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The conversion of a climate-change skeptic

Rich Muller, professor emeritus of physics | July 30, 2012

Call me a converted skeptic. Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming. Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I’m now going a step further: Humans are almost entirely the cause.

My total turnaround, in such a short time, is the result of careful and objective analysis by the Berkeley Earth Surface Temperature project, which I founded with my daughter Elizabeth. Our results show that the average temperature of the earth’s land has risen by two and a half degrees Fahrenheit over the past 250 years, including an increase of one and a half degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases.

These findings are stronger than those of the Intergovernmental Panel on Climate Change, the United Nations group that defines the scientific and diplomatic consensus on global warming. In its 2007 report, the I.P.C.C. concluded only that most of the warming of the prior 50 years could be attributed to humans. It was possible, according to the I.P.C.C. consensus statement, that the warming before 1956 could be because of changes in solar activity, and that even a substantial part of the more recent warming could be natural.

Our Berkeley Earth approach used sophisticated statistical methods developed largely by our lead scientist, Robert Rohde, which allowed us to determine earth land temperature much further back in time. We carefully studied issues raised by skeptics: biases from urban heating (we duplicated our results using rural data alone), from data selection (prior groups selected fewer than 20 percent of the available temperature stations; we used virtually 100 percent), from poor station quality (we separately analyzed good stations and poor ones) and from human intervention and data adjustment (our work is completely automated and hands-off). In our papers we demonstrate that none of these potentially troublesome effects unduly biased our conclusions.

The historic temperature pattern we observed has abrupt dips that match the emissions of known explosive volcanic eruptions; the particulates from such events reflect sunlight, make for beautiful sunsets and cool the earth’s surface for a few years. There are small, rapid variations attributable to El Niño and other ocean currents such as the Gulf Stream; because of such oscillations, the “flattening” of the recent temperature rise that some people claim is not, in our view, statistically significant. What has caused the gradual but systematic rise of two and a half degrees? We tried fitting the shape to simple math functions (exponentials, polynomials), to solar activity and even to rising functions like world population. By far the best match was to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice.

Just as important, our record is long enough that we could search for the fingerprint of solar variability, based on the historical record of sunspots. That fingerprint is absent. Although the I.P.C.C. allowed for the possibility that variations in sunlight could have ended the “Little Ice Age,” a period of cooling from the 14th century to about 1850, our data argues strongly that the temperature rise of the past 250 years cannot be attributed to solar changes. This conclusion is, in retrospect, not too surprising; we’ve learned from satellite measurements that solar activity changes the brightness of the sun very little.

How definite is the attribution to humans? The carbon dioxide curve gives a better match than anything else we’ve tried. Its magnitude is consistent with the calculated greenhouse effect — extra warming from trapped heat radiation. These facts don’t prove causality and they shouldn’t end skepticism, but they raise the bar: to be considered seriously, an alternative explanation must match the data at least as well as carbon dioxide does. Adding methane, a second greenhouse gas, to our analysis doesn’t change the results. Moreover, our analysis does not depend on large, complex global climate models, the huge computer programs that are notorious for their hidden assumptions and adjustable parameters. Our result is based simply on the close agreement between the shape of the observed temperature rise and the known greenhouse gas increase.

It’s a scientist’s duty to be properly skeptical. I still find that much, if not most, of what is attributed to climate change is speculative, exaggerated or just plain wrong. I’ve analyzed some of the most alarmist claims, and my skepticism about them hasn’t changed.

Hurricane Katrina cannot be attributed to global warming. The number of hurricanes hitting the United States has been going down, not up; likewise for intense tornadoes. Polar bears aren’t dying from receding ice, and the Himalayan glaciers aren’t going to melt by 2035. And it’s possible that we are currently no warmer than we were a thousand years ago, during the “Medieval Warm Period” or “Medieval Optimum,” an interval of warm conditions known from historical records and indirect evidence like tree rings. And the recent warm spell in the United States happens to be more than offset by cooling elsewhere in the world, so its link to “global” warming is weaker than tenuous.

