How much is most?

A growing body of research is showing that humans are likely causing more than 100% of global warming: without our influences on the climate, the planet would actually be cooling slightly.

In 2007, the Intergovernmental Panel on Climate Change published its fourth assessment report, internationally regarded as the most credible summary of climate science to date. It concluded that “most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations”.

A clear question remains: How much is “most”? 51%? 75%? 99%? At the time that the IPCC report was written, the answer was unclear. However, a new frontier of climate research has emerged since, and scientists are working hard to quantify the answer to this question.

I recently attended the 2011 American Geophysical Union Fall Meeting, a conference of over 20 000 scientists, many of whom study the climate system. This new area of research was a hot topic of discussion at AGU, and a phrase that came up many times was “more than 100%”.

That’s right, humans are probably causing more than 100% of observed global warming. That means that our influences are being offset by natural cooling factors. If we had never started burning fossil fuels, the world would be cooling slightly.

In the long term, oscillations of the Earth’s orbit show that, without human activity, we would be very slowly descending into a new ice age. There are other short-term cooling influences, though. Large volcanic eruptions, such as Mount Pinatubo in 1991, have thrown dust into the upper atmosphere where it blocks a small amount of sunlight. The sun, particularly in the last few years, has been less intense than usual, due to the 11-year sunspot cycle. We have also experienced several strong La Niña events in the Pacific Ocean, which move heat out of the atmosphere and into the ocean.

However, all of these cooling influences pale in comparison to the strength of the human-caused warming influences. The climate change communication project Skeptical Science recently summarized six scientific studies in this graphic:

Most of the studies estimated that humans caused over 100% of the warming since 1950, and all six put the number over 98%. Additionally, most of the studies find natural influences to be in the direction of cooling, and all six show that number to be close to zero.

If you are interested in the methodologies and uncertainty ranges of these six studies, Skeptical Science goes into more detail, and also provides links to the original journal articles.

To summarize, the perception that humans are accelerating a natural process of warming is false. We have created this problem entirely on our own. Luckily, that means we have the power to stop the problem in its tracks. We are in control, and we choose what happens in the future.


38 thoughts on “How much is most?

  1. Thanks Kate. By the way, somebody pointed out to me today that in the figure above, the “G11” label should read “G12”. I’ll be correcting the figure tonight, for anyone anal enough to be bothered by such details :-)

    Sounds great. If the image file name is the same, it will get updated here too – let me know if it changes. Thanks for creating the awesome graphic! -Kate

  2. Paul, here is the Met Office’s response, which begins, “[The Daily Mail] article includes numerous errors in the reporting of published peer reviewed science undertaken by the Met Office Hadley Centre and for Mr. Rose to suggest that the latest global temperatures available show no warming in the last 15 years is entirely misleading.”

    Here is Deltoid taking David Rose apart on some earlier pieces:

    And NASA never said anything about the Thames freezing over. Rose just made that bit up.

    Finally, here is a chart of global temps from HadCRU:

    From this, it is pretty clear why Rose chooses 15 years as his starting point: 1997-1998 was the time of the largest El Nino ever recorded, resulting in a huge temperature spike. Using that as the starting point for a temperature comparison is absolutely classic cherry-picking.

    And in any event, you can’t say anything about trends in noisy data by simply comparing two arbitrary points. That is not a valid way to analyze the data (especially if you pick an obvious outlier as your starting point!). It is like trying to say whether the tide is coming in or going out by looking at the height of two waves. It just doesn’t work that way. You have to look at the long-term trend to remove the noise.

      • Sure.

        The chart shows the global temperature data since 1980 from HadCRU, which is a joint effort by the British Met Office’s Hadley Centre and the University of East Angli’s Climatic Research Unit–so it’s HadCRU, not FadCRU.

        There are several other global temperature data sets I could have used. They all show very similar trends, but I showed HadCRU because that’s the one referenced by the Daily Mail article you linked to.

