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Aerosols have partially masked human-caused global warming

What the science says...

The global dimming trend reversed around 1990 - 15 years after the global warming trend began in the mid 1970's.

Climate Myth...

It's aerosols

Is a Thinning Haze Unveiling the Real Global Warming? (Kerr 2007) points out that the sunlight-reflecting haze that cools much of the planet seems to have thinned over the past decade or so. If real, the thinning would not explain away a century of global warming but it might explain the unexpectedly strong global warming of late, the accelerating loss of glacial ice and much of rising sea levels.

The global dimming trend reversed around 1990 - 15 years after the global warming trend began around 1975. So it can't explain what began the global warming trend. Aerosols have a cooling effect on Earth's climate. When aerosols thin, the result is a lack of cooling, not a warming effect. That's not just semantics - take aerosols out of the equation and in the absence of any other forcings, global temperatures would remain steady.

So what is driving the warming? In the past, solar variations have been the main driver in climate change. A comparison of solar activity and temperature over the past 1150 years shows a close correlation between solar activity and temperature. However, the correlation ends around 1980 when temperatures started rising but solar levels remained steady.

Another suspect in climate change is cosmic radiation which is thought to increase cloud cover (hence cooling the earth). However, again there has been no correlation between temperature and cosmic ray flux since 1970. In fact, all the usual suspects in natural climate change - volcanic activity, orbit wobbles, solar variations are conspicuous in their absence over the past 30 years of long term global warming.

The only forcing that causes warming and also correlates with current temperature rises is atmospheric CO2. It's risen 100 parts per million over the past 120 years - in the past, that kind of change has taken 5,000 to 20,000 years. As CO2 rose over the 20th century, the only mystery has been why global temperatures actually cooled from 1950 to 1980. I even read one study in 1980 where the researcher posed the question "why aren't we seeing any global warming with all this CO2 in the air?"

The answer is now apparent with recent studies in aerosol levels and global dimming. Atmospheric aerosols caused a global dimming (eg - less radiation reaching the earth) from 1950 to 1985. In the mid-80's, the trend reversed and radiation levels at the Earth's surface began to brighten. From 1950 to the mid-80's, the cooling effect from aerosols was masking the warming effect from CO2. When aerosol cooling ended, the current global warming trend began.

Intermediate rebuttal written by John Cook

Update August 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


This rebuttal was updated by Kyle Pressler in September 2021 to replace broken links. The updates are a result of our call for help published in May 2021.

Last updated on 4 August 2015 by MichaelK. View Archives

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Comments 1 to 21:

  1. "...That's not just semantics - take aerosols out of the equation and in the absence of any other forcings, global temperatures would remain steady." This doesn't make any sense--when you "take aerosols out of the equation" doesn't that mean more EM radiation is reaching Earth, warming it?
  2. Which aerosols are you referring to? Man-made presumably? Aerosols are also produced by the biosphere and the atmospheric concentrations vary considerably. For example, many plant species produce aromatic oils (terpenes) which generally persist for a few minutes to a couple of days. Species include pines,eucalypts, beech,citrus, as well as the 'herbs'... rosemary, thyme, sage etc. These oils degrade in the atmosphere and form aerosols. A recent study indicates that plant aerosols can affect cloud cover : "The team found the terpenes react in the air to form tiny particles called aerosols. The particles help turn water vapour in the atmosphere into clouds. Spracklen said the team's computer models showed that the pine particles doubled the thickness of clouds some 1,000m above the forests, and would reflect an extra 5% sunlight back into space. He said: "It might not sound a lot, but that is quite a strong cooling effect. The climate is such a finely balanced system that we think this effect is large enough to reduce temperatures over quite large areas. It gives us another reason to preserve forests." The research, which will be published in a special edition of the Royal Society journal Philosophical Transactions A, is the first to quantify the cooling effect of the released chemicals. The scientists say the findings "must be included in climate models in order to make realistic predictions". Because trees release more terpenes in warmer weather, the discovery suggests that forests could act as a negative feedback on climate, to dampen future temperature rise. The team looked at forests of mainly pine and spruce trees, but Spracklen said other trees also produce terpenes so the cooling effect should be found in other regions, including tropical rainforests."
  3. Here is a NASA study about aerosols affecting the Arctic: "Clean air regulations passed in the 1970s, for example, have likely accelerated warming by diminishing the cooling effect of sulfates" So without our pollution in the sixties global warming would have started much earlier.
  4. Is it possible that the IPCC underestimates the growth of aerosol concentrations from developing countries for the coming century? If aerosol growth is underestimated, then their temperature projections might be too high. And could the cooling effect of aerosols be stronger than climate models suggest?
  5. While it's not an answer to your questions, there's more useful background information on aerosols at How much did aerosols contribute to mid-20th century cooling? See also treatments at RealClimate: An Aerosol Tour de Forcing Aerosol formation and climate, Part I Aerosol effects and climate, Part II: the role of nucleation and cosmic rays
  6. This paper probably goes more to the heart of your ponderings, Karamanski: Air pollution, greenhouse gases and climate change: Global and regional perspectives As well, see: Uncertainties in climate stabilization
  7. Gilles "was there a sudden burst of sulphate aerosols in the 40's ?" - Yes. If you were to read the article I linked to, you would have seen that as the second figure: Since such aerosols are fairly short-lived (removed by precipitation), their influence dropped off rapidly once emissions started reducing. Not that this was the only factor, but it was a major one, and also explains hemispheric differences in the temperature anomaly record: All credit goes to Tamino for these images and data.
  8. I don't see any quantitative fits of the variation causes by aerosols. The break of temperature curves occured in the 40 , not in the 50's. And it's much more obviously correlated with PDO than with aerosols. My question was : maybe PDO is caused by aerosols ?
    Response: [DB] "maybe PDO is caused by aerosols" You can't possibly be serious...Pirates, maybe, Leprechauns, certainly, but aerosols???
  9. Just in case anyone's still interested in this. A new video of Hansen discussing aerosols, models, sea rise etc. via ClimateProgress a good watch.
  10. Rob Painting you got the right graph from here( Quoting "After a rapid rise in global surface air temperatures during the late 1970s to 1990s" Notice that the Sulfur emissions are higher in 1980 than they are today. ie if Sulfur emissions reflect sunlight then we should have been freezing in 1980 and cold today. Not the other way around. ie that graph blows the premise of the article. As for CFC thing it had the other forcing value in 2007 according the IPPC however it has been scrubbed apparently. I'll see if I can find a link on it to show it. Until then I won't discuss it further. I did find this link which claims CFC cools in the upper atmosphere. under Ozone. As a side note in the same section they claim that that "This paradox occurs because the atmosphere thins with height, changing the way carbon dioxide molecules absorb and release heat." I have never seen a physics experiment that shows that temperature and/or pressure effects the way CO2 absorbs and releases heat. Nor CFCs for that matter. A plausible claim again with no proof I've seen. I like your links scaddenp. The Aerosol one doesn't clearly prove that aerosols actually reflect any light from a physics perspective. The crux sentence seems to be here "Here we use state-of-the-art satellite-based measurements of aerosols6, 7, 8 and surface wind speed9 to estimate the clear-sky direct radiative forcing for 2002, incorporating measurements over land and ocean" The satellites are used to measure the amount of aerosols *not* the amount of reflection by them. Notice the word "estimate." It doesn't mean Aerosols don't reflect light. Just that article doesn't prove that property. Not wrong just not proof. To give an analogy. Measuring forcing from the quantity of aerosols present and known atmospheric behavior would be like trying to figure out the speed of a bus by knowing the velocities and locations of the air molecules around it. You might get an reasonable result, but it probably would be a lot more accurate to measure the speed of the bus itself.

    [DB] Note:  This comment was moved from the Myth of the Mini Ice Age thread to this one, where it is much more on-topic.

