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How much did aerosols contribute to mid-20th century cooling?

Posted on 16 September 2010 by dana1981

There was a very slight cooling in the average global surface temperature from about 1940 to 1975.  Although the global temperature only decreased by approximately 0.1°C, this period represents a divergence from the warming periods of 1915 to 1940 and 1975 to Present.

Figure 1: Global temperature anomaly and distinct linear trends over the 21st century (Open Mind)

Climate scientists believe that the primary cause of this mid-century cooling was an increase in atmospheric aerosols due to anthropogenic emissions (primarily from the burning of fossil fuels).  Aerosols have a complex effect on the climate, because they have both direct and indirect impacts.

Direct Effect

The direct effect of aerosols on climate is the mechanism by which aerosols scatter and absorb shortwave and longwave radiation (a.k.a. "global dimming"), thereby altering the radiative balance of the Earth-atmosphere system. The key parameters for determining the direct aerosol radiative forcing are the aerosol optical properties and distribution in the atmosphere (IPCC 2007).

Indirect Effect

The indirect effect of aerosols on climate is the mechanism by which they modify the microphysical and, therefore, radiative properties, amount, and lifetime of clouds.  A key parameter for determining the indirect effect of aerosols on the global surface temperature is the effectiveness of an aerosol particle to act as a cloud condensation nucleus - a function of the aerosol size, chemical composition, mixing state, and ambient environment (IPCC 2007).


Figure 2: Diagram of direct and indirect aerosols' radiative mechanisms (IPCC 2007)

Radiative Forcing

We can attempt to quantify the impact of anthropogenic aerosol emissions on the average global temperature by first examining the associated radiative forcing.  The radiative forcing is a measure of the influence that a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system.

According to the IPCC, the total (direct + indirect) radiative forcing due to anthropogenic aerosols could range anywhere from -0.4 to -2.7 Watts per square meter (W/m2), although the most likely value is -1.2 W/m2.


Figure 3: Radiative forcing from various sources in 2005 as compared to 1750 (IPCC 2007)

The aerosol radiative forcing, particularly the indirect cloud albedo effect, is the largest source of uncertainty among all anthropogenic radiative forcings.  However, keeping the range of possible values in mind, we can still estimate the cooling effect caused by these forcings during the mid-20th century.  The majority of the increase in atmospheric sulfates occurred during this period:

Figure 4: Atmospheric sulfate aerosol concentration in parts per billion (ppb) from the GISP2 ice core (black) vs. anthropogenic sulfate emissions (red) since 1800 (Open Mind)

The general natural baseline is around 30 ppb, with a rise to about 65 ppb in the early 20th century.  Then from 1945 to 1970 (the mid-century period in question), atmospheric sulfate concentration increased from 65 to 120 ppb.  This tells us that approximately 60% of the total sulfate increase since 1800 occurred during the mid-century cooling period.

Direct Aerosol Forcing

The change in reflected solar flux is proportional to the optical thickness of the aerosol layer (Charlson et al. 1991).  


where FR is reflected solar flux, Qo is the solar constant (total solar irradiance), Ti is the atmospheric transmittance, Ac is the fractional coverage of clouds, β is the fraction of upward scattered flux, and δ is the optical thickness.

Using Qo = 1366 W/m2, Ac = 0.61 for globally averaged cloud cover, Ti = 0.76, and Rs = 0.15 (see Charlson 1991 for references to the sources of these values), FR is approximately 32 times the optical thickness δ.  Charlson et al. estimate the optical thickness as the product of the aerosol burden and mass scattering coefficient, and find for cloudless skies, the optical thickness is approximately 2.8x10-2.  Thus from the formula above, the reflected solar flux is approximately 0.9 W/m2, which is on the high end of the IPCC range of values for the direct aerosol forcing.

The indirect sulfate effect on cloud formation is more complex, but we'll proceed under the assumption that the total sulfate radiative forcing is approximately proportional to the atmospheric sulfate aerosol concentration, in order to come up with a ballpark estimate regarding its impact on mid-century cooling.  In other words, we'll assume that a 60% increase in atmospheric sulfate concentration corresponds to a 60% increase in radiative forcing.  Also note that sulfates are not the only anthropogenic aerosols, but do comprise the majority of the radiative forcing.

