Saturday, November 29, 2014

The Known Knowns of Climate Change

POTSDAM – The philosopher Daniel Dennett once compared science to the construction of a huge pyramid. Its base comprises the mass of well-established knowledge – no longer controversial and seldom discussed outside academia. More recent research is piled toward the top of the pyramid, where most public debate takes place. It is an apt metaphor for climate-change research, and one worth bearing in mind with the publication of the latest report of the United Nations Intergovernmental Panel on Climate Change (IPCC).

The IPCC’s fifth report, the product of several years of work by hundreds of climate scientists around the world, reviews our established understanding of climate change and explains more recent findings. The media understandably tend to focus on the latter – like the much higher sea-level rise predictions compared to the previous IPCC report of 2007. But let us step back from the news cycle to look at the solid knowledge base of our pyramid.

Climate research dates back at least two centuries, to Joseph Fourier’s discovery of the effects of greenhouse gases on planetary climates; in 1859, John Tyndall demonstrated in his laboratory which gases cause this effect. Detailed radiation measurements on the ground and from satellites have since proved the greenhouse effect’s existence.

We also know beyond doubt that emissions from human activities have substantially increased the amount of greenhouse gases (especially carbon dioxide) in our atmosphere. When the first IPCC report was published in 1990, the concentration of CO2 in the atmosphere had reached 354 parts per million (up from the preindustrial baseline of 280 ppm). This year, the concentration of atmospheric CO2 crossed the 400 ppm line for the first time. CO2 levels are already far higher than they have been in a million years, as ancient air bubbles trapped in the Antarctic ice show.

We know that the amount of greenhouse gases is rising due to our emissions, and we know that this is causing warming. But how much? The most telling number here is the “climate sensitivity” – that is, the degree of global warming caused by a doubling of atmospheric CO2. IPCC reports have consistently given a range of 1.5-4.5ºC (with a minor exception in the fourth report, which gave a range of 2.0-4.5ºC). Natural climate changes in the past – for example the great ice ages – attest to the sensitivity of our planet’s climate to disturbances, and analyzing them is one method used by scientists to estimate this sensitivity.

An extraordinary, if underappreciated, feature of the IPCC’s reports is that, though many different scientists have worked on them over the past 23 years, the fundamental conclusions have not changed. This reflects an overwhelming consensus among scientists from around the world. Polls of climate researchers, as well as analysis of thousands of scientific publications, consistently show a 97-98% consensus that human-caused emissions are causing global warming.

Yet these conclusions need to be reaffirmed in the face of efforts by well-funded special-interest groups to sow doubt among the public. Indeed, these efforts have been so successful that few members of the public are aware of the scientific consensus on the fundamentals of climate change. Many believe that there is controversy where there simply is not.

The past can serve as a guide to the consequences of the warming that we are causing. Scientists studying paleoclimate – the climates of the ancient past – have documented the massive impact of earlier climatic changes. At the end of the last Ice Age, for example, global temperature rose by about 5ºC over a period of 5,000 years. This was enough to transform the Earth’s vegetation cover, melt two-thirds of the continental ice masses, and raise sea levels by more than a hundred meters. Slowly but surely, sea levels are inching up once again. A key conclusion of the new IPCC report is that sea-level rise has accelerated.

But, before millions of people are submerged, many will be struck by extreme weather events. Record-breaking hot months now occur five times more frequently than they would in a stable, unchanging climate; these heat waves cause droughts, wild fires, poor harvests, and, inevitably, loss of life.

The latest IPCC report describes our current predicament with disturbing clarity: global temperatures are climbing, mountain glaciers and polar ice caps are melting, sea levels are rising, and extreme weather events are becoming more frequent and more severe.

The details near the top of the knowledge pyramid can and should be intensely debated. But our solid understanding of the fundamentals of global warming – the base of our knowledge of climate science – should provide reason enough to press on with the implementation of carbon-free energy technologies. With a rapid reduction in emissions, it is still possible to keep warming within safe bounds (estimated at below 2ºC); but the task is becoming increasingly difficult. Failure to act quickly and globally will leave our children and grandchildren struggling to adapt to rapidly rising seas and devastating weather.

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    1. CommentedWayne Davidson

      The paradox that is climate change will finds its denouement only in catastrophe, there will be no immediate or final solution, only the tipping point of unimaginable consequence. There is no dichotomy of deceiver and deceived, only the human condition for self-deception.