  11. James, responding where this should be. "Notice that the Sulfur emissions are higher in 1980 than they are today. ie if Sulfur emissions reflect sunlight then we should have been freezing in 1980 and cold today. Not the other way around. ie that graph blows the premise of the article." Nonsense - because aerosols are only one of the forcings at play. GHGs are another. See the recent article on Lean and Rind 2008 which look at temperature as function of all forcings. A similar study would be Benestad and Schmidt 2009. "it had the other forcing value in 2007 according the IPPC however it has been scrubbed apparently" This is very close to accusations of fraud. Which is more likely - that IPCC managed to change what they said in printed copies of reports in libraries round the world - or that you misunderstood what it said. Looking at our bound copies, I see that in First Assessment report 1990, CFCs were reported at having positive forcing - a theme repeated in every report since that. Since the IPCC reports are summaries of the published science, this is no surprise since there is no published science that says otherwise. As to effect on stratosphere, well funny you mention that. A warming surface and cooling stratosphere is a fingerprint of GHG warming. CO2 is actually the main player in cooling the upper stratosphere. As to why, that is not so straightforward to understand. In one sense it can directly calculated from the Radiative Transfer Equations (RTEs), but for a qualitative explanation, see here. Whether you understand it or not, the actual measurements of temperature and spectra closely confirm the theoretical prediction from solving the RTEs. As to that paper on aerosols that I linked, read more closely. The satellite does not directly sample aerosols (it couldnt). Instead it deduces them from Aerosol Optical Thickness - which is of course the radiative effects from the aerosols. More detail on this can be found here. Note the references for studies on checking and calibration of the method. I would like to point out that so far you have insisted that aerosols warm based on no evidence at all that I can see, while refusing to accept laboratory and direct emperical evidence ( eg satellite or say Ball R J and G D Robinson, 1982 "The origin of haze in the Central United States and its effect on solar radiation") to the contrary.
  12. JamesWilson#10: "I did find this link which claims CFC cools in the upper atmosphere. under Ozone." From that link: "Ozone is actually a greenhouse gas, and so are CFCs, meaning that their presence in the troposphere contributes slightly to the heightened greenhouse effect." In the stratosphere, CFC's do lower ozone levels which then lowers temps there, but this is only a small part of the cooling that has happened in the stratosphere. Most of the cooling is from rising CO2, and this cooling is a direct fingerprint of AGW. More importantly, this is yet another example of CFC's being called GHG's and direct warming agents. This goes against your claim that CFC's were considered to cause net cooling, and that aerosols were substituted as cooling agents only very recently. The later claim is absurd, since the cooling effects of aerosols have been known for many decades now and were at the center of the debate in the 60's and 70's over whether cooling from aerosols would outweigh the warming from rising GHG's. Why can't you just admit you messed up?
  13. This article conflicts with the one I was commenting on here Quoting from there "After a rapid rise in global surface air temperatures during the late 1970s to 1990s" Quoting from here "In the mid-80's, the trend reversed and radiation levels at the Earth's surface began to brighten." So there the temperature went up from 1970 to 1990. Here it was cool and then went up in 1985. Both are because of Aerosols... and proven... and conflicting... Right? Then if you go back to the temperature graph here you see that there is no cooling period pre 1985. (This is the modern temperature graph.) Conclusion: Both articles conflict. The one with a modern temperature graph conflicts with its own SO2 emissions graph. Not a very scientifically convincing solution to the 2000-2010 flat temperature problem.

    [DB] Fixed bad html.

  14. JamesWilson #13: "So there the temperature went up from 1970 to 1990." No, it said "late 1970s to 1990s". You should know this because that's what you quoted from the article. That is not 1970-1990. And it didn't say that warming stopped after the 90's, btw, it said "the rate of global warming in the last decade or so has slowed." "Here it was cool and then went up in 1985." No, that is not what this article said: "The global dimming trend reversed around 1990 - 15 years after the global warming trend began around 1975. So it can't explain what began the global warming trend." See? The warming started in about 1975 in both articles. The trend that reversed in the 80's was the global dimming trend. There is no conflict.
  15. Can we get this clear? Climate responded to sum of all the forcings. Graphs of just one arent useful by themselves. Volcano aerosols are also important. Please read the Lean and Rind paper.
  16. If you add the confidence intervals for the radiative forcing due to aerosols presented in the 4AR, you get approximately {-2.5,-0.5}W m^-2. This seems to account for the majority of the uncertainty of the total RF. To what extent has the aerosol uncertainty reduced since 2006/7?
  17. Does a higher atmospheric aerosol ( in the cloud condensation neuclii size range ) content result in a dryer atmoshere? Considering the particle size of dirty emissions between 1940 and 1970 and the cooling during that period, then the concerted effort to clean up emissions post 1970 coinciding with a rise in atmospheric water vapour content, should this correlation be considered as being part of the cause of the post 1970's warming or 1940 - 1970 cooling?