Approximation of sulfate aerosol cooling effect

With these assumptions in mind, approximately 60% of the -0.4 to -2.7 W/m2 net aerosol forcing occurred during mid-century, or -0.24 to -1.62 W/m2, with a most likely value of -0.72 W/m2. To convert this to a surface temperature change, we need to multiply by the climate sensitivity factor (λ), which is 0.54 to 1.2°C/(W/m2) (IPCC 2007).  The relationship between surface temperature change (dT) and radiative forcing (dF) is:

dT = λ*dF = (0.54 to 1.2)*(-0.24 to -1.62) = -0.13 to -1.9°C with a most likely value of -0.58°C

We can also compare this to the temperature change caused by anthropogenic CO2 during the period in question:

dT = λ*dF = (0.54 to 1.2)*5.35*ln(331/308) = 0.21 to 0.46°C with a most likely value of 0.31°C

In addition to CO2, other anthropogenic greenhouse gas emissions and solar output during this period caused an additional ~0.15°C of warming.  Therefore, using the most likely values calculated above, we would expect to see an approximately 0.1°C cooling of global average surface temperatures from 1940 to 1975.  And indeed that is what we observe:


Figure 5: Average global land and ocean surface temperature (NASA GISS)

Days cooled, but nights warmed

Another signature of aerosol cooling offsetting greenhouse gas warming is the continued increase in daily minimum, nighttime temperatures while the maximum, daytime temperatures drop.  This is because aerosols cause global dimming by scattering sunlight; an effect which is much more influential during the day when solar radiation is bombarding the Earth's surface.  At night, greenhouse gases continue to absorb and re-radiate thermal radiation from the Earth's surface, which causes the nighttime surface temperatures to continue warming despite the cooling daytime temperatures due to the aerosol dimming.

Wild et al. (2007) investigated these effects and found that from 1958 to 1985, daytime land surface temperatures cooled at a rate of -0.04°C per decade, but nighttime temperatures warmed at a rate of 0.11°C per decade.  From 1982 to 2002, they found that daytime land surface temperatures warmed at a rate of 0.37°C per decade, while nighttime temperatures warmed by 0.40°C per decade.  

The fact that nighttime temperatures continued to warm while daytime temperatures cooled mid-century is a strong indicator of the combined effects of anthropogenic aerosol and greenhouse gas emissions, while the increased rate of warming over the past 3 decades reflects the increasing atmospheric concentration and radiative forcing from greenhouse gases.

What happened since 1975?

Clearly since about 1975, global surface temperatures have trended rapidly upwards (at a rate of nearly 0.2°C per decade).  So what caused the mid-century cooling to end?

The main cause of the sudden shift in global temperature trends was the passage of Clean Air Acts by various countries in response to air pollution and acid rain.  The USA, for example, first passed its Clean Air Act in 1970, with amendments in 1977 and 1990.  Coincidentally, the US Supreme Court (in Massachusetts v. EPA) and EPA (in an endangerment finding) also recently decided that greenhouse gases qualify as 'air pollutants' in the Clean Air Act and must be regulated accordingly.

Under the Clean Air Acts, sulfate emissions were regulated, and as a consequence their rapid atmospheric increase was stabilized right around 1975:


Figure 6: Global anthropogenic sulfur emissions (Pacific Northwest National Labs)

Meanwhile anthropogenic greenhouse gas emissions have continued to increase.  Since 1975, the atmospheric CO2 concentration alone has increased from 331 to 392 parts per million by volume, which corresponds to a temperature increase of about 0.7°C, though we've only seen about 0.55°C warming over that period due to the thermal inertia of the oceans and 'warming in the pipeline'.

Mid-century cooling was primarily anthropogenic

To sum up, anthropogenic sulfur emissions appear to be the main cause of the mid-century cooling.  These emissions decreased the mean global surface temperature by approximately 0.5°C during this period, while anthropogenic greenhouse gas emissions caused a warming of approximately 0.4°C.  Therefore, even though greenhouse gas emissions continued to have a warming effect during this period, it was more than offset (hidden) by anthropogenic aerosol emissions, until those emissions were brought under control by government intervention while greenhouse gas emissions continued to increase unabated.  In other words, the mid-century cooling is actually an expected result based on our current understanding of climate science, and is successfully hindcasted by climate models (Meehl 2004). 


Figure 7: Anthropogenic plus natural vs. just natural radiative forcing temperature change vs. observed global surface temperature increase (Meehl 2004)

However,  the overall impact of sulfate aerosols, particularly due to their indirect effects via cloud formation, remain a significant source of uncertainty.  Despite this uncertainty, they remain the likely dominant cause of the slight mid-20th century cooling.

This post is the Advanced version (written by dana1981) of the skeptic argument "It cooled mid-century". If reading this caused your brain to overheat, you can always wind down with the Basic version by Anne-Marie Blackburn or the Intermediate Version by John Cook.