    2. CommentedDavid Goldstein

      Dear Dr. Rahmstorf: Greetings from Eugene, Oregon. My name is David Goldstein and I am a climate activist and writer. Thank you for your article. Since you are in my 'pantheon' of hero climate scientists, I want to enclose a copy of my latest article if you would like to give it a quick read: Thanks again!

    3. CommentedLeo Arouet

      Congratulations... a great article. Me ha gustado el recuento sobre los conocimientos sólidos que se ha adquirido respecto al cambio climático. Los lobbies de las empresas petroleras y demás magnates del carbón son inmensos y eso dificulta obstaculiza y degrada el juicio justo sobre la contaminación y del cambio climático.

    4. CommentedAndrew Urban

      The obvious reason for the deep divide on this subject is that the IPCC offers modeling which cannot be tested for decades ahead. What little modeling is now test-prone has been shown to be inaccurate. Science is a mode of enquiry, and only the base of the scientic pyramid can enjoy genuine consensus (although even there occasional revelations can surprise). The consensus of which Mr Rahmstorf speaks is itself in dispute: the "97 per cent of climate scientists say…." meme – is expertly covered in a new paper for the Global Warming Policy Foundation by Andrew Montford.

      My view is that the debate should be re-framed: never mind trying to reduce emissions, it's futile and very expensive. Better to engage in highly resourced research which should include more on nuclear energy potential. Whatever the reason for finding renewable energy, we can all agree on wanting it and stop wasteful and acrimonious argument.

        Portrait of Stefan Rahmstorf

        CommentedStefan Rahmstorf

        Being part of the scientific community for over twenty years now, I can assure you from my own experience that the overwhelming consensus on the basic science of global warming does indeed exist, as has been shown by several surveys published in the peer-reviewed literature. But of course you are free to believe the claims by a lobby group, if those conform better to your world view.

    5. CommentedStamatis Kavvadias

      Not really convincing, because of great contradiction in data that appears in the press, and cryptic "scientific" language.

      The author states
      "Slowly but surely, sea levels are inching up once again. A key conclusion of the new IPCC report is that sea-level rise has accelerated."

      In project syndicate's article by Bjørn Lomborg we find:
      "For sea-level rise, the IPCC now includes modeling of glacier responses of 3-20 centimeters, leading to a higher total estimate of 40-62 cm by century’s end – much lower than the exaggerated and scary figure of 1-2 meters of sea-level rise that many environmental activists, and even some media outlets, bandy about."

      Also, the author says "polar ice caps are melting," but elsewhere ( we find images of 60 per cent increase of north polar ice caps!

      I am sure these are all facts that *do not contradict*. But, really, can even the scientists make a meaningful case of these data? Is there a measure or vector of "total impact" that actually deteriorates?

      It cannot be the case that there is not more serious science than the two articles I mention, from project syndicate's otherwise esteemed forum.

        CommentedStamatis Kavvadias

        Prof. Rahmstorf, thank you for your reply.
        I apologize for my extreme expression of discontent before.

        Thank you very much for the links you provide, especially and (Fig. 3)

        I really believe, if those had been referenced in the article, there would have been much less controversy. If scientific data is as much as you say, their reference would help very much the public discussion, provided absence of contradictory sources.

        I also feel compelled to point out that some of your arguments are oversimplifying, and do not really convince of the bottom-line conclusion --at least not broadly. For example, in the text on CO2 you write:

        "We also know beyond doubt that emissions from human activities have substantially increased the amount of greenhouse gases (especially carbon dioxide) in our atmosphere."

        Then, you present increased levels of CO2 and do not refer to why these increases should be accounted to humans, despite your original claim. Instead, you could reference the work on isotope measurements that I find in the comments, and any more recent measurements about it than the 1960's work you mention.

        Nevertheless, thank you for your article. Eventually, I feel I learned more concrete things on climate change than ever before.

        Portrait of Stefan Rahmstorf

        CommentedStefan Rahmstorf

        Firstly, Lomborg does not cite the full range of sea-level rise given in the upcoming IPCC report - check it out next Friday when it is public. And Lomborg is not a climate scientist but someone who has a long history of downplaying sea-level rise. I am a scientist studying sea-level rise and have published several papers on the topic which are amongst the most-cited in the scientific literature. You decide whom to trust.

        I called Lomborg out a couple of times on this in the Guardian, see:

        The data have proven me right: Lomborgs claims of a slow-down of sea-level rise were simply unfounded and wrong. You can always look at the current sea-level data here:

        Your other point on Arctic ice: just look at the original data please, rather than what a tabloid newspaper is telling you about it: (Fig. 3). Firstly, the Daily Mail number was just wrong. And secondly this year's sea ice cover is only an increase when compared to last year's record low. This year's sea ice cover is less than in any year before 2007 - it simply is part of the long-term sea ice decline, which of course is superimposed by year-to-year variations. Again my plea: get your information from reliable scientific sources if you are intersted in what is really going on.

