Climate science and Hansen misrepresented – aerosols disguise real global warming

[Published on The Drum 20 May]

Recently climate sceptics Anthony Cox and David Stockwell published an opinion piece on the ABC’s The Drum claiming climate scientist James Hansen had “admitted” climate models have been wrong, and that human-caused global warming was therefore in doubt.  The article contains basic misrepresentations of Hansen and of the substance and implications of a draft paper by Hansen.

The Hansen paper does not weaken the case that humans are the main cause of global warming.  On the contrary, it suggests we have unwittingly and temporarily shielded ourselves from the full effects of our activities.

Professor James Hansen, of the NASA Goddard Institute for Space Studies, recently (18 April) posted a draft scientific paper (pdf, 1.5 Mb) for comment.  The paper argues that most computer models of climate have over-estimated the rate at which heat is being absorbed by the oceans.  Because there is a trade-off in the models between heat entering the oceans and heat being reflected back into space by aerosols in the atmosphere, the implication is that the cooling effect of aerosols has been under-estimated.

This is not good news, because it implies aerosols have been partially shielding us from the full warming effects of carbon dioxide emissions.  As and when aerosol pollution is cleaned up, global warming will get worse.

The Cox-Stockwell commentary misrepresents Hansen’s role and his intention in posting the draft paper.  It uses prejudicial language.  It claims Hansen merely assumed the warming effect of greenhouse gases is known, when in fact Hansen has clearly spelt out the independent evidence for this.

The commentary begins by claiming Hansen “published” his draft paper “free from the restraints of peer review” on a personal web site, implying that Hansen was engaging in a sly trick.  In fact Hansen was quite clear that the paper was a draft and that critical comment was welcome.  Posting is not at all the same as publishing.

The commentary characterises Hansen as “the world’s most prominent expert on the use of computer models for understanding of the Earth’s climate”, which gives the impression that Hansen is a great believer in the accuracy of computer models.  In fact Hansen’s group is not one of the main computer modelling groups, and Hansen is quite clear, in his book Storms of my Grandchildren, that computer models are a useful tool to improve understanding but are not to be exclusively relied upon.

Cox and Stockwell repeatedly characterise the Hansen paper as a “frank admission” and a “concession” that climate models have had a significant inaccuracy.  In fact Hansen is just doing normal science.  He is trying to improve our understanding of climate and to improve the accuracy of climate models.  All climate modellers know there are inaccuracies and poorly-constrained factors in the models.  There has been no conspiracy to exaggerate the models’ accuracy and there is nothing to be “admitted”.  Nor is there any implication that the case for human-caused global warming is weakened (more on that later).

Cox and Stockwell erroneously characterise a principal conclusion of the Hansen paper when they say that the models’ inaccuracy “has the effect of exaggerating the net AGW [anthropogenic global warming] in models”.  AGW refers to the rise in temperature (the warming), and there is in fact no implication that the models have exaggerated the projected rise in temperature.

Rather, the implication stated by Hansen is that, in the models, “the corresponding net human-made climate forcing is unrealistically large” [emphasis added].  The forcing refers to the extra energy trapped in the atmosphere, not to the resulting temperature rise.  The two major human contributions to the energy budget are from carbon dioxide, which is positive and accurately known, and from aerosols, which is negative and not accurately known, as Hansen has been saying for decades.  In the models too much heat energy has been absorbed by the oceans, so not much aerosol cooling was implied.  Hansen argues that in reality less heat is going into the oceans so the aerosol cooling must be larger and the net effect (carbon dioxide warming effect minus aerosol cooling effect) smaller.

Mainstream scientists and sceptics alike have been aware that the aerosol effect is complex and not accurately known.  Hansen has for decades advocated a dedicated satellite to measure the aerosol effects, but this was not attempted until the past year, and the launch failed.  Lacking direct measurements, the magnitude of the aerosol effect has been inferred after other energy terms have been accounted for.  The total energy is implicitly constrained by requiring models to match the observed temperature record of the past century.

It is a widespread misconception, evidently shared by Cox and Stockwell, that the case for human-caused global warming rests mainly on computer models and is therefore vulnerable to the aerosol uncertainty.    This misconception is part of the disinformation put about by the professional deniers funded by the likes of ExxonMobil.

In fact Hansen’s main argument is based on paleoclimate records, records of past climate variations extracted from ice and rocks.  Cox and Stockwell would know this if they had properly read Hansen’s book, and a more recent paper also listed on Hansen’s web site (Paleoclimate Implications for Human-Made Climate Change, January 18).

