By RICHARD A. MULLER
Published: July 28, 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 atBerkeleyEarth.org. 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.
Richard A. Muller, a professor of physics at the University of California, Berkeley, and a former MacArthur Foundation fellow, is the author, most recently, of “Energy for Future Presidents: The Science Behind the Headlines.”
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Climate change study forces sceptical scientists to change minds (The Guardian)
Earth’s land shown to have warmed by 1.5C over past 250 years, with humans being almost entirely responsible
guardian.co.uk, Sunday 29 July 2012 14.03 BST
The Earth’s land has warmed by 1.5C over the past 250 years and “humans are almost entirely the cause”, according to a scientific study set up to address climate change sceptics’ concerns about whether human-induced global warming is occurring.
Prof Richard Muller, a physicist and climate change sceptic who founded the Berkeley Earth Surface Temperature (Best) project, said he was surprised by the findings. “We were not expecting this, but as scientists, it is our duty to let the evidence change our minds.” He added that he now considers himself a “converted sceptic” and his views had undergone a “total turnaround” in a short space of time.
“Our results show that the average temperature of the Earth’s land has risen by 2.5F over the past 250 years, including an increase of 1.5 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,” Muller wrote in an opinion piece for the New York Times.
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The team of scientists based at the University of California, Berkeley, gathered and merged a collection of 14.4m land temperature observations from 44,455 sites across the world dating back to 1753. Previous data sets created by Nasa, the US National Oceanic and Atmospheric Administration, and the Met Office and the University of East Anglia’s climate research unit only went back to the mid-1800s and used a fifth as many weather station records.
The funding for the project included $150,000 from the Charles G Koch Charitable Foundation, set up by the billionaire US coal magnate and key backer of the climate-sceptic Heartland Institute thinktank. The research also received $100,000 from the Fund for Innovative Climate and Energy Research, which was created by Bill Gates.
Unlike previous efforts, the temperature data from various sources was not homogenised by hand – a key criticism by climate sceptics. Instead, the statistical analysis was “completely automated to reduce human bias”. The Best team concluded that, despite their deeper analysis, their own findings closely matched the previous temperature reconstructions, “but with reduced uncertainty”.
Last October, the Best team published results that showed the average global land temperature has risen by about 1C since the mid-1950s. But the team did not look for possible fingerprints to explain this warming. The latest data analysis reached much further back in time but, crucially, also searched for the most likely cause of the rise by plotting the upward temperature curve against suspected “forcings”. It analysed the warming impact of solar activity – a popular theory among climate sceptics – but found that, over the past 250 years, the contribution of the sun has been “consistent with zero”. Volcanic eruptions were found to have caused short dips in the temperature rise in the period 1750–1850, but “only weak analogues” in the 20th century.
“Much to my surprise, by far the best match came to the record of atmospheric carbon dioxide, measured from atmospheric samples and air trapped in polar ice,” said Muller. “While this doesn’t prove that global warming is caused by human greenhouse gases, it is currently the best explanation we have found, and sets the bar for alternative explanations.”
Muller said his team’s findings went further and were stronger than the latest report published by the Intergovernmental Panel on ClimateChange.
In an unconventional move aimed at appeasing climate sceptics by allowing “full transparency”, the results have been publicly released before being peer reviewed by the Journal of Geophysical Research. All the data and analysis is now available to be freely scrutinised at the Bestwebsite. This follows the pattern of previous Best results, none of which have yet been published in peer-reviewed journals.
Early indications suggest that climate sceptics are unlikely to fully accept Best’s latest results. Prof Judith Curry, a climatologist at the Georgia Institute of Technology who runs a blog popular with climate sceptics and who is a consulting member of the Best team, told the Guardian that the method used to attribute the warming to human emissions was “way over-simplistic and not at all convincing in my opinion”. She added: “I don’t think this question can be answered by the simple curve fitting used in this paper, and I don’t see that their paper adds anything to our understanding of the causes of the recent warming.”
Prof Michael Mann, the Penn State palaeoclimatologist who has faced hostility from climate sceptics for his famous “hockey stick” graph showing a rapid rise in temperatures during the 20th century, said he welcomed the Best results as they “demonstrated once again what scientists have known with some degree of certainty for nearly two decades”. He added: “I applaud Muller and his colleagues for acting as any good scientists would, following where their analyses led them, without regard for the possible political repercussions. They are certain to be attacked by the professional climate change denial crowd for their findings.”
Muller said his team’s analysis suggested there would be 1.5 degrees of warming over land in the next 50 years, but if China continues its rapid economic growth and its vast use of coal 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,” wrote Muller. “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.”