What happened to greenhouse warming during mid-century cooling?

And could global brightening be causing global warming?

Physicist John Cook of Skeptical Science has two good pieces on global dimming and global brightening I’m combining and reposting here.

A few weeks back while researching global brightening, I came across a gem of a paper: Impact of Global brightening and dimming on global warming (Wild et al 2007). The paper examines temperature trends over the second half of the 20th Century, including the cooling period in the middle of the century. From the 1950s to early 1980s, while CO2 levels were rising, global temperatures cooled slightly. How can this be if CO2 causes warming? Wild 2007 found there was greenhouse warming during this cooling period and they find it in an interesting place…

The paper looks at trends in the amount of sunlight reaching the ground over the latter 20th century. Various factors can affect how much sunlight gets through to the Earth’s surface, with the amount of aerosols in the atmosphere being the main contributor. And of course, the amount of sunlight reaching the surface will have an effect on global temperatures. Wild 2007 attempts to disentangle just how much contribution this surface dimming and brightening has on global temperature.

They start by looking at measurements of surface radiation from 1958 (when widespread measurements began). They find a period of “global dimming” from 1958 to 1990 where surface radiation fell. Afterwards, the dimming levels off and transitions to slight brightening from 1985 to 2002. While the warming during the period of solar dimming is moderate, the warming is more rapid in the last two decades where dimming was no more present.

Temperature change over global land surfaces from 1958 to 2002.

How much does global dimming and brightening contribute to the temperature trends. To disentangle the effects of dimming and brightening from greenhouse warming, Wild digs a little deeper into the temperature record by looking at the daily temperature cycle. Sunlight affects the daily maximum temperature more than the nightly minimum, which is affected more by the greenhouse effect. What they find is from 1958 to 1985, during global dimming, the maximum daytime temperature falls. This makes sense as less sunlight is reaching and warming the Earth’s surface. The interesting result is that over this period, the nighttime minimum temperature increases. While global dimming was cooling temperatures in the daytime, the increased greenhouse effect was warming in the nighttime.

From 1985 to 2002, the warming trend during the daytime increases significantly and almost catches up to the nighttime warming trend (almost but not quite). This is consistent with the surface radiation measurements which find global dimming levels off or transitions to brightening in the mid 1980s. Global dimming masked greenhouse warming until the 1980s. Once the atmosphere cleared and the dimming was removed, global warming came into its own.

Does this mean global brightening could be responsible for global warming? Not quite. While there has been some global brightening since 1985, the amount of sunlight hitting the Earth hasn’t reached 1960 levels yet. Sunlight has fallen since 1960 while global temperatures have risen 0.8°C. The daily temperature cycle indicates it’s greenhouse warming that has driven the warming. Yet another human fingerprint!

Where did CO2 warming go during mid-century cooling? Global dimming caused by pollution masked the increased greenhouse effect. Nevertheless, the CO2 warming was still percolating away while we were sleeping.

JR:  Here’s the global brightening post.

One skeptic argument, employed by Christopher Monckton in his testimony to US Congress, is that global brightening is the cause of global warming. From 1983 to 2001, the amount of sunlight hitting the Earth’s surface has increased by 1.9 W/m2. Monckton compares this to the radiative forcing from manmade influence since pre-industrial times, estimated at 1.6 W/m2 (IPCC AR4). Monckton argues that these numbers prove global brightening is responsible for recent global warming. But is this the full picture?

Monckton’s numbers come from Do Satellites Detect Trends in Surface Solar Radiation? (Pinker et al 2005). This study analyses satellite measurements of solar radiation, upward radiation from the Earth and cloud cover fraction to model the amount of sunlight reaching the Earth’s surface. They calculate an overall increase in surface solar radiation of 0.16 W/m2 per year. Once the satellite data is corrected to remove an orbital decay bias (ERBE 2005), Monckton calculates a net increase in surface radiative flux of 1.9 W/m2.