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Comments 1 to 22:

  1. None of the your figures show the obvious correlation with mid 20th century cooling and the PDO cool phase of the mid 20th century.

    Aerosols are currently increasing in SE Asia significantly, but this doesnt correspond to any cooling (?).

    Aerosols 0. PDO 2.
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  2. thingadonta.

    The chindian aerosols are one of the nasty effects in the wings. Anyone who says that CO2 emissions aren't lining up with the observed temperature increases should look at what could happen as aerosols reduce in the same way as those in advanced economies.
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  3. thingadonta - yes, PDO was another small contributor to the cooling during this period. And so were volcanoes, and black carbon caused some warming, etc. etc. I can't address every single global temperature influence in one post, so I covered the big ones.

    Your claim that aerosols are not causing any cooling in SE Asia is incorrect. Their local cooling effect is overwhelmed by anthropogenic warming, but that doesn't mean it doesn't exist. That's like saying 2 + (-1) = 1, therefore -1 isn't negative.
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  4. Hmm. Globally, I thought aerosols were flat or declining

    Asia emitting more as industry moves there while reducing in the west? Anyway, remember that climate is not single-factor. The overall effect is sum of all forcings, positive and negative. Claiming PDO as cause may be mixing cause and effect. The PDO argument is somewhat moot anyway because there is no trend.
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  5. This is a good post except for one thing. Drawing the blue lines in the top curve is not helpful. I realize this is often done to "guide the eye", but I think especially to non-scientists, it looks like you are trying to make more of the data than is there: nothing happened up to 1920, then something happened, linearly, changed completely all at once in 1940, and again in 1975. I know you don't claim to say this, but to the non-scientist it seems like you are trying to prejudice the reader into seeing lines when in fact there is only noisy data. Leave the data alone and readers can see the trends for themselves.
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  6. Re: Rab (5)
    "Drawing the blue lines in the top curve is not helpful. I realize this is often done to "guide the eye", but I think especially to non-scientists, it looks like you are trying to make more of the data than is there: nothing happened up to 1920, then something happened, linearly, changed completely all at once in 1940, and again in 1975."
    Are you referring to Figure 1?

    If so, this is from a post by Tamino over at Open Mind. Tamino is a professional time-series analyst. He used this figure to illustrate how a time-series dataset can show natural "break points" in the data where a "tipping point" can be demonstrated to have occurred. The purpose of the blue line is not to "guide the eye". The red line is a loewess smooth to show the trend with less noise.

    I recommend following the link to Open Mind for further detail.

    The Yooper
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  7. #! Thingadonta

    Aerosols are currently increasing in SE Asia significantly, but this doesnt correspond to any cooling (?).

    Please stop indulging in argument by assertion, and start providing credible evidence for your claims.

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  8. #1: "Aerosols 0. PDO 2. "

    I'm mystified why so many people throw around these ocean cycles as an explanation for just about everything. Clearly I do not understand enough about ocean dynamics. But it is apparent just by inspection that there is no long term trend to these cycles:
    From The PDO

    From The AO and the NAO

    So I have to ask: How can cycles with no long term coherency be the causes for an effect (increasing global temperatures) that has a long term trend? Wouldn't those cycles more likely be the causes of the short-term low-amplitude variations?
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  9. And here's someone answering my question on a related thread. Gotta love skepticalscience!
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  10. It's valid to point out that during the mid-century cooling period, PDO was negative essentially the whole time (though still a relatively small impact on average global temperature). But PDO does not cause long-term temperature trends, as discussed in the rebuttal linked by muoncounter in #9. Nor can negative PDO explain the nighttime warming trend discussed above.
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  11. Daniel #6 (in reply to Rab #5

    Thanks for the coherent explanation of that particular piece of erroneous thinking. You made me realise that this was another example of so-called climate sceptics (sub-consciously?) using the often uninformative technique of philosophical solipsism in an attempt to maximise perceived uncertainty.
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  12. #10: "during the mid-century cooling period, PDO was negative essentially the whole time"

    So I looked at a PDO data file and graphed it with global LOTI for the same period (1900-current). The mid-century negative certainly looks like the mid-century cooling, but the PDO seems to be touching negative territory since the late '90s: with no sign of any cooling.

    PDO: Pink is summer 3 month average; light blue is winter. LOTI (dark blue) on the right-hand scale.

    You can't have it both ways. Perhaps there is no real coupling between PDO and temp and the mid-century was mere correlation without you-know-what? Or perhaps any such coupling was faint and is now swamped by CO2?