During the ice ages the global mean temperature rose and fell by about 5 degrees Celsius every hundred thousand years or so.  The measured variations of temperature and atmospheric carbon dioxide during the ice ages accurately constrain the so-called climate sensitivity to 3 degrees per doubling of carbon dioxide, plus or minus about 0.3 degrees.  Because this number is based on a natural experiment, it accounts for many complicated subsidiary effects and feedbacks, such as the changes in water vapour and methane content in the atmosphere and changes in cloud cover, that accompany changes in temperature.

A side issue here is another widespread misconception, that carbon dioxide cannot be causing global warming because during the ice ages temperature changes occurred several hundred years before carbon dioxide changed.  The reason for this is well understood, if not widely appreciated outside the climate science community.  During the ice ages the trigger for temperature changes was small changes in the heat received from the sun.  On their own these would have caused less than a degree or so of temperature change, but temperature rises cause carbon dioxide to be released from the ocean and soils, and this greatly magnifies the effect on temperatures.  On the other hand the present warming is being triggered by human releases of carbon dioxide, not by variations in the sun’s heat, so the carbon dioxide rise is leading the temperature rise, rather than lagging behind as it did in the ice ages.

Thus Cox and Stockwell are incorrect when they claim “the effects of CO2 and feedbacks are also unmeasured”, and go on to propose that “another way to bring the energy into balance might be to reduce the presumed warming effect of greenhouse gasses”.  Rather, the warming due to greenhouse gases (carbon dioxide, methane, etc.) is well constrained by paleoclimate, satellite and laboratory measurements (not all of which I have recounted here), and the net energy forcing is constrained by the observed temperature record of the past century, which includes the significant warming of the past few decades.  The conclusion that human emissions are causing most of the recent warming remains strong.

The other recent paper of Hansen’s mentioned above, on paleoclimate, gives even more reason to be very concerned about our present situation.  Hansen notes that a couple of the past interglacial periods were very close to the present temperature (which has been raised about 0.6 degrees by human causes) and that sea level was then several meters higher than at present.  This implies that if the present temperature were maintained we would suffer a serious sea level rise, probably over the next century or so.

Of course the temperature will rise by several degrees more if we do not quickly change our ways.  Paleoclimate records from 3 million years ago show that the temperature was only 1-2 degrees Celsius higher and sea level was around 25 meters higher than recent pre-industrial levels.  A sea level rise of this magnitude, occurring at rates of meters per century, as observed in paleoclimate records, would destroy many coastal cities and much low-lying land.  It would be very difficult to rebuild and maintain port infrastructure.  Along with major changes in climatic zones, storms, vegetation, and food and water supplies, it is hard to see how our industrial civilisation could survive in anything like its present, globally connected form.

Thus Hansen’s recent work warns us that we seem to be much close than previously realised to a threshold above which global warming and sea level rise will run out of control, and the world will tip into a quite different climate regime.  This is why he argues that the temperature rise should be limited to only 1 degree more, not the 2 degrees commonly assumed by  politicians, and brought down again as quickly as possible.  This requires the atmospheric carbon dioxide to be brought down as quickly as possible from the present 390 parts per million to no more than 350 parts per million.  If aerosols have been limiting the warming we are causing, then the task is even more urgent.

The writings of climate “sceptics” typically betray a patchy and incomplete understanding of climate science, and the Cox and Stockwell commentary is no exception.  This is symptomatic of a tendency to cherry-pick the science for uncertainties they hope will discredit the main conclusions.  Their poor scholarship does our society a grave disservice.

16 thoughts on “Climate science and Hansen misrepresented – aerosols disguise real global warming

  1. Ronald Bastian

    Geoff –

    Understanding clearly my own limitations of this particular subject I immediately ask myself, if “sceptics” such as Cox and Stockwell, including the plethora of “others” who appear on the world stage spruiking similar messages, did not choose to challenge the “science”, then what/who would they be?….I respectfully suggest that they would never be heard of in scientific circles or any other platform of significance. “Merchants of Doubt”, a book by Naomi Oreskes explains the tactics and skills used by skeptics/denialists over many years which challenged the scientifically proven effects of tobacco and climate change, as well as anything else that might reduce the profits of their investors.
    Debate is necessary and healthy on all matters of national and global significance, however, it is unforgiveable to extract select pieces of information from any published (or unpublished) works to plant the seeds of doubt in the minds of those trying to make a decision on what to believe, but this is the aim as” Oreskes” suggests, it is a skill and a game played to ultimately confuse the masses and cause chaos regardless of how much damage it causes to the well-intenioned efforts of Governments and the future health of the planet. Our current media feed off these egotists, hanging off every word just to flog their product with “sensationalism” as their only salespitch, truth has little value it seems to those who choose to flog misery…………….