        CommentedBrian Hoffer

        In answer to the issues Stamatis Kavvadias brings up:

        Lomborg is comparing the IPCC's latest projection on sea-level rise to a figure that "many environmental activists, and even some media outlets, bandy about," with a link merely to his Facebook page as a citation. The IPCC's first projection in 1990 was for a rise of 6cm per decade, with an uncertainty in the range of 3-10cm per decade. The latest projection still falls within that range. This comparison is a classic Lomborg straw man: politely ridiculing the science in the consensus IPCC report by holding it against what "many" say.

        As to the Arctic icecap, the Daily Mail conveniently fails to mention that the 60% increase in 2013 over 2012 still leaves the icecap coverage 20% lower than it was in 1978, when NSIDC's record-keeping on the extent of Arctic ice began.

        So, yes, scientists are very much able to make a meaningful case for such data, and they do so quite lucidly in the IPCC assessments and in the NSICD's annual arctic ice extent reports. The question should be, can Bjørn Lomborg and media outlets like the Daily Mail be trusted to do so?

    6. CommentedMike Nelson

      The new IPCC report dials down the risks associated with global warming and concludes that climate models have been too pessimistic. That is hard to admit if the funding for your research depends on perpetuating climate alarmism. This article is not written by an independent analyst but by someone who is dedicated to proving that global warming is man-caused and that it is deadly. What is truly concerning is that all dissent has been crushed by the climate dogmatists to the point that no research can be published that differs from the prevailing dogma. This article clearly states this. The rigid enforcement of monolithic academic thought means that true science has been replaced by ideological conformity. What is sad is that the academics are completely unaware that this is plainly seen by objective outsiders. The king has no clothes.

        Portrait of Stefan Rahmstorf

        CommentedStefan Rahmstorf

        Nice conspiracy theory, but (a) it is rather unrealistic: trying to get a bunch of scientists to agree on anything is like herding cats. Thousands of academics from many countries and with all kinds of political views, all conspiring to deceive the public? And (b) it is self-contradictory: the IPCC report is written by climate scientists, so why should a climate scientist like myself not want to admit what the IPCC report says? I think it is an excellent document.

        And re it "dialling back" - have you read it, or have you just heard media reports, some of which have grossly misrepresented it? You should read the original, it will become public next Friday. I have actually read it, and I recommend this New York Times article for a decent summary of what will be in it:

    7. CommentedKir Komrik

      Thank you for your take on this,

      You wrote:
      "We know that the amount of greenhouse gases is rising due to our emissions"

      How do "we" know that? My concern isn't whether it is occurring, just the anthropogenic part.

      "The past can serve as a guide to the consequences of the warming that we are causing."

      Are we?

      Primary sources with data and methodology would probably be most useful.

      "...solid understanding of the fundamentals of global warming..."

      Is it?

      Given the circus act at East Anglia/PA and the cheap watercolor "Hockey Stick" I don't know what to believe anymore, but good references to real data would help.

      I certainly would not, as an objective reader, feel that you've made a case for the "known knowns" if all the statements regarding causality (only 2) are naked assertions with no data or references to _existing_ data (or, in this case, any data at all). It may be that I'm partial to the contrarian view because I have such an aversion to ideology and histrionics, and this subject seems to have an overdose of it.

      On the other hand, if it is "our fault" then it might not matter because we've just about used up all our petroleum tokens anyway. ;-)

      Realistically, and for now, I do agree that we should anticipate these changes and work to mitigate them to the extent we're able, whatever the cause.


      - kk

        Portrait of Stefan Rahmstorf

        CommentedStefan Rahmstorf

        Dear Kir, I checked this, and think you mixed up Mike Mann with Phil Jones, about whom "climate skeptics" indeed made such claims. They are bogus, though: this was about the weather observations that go into the CRU global temperature series, and these primary data of course come from the weather services of the various countries, who are charged with collecting and archiving them. An overview over this and other myths and falsehoods spread by "climate skeptics" was compiled by the media watchdog organisation Media Matter for America here:

        CommentedKir Komrik

        Thanks Stefan,

        "Your statement about destroyed primary data is bizarre - where did you get that from?"