    In either case, its now PDO 0, aerosols 1.
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    Response: Note: added "width=450" to your image. I get a little grumpy when people break my web design. Just letting you know for future reference :-)
  13. That was an interesting point in the section "Days cooled, but nights warmed". Can anyone point to a global anomaly graph that tracks the night time temperature minimums and C02 since the industrial revolution? It would make an excellent centrepiece for debunking CO2 scepticism.
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  14. A silly question:

    The 1940s coincided with some historically cataclysmic events - lots of oil spills from sunken ships, lots of dust from high explosive devices, cities burned up by conflagrations, industrial production soaring to meet the demands of the military world wide, aircraft flying in vast armadas across European skies, and so the list goes on.

    Any chance this might have had some climatic impacts?
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  15. Hi John,

    Fascinating post, this is something that I have often pondered. I was wondering, is there any evidence form observatories which indicate/suggest that the luminosity of certain stars decreased during the time in question? If so, that would be another confirmation that a contributing factor to the cooling was increased aerosol loading.

    I know incredibly little about astronomy, so apologies in advance if this is a daft question.
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  16. macoles - greater warming at night than day is on my list of anthropogenic global warming fingerprints, though the particular study I referenced only went back to 1951.
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  17. @Daniel Bailey: I saw the post at Open Mind but neglected to respond. As Tamino and the present post state, the influences change smoothly, not discontinuously, so there is no warrant for drawing straight lines. Tamino says: "But it’s still a pretty good approximation to model global temperature as a piecewise-linear function." I say: "It's an unwarranted approximation." You may disagree. You may want to cherish the notion that Tamino is more qualified than I am.
    @kdkd: So now I'm a sceptic? My friends (and enemies) would find that hilarious. Try not to jump to conclusions. I am not disputing any of the results. I am complaining about the way they are communicated to non-scientists. Isn't such communication the main purpose of this blog? The blue lines hinder rather than help.
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  18. krab:

    I quite like the blue line - it shows the relationship to the smoth and the raw points quite nicely. It's possible the raw points shouldn't be joined though ;)
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  19. #14: "The 1940s coincided with some historically cataclysmic events - Any chance this might have had some climatic impacts?"

    The sulphate emissions curve (red) on the graph above has a bump which appears to be WW2, as does the carbon emissions graph from CDIAC data shown below.

    But the scale of the emissions during the more recent decades seems to overwhelm.

    Note to John C.: Sorry, I was using width=480; I will downsize in the future.
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  20. I watched this movie:

    about global dimming. Peter Cox of the UK MET office seems to thing it is (still) playing a significant role in suppressing rises in temperature that would otherwise be much higher.

    My question is:

    Could it be that (part of) the disparity in warming between the arctic temps and the lower latitudes is due to more global dimming clustered around the equator as opposed to cleaner air around the poles???

    I would appreciate any thoughts!
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  21. Re: Gail

    Thanks for the video link (nice site, BTW; I've read your posts on the issues with the trees...I see much the same patterns here in the Midwest).

    Cox is spot-on in regards to the protective effects (thus far) of the global dimming the video documents. This post covers that fairly well. Thus, as we clean up our emissions and therefore the air, we restore more fully the true heating capability of the sun, masked to some degree for decades. Add in a wakening sun (coming out of its dormancy as it climbs to the next solar maximum...

    As to your last question, no. Arctic amplification of the high latitudes is the primary driver here. Romm over at Climate Progress touches on it in posts here, here and here as well (see also this related post). Basically, the warming of the Arctic, shifting of the polar jet and the expansion of the Hadley cells has reached the point where the entire circulation patterns of the northern hemisphere are being reorganized.

    The world we were born into will not resemble the world we will leave behind.

    But everything's hunky-dory as long as the Bills don't win the Super Bowl*...

    *OK, wasted a lot of time looking for a vid on that so hear's the quote:
    Smoking Man: "What I don't want to see is the Bills winning the Super Bowl. As long as I'm alive that doesn't happen."
    Third Man In Black: "Could be tough, sir. Buffalo wants it bad."
    Smoking Man: "So did the Soviets in 80."
    Third Man In Black: "What? You saying you rigged the Olympic hockey game?"
    Smoking Man: "What's the matter? Don't you believe in miracles?"
    Fourth Man In Black: "The boss gave the Russian goal tender a little pre-game good luck pat on the back. Unseen novacaine needle on a bogus wedding ring. Goalie's a little slow on the stick side, 4-3 home team."
    Smoking Man: "Payback's a bitch, Ivan."
    Smoke 'em if ya gots 'em...

    The Yooper
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    Moderator Response: [Daniel Bailey] You're welcome!

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