Figure 1: Changes in solar radiation at the Earth’s surface from 1983 to 2001. Solid line is linear fit, dotted line is quadratic fit. The linear slope (solid line) is positive at 0.16 W/m2 per year (Pinker et al 2005).

Is it valid to compare changes in surface solar radiation to radiative forcing? A good person to answer this is Rachel Pinker herself who in responding to Monckton’s argument, said the following:

‘The CO2 ‘radiative forcing’ value that Mr. Christopher Monckton is quoting refers to the impact on the Earth’s Radiative balance as described above. The numbers that we quote in our paper represent the change in surface SW due to changes in the atmosphere (clouds, water vapor, aerosols). These two numbers cannot be compared at their face value.”

Why can’t you compare the two numbers? Radiative forcing refers to a disturbance in the planet’s energy balance. Forcings change the balance between incoming sunlight and outgoing radiation at the top of the atmosphere, causing the planet to lose or gain energy. Global temperatures will only respond to surface brightening if the total amount of solar energy absorbed by our climate system changes. To determine this, we need to understand what’s causing global brightening.

There are three major contributors: a reduction in cloud cover, a reduction in scattering aerosols such as sulfates and a reduction in absorbing aerosols like soot (Wild 2009). Scattering aerosols reflect incoming sunlight, preventing it from reaching the Earth’s surface. As the amount of sulfate pollution in the atmosphere lessens, more sunlight reaches the surface. If this was the sole cause of global brightening, then the increase in surface solar radiation would equal the extra energy absorbed by our climate (eg – the radiative forcing).

However, changes in cloud cover and absorbing aerosols also contribute to global brightening. As well as reflect sunlight, clouds trap infrared radiation coming up from the surface. So while less clouds allow more sunlight to reach the surface which has a warming effect, they also let more infrared radiation escape to space which has a cooling effect.

Similarly, a decline in absorbing aerosols like soot means more sunlight reaches the Earth which has a warming effect. But they also absorb sunlight which warms the atmosphere so a decline in absorbing aerosols also has a cooling effect. Absorbing aerosols like black carbon have shown a large decreasing trend since the 1980s (Wild 2009).

To focus solely on the amount of sunlight hitting the Earth doesn’t give you the full picture of global brightening. As absorbing aerosols and clouds are contributing factors,  the change in surface solar radiation is expected to be much more than the net radiative forcing from global brightening. To gain a fuller understanding of climate, we need to consider all the various forcings together rather than take one small piece in isolation. These include the direct effect from reflective aerosols, the indirect effect of aerosols on cloud cover and the effect of absorbing aerosols (black carbon) to name just a few.

Figure 2: Separate global climate forcings relative to their 1880 values (

Figure 2 demonstrates that CO2 is not the only driver of climate. Nevertheless, it’s clear that man-made greenhouse gases (of which CO2 is the greatest contributor) is currently the most dominant forcing and  increasing faster than any other forcing.


17 Responses to What happened to greenhouse warming during mid-century cooling?

  1. BB says:

    Does that (GISS) graph indicate that Global land use is actually a mitigating factor in radiative forcings? I would have thought differently.

  2. homunq says:

    BB: grass and cement are lighter colors than dense forests, so yes. But the CO2 consequences of deforestation are about 15% of that upper green line, so overall land use changes are net-warming.

  3. Christopher S. Johnson says:

    As a lay person I can see that those dates for when dimming becomes less of an issue correlate with the implementation of the Clean Air Act and the invention of the catalytic converter, and emission controls from the 70’s and into the 80’s. That can’t be a coincidence. And it provides an example of regulation and laws having an effect on the climate. A useable example.

  4. Prokaryotes says:

    “The interesting result is that over this period, the nighttime minimum temperature increases. While global dimming was cooling temperatures in the daytime, the increased greenhouse effect was warming in the nighttime.”

    Global dimming
    From Wikipedia, the free encyclopedia
    Deliberate manipulation of this dimming effect is now being considered as a geoengineering technique to reduce the impact of global warming.