The careful analysis by our team is laid out in five scientific papers now online at That site also shows our chart of temperature from 1753 to the present, with its clear fingerprint of volcanoes and carbon dioxide, but containing no component that matches solar activity. Four of our papers have undergone extensive scrutiny by the scientific community, and the newest, a paper with the analysis of the human component, is now posted, along with the data and computer programs used. Such transparency is the heart of the scientific method; if you find our conclusions implausible, tell us of any errors of data or analysis.

What about the future? As carbon dioxide emissions increase, the temperature should continue to rise. I expect the rate of warming to proceed at a steady pace, about one and a half degrees over land in the next 50 years, less if the oceans are included. But if China continues its rapid economic growth (it has averaged 10 percent per year over the last 20 years) and its vast use of coal (it typically adds one new gigawatt per month), then that same warming could take place in less than 20 years.

Science is that narrow realm of knowledge that, in principle, is universally accepted. I embarked on this analysis to answer questions that, to my mind, had not been answered. I hope that the Berkeley Earth analysis will help settle the scientific debate regarding global warming and its human causes. Then comes the difficult part: agreeing across the political and diplomatic spectrum about what can and should be done.

Cross-posted from The New York Times Opinion pages.

Comments to “The conversion of a climate-change skeptic

  1. Nice article…I wish Prof. Rich Muller presents his converted skeptic theory in Indonesian university like Environmental Engineering Department, Sepuluh Nopember Institute of Technology. This faculty concern in Climate change matter in years. maybe, it will bring a brilliant solution to keep earth from climate change. I’m student of this faculty and wish to know Prof Rich Muller and discussed some environmental problems. Thanks.

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  2. Dr. Muller’s position on global warming changed over time, from pure skeptic, to one that saw good evidence for global warming, then , following close examination of the available data with the Berkeley Earth Surface Temperature Project, to one that also saw much evidence for global warming caused by mankind.

    The science of global warming is difficult, often relying on observational data, on multidimensional data sets, and on sampling procedures that are far from ideal. Data, such as those presented following 9-11, in which the absence of aircraft contrails reduced the earth’s surface temperature ( have to be interpreted with care and caution.

    However, the overwhelming stack of data that now exits leads me to conclude that global warming does exists, and that man is probably the cause. Now to find solutions to the problem; an equally complex problem. Natural gas, found in abundance throughout the USA and Canada, can be a short term solution in our long-term goals for clean energy solutions. For natural gas to work, the industry will need to use clean fracking methods, and the government will need to be vigilant in regulating the methodology and its implementation.

    In the mean time, government, industry, and academia (The Triad) must work together to devise our long term strategies for clean, sustainable energy, and the efficient use thereof, including efficient transportation, smart-grids, and energy-efficient buildings. Berkeley will continue to be a leader in developing these new technologies.

    • Some of you need to go read stuff, at, starting with Veron 2008, which describes coral studies, showing how a rapid rise in CO2 precedes each of five mass extinction events, and given CO2 AND CH4 AND industrial GHGs are all rising, in atmospheric concentration, faster than ever, before, why should we doubt, how we are in the opening stage, of Mass Extinction Event 6?

      The NH is heating up, faster than is the SH. The Arctic ice cap is closer, to failing, every summer, so by late in the northern summer, more solar energy is getting into the climate system, every year. This will eventually combine, with GHG concentrations, to finish the Arctic ice, every northern summer, which will cause an acceleration of all dire factors, warming, melting of ice and methane clathrates, sea level rise, and acidification of waters.

      As the rain passes through CO2, NO2, or SO2, it becomes more acidic, on the way down. When rain or melt or runoff water gets into the ocean, and over-fishing continues, the days of die-offs and decimations draw closer, to causing extinction of species and families, in the oceans, whereupon jellyfish and algae will rule the oceans.

      I see a guy commented, who doesn’t believe in the greenhouse effect, based on Stefan-Boltzman and vague 2nd Law of Physics references. I don’t know what to tell guys like that. IR tends to get trapped, in the 50 or so miles of atmosphere, more so, if molecules of three atoms or more become concentrated.

