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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Why did climate cool in the mid-20th Century?

What the science says...

Select a level... Basic Intermediate Advanced
Mid-century cooling involved aerosols and is irrelevant for recent global warming.

Climate Myth...

It cooled mid-century
"It was the post war industrialization that caused the rapid rise in global CO2 emissions, but by 1945 when this began, the Earth was already in a cooling phase that continued until 1975. With 32 years of rapidly increasing global temperatures and only a minor increase in global CO2 emissions, followed by 33 years of slowly cooling global temperatures with rapid increases in global CO2 emissions, it was deceitful for the IPCC to make any claim that CO2 emissions were primarily responsible for observed 20th century global warming." (Norm Kalmanovitch)

Although temperatures increased overall during the 20th century, three distinct periods can be observed. Global warming occurred both at the beginning and at the end of the 20th century, but a cooling trend is seen from about 1940 to 1975. As a result, changes in 20th century trends offer a good framework through which to understand climate change and the role of numerous factors in determining the climate at any one time.

Early and late 20th century warming has been explained primarily by increasing solar activity and increasing CO2 concentrations, respectively, with other factors contributing in both periods. So what caused the cooling period that interrupted the overall trend in the middle of the century? The answer seems to lie in solar dimming, a cooling phenomenon caused by airborne pollutants.

The main culprit is likely to have been an increase in sulphate aerosols, which reflect incoming solar energy back into space and lead to cooling. This increase was the result of two sets of events.

  1. Industrial activities picked up following the Second World War. This, in the absence of pollution control measures, led to a rise in aerosols in the lower atmosphere (the troposphere).
  2. A number of volcanic eruptions released large amounts of aerosols in the upper atmosphere (the stratosphere).

Combined, these events led to aerosols overwhelming the warming trend at a time when solar activity showed little variation, leading to the observed cooling. Furthermore, it is possible to draw similar conclusions by looking at the daily temperature cycle. Because sunlight affects the maximum day-time temperature, aerosols should have a noticeable cooling impact on it. Minimum night-time temperatures, on the other hand, are more affected by greenhouse gases and therefore should not be affected by aerosols. Were these differences observed? The answer is yes: maximum day-time temperatures fell during this period but minimum night-time temperatures carried on rising.

The introduction of pollution control measures reduced the emission of sulphate aerosols. Gradually the cumulative effect of increasing greenhouse gases started to dominate in the 1970s and warming resumed.

As a final point, it should be noted that in 1945, the way in which sea temperatures were measured changed, leading to a substantial drop in apparent temperatures. Once the data are corrected, it is expected that the cooling trend in the middle of the century will be less pronounced.

Last updated on 21 August 2010 by Anne-Marie Blackburn.

Printable Version  |  Offline PDF Version  |  Link to this page

Further reading

Open Mind has an interesting post Hemispheres that compares temperature records for both hemispheres over the past century and the significance for mid-century cooling.

Comments

Comments 1 to 18:

  1. John, recently on Irregular Climate you mentioned that in the mid-20th century, although the daily maximum temperature decreased, the daily minimum temperature increased. Can you add links to that evidence here? Thank you.
    Response: Always making work for me, aren't you James? Here's the original blog post about daily minimum temperatures increasing during mid-century cooling. I've integrated the blog post into the above info. Thanks for the suggestion (your ideas while making work for me are always good ideas).
  2. How does this relate to this recent article?

    http://www.physorg.com/news204381778.html

    The story suggests that sea temperatures actually did decrease sharply around this period of time, if I'm reading it right. I haven't read the details in Nature, so I haven't seen the details. It seems to contrast with some of what you have here.
  3. GSwift7,
    that paper may add something to the mechanism illustrated here, they're not mutually exclusive.
  4. from this site, above:

    "As a final point, it should be noted that in 1945, the way in which sea temperatures were measured changed, leading to a substantial drop in apparent temperatures. Once the data are corrected, it is expected that the cooling trend in the middle of the century will be less pronounced."

    From the other, newer story:

    "The international team of scientists discovered an unexpectedly abrupt cooling event that occurred between roughly 1968 and 1972 in Northern Hemisphere ocean temperatures. The research indicates that the cooling played a key role in the different rates of warming seen in the Northern and Southern Hemispheres in the middle 20th century"

    That seems prety much mutually exclusive to me.
  5. These both seem to be pro-global warming stories, so it doesn't really matter which one is right as far as I'm concerned. The matter of increased night-time temperatures also seems to be inconsistent with an actual drop in sea temperature. I actually wonder if the guys in the new story are serious. The theory here on this site seems to make more sense and seems to be more supported by known facts.
  6. GSwift7,
    if you're looking for pre-cooked anti-AGW informations, you're in the wrong place. If you're not interested in the science, again you're in the wrong place. And given that you definitely are in the wrong place, I do not understand why you ask questions.
  7. The temperature variations according to NOAA/NCDC dataset from 1880-2010 vs rising CO2 levels indicate no significant impact from CO2. However, the day/night variations explained above appear to be legitimate science and I applaud them for their work. The real worry may not be so much CO2 but a possible sudden release of Methane Hydrate from the sea bottoms and tundra bogs of Siberia.
  8. #7: "show no significant impact from CO2"

    That's absolutely incorrect, as the graph below (from NOAA/NCDC clearly shows). Where do you come up with such silliness?