    In a blog post, Gavin Schmidt stated that “Ideas that we should increase aerosol emissions to counteract global warming have been described as a ‘Faustian bargain’ because that would imply an ever increasing amount of emissions in order to match the accumulated greenhouse gas in the atmosphere, with ever increasing monetary and health costs.”

  5. john kearns says:

    Reid Bryson was contending in the early 1960’s that the cooling effects atmospheric particles was greater than the warming effects of increased CO2; at that time, according to figure 2, he had it about right, according to the measurements available. The London and Donora PA deaths in the fifties eventually led to cleaner safer air in the cities.

    Earlier, measures to prevent dust storms, the horrific “black blizzards” of the thirties were effective and are likely still the best example of successful deliberate large-scale weather modifications.

  6. Lou Grinzo says:

    So, ask yourself what would happen if every enviro-geek (including me) got his or her fondest wish, and all the coal plants in the world instantly morphed into zero carbon renewable sources. Atmospheric CO2 would keep climbing, thanks to all the oil and natural gas we burn, among other sources, and the effective forcing from sulfates would decline very quickly to a new level less than half of the current cooling effect. (The best numbers I could find say that in the US coal plants account for about 60% of our sulfur emissions. I’d guess the percentage is even higher for other countries.)

    Is it me, or does everyone else here feel a little, um, trapped…?

  7. villabolo says:

    @#6. Lou Grinzo says:

    “Is it me, or does everyone else here feel a little, um, trapped…?”

    I’m just a lay person but, based on what I’ve been learning in the past year, I feel very trapped.

    What I would like to know is this. What increase in temperature would we have if all emissions were to stop and:

    1) Thermal lag during the next 30 years (1F?) were included?
    2) Sun activity would be at maximum?
    3) The Arctic Sea would be ice free?

    Or is it silly to ask such questions? :(

  8. David B. Benson says:

    Global dimming is part of the answer, but also CO2 concentrations didn’t grow by much during the 1940S and 1950S. Also, during the decades in question internal variability was, in of itself, causing a decline. All these factors added together produced the slight temperature dip mid-century.

  9. Tranche Demerde says:

    Ramanathan has been writing good papers on the effect of aerosols for years. One aimed at policy makers published in the runup to Copenhagen is “On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead” Available here:

    where he points out that according to IPCC data not normally extracted and presented in the way he is using it, Earth is already committed to DAI (dangerous anthropogenic interference) even if no more additions are made to the GHG present in the atmosphere as of 2005.

    This is because the generally agreed understanding of aerosols, because they act to cool the planet and there are enough of them, is that they are masking the effect of the GHG that is already in the atmosphere, and they are not likely to be in the atmosphere in the relatively near future in anything like the concentration they are today.

    The policy implication he wanted people to be aware of is that because it is easier and cheaper by far to remove the aerosols from emission sources, and history shows it has been done in some rich regions, therefore they are likely to be removed globally in the relatively near future. The classic scenario already playing/played out in the developed world is to get rich by filling the air with aerosol and GHG, then cut the aerosols while leaving the GHG still accumulating as the population demands things be “cleaned up”. Because the lifetime of aerosols in the atmosphere is only a few weeks, once the sources stop the concentration drops. It is therefore very likely the concentration of aerosols in the atmosphere will actually be drastically reduced, as opposed to GHG concentration, which looks out of control headed inexorably upward.

    Which means we will see far more effect from the GHG already present than any modellers are presenting. Hansen made a similar point in his own inimitable way in his Bjerknes Lecture of 2007

    Hansen made the further point that although there are good reasons why the general consensus is that aerosols have the cooling power they are calculated to have, he says it is almost as likely that they have twice as much power. This is what he is doing with the pictures of his grandkids in the paper, the section where he asks them what do they think, is the effect 1 or 2. He’s trying to get us to see that the consensus about how powerful these aerosols are is about as well founded as his grandkid picking a 1 or a 2. In other words, if the general consensus is wrong and one of his grandkids is right, as aerosol sources are cleaned up and their total concentration starts declining we will discover to our horror what the effect of the GHG that is already in the atmosphere really is. This is why Hansen is wandering around talking about we must aim to reduce the GHG concentration, never mind all this cap it at 550 or 450.