      I must admit, I’m stumped, if the perennial ice is melting, out-gassing is going on, to beat the band, and the same thing happens, every time GHGs shoot. Of course, temps and GHGs lead each other, but given the way rise in CO2 leads to out-gassing, of CH4, CO2 is still the leading forcer, especially if it leads, like it is doing, today, in such a way, that forests succumb, to the chainsaw and climate change, to get finished, by beetles and fire. CO2 and CH4 have to keep out-gassing.

      The Anthrocene Thermal Maximum lies a few hundred or a few thousand years away, no more. On the way, most life on Earth can die out, as if we were all afraid, of some bollide impact, which may happen, if humans are stupid enough, to simply exploit more sequestered CO2 media.

      FYI, if we burn coal, we need Fischer-Tropsch plants, with scrubbers and media, to compress CO2, which can be piped, to make methanol and to grow ALGAE, nearby. We then process any excess, with oxides, to make carbonates. Don’t pipe it way out somewhere, to mess up the cost structure of the project.

      Burn natural gas or brewed methane, with O2, capture the CO2, and make more methanol and algae. Refine the algae and blend biofuels right nearby, for efficiency.

      People who won’t get biofuels efficiently aren’t keeping up, with the Navy, which has a Green Fleet, already, or with DARPA, which will have algae refineries, at forward bases, before some people figure out human habitat is threatened, if we don’t re-green deserts and polluted areas, BOTH.

  3. I see once again a physicist has jumped on the bandwagon of an anthropogenic cause of a global warming. Any one who knows physics cannot explain how a slow moving molecule of CO2 can absob a photon and heat another molecule with that energy in the 12 nanoseconds it is excited.

    They also fail to take into consideration the tremendous amount of energy released by the combustion process. That alone has been enough to heat the atmosphere by 1.68 degrees C since 1800. It is our use of energy which once released has to be stored somewhere since it cannot be destroyed. It gets stored in the atmosphere and the oceans and some melts some ice. I have yet to find anyone in the field of climatology who actually understands the chemistry of the combustion process or the physics of what happens.

    There has never been a paper published which looks at the extra sunlight which has been absorbed by the hundreds of thousands of square miles of asphalt roads and roofs around the world. The calculations are easy area X energy flux X time = total energy. This alone should have added more than 10 degrees Celsius to the air temperature but it hasn’t. This is because as the atmosphere warms up more water is evaporated and it radiates energy to space keeping us nice and comfy.

    The other aspect of global warming is that it is impossible to actually the average temperature of either the atmosphere or the oceans due to their vast sizes. To make the statement that the temperature has warmed by 0.78C +- 0.26 C is a bunch of malarkey and was only done to try and stem the tide of skeptics.

    I must say I am very disappointed by Prof. Muller. We may be causing a warming but it is not possible to prove one way or another. We do know that it has been colder and warmer during the past 1000 years and we have no reasonable answer as to why there has been such variation.

  4. In 2008 Prof Muller said in an interview with, “There is a consensus that global warming is real. There has not been much so far, but it’s going to get much, much worse.”

    This is not the statement of a global warming skeptic. I suggest that his claims should be treated with skepticism.

    • There is quite a difference, between skeptic and denier. There is also quite a difference, between climate disasters NOW and climate disasters, of the near future, which will be much, much worse. (Put THIS comment up. It’s better than my other one.)

      Today, we have melting ice cooling the runaway warming, while oceanic acidification is a sign, how the oceans will one day stop absorbing CO2, when some seas are caustic, and the warm temperatures make really sure CO2 out-gasses, with any CH4, left in the oceans or on the sea bottom.

      CO2 should max out, at 280 ppm, to turn down, to force global cooling. But CO2 is 400 ppm, headed for 1000 ppm, while CH4 out-gasses, so it is no longer measured, in ppbillion.

      The oceans have been absorbing CO2, to keep GHGs at a minimum, while the perennial ice melts, to keep temperatures at a minimum. Even so, we have just seen thousands of high temp records fall, and here comes El Nino, expected at the end, of Summer 2012.