        The red line is the monthly average temperature for each month from January 1980 to the most recently available, which is currently December, 2011. The green line is the least-squares linear trend for the same period. It shows that on average over that period, the temperature is increasing at about 1.5 degrees C per century.

  3. Regarding the WSJ article

    They misrepresented the work of William Nordhaus –

    “The piece completely misrepresented my work. My work has long taken the view that policies to slow global warming would have net economic benefits, in the trillion of dollars of present value. “

      • It’s from an interview with Andrew Revkin (also quoted in my post on SkS):

        Though Nordhaus tends to be conservative about the costs of climate change, he’s also always been clear that a price on carbon emissions will help the economy. So when the WSJ piece misrepresented his work as supporting inaction, he was not a happy camper, understandably.

      • dana1981
        Thanks again for your SkS post. 24 hours later, I am still slightly amazed to find out that William Nordhaus has (for some years it seems) accepted that Sir Nicholas Stern was actually right to warn that climate change must be tackled.

        Presumably, he would also agree with the International Energy Agency (IEA) saying that time to act is running out? That is to say, as the IEA did in last year’s World Energy Outlook, failure to move away from fossil fuels by 2020 will quadruple the cost of doing so.

      • Martin,

        Nordhaus has advocated for an internationally harmonized carbon tax the entire time he has worked in this field; see, e.g., Nordhaus (1992) for an early paper. You should read his latest book, A Question of Balance (draft online), to see what he thinks.

        His main objection to Stern was not that Stern wanted to tackle climate change, but that Stern’s calculations use a too-high discount rate (in Nordhaus’s opinion) that over-weights future climate damages. In turn, others have criticized Nordhaus for under-valuing the welfare of future generations.

        As for “time to act running out”, Nordhaus finds (Table 5-3 in the above book) that to act now would have $5.23 trilion (2005 USD) in benefits and $2.16 trillion in costs, whereas to wait 50 years and then act would have $3.69 trillion in benefits and $1.55 trillion in costs. The net benefit is $3.07 trillion for acting now vs. $2.14 trillion vs. acting in 50 years; they have respective benefit/cost ratios of 2.42 and 2.38. This of course is just a scenario analysis and not a comprehensive sensitivity analysis to assumptions.

      • Thanks for taking the time to reply to me, Nathan. However, I was aware of the nature of Nordhaus’ crticism of Stern; which climate scientists see as deeply flawed because the costs of delaying mitigation are rising exponentially such that no amount of economic trickery (net present value, etc.) can reign them in. Therefore, I guess, I should clarify that I am pleased to learn Nordhaus is – and always has been – an advocate of Carbon Taxes (even if he flat wrong when it comes to accountancy).

      • Martin,

        I don’t think climate scientists are particularly qualified to say anything about the costs of mitigation. But for what it’s worth, as a climate scientist (who isn’t particularly qualified in climate economics either), I don’t see much justification for your claim that the costs of mitigation are rising “exponentially” with time. They do rise, of course, as there’s more to mitigate, but I wouldn’t say they’re rising “exponentially”, nor would I say that discounting is irrelevant to the question of cost-benefit analysis.

      • Nathan, as a geologist-turned-environmentalist, I will always defer to the expertise of climate scientists (i.e. yourself, Kate, dana1981, et al.). So you will have to forgive me for my shorthand and/or unusual choice of verb tense (i.e. “increasing). What I was alluding to is the near-certainty that, unchecked, the runaway greenhouse effect will result in accelerating growth in adaptation costs and/or negative consequences. In this respect, I believe climate scientists should feel able to speak out about the economic consequences of failing to act.

      • There is some good work out there on “the regrets of procrastination” in climate policy. Certainly anyone who finds a positive economic value for mitigating now will also find a loss of utility in procrastinating. Climate scientists should be able to freely cite this economic literature, but make it clear that this is the work of professional economists, and not their own unqualified opinion.