  18. @ 7, first graph. To get a better picture of how changes in emissions between the 70's and more recent years would require a profile of the changing emissions particle size over that period. Below CCN size, smaller average particle size would result in longer residence time in the atmosphere, while they grow in size, raising the opportunity for the particles to bypass water vapour to higher altitudes leaving a higher WV content at lower altitudes. An examle of how raising emission standards can affect particle size can be seen in the following study " Measurement of Engine Exhaust Particle Size" by David B. Kittleson, ( pages 7 and 8 in particular ) where they describe the effect of new standards ( introduced in 1990 ) on deisel emissions.

  19. carbtheory,

    Most atmospheric aerosol is secondary in nature, meaning it is formed in the atmosphere through gas-to-particle conversion, not via direct emissions. Most direct emissions are coarse mode emissions of sizes >2-5µm, which have relatively short atmospheric lifetimes. Accumulation mode particles, which contain most CCN, have longer lifetimes, but are formed through gas-to-particle conversion, mostly involving sulfur dioxide (and resulting sulfuric acid) and ammonia. Thus, their abundance is largely driven by SO2 and NH3 emissions. The first is dominating the graph you refer to. SO2 emissions, dominated from coal combustion, have dropped since the 80s, but have risen again in the 00s due to coal combustion in East Asia.

    The net effect of these aerosols is cooling because they increase Earth's albedo.

    btw, CCN are not defined by size, but, as the name "Cloud Condensation Nuclei" says, they have the ability to take up a lot of water ultimately growing/leading to (cloud) droplets. Their existence (or increase) does not lead to drying in the atmosphere, but to a redistribution of cloud water content from larger to smaller droplets, also known as the Twomey effect, which makes clouds "brighter" (as seen from space), which has a cooling effect (search "ship tracks").

    The emisisons standards induced changes most likely have only a small and regional/local effect. The biggest large-scale changes come from how much coal is burned and what end-of-the-pipe technology is used to clean the stack gas from SO2.

  20. @ 19, first sentence, quite believable.

    Second sentence: take a look at page 3 of the study I refered to, by mass the majority of emissions are between 0.1 um and 1 um. By number the majority are substantally smaller than 0.05 um for typicle deisel engine particle size. So, with an extremely high number weighting in what they describe as nueclii mode particles where does that leave us for potential CCN from those emissions once they start mixing with the atmosphere? This is for an internal combustion engine. However petroleum accounts for a higher % energy usage than coal.

    Third sentence: "Accumulation mode particles", the name suggests they are growing in size, mass or both. Wikipedia state a 'typical' CCN size of 0.2 um. I'm under the impression that the accumulation mode particles that become CCN need to reach a minimum size which is dependant on how hydroscopic they are ( due to their chemical composition ) and the atmospheric conditions they exist in. Is this assumption correct?

  21. carbtheory @20, first of all, you should not place all your argument on one publication, even if it turns out correct. That said, I presume you are talking about this presentation, given 14 years ago:

    It shows on said page three (not including the front page), a "typical" diesel engine exhaust PM mass and number distribution. These (ultrafine and fine) particles are actually generated from gas-to-particle conversion, often inside the exhaust pipe. And yes, such traffic related emissions are responsible for a significant amount of fine PM in urban areas (not globally). The, most abundant in terms of numbers, ultrafine particles coalesce within hours to at most days into the accumulation range (the size range in the middle of that graph, labelled). They do not act as CCN themselves.

    The accumulation  mode is named such because that is where atmospheric PM "accumulates" after coalescence (i.e. from smaller particles) or deposition (i.e. removal of larger particles) over time. Due to steep number reductions (do not mistake emissions with ambient abundances), they do not significantly grow by coalescence any more, but mostly by condensation, i.e. more gas-to-particle conversion, which limits the overall growth rate and total size achievable. Their lifetime in the atmosphere is on the order of a week and thus they do not mix throughout the troposphere.

    And yes, not all accumulation mode particles are good CCN. They have to be hygroscopic. Köhler theory describes what you are looking for. You can try this also.

    The "energy usage" is not what matters in terms of how much PM is formed. Sulfur content is much more relevant. That is why it was mandatorily reduced in diesel a few years ago.

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