      When northern ice albedo (reflectivity) fails, northern summers will be a nuisance, as the Earth absorbs energy, probably from a Sun, which radiates more energy, than it does, today. Warming of oceans, lands, and atomosphere will accelerate, past either the PETM or P-T extinction hot-houses.

      We are due, to have more heat and water, in the climate system, which will yield more disasters.

      Proposing carbon credits is like trying to stop the spread, of rampant HIV, at the now-closed bath-houses, as if instead of closing this nuisance, authorities simply allowed owners to charge more admission.

      Proposing fracking or abuse of natural gas is like trying to sell bath-house patrons a cleaner form of methamphetamines, so when they shoot their speed, HIV is spread marginally slower.

      The only possible solution is to legalize hemp, minimize the carbon signature of all government wars, and grow hemp, with switchgrass and algae, to process ethanol, with ultrasound, and make the hemp into darned near anything, including indestructible plastic (see YouTube, Ford’s 1941 plastic car). Ford was making hemp-ethanol and plastic, since the Model T.

      We need immediate research, into botanical candidate plants, to re-green deserts and polluted areas, OR WE WILL LOSE HUMAN HABITAT, by any fudging and hedging. It’s re-green time, or dying time will follow, in a deadlier Mass Extinction Event 6.

  5. Prof Muller there is an awful lot of evidence that shows you are far from a reformed sceptic but an active member of the pro-CAGW proponents. Have you any comment on that?

  6. Here’s what most of you need to notice (some of you know this):

    1. Solar intensity has been mild, for several decades, including our current cycle 24, which will peak, averaging 90 sunspots per day, 2013;
    2. Ice has been melting, so the heat exchange should provide a forcing factor, against high average global temperatures;
    3. Average global temperatures are up, anyway, and many thousands of high temp records were broken, in the last year;
    4. Oceanic acidification from CO2 absorption is killing corals, oysters, little fish, eggs, and plankton;
    5. Sea level rise is getting out of control, on the east Coast US, likely since ice melts are disrupting trade currents;
    6. A lot of CH4 is out-gassing, from warming lands and waters, noticeably in the Arctic, which with the recent Greenland sheet ice melt represents tipping point media, which need to be watched.

    IF tipping points toward runaway warming all get passed, we will need radical re-greening, to have any hope of keeping a human habitat.

    By the time tipping points degenerate, to warmer, heavier tides massaging plates, faults, and magma chambers, our survivors will suffer volcanism, while warmer seas will be full of jellyfish, with NS2 respirators, on the sea floor.

    But cod? Tuna? Sharks? Whales? Tilapia? What could you be thinking? Nooooo, those won’t be around, anymore. We are looking at losing most ocean species and families, and a lot of die-offs will affect even survivors, on land.

    We would rather cut the carbon footprint of corruption AND re-green, don’t you think?

  7. Prof. Muller, your efforts against and for global warming have proven the paramount truth that it is the dichotomies we create that shall be the root cause of our self-destruction. In spite of all the education and scientific advances we have produced, our brain is our worst case scenario weak link.

  8. I was just wondering – this ‘excellent analysis’ and ‘job well done’, why was it ‘disappeared’ from both The Age (, and from Sydney Morning Herald ( within few days of publication?
    Any idea what reasons they had to disown something they published so eagerly?
    Didn’t take long to recognise what it’s worth, and treat it accordingly, did it.

  9. Temperatures are not rising as much as people believe. Certainly not enough to justify halting the production of cheap, efficient and abundant fossil fuel that is needed to keep our economy growing and people working. People who believe that something must be done quickly to “stop global warming” need to scrutinize the temperature stations used to measure the perceived rise in temperatures. If they did they would discover that at least one half of the temperature stations used to measure the rise in temperatures are located next to heat producing sites, (for example airports) which artificially raise their readings. Anthony Watts from the web site Watts Up With That has just completed the aforementioned analysis which you can find on his site.