  4. Nice one, Kate (and/or 20,000 scientists at GAU’s Fall Meeting).

    It may just be another way of describing “global dimming” or, because it forces the reader to think through what is being communicated, “humans are likely causing more than 100% of global warming” could just be the key to making a sceptical world wake-up to what is actually happening.

    We can but hope.

  5. “That’s right, humans are probably causing more than 100% of observed global warming. That means that our influences are being offset by natural cooling factors.”

    What this leaves out is that humans are also causing global cooling (mainly through sulphates from burning coal). So the warming we’re causing offsets not just natural cooling factors but also artificial ones.

    As I understand it, the relatively slow rate of warming in the last decade or so (see the comments above) is likely, at least in part, to be caused by increased emissions from coal burning (particularly in China and, to a lesser extent, in India).

    The distinction between human-caused warming which is offset by natural cooling vs human-caused warming which is offset by human-caused cooling is important because of the differing time scales; CO₂ lasts decades to centuries whereas the sulphates causing cooling only last fractions of a year.

    • True, if you read my entire post, I’ve also got graphics breaking down the human and natural components. GHGs have caused well over 100% of the observed warming in almost every study over every timeframe (one exception, Stone 2007 putting the GHG contribution at 100% over the past century). Anthropogenic aerosols are also the second-largest factor influencing global temperatures in every study.

      Here’s the above graphic broken down into individual components:

  6. Wow, thank you all for those replies. Much more than I expected. I’m going to work up a Post on Learning from Dogs around my question and the replies. They are worthy of re-publication. Much appreciated, Paul

  7. **Most of the studies estimated that humans caused over 100% of the warming since 1950, and all six put the number over 98%. **

    The key word is “estimated”.
    I am still looking for the term “measured”.

      • No, I am not looking for 0% error. I am looking for some semblance of a scientific measurement, but see none, only theory, modelling, and estimates.

      • Gerald, I know it is not considered to be good form to indulge in self-publicity but, if no-one did it there would be a lot less people on prime-time television! :-)

        Therefore, having spent a long time thinking through why it is that so many people find it so hard to shake of their so-called “scepticism” regarding climate science, I have managed to reduce my answer to just 104 words. You will find these in the footnote to today’s post on my blog:
        Climate science in a nut fragment

        I would also recommend reading the discussion between me and a certain John Kosowski in the comments section; although you may have to skim over those by John – as he writes much but says very little (IMHO).

      • Gerald, your phrase “only theory” is pretty revealing. Theory is what science aspires to. Theory is what you end up with when your hypotheses are confirmed and there is broad agreement among experts that you’ve provided an accurate and correct explanation for observations. In science, it doesn’t get any better than that.

        Sorry, but people who say, “Well, it’s only a theory” typically don’t have a very firms grasp on what science is and how it works.

  8. “To summarize, the perception that humans are accelerating a natural process of warming is false. We have created this problem entirely on our own. Luckily, that means we have the power to stop the problem in its tracks. We are in control, and we choose what happens in the future.”

    I jumped off this cliff entirely on my own. There was no wind or rockfall that made me slip. Luckily, that means I have the power to stop my free-falling descent in its tracks. I am in control and I choose what happens in the future as I plummet to the rocks below.

    Unfortunately, not every human action is humanly reversible.

  9. In turn, others have criticized Nordhaus for under-valuing the welfare of future generations.

    Nathan yes I’ve heard that too, and that critique seems a bit unfair. I have a positive impression of Nordhaus’ intellectual integrity.

    Instead, his argument is apparently that of “tactical spending alternatives”: when you have a certain amount of money to use, you may use it
    1) to mitigate (i.e., prevent part of) climate change by spending it now, or
    2) invest it productively, so you’ll have more resources to work with when the time comes to adapt to the change you didn’t prevent.

    There is no valuing, even implied, of the welfare of future generations compared to today’s.