        Emails written by Michael Mann. I'll go back and look over those emails, which came out subsequent to that controversy with East Anglia and the Wikileaks publication of emails. In those emails Mann refers to the data used as destroyed primary source data and that he would publish results anyway. I'll look into this again and see if I can figure out what is going on. See what I mean? Why does the general public have to spend this much time and effort trying to validate basic information? The emails came from Wikileaks, the emails were from Michael Mann. Something is indeed bizarre here.

        The primary source material you reference is just a copy of that data, correct? I'm talking about _primary_ sources. Normally, this wouldn't be an issue, but as you can see, it apparently is for any lay person trying to assess it. If these emails are erroneous (I can post them at my blog if need be) then a true copy of the primary source material posted online would be a great benefit.

        Thanks again,

        - kk

        Portrait of Stefan Rahmstorf

        CommentedStefan Rahmstorf

        Dear Kir, we know that the anthropogenic increase in greenhouse gases in the atmosphere is by far the dominant climate forcing over the last hundred years. The increase in CO2 to date causes a climate forcing of 2 Watts per square meter of Earth's surface. Compare this e.g. to solar variations, which have a peak forcing (difference between solar max and solar min) of only 0.2 Watts per square meter. (And this is a cycle, not a trend.) All forcings, natural and anthropogenic, are always compared in one of the very first diagrams in the IPCC reports - have a look at the upcoming one as soon it is published, or at the last one, you find them online at

        Your statement about destroyed primary data is bizarre - where did you get that from? The primary paleoclimate data used in the hockey stick are available here:
        A list of many further data sources is found on the Realclimate website here:
        NASA maintains a nice overview site with key climate indicators (CO2, temperature, sea level, sea ice , land ice):
        There is probably no field of science were so many data (and computer codes for their analysis) are freely available to everyone as in climate research. (Or do economists lay open their data and model codes?) Don't get confused by the political propaganda found in some media. All the information you need is available freely from reputable scientific sources.

        CommentedKir Komrik

        Stefan, thanks for that great reply,

        I guess what I was really asking about with the CO2 isotopic measurements was more about causality: yes, we know CO2 is being emitted but is its contribution to climate change beyond negligible? Are there other causes? What is the individual impact of all causes?

        I particularly appreciate the second part of your reply as I do feel like I'm constantly playing defense on the data and don't know who to believe. You are right, there is a lot of zealotry out there; and for all sorts of causes.

        I have been led to believe that the primary source data cited by East Anglia and the University of PA was all destroyed and that the hockey stick and virtually all the anthropogenic source data was based on it. Is this accurate?

        It concerns me that, if this is truly anthropogenic, some significant portion of CO2 must be coming from petroleum consumption. Limiting that will be painful. Transitioning to alternative power sources will be long and costly but the silver lining is that there is good reason to do it not just because of anthropogenic climate change.

        - kk

        Portrait of Stefan Rahmstorf

        CommentedStefan Rahmstorf

        Your question is worth responding to because it highlights two major problems in the public climate discourse: (1) lack of scientific knowledge and (2) the disinformation campaign by interest groups.
        (1) That the CO2 rise in the atmosphere is caused by human emissions is well established since the early 1960s from isotope measurements, as the isotope composition of cabon in fossil resources (coal, oil...) differs from that of carbon circulating in the natural Earth system. If isotope analysis is unfamiliar to you, you can also look at a simple budget calculation. We know how much we emit, and it is more than enough to explain the entire CO2 increase in the atmosphere. In fact, the accumulation in the atmosphere represents only 57% of our emissions. So there can be no question about this CO2 increase coming from somewhere in the natural Earth system, because to the contrary, the natural Earth system is taking up almost half of our emissions. This is all basic knowledge that you can look up e.g. at Wikipedia if you are really interested in knowing this: Or check out the carbon budget here:
        (2) What exactly is wrong with the "hockey stick" climate reconstruction in your opinion? It has been confirmed many times over by independent groups of scientists. The latest massive effort in paleoclimatology is the PAGES 2k project which has produced a global temperature reconstruction for the past two millennia - check out my article on it here: Their results look identical to the original "hockey stick" reconstruction of 1999. The only thing that is so special about the "hockey stick" is that it was subject to a massive disinformation campaign by politically motivated "climate skeptics" - but all their allegations have long been disproven. The authors of the original "hockey stick" reconstruction and their work are extremely highly regarded in the scientific community, as seen from the fact that they won important awards from the world's two largest geoscience organisations, the American Geophysical Union and the European Geosciences Union. Again, you can get solid information from Wikipedia if you are really interested:

        The key thing about the climate discussion is: be very careful where you get your information, since there are many who dislike the political consequences of confronting climate change and they are working very hard to cloud the facts.