A weekly UPI series examining the potential human impact on global climate change.
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BOULDER, Colo., Jan. 26 (UPI) -- Gamblers have a maxim: Don't bet against a streak.
In the world of climate science, controversies often center around whether there is a streak at all.
The latest involves a Harvard University astronomer who claims he has been unable to reproduce rising surface temperature trends at the end of the 20th century. That assertion places him in conflict with both the Intergovernmental Panel on Climate Change and Michael Mann, a leading climate researcher with the University of Virginia's department of environmental sciences. Both have published well-circulated reports concluding that the rising trends are real and proceeding.
In a paper to be published in the Jan. 27 edition of the journal Geophysical Research Letters, Willie Soon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., used what he called "standard approaches to data estimation" to try to replicate the results published by Mann, the IPCC and others.
"We were successful in replicating the instrumental surface temperature trends in several previously published results," Soon wrote. "However, we failed to reproduce the long term Northern Hemisphere surface thermometer temperature trends in ... IPCC TAR (Third Assessment Report) and in ... Mann and Jones."
Because scientific results ought to be reproducible, Soon told United Press International, he has been trying to do just that with the warming estimates other investigators have presented. He said although he is not denying the reality of global warming, "the most important theme here is to suggest that defining a trend in the data ought to be more carefully done, because the process is subjective. We're not able to verify some of the data being presented, specifically from the IPCC report, as well as the latest publication of Michael Mann."
Asked about the paper, Mann responded that his trends were derived using standard mathematical tools and he would have been happy to make the methods available to Willie Soon -- had Soon simply asked him.
"The researcher has produced very poor work in the past, and isn't taken seriously in the climate community," Mann told UPI. "This sounds like another in their installation of just bad work." He added: "I'm amazed this paper got into print. They don't even try to determine what method we used ... Our method was described in more detail in other papers."
At issue is a technical mechanism that can indeed be open to subjective interpretation -- but one that has real-world implications.
Latest estimates are that the Earth's composite air and sea surface temperature increased by about 0.6 degrees Celsius over the 20th century, but the estimates from the IPCC and Mann indicate the pace of warming accelerated at the end of the century -- reaching 0.1 degree to 0.2 degree C. per decade.
Climate researchers attempt to detect contemporary trends by using what are called long-term moving averages, in which they use previously compiled data to predict what current data are likely to show when they eventually are compiled and verified. Researchers smooth and combine many years of data -- 40 years in the case of these estimates -- to extrapolate current trends. The moving average data show global temperature increases accelerating, beginning about 1960.
Just about everyone's data -- Soon's, IPCC and Mann -- agree on this. The mathematical point of contention arises when the 40-year averages hit 1980. After that, climate scientists must base their estimates on less and less certainty because the full 40 years are no longer available.
Mann said he agrees with Soon about the degree of uncertainty, but added his work is based on techniques that have been used by time-series experts over the past decade or more.
"There are three different ways you can smooth a time series," he said. "Or put another way, the smoothing is non-unique because you don't know what happens to the series outside of the interval where you have data."
When a moving-average series bumps into the end of the data, Mann said, "there are additional constraints, called an inverse problem. You don't know exactly what happens, but you can invoke various constraints" to estimate the continuing trend.
Such constraints include:
-- Setting the missing data points at zero;
-- Setting the missing data points at the mean of the available points, and
-- Allowing the missing points to preserve some characteristics of the earlier trend.
These mathematical devices are called, respectively, minimizing zero, minimizing the first derivative and minimizing the second derivative.
"In past published work, I and other people have described methods where you can use each of these constraints," Mann said. "What you're trying to do is minimize the misfit with the raw data. You can try each of these constraints and see which one minimizes the misfit of the smooth series from the raw series. And the method in the paper -- Mann and Jones 2003 -- where (Soon and colleagues) couldn't figure out which method we used, that's the method we used ... It's an objective method. It finds which of those three choices minimizes the misfit."
Mann called Soon's analysis of his and the IPCC's work "an intellectually dishonest approach ... The intellectually honest approach would be to consult with us, ask us, 'Well, what approach did you use?' But instead they didn't do that. They just said, 'Oh well, we can't reproduce it and it must be wrong.'"
Soon replied that when his paper becomes publicly available he will get in touch with Mann and his co-authors. "I will communicate with them when the data is published," he said.
"I did not really come up with a reasonable recommendation except to say that I disagree with the approach of defining a trend," Soon said. "To get even close to what they are trying to say, they have to pad in a lot of data, and that procedure ought not to be recommended. To get roughly what they get, you have to define a point in 2000 and fill in the data up to 2020. That is a totally arbitrary process ... The quantitative information is highly uncertain, because it depends on the future," Soon said. "I stop the red line at 1980. I don't make any extrapolation. This is the most conservative."
Though Soon admitted his assertion seems confrontational, "I don't mean to be. I'm very confident of my result. The rest of it will be up to the community to decide."
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Dan Whipple covers the environment for UPI Science News. E-mail [email protected]