    If course it is fair to say that questions come to mind to using this approach, like
    1) not all impacts of climate change offer the alternatives of mitigation and adaptation; for some, the alternatives are mitigation and suffering. The more severe the projected climate change, the larger this part is likely to be. How does the monetary equivalence for “suffering” — a trivial example, the loss of coastal wetlands and their enjoyment as outdoor environment — behave as GDP goes up?
    2) Prices will change within the economy when GDP goes up. E.g., “on-the-spot products” like houses will get relatively more expensive, being not as readily automatable/outsourcable. Many adaptation tools like levees, or the transfer of infrastructure to higher ground, or keeping desertifying regions viable for agriculture, are of this nature.

    I’m certainly no economist, but I hope the real ones know what they are doing :-)

  10. There are two key numbers in doing climate action cost/benefit analyses. One is the discount rate (discussed by a few commenters here), which I think is rather subjective, but generally ranges from 2 to 5%. Stern used 1.4%, which some have criticized as too low, whereas Nordhaus has (at least in some recent research) used 5%, which could certainly be criticized as too high.

    From what I’ve seen, 2-3% is the most generally-accepted discount rate range. I think governments often use 3%.

    The second key number is the social cost of carbon, which is basically an estimate of the direct effects of carbon emissions on the economy, but also depends on the discount rate (higher discount rate means less effective future economic cost, and thus lower social cost of carbon).

    I think Nordhaus tends to be quite conservative on both values. The first link above compares recent papers by economists Epstein and Nordhaus (and co-authors). Epstein’s social cost of carbon estimate was about 4 times higher than used in the Nordhaus paper ($7.36 vs. $30 per ton of CO2). Some have argued that even Epstein’s social cost of carbon estimate is far too low.

    Click to access Climate_Risks_and_Carbon_Prices_executive_summary+full_report+comments.pdf

    U.S. government agencies generally estimate it at between $19 and $68 per ton of CO2, consistent with the Epstein estimate. $7 per ton of CO2 is certainly at the low end of plausible values. Depite being so conservative, Nordhaus argues for taking action to address climate change by putting a price on carbon emissions. That says a lot – there’s just no economic justifcation for failing to take action to reduce CO2 emissions.

  11. It’s me again! The fabulous replies to my original question resulted in two posts on Learning from Dogs, here and here

    Then I passed these links to a good friend of over 40 years who is a skeptic. Here is his reply:

    Politicization of the science of natural events to benefit either side is beyond reproach. Whether you call it global warming or euphemistically “climate change”, there is a lot of public money at stake to save the world. Manmade influence causes some weather influence, albeit very small in terms of impacting the entropy surrounding us. Warming and cooling will continue to fluctuate as long as it has in the past. Again, one little volcano…….

    Like global over-population, any anthropogenic “climate change’ will take care of itself. We will eventually run out of fossil fuel deposits. Alternative energy sources can be competitively introduced without any political self-interest, creation of new regulatory agencies or forced government(s) redistribution of tax base revenues.

    In the end, as I have said many times, there are so many more great causes to promote that can have immediate and positive impacts on the human condition.


    He then passed me a link to

    Any thoughts?

    • Well, the nice way to put it is that your friend is an agnotologist.

      The not-so-nice way to put it is that your friend is begging for people to tell him lies that he can believe. Really, if after all your debunking of lies, he still thinks the lies don’t matter, then what other conclusion is there?

      Also, out-of-context e-mails are … out-of-context. Without the context to fill in what the e-mails are really about, they prove nothing.

      — frank

      • Actually Frank, they are not really out of context. The dirty denialist mind fills in the blanks. Projection.

    • Again Paul, I have been very slow to spot your return to this page.

      The Barry Bickmore presentation I was telling you about (now re-posted on my blog with his permission), has a very good section on why we should all pay no attention to the likes of Spencer, Michaels or Lindzen (view from about 19min 18sec).

      The stupidity of Climategate 2.0 is that, by publishing a second tranche of emails data mined prior to 2009, the perpetrators proved their intent was merely to discredit climate scientists and prevent UNFCCC progress. I very much doubt they will do it again but,if they do then they are more stupid than they already appear.

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