hysterical proclamation.12

There seems little doubt that the present period of unusual warmth will eventually give way to a time of colder climate, but there is no consensus with regard to either the magnitude or rapidity of the transition. The onset of this climatic decline could be several thousand years in the future, although there is a finite probability that a serious worldwide cooling could befall the earth within the next hundred years.

What is the finite probability? If the end of the current interglacial is episodic, and therefore abrupt, then “as each 100 years passes, we have perhaps a 5% greater chance of encountering its onset,” Or if the changes are sinusoidal, or more cyclical in nature, then “the climate should decline gradually over a period of thousands of years.”

But were they predicting an ice age? “These climatic projections, however, could be replaced with quite different future climatic scenarios due to man’s inadvertent interference with the otherwise natural variation.” They cite rising atmospheric CO2, aerosols, and waste heat from civilization.

They continue:

Such effects may combine to offset a future natural cooling trend or to enhance a natural warming. This situation serves to illustrate the uncertainty introduced into the problem of future climatic changes by the interference of man and is occurring before adequate knowledge of the natural variations themselves has been obtained. Again, the clear need is for greatly increased research on both the nature and causes of climatic variation.

The idea that the National Academy of Sciences was hysterically predicting an imminent ice age is the fantasy of skeptics like Singer, and journalists who didn’t bother to actually read what the report said.

Pacemaker of the ice ages

The uncertainty expressed in the 1975 report regarding the natural end to the current interglacial period has been dramatically reduced. A year later, a seminal paper on the subject was published by Hays et al, titled “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages.”13 They looked at the orbital variations of the earth, and they made a forecast as to when the next ice age would occur. In a column summarizing global cooling talking points, commentator George F. Will says that the Hays et al paper “warned of extensive Northern Hemisphere glaciation.”14

In reality it says, “the results indicate that the long term trend over the next 20,000 years is toward extensive Northern Hemisphere glaciation and cooler climate.” For context, it has been 10,000 years since the last ice age, so we are about a third of the way through the current interglacial. All of civilization has developed in one-fifth the time, so 20,000 years is hardly a “warning.”

The paper also qualifies such forecasts.

First, they apply only to the natural component of future climatic trends-and not to such anthropogenic effects as those due to the burning of fossil fuels. Second, they describe only the long-term trends, because they are linked to orbital variations with periods of 20,000 years and longer.

In other words, don’t mix this prediction with those concerning anthropogenic causes because they are not related, and don’t confuse them with short term climate change. George Will did both.

The verdict

In this section, we’ve talked extensively about what the National Academy of Sciences did and did not say about warming and cooling. Two years after the 1975 report, a more conclusive report was issued, Energy and Climate: Studies in Geophysics. It concluded:15

  • “If the [...] increased particulate emissions are properly controlled, there should be little global effect on climate from an increased atmospheric burden of aerosols.”

  • “The climatic effects of carbon dioxide release may be the primary limiting factor on energy production from fossil fuels over the next few centuries.” That is, the climate, not necessarily availability of fossil fuels will eventually dictate their continued use

  • And it notes the significant uncertainties that remain, “These uncertainties can be resolved only by a well-coordinated effort of extraordinarily interdisciplinary character,” effectively a harbinger of the IPCC.

They made two recommendations

  • “The possibility of modification of the world’s climate by carbon dioxide released in the production of energy from fossil fuels should be given serious prompt consideration by concerned national and international organizations and agencies,” and

  • “a worldwide comprehensive research program should be undertaken.”

In 1977, the National Academy of Sciences was warning that CO2 emissions “be given serious prompt consideration.” Since then, these same conclusions have been repeatedly affirmed and are now mainstream science.

Parting advice from a young(er) S. Fred Singer

In 1970, Singer provides this parting advice in his book Global Effects of Environmental Pollution, which is the source of the first Mitchell paper that we discussed.16


  1. (Gwynne, 1975) Online here

  2. (Mathews, 1976)

  3. (Peterson, Connolley, & Fleck, 2008) Online here. Former British Antarctic Survey Scientist William Connolley has made debunking the “global cooling consensus” something of a hobby. Much of the content of this section owes itself to his work, summarized on his website (Connolley), on his blog (Connolley, Stoat: Taking Science by the Throat) and on realclimate.org (Connolley, RealClimate.org, author_name=william)

  4. (NASA GISS, 2008) Online here

  5. (NASA GISS, 2008) Online here

  6. (Rasool & Schneider, 1971) Abstract here. A recent “controversy” has erupted over the contributions of James Hansen to this paper. At this time, Hansen was studying Venus, which has an atmosphere over 96% CO2 and temperatures above 400 °C. Hansen contributed “mie scattering code” to the authors (both working for NASA at the time) which is computer software used to calculate the scattering of light caused by small atmospheric particles. Because of this, skeptics accuse Hansen of endorsing the conclusions of the paper, which depended on CO2 sensitivity calculations that were separate from the effects of aerosols and far too low to be accurate. The use of that code no more implies that Hansen was predicting an ice age on Venus. The first paper Hansen wrote on the climate of the Earth was published in 1976. The current NASA climate model that Hansen oversees has one of the better treatments of aerosols of all the computers models.

