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The main culprit in mid-century cooling

Posted on 21 August 2010 by Anne-Marie Blackburn

This post is the Basic version (written by Anne-Marie Blackburn) of the skeptic argument "It cooled mid-century".

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.

Note: we're currently going through the process of writing plain English versions of all the rebuttals to skeptic arguments. It's a big task but many hands make light work. If you're interested in helping with this effort, please contact me.

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Comments 1 to 11:

  1. Tamino over at Open Mind has a nice post showing just when (1975) the warming "resumes" (pollution controls kick in, allowing the true warming signal to no longer be masked/inhibited by the aerosol cooling effect). The CO2 concentration/temperature climb continues unabated to this day. Good post, Anne-Marie The Yooper
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  2. Yes, good post. Something I am mildly curious about is why the post 1975 heating doesn't converge on the trend line up to 1940, but runs parallel and offset down. Is it because there is still substantial (even if lower) aerosol presence?
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  3. Focusing on the cooling period as indicated above, 1945 to 1975, that period began with 2 declared El-Nino years that were much stronger than normal. Including those 2 years, a total of 10 years within the nominated period were declared El-Nino years. There was also 10 La-Nina years within the same period with the 3 final years being successive declared La-Nina years. HOWEVER, virtually the whole period, in fact the exact period was dominated with the SOI predominately and quite strongly in the positive (wetter) phase, and the IPO entirely and also very strongly in the corresponding negative (wetter)phase. The IOD was in the postive phase(drier) 3 times whilst in the negative (wetter) phase 4 times. The negative phase of the IOD is a pattern with cool SST in the Indian Ocean west of Australia, and warm SST in the Timor Sea to the north. For Australia overall, this cooling period was also a period of generally higher rainfall, so irrespective of what else may have been happening in the atmosphere, the amount and distribution of cloud cover would have had a determining influence on both how much the SST's warmed and cooled, and where, as well as where the moisture that formed into those clouds was carried to and deposited. Clouds also affect temperatures, causing lower temperatures during the day and warmer at night, so more clouds are not only indicated by the generally higher rainfall, but consistent with how temperatures varied also.
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  4. Could aerosols from industrialisation in China and coal fired stations elsewhere be damping down the warming trend over the last decade?
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  5. Hi Nigel. Yes, if you look at all the factors affecting climate change, aerosols have had a cooling effect and have therefore masked some of the warming expected from increasing levels of greenhouse gases. Some scientists have even suggested that we could inject huge quantities of sulfate aerosols into the atmosphere to keep global warming under control. Though technological and environmental issues mean that it's unlikely at the moment.
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  6. #4: very good point regarding China. From the late 90's to around 2005, the industrialized southern part of China (Shenzhen, etc) was covered in a dense haze, caused by emissions. Airborne dust and particles made it very uncomfortable to be outside. This has cleared up a lot over the past few years since around 2006. It is very possible that the Asian "brown cloud" helped mask the temperature increase on the early 00's, then, as it improved towards the end of the decade, temps started climbing faster again.
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  7. I'm not sure if this is on topic, but I feel it is pertinent to AGW. Does this experiment prove that CO2 is not a greenhouse gas? And if not, why not? Michael
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    Response: Does this experiment prove that CO2 is not a greenhouse gas? And if not, why not?

    On the contrary, the behaviour of temperatures during the mid-century cooling period actually confirm the greenhouse warming effect. What we find is that daytime temperatures cooled from the 1940s to the 1970s. This is consistent with measurements of solar radiation at the Earth's surface which also fell over this period - "global dimming" due to rising sulfate pollution. But interestingly, nighttime temperatures actually increased over this same period. This is consistent with an increasing greenhouse effect. Even while sulfate pollution was cooling the Earth, greenhouse warming was still percolating away while we were sleeping.
  8. What happened to the link ? My comment #7 doesn't make sense without the link
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    Response: Well, your comment does make more sense now that you've posted that link which wasn't included in the initial comment. And yes, now I see that your comment is off-topic :-)

    To discuss whether there's a greenhouse effect, I suggest you either go to the empirical evidence for the enhanced greenhouse effect or Has the greenhouse effect been falsified?.
  9. Ok and thanks Michael
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  10. I have posted a rebuttal to in this thread.
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  11. What about the effects of nuclear testing? I know it's unfashionable to talk about "nuclear winter" but the fact remains that there were multitudes of atmospheric nuclear tests during the period 1945 to 1970s (France 1974, China perhaps even later). Most of these injected quantities of aerosols into the stratosphere where it evidently hung around for a few years. Many Russian tests were in the arctic where the stratosphere is much lower than elsewhere, so even more dust would have been injected there.
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