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  3. Amy Mullins

    Congrats on your Drum article today Geoff. I’m no expert on the science, but I’m really glad people like you are countering the nonsense that so easily gets accepted by the public as truth. If we all keep chipping away at it, I’m sure we’ll get there eventually 😉


  4. Geoff Davies Post author

    Thanks Amy. I’m pleased to see the level of positive and constructive response it’s getting too.


  5. Gordon & Suesie

    I like the way you think. Keep up the good work.

    And while I haven’t read all your material yet and so maybe my suggestion will be void, can I suggest you try to explain, simply and with clarity that science is not about “proving” anything. Science merely improves the likelihood of a hypothesis (or disproves it). I note so often people talk of proof being required before belief. This way of thinking is seriously limiting to be polite!
    You have touched on this point within some of your other material but I feel it deserves a focussed attempt to correct the misconception of so very many people that science “proves” this or that.


  6. Geoff Davies Post author

    G&S –
    I totally agree. Chapter 6 of Economia develops this point at some length. Not sure I’ve spelt it out much on this blog. Perhaps sometime.


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  8. Harquebus

    Nothing posted to the ABC was abusive, that’s the problem. Nothing was posted.
    Climate change is not mankind’s greatest threat. Oil depletion is. We ain’t gonna feed seven billion people.
    There is no such thing as renewable energy. Renewable energy generators only provide diminishing energy returns.
    There were only three things we could’ve and should’ve done. Depopulated, consumed less using quotas, not taxation and planted lots and lots of trees. Everything else is a con and the survivors are going to get tonnes of free stuff.
    That’s it in a nutshell. Wish I could’ve debated you properly at the ABC.
    Sorry about the brain in the bum bit.
    Do you have an article on peak oil?


  9. Paul W

    The ABC blog post you made was great. The distortion and fraud involved in the post from Cox and co needed a response.

    The responses from the people who had an understanding of climate science started as sincere and I found interesting.

    What was remarkable was the break with reality, objective data and science that some of the climate change deniers made as the responses developed!

    What I think you have achieved now is a clear demonstration of the “aboration depth” or commitment to fantasy that became apparent with the climate change deniers.

    I’ve spent 30 years leading counselling groups that men and women to work through issues limiting their lives. Only a small number of times have I seen the kind of delusional commitment to unreality that is now displayed in the responses to your article!

    I guess that at points where intense politics have had realities colliding such public displays of distorted stuff have been similar. The military dictatorships of south america, second world war, cold war anti communism.

    We have a challenge ahead!


  10. Sundance

    It is clear to me that they identify that a doubling of aerosol cooling would be necessary to compensate for the lack of warming . It’s right here:

    “As aerosol cooling in GCMs is typically less than 1 Watt per square meter, if the heating due to CO2 is kept constant, the rate at which aerosols are cooling the planet must be doubled in order to compensate for the reduction in ocean mixing.”

    Those that understand the science know it isn’t that simple though. As NASA’s Shindell indicated in their paper on aerosols, 1/2 the current Arctic warming is due to aerosols. Aerosols can warm or cool the planet and in 2009 measured evidence indicated that the cooling feedback for aerosols was overstated and that aerosols were really warming the planet.
    The ratio of large to small aerosols (higher ratio means more warming) is 2 to 8 times larger than the modeled ratio based on recent analysis. Hansen has not accounted for the added heat from larger ratio aerosols which would diminish the amount of recent warming caused by CO2. Hansen doesn’t know for sure and that is why the Glory mission was so important which is discussed here.

    “While Hansen argues that energy balance can be achieved by increasing the presumed cooling effect of aerosols, he notes that: “climate forcing caused by changing human-made aerosols is practically unmeasured”. Sadly, as Hansen notes [page 46], the Glory mission satellite that would have provided that capacity to measure the effect of aerosols failed to reach an Earth orbit earlier this year.”

    Just to reiterate Hansen doesn’t know the positive feedback of the warming aerosols or the cooling but recent research indicates that the positive feedback is greater than the negative feedback from aerosols.

    The next part is extremely complicated and so far the only confirmation I’ve seen on how much potential cloud forcing has, is based on data from the cloudsat satellite on NASA’s A-train satellite array. The NASA A-train data measured a 16% reduction in Arctic cloud cover in 2007 which was calculated to have been the “primary” factor of the record ice minimum in 2007. More study needs to be done on clouds.