    What is surprising to me is why this type of analysis was not done from the very beginning. All of those so called peer reviewed reports claiming temperatures are rising at catastrophic rates were based on erroneous temperature data. Which makes me conclude that the people who peer reviewed these catastrophic\ global warming reports neglected the basics by not bothering to evaluate the accuracy of the temperature data in said reports.

    • Mary, you seemed to ahve missed the point of the Professor’s work. He’s demonstrated that the temperature data IS correct. (Granted, he’s a little late to that party, but better late than never.)

      And as for Watts, read up a little. The amount of the error he has claimed to discover happens to match quite nicely with the adjustment one has to do for changes in how individual stations report. (Known as the TOBS adjustment.) What Watts has done is proven why the TOBS is important: because without it you end up with temperature data that is incorrect. (For example, if Watts was right, then the surface record would contradict what we known from satellite measurements.)

  10. It is good to see a careful, skeptical scientist follow the evidence and eventually conclude that the hypothesis stands up to his own rigorous testing. As a physicist Dr Muller probably had to invest some effort in becoming familiar with the methods and pitfalls of analysing global climate records. Some expert climatologists, who had been analysing the same or similar data, had come to similar conclusions more than 20 years ago. This shows that the scientific method, with all its warts, does have some credibility.

    On the other hand, it is interesting that Dr Muller is still poo-pooing some of the other implications of climate change about which he is not an expert. So, he does not mention the warming to which the planet is already committed because of the thermal lag effect of global oceans. Nor does he discuss the potential effects of feedbacks (winter albedo reduction, forest fires, permafrost melting, methyl clathyrates) which could accelerate the rate of warming above that due to anthropogenic GHG emissions alone. Nor does he acknowledge IPCC scientists who have spent many person-years of effort looking for evidence of climate change impacts on extreme weather (and have not found it in tropical storm frequency or intensity). Nor do the many wildlife scientists studying arctic ecology gain any recognition, even though some of them are monitoring the demise of some (but not all) polar bear populations due in part to the rapid and progressive loss of summer ice from the Arctic Ocean.

    It is good to see a climate change skeptic stand up for good science rather than for what is politically expedient. And there is no question that doing the science is the easy part: deciding what to do about it is infinitely harder! But it would also be nice to see a little more humility and respect for the efforts of thousands of other scientists each of whom has been doing their part to chip away at the climate change problem and all its implications — long before Dr Muller came to see the light.

  11. This would be more credible if you responded to the doubts expressed by Dyson. The arguments presented may convince you but the logic is convoluted. The mere fact that Global Warming is now called Global Climate Change should alert you that this is more likely a fad of the guild than science. Remember plate tectonics and the big bang? – what a hoot for the scientific consensus, until proven correct. We don’t have enough data points to make positive assertions about a planetary chaos system over thousands of years. Humans may be involved, but the best argument is a moral one: pollution is repellent – you don’t need faux science. I’d suggest you are simply trying to avoid isolating yourself from the club.
    No one really knows what’s going on with current weather shifts.

  12. Professor, I took your L&S C70V in 2008 and you were obviously far from a skeptic then. Why are you claiming to have been one?

  13. Professor Muller, I am most disappointed to learn of your defection to the populist camp of those who have the arrogance to believe that Man is soley responsible for climate change. From your time as a skeptic ( or as we in the UK would say, sceptic) you should know that climate change has been a constant since the Earth was formed. I would not say that given our gift for polluting the atmosphere with our industrial discharges that we have not made a contribution but in the overall picture over millions of years it is a mere pinprick. To suggest otherwise is an insult to our solar system which will continue to evolve climate long after Man has disappeared from the planet.


    Clive Millman

  14. Excellent analysis and a job well done. I hope this will settle once and for all the real cause of global warming.

  15. So what should be done? How are you and your team of scientists going to convince “the red team” that action needs to happen very quickly to stop the global temperatures from continuing to rise through the release of carbon dioxide emissions? Here is where I am a skeptic. I do not think you and an army of scientists can do it! Maybe your daughter Elizabeth can convince her generation to take action, but then that may be too late.

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