    And of course, you're off topic. Find the appropriate threads using the search function.
  9. muoncounter : I would call this figure an unconvincing correlation, since the slope in the 1900-1940 period is quite comparable to the modern one, although CO2 was very different.
    Response: [Dikran Marsupial] yes, indeed, but CO2 is not the only forcing, not all warming is caaused by CO2 and nobody claims it is.
  10. Correlation isn't what it is about. The question is whether modelled temperatures using estimated forcing match the actual record. See IPCC WG1 for the answer.
  11. I agree , that's the point. And if I look at the comparison between models and data, I'm not convinced again that the agreement during the first half is so good



    Of course at first glance it seems that the match is almost perfect. But if you look carefully at the first half of the century, and if you look also carefully at the methodology used to produce these curves, you will notice that
    a) curves are generated by a variety of different models, which is really strange, since it means that different modelling can produce the same kind of visual output - this really means that observations are only LOOSELY constraining models - which is the opposite of a scientific validation.
    b) the models contain parameters , especially for clouds, so there is an obvious selection bias due to the fact that bad models are simply not selected here. In other words, adding a superposition of approximate models with a selection of parameters giving results close to the data, will ALAWAYS produce an interval, a corridor , containing these data - it's almost certain and doesn't prove much. Note how curiously the black observed curve travels throughout the corridor and never crosses the border : is it not surprising that a unbiased set of models just reproduces exactly the range of natural variability, without any "lost space" in the yellow interval or without the black curve goint out of it? this cannot for sure be obtained without a selection of the sample.
    c) models produce temperature that are not precisely matching the reality in absolute. What is displayed here is ANOMALIES. Anomalies with respect to which baseline ? you have to read carefully the report to find the answer : with respect to the 1900-1950 period. So the agreement at least on the central point of the first half is automatically insured - no surprise here.
    So the real test of the preanthropic period is not the average value, but the details of the shape around this value. Is it well reproduced ? not so much. The break around 1940 is NOT reproduced in models - it just the width of the interval that blurs out the comparison. The only break in the models are in major volcanic eruptions - first Agung in 1963. Note also that volcanic eruptions are NOT so conspicuous in data. Actually if you look only at data, you couldn't say when these eruptions occured, contrary to the models. So it seems that models "play" with eruptions to try to reproduce breaks that are not really at the right place - playing with a whole interval of parameters to blur out the disagreement.

    That is not, by far, what I would call an accurate fit of data.
  12. Giles@11:

    (a) No, the use of multiple models is not "really strage", it is not even unusual. There is uncertainty in the details of the physic is involved and in the parameters, the use of multiple models captures some of that uncertainty. Secondly if different models give similar results, that indicates that the uncertainty in the physics is small and the climate projections are not greatly sensitive to them. That is a good thing from the modelling point of view, not a bad one. If anything it actually means the data do constrain the models relatively tightly as it constrains them all to say the same basic thing. As to scientific validation, you obviously don't understand there is no such thing as scientific validation, only scientific invalidation. You can't prove a theory right, only disprove it.

    (b) Complaining that bad models are not selected is pretty daft, if the model is inconsistent with reality it means the assumptions underpinning that model are incorrect, so why should we look at it. The CMIP ensemble were not selected in that way, it is an ensemble of models from leading modelling groups, so your objection is incorrect anyway. Complaining that the black line doesn't go out of the corridor is basically saying "the models must be wrong because they give the right answer"!

    (c) The models should not expect to produce temperatures that precisely match the observations, that comment shows a complete lack of understanding of Monte Carlo simulation methods. We can't predict the chaotic weather, so the model runs will always be different. The model mean won't match the observations either as it is an estimate of only the forced component of climate, not the unforced response - the observed climate has both components so there is no reason to expect that close a match.

    "That is not, by far, what I would call an accurate fit of data" well possibly that is because you don't understand the effects of the major sources of uncertainty. Given internal climate variability (which models cannot be expected to be able to model), the hindcast is pretty impressive.

    However, I suspect this should be discussed further on another thread.
  13. 12: Dikran Marsupial
    However, I suspect this should be discussed further on another thread.

    and, indeed, was touched on previously in the Climate Sensitivity: The Skeptic Endgame where Giles didn't understand it either.

    Someone might do an intro to modeling, simulation, log-likelihoods and MC methods...
    Response: (Daniel Bailey) Are you offering...?
  14. And try comparing vs something else than GISS which is showing less cooling in mid 1900's and also less warming pre 1940. Like Hadcrut which has not been so heavily adjusted by the modelers.

    For example, look at the Model E outputs. Aerosols clearly do not explain the mid century cooling. GISS vs ModelE vs Hadcrut:
    http://i.picasion.com/pic38/0f6666b2060569d680da06c477670f9b.gif
  15. Between 1944 and 1980 there were more than 1800 nuclear explosions were conducted - many above ground - including the biggest H-bombs. This time period was a mini-nuclear winter.
  16. Hmm, I'm skeptical. Any evidence that aerosol loading from tests and large enough and continuous enough to have a significant effect on aerosol loading compared to industrial emissions?
  17. "Any evidence that aerosol loading from tests and large enough and continuous enough to have a significant effect..."

    Not much. It takes a yield of at least 50 ktons to make a cloud tall enough to reach the stratosphere. Large yield testing didn't begin until 1952; mid-century cooling started several years prior to that (there's a lag problem).

    And the USGS shows that it is not dust as much as sulfate aerosol that causes detectable multi-year cooling.

    About the only significant climate-scale result from nuclear testing is the C14 spike. And that doesn't make a blip in the older cosmic ray records.
  18. I have heard it said that the Pacific decadal oscillation superimposes a sine wave of variation onto the underlying warming trend line, http://en.wikipedia.org/wiki/File:PDO.svg and that this correlates to the multidecadal variation of the rate of rise.  Is there research that supports this?

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