    “The indirect effect of aerosols on clouds may be the missing factor. Hansen admits: “Indeed, most IPCC GCMs exclude indirect aerosol forcing, ie, the effect of human-made aerosols on clouds.” Some argue that clouds dwarf the radiative forcing of CO2.”

    All in all I find the Drum article to be less offensive than you do. I did review the entire 53 page paper by Hansen and recommend it to anyone who has studied the radiative properties of Earth’s energy budget. One last note, in 2009 CO2 was determined to be responsible for 43% of warming and black carbon accounted for 12% of warming as was candidly revealed on page 5 of this Newsweek article.
    Based on more recent aerosol research the 2009 figure of 12% contribution to recent warming by black carbon and other aerosols is likely far too low.


  11. graemebird

    “….The reason for this is well understood, if not widely appreciated outside the climate science community.� During the ice ages the trigger for temperature changes was small changes in the heat received from the sun.� On their own these would have caused less than a degree or so of temperature change, but temperature rises cause carbon dioxide to be released from the ocean and soils, and this greatly magnifies the effect on temperatures……. ”

    Stop right there. There is just no evidence for this. This is a brazen lie. Any graph you have looks more like CO2 is causing a negative feedback.


  12. Geoff Davies Post author

    Graeme –
    If you want to post on this blog you will have to treat others here with respect. That means granting that they have reasons for their views and they are not dishonest. Any more accusations like “brazen lie” and you will be banned. Also any direct insults to others will get your comment deleted, and you will be banned if you persist.

    If you want to make arguments and state your evidence you are welcome to comment.

    I would not have made those statements if there were not evidence. I can’t imagine how you infer negative feedback from a graph like Figure 4 in the Hansen paper, which shows temperature, CO2 and sea level swinging up and down pretty much together. That translates to a clear positive correlation.


  13. graemebird

    You don’t have any evidence since a disputed model cannot be shown to be true by reference to itself. The CO2 is simply assumed to be the missing factor. Why? Because their model assumes that the CO2 is doing the amplifying. But other things could be doing the amplifying, why CO2? Why do we assume amplification in the first place when joules can accumulate and decumulate?

    I can hold my tongue should you decide to lift your game.


  14. Geoff Davies Post author

    Graeme – here is a brief summary of the line of argument as I understand it.

    The physics of CO2 causing greenhouse warming of the atmosphere is well-established, based on the atmospheric temperature profile, re-radiation, CO2 absorption bands and the way CO2 modifies the temperature profile and re-radiation. The question Hansen is addressing is whether other things are also affecting the heat budget, such as methane, water vapour, clouds, aerosols etc. The ice ages give us a natural experiment in which water vapour, clouds, natural aerosols etc. do all their complicated things and we can see the net effect.

    That is a sensible hypothesis and Hansen is pointing out the result of that interpretation. As always in science, you can dream up other hypotheses to your heart’s content, but you need to test all their parts to establish them as plausible. There is already a substantial body of tests of the parts of Hansen’s story that makes it plausible.

    Hansen is also arguing that artificial aerosols may be cooling us much more than previously realised. If so, the underlying global warming is stronger than previously realised, which is potentially extremely dangerous.

    Science is never about truth and “proof”. Hansen’s interpretation of the evidence (many kinds, he’s unusually well-versed in diverse topics) is plausible and cautionary. Given the dire consequences if he’s right, we are wise to pay attention.

    Other scientists will (and do) critically examine the parts of his story. Many think he may be overstating the case or simplifying some things too much. Nevertheless his work has stood up very well over the long term (despite heated assertions on blog sites). Most pertinently, his prediction in the 1980s that the greenhouse effect on temperature would emerge from the noise around 2ooo was correct. If you have serious criticisms with a sound basis, you can join in the critical examination of his work. Yelling on blog sites is not very constructive in the long run.


  15. graemebird

    “The physics of CO2 causing greenhouse warming of the atmosphere is well-established, based on the atmospheric temperature profile, re-radiation, CO2 absorption bands and the way CO2 modifies the temperature profile and re-radiation.”

    I absolutely disagree. I hear skeptics confirming this view but they have been so put upon, that if it sounds reasonable to them they don’t want to appear churlish. The reasoning is for black bodies and phantom gas atoms/molecules that don’t move. The inference comes from a comparison with an anomaly that we cannot even so much as attribute to greenhouse or “backradiation”. The 33 odd degrees anomaly is held to be mostly due to greenhouse for god-of-gaps reasons.

    Even the most basic assumptions in this debate are wrong. Even the bipartisan basic assumptions are baseless. The natural world laughs at us when we agree in error.



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