    Schellnhuber commented on Ramanathan’s paper in his “Global WArming: Stop Worrying, start panicking?” PNAS Commentary here:

    sums up Ramanathan saying his analysis is “excellent”, and a “harsh wake-up call” because it stated GHG emissions of the past may have already committed Earth to “disastrous global warming”. He wrote the paper to say there is still a “may” we can believe in.

    Schellnhuber, perhaps Germany’s most distinguished climatologist, co-author of the highly cited paper that spawned the usage of “tipping point”, i.e. “Tipping elements in the Earth’s climate system”

    cautioned that Ramanathan could be overstating a few points in the case and said if “an industrial revolution for sustainability starting now” was implemented, there was a chance we may not be committed to disaster already.

    And now, after this refreshing and entertaining light interlude, we return to more serious activities.

  10. Gladys says:

    What is the solution to man-made green house gases? Well with politics raging little will be accomplished in our life time-Future generations are going to experience extreme climate.

  11. M says:

    Re: An old post: “”

    I think that the experiment might be flawed. See:

    eg, the warming properties of CO2 are real, but this particular experiment doesn’t properly control for the non-radiative properties (eg, convective & conductive properties). I’m still looking (in my spare time) for a better experiment to produce a video that gets the right result for the right reasons…


  12. Fred Heutte says:

    There is a big, big difference in the average duration of the various forcings. Since CO2 has an average atmospheric lifetime of at least several hundred years (with significant proportions staying aloft 10,000 years and longer), yes, it is a good idea to stop the CO2 emissions now from coal plants, industries and vehicles, even if there is a short-run temperature increase due to the removal of the sulfates and other cooling co-emissions.

  13. cr says:

    Lou Grinzo @6:

    The more I read the more trapped I feel.

  14. David Lewis says:

    Grinzo: what you are talking about is one reason natural gas is a bridge to nowhere.

  15. Lewis Cleverdon says:

    Lou –

    With regard to ‘being trapped’, given that the US is still stonewalling the UN treaty negotiations, I’d point out that we’ve yet to actually try applying the requisite trio of options to resolve our predicament. Viz:

    1/. ending global GHG emissions ASAP;
    2/. actively reducing airborne GHG concentrations to pre-industrial levels;
    3/. temporarily restoring albido in order to control global temperature while items 1/. & 2/. are achieved, and thereby to both decellerate the feedbacks and prevent the foreseeable post-fossil-aerosols’ temperature boost.

    Yet we are certainly trapped in some regards. Viz:

    – The US govt.’s conviction that the inherited Bush policy of a ‘brinkmanship of inaction’ with China, which precludes any and all significant domestic action, will somehow yield national benefits that will outweigh the damages of global climate destabilization;

    – the widespread misperception that US domestic inaction reflects some mixture of incompetence and fear of the political opposition, to the extent of precluding even the most basic essential programs of public education and of official exposure of the corrupt funding of denialism;

    – the equally widespread misperception that ending global emissions that add to the problematic airborne GHG stocks is the sufficient strategy for addressing the problem, and that Geo-E in all its forms is essentially the tool of the corrupt status quo.

    These surely are the traps of myopia that must be broken by Americans if the nation is to avoid leading the world to perdition.



  16. Icarus says:

    M: I think it should be sufficient to show that introducing extra CO2 into a transparent container of air reduces the level of infrared light passing through it. It’s not necessary to construct an experiment that actually produces warming, as the warming is a side-effect of the more fundamental property of CO2’s transparency to infrared. It then only needs to be explained how this property causes the greenhouse effect in the atmosphere. You could probably do the experiment with a TV remote control, a suitable glass container and a mobile phone video camera.

  17. Jeffrey Davis says:

    “Future generations are going to experience extreme climate.”

    As opposed to the current Chinese, Russians, Pakistanis, and Iowans?