  7. (Schneider, 1975) Online here. (Manabe & Wetherald, 1967) Online here. (IPCC, 2007) Online here.

  8. (Mitchell, 1970) This publication of Mitchell’s is particularly ironic. It was published in a book summarizing a symposium from 1968. S. Fred Singer both organized the symposium and edited the resulting book (Global Effects of Environmental Pollution). A version of this paper from the second edition of Singer’s book was one of the principal sources for the discussion of aerosols in the 1975 NAS report, which Mitchell co-authored. The symposium also featured work from Manabe (see section 1), and Reid Bryson who is a global warming skeptic. Bryson maintains that people laughed at him when, during this symposium, he suggested that mankind was capable of changing the climate (in his case cooling it due to the “human volcano” of manmade particulate emissions). There were three other papers (Mitchell, Manabe, and another discussing pollution’s effect on clouds) from the same symposium suggesting that very thing!

  9. (Stern, 2005) Abstract here

  10. (Mitchell, 1972)

  11. (Singer, 1998) Online here. Singer wrote the editorial to defend the Oregon Petition’s use of the PNAS format (described in section 2).

  12. (National Academy of Sciences, 1975)

  13. (Hays, Imbrie, & Shackleton, 1976) Abstract here

  14. (Will, 2006) Online here

  15. (Geophysics Study Committee, Geophysics Research Board, Assembly of Mathematical and Physical Sciences, National Research Council, 1977) Online here

  16. (Singer, 1970)

Sources cited in A New Ice Age

Connolley, W. (n.d.). RealClimate.org, author_name=william. Retrieved from RealClimate.org: http://www.realclimate.org/index.php?author_name=william

Connolley, W. (n.d.). Stoat: Taking Science by the Throat. Retrieved from ScienceBlogs: http://www.scienceblogs.com/stoat

Connolley, W. (n.d.). Was an imminent Ice Age predicted in the '70's? No. Retrieved June 1, 2008, from William Michael Connolley: http://www.wmconnolley.org.uk/sci/iceage/

Geophysics Study Committee, Geophysics Research Board, Assembly of Mathematical and Physical Sciences, National Research Council. (1977). Energy and Climate: Studies in Geophysics. Washington DC: National Academy of Sciences.

Gwynne, P. (1975, April 28). The Cooling World. Newsweek , p. 64.

Hays, J. D., Imbrie, J., & Shackleton, N. (1976). Variations in the Earth's Orbit: Pacemaker of the Ice Ages. Science , 1121-1132.

Manabe, S., & Wetherald, R. T. (1967). Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity. Journal of the Atmospheric Sciences , 24 (3), 241-259.

Mathews, S. W. (1976, November). What's happening to our climate? National Geographic , pp. 576-615.

Mitchell, J. M. (1970). A preliminary evaluation of atmospheric pollution as a cause of the global temperature fluctuation of the past century. Global Effects of Environmental Pollution (pp. 139-154). New York: Springer-Verlag.

Mitchell, J. M. (1972). The natural breakdown of the present interglacial and its possible intervention by human activities. Quaternary Research , 436-445.

NASA GISS. (2008, January 11). GISS Surface Temperature Analysis: Analysis Graphs and Plots. Retrieved June 1, 2008, from Goddard Institute for Space Studies: http://data.giss.nasa.gov/gistemp/graphs

National Academy of Sciences. (1975). Understanding Climate Change: A Program for Action. Washington, DC: National Academy of Sciences.

Peterson, T. C., Connolley, W. M., & Fleck, J. (in press). The myth of the 1970s global cooling scientific consensus. Bulletin of the American Meteorological Society .

Rasool, S. I., & Schneider, S. H. (1971). Atmospheric Carbon Dioxide and Aerosols: Effects of Large Increases on Global Climate . Science , 138-141.

Schneider, S. H. (1975). On the Carbon Dioxide-Climate Confusion. Journal of the Atmospheric Sciences , 32, 2060-2066.

Singer, S. F. (1970). Epilogue. Global Effects of Environmental Pollution (pp. 205-206). New York: Springer-Verlag.

Singer, S. F. (1998, May 5). Scientists add to heat over global warming. Retrieved June 1, 2008, from Science & Environmental Policy Project (Reprinting Washington Times commentary): http://www.sepp.org/key%20issues/glwarm/sciaddheat.html

Stern, D. I. (2005). Global Sulfur Emissions from 1850 to 2000. Chemosphere , 163-175.

Will, G. F. (2006, April 2). Let Cooler Heads Prevail. Retrieved June 1, 2008, from The Washington Post: http://www.washingtonpost.com/wp-dyn/content/article/2006/03/31/AR2006033101707.html