by Elijah Wolfson, Newsweek, May 7, 2014
In November 2009, climate skeptics had their day in the sun when an anonymous hacker posted years' worth of emails sent back and forth between some of the world’s most prominent climate scientists, working with the Climate Research Unit (CRU). Buried within these thousands of emails were a handful of fragments that skeptic seized on as proof positive that climate scientists had fabricated the idea of man-made global warming. After the emails were made public, a media frenzy ensued; “Climategate” culminated in deep investigations into the legitimacy of the science that backed of the theory of a man-made global warming. Ultimately, the CRU scientists were acquitted of any malfeasance, and their science was verified to be clean, but the events had a significant impact on public perception.
Five years later, a new study published in Nature Communications may help cool the lingering fallout from Climategate. The study, led by Alexander Stine of San Francisco State University and Peter Huybers at Harvard University provides a compelling scientific explanation for the issue at the center of the 2009 controversy: the tree-ring divergence problem.
Scientists are nearly 100% sure of the world’s daily temperatures for the last 150 years or so—which is when we started to keep daily weather records. But before that, it’s a little less clear. So many climate scientists, on both sides of the debate, have come to use tree rings, in conjunction with other proxy measurement tools like coral growth, isotope variation in ice cores, ocean and lake sediment records, and more, in order to reconstruct temperatures changes over years.
“The tree-ring records are in many ways the best record we have,” says Stine. “There are trees all over the Earth, and they have this annual resolution, which very few of our proxies do.” Trees reflect a more human time scale; ice core records show temperature changes across time periods on the order of millions and millions of years, while trees rings track changes year by year. In temperate climates (like much of the U.S.), trees only grow during the part of the year referred to simply as the growing season. In the Northern Hemisphere, this is the spring and summer. In a good growing season—where there’s plenty of precipitation and warm temperatures for months on end—the resulting growth ring will be relatively wide. In poor growing seasons, the rings will be narrow.
It’s not perfect, but it’s a pretty decent proxy for those years before thermometers and recordkeeping.
The problem, though, is that since the 1960s, Arctic tree-ring growth seems to not match the actual temperature records we have. Thermometer measurements show a sharp jump in temperature in the late 20th century. This is one of the primary arguments for man-made climate change: as the global population increased, and we began to build bigger and more industrial cities, and rely more on fossil-fuel based travel in cars and planes, humans greatly sped up the warming of the planet. But Arctic tree-ring records don’t back this up. In fact, they seem to indicate a decline in temperatures during those years. This is called the tree-ring divergence problem, and has been recognized, accounted for, and written on extensively—not the least notably by Keith Briffa, one of the CRU scientists singled out as supposedly burying data after the emails were leaked, in a paper published almost a decade before Climategate.
Briffa had put forth—and the scientific community had long-since accepted—the notion that this “divergence issue” was actually a strong decline in tree sensitivity to temperatures after 1960—and not the temperature themselves. In fact, Stine says, if you look more closely at the numbers you can see that year to year, the rings on an individual tree in the Arctic Circle still do change size with year to year temperature changes—there is just a “systematic underprediction by the tree rings” that began in the mid-20th century. “And when you average all the trees, everything goes away except for that systematic underprediction.”
In 2009, critics from latched on to one email in which Phil Jones, at the time the head of the CRU, wrote that in a recent graph (prepared for a World Meteorological Organization Report) he used “Mike's Nature trick of adding in the real temps to each series for the last 20 years (i.e., from 1981 onwards) and from 1961 for Keith's to hide the decline.”
It was unfortunate phrasing. Jones was referring to the famous “hockey stick” graph used by Michael Mann, the director the Earth System Science Center at Pennsylvania State University, in an article published in the journal Nature, in which he combined a number of different data sources to show overall global temperature increase over the years. According to Mann, the criticism was simply a “cynical misrepresentation” of Jones’s email, which he tells Newsweek “spoke of something that was already well known to the scientific community—that tree-ring density data should not be used to depict temperatures after 1960.” Following the release of these emails, Mann was extensively investigated for any scientific misconduct—and completely acquitted.
Today, Mann says that the whole Climategate fiasco was based around the misconception that the only evidence there is for climate change from the paleoclimate record (i.e., the record of the climate change throughout the entire history of earth) was from tree rings. “Nothing could be further from the truth,” he says. “[There are] multiple independent lines of evidence for human-caused climate change. And the most important evidence for modern warming comes from instrumental measurements—thermometers—not paleoclimate data.”
Stine points out that in most of the scientific papers at the time, researchers would typically plot tree-ring records and thermometer records next to each other—then make an argument about the gaps between the two. “I don’t think you need to argue too hard to convince me that thermometers are better than tree-ring records,” says Stine. “What got turned into a big hoopla was the putting together of a summary graph, to say ‘this is our best interpretation of what the numbers have been.’ ”
Nevertheless, Stine and Huybers set out to explain the tree-ring divergence data—mostly, Stine says, because he wanted to get a better sense of what the Earth’s most ancient trees are actually able to tell us. Their solution is simple, elegant, and intuitive: global dimming. Since the 1960s—exactly when tree-ring data started to go awry—“there’s been large scale decreases in the amount of light that’s reaching the earth,” says Stine. It’s fairly easy to see why, too. In rapidly industrializing parts of the world with fewer emissions laws—like Southeast Asia—the light decline is particularly steep, and continues into the 21st century. On the other hand, in areas like the U.S. and Europe, you see a rapid decline in the middle of the 20th century, but then light levels steady themselves later on—right around the time most air pollution laws were put into place.
Trees of course, need light to grow—they use photosynthesis to turn light energy into chemical energy to fuel all their activities, including their growth. “My hypothesis,” says Stine, “is that the light is directly affecting tree growth: You turn down the lights, you turn down tree growth.
In the scientific community, skeptics of human-induced climate change have mostly been silenced; the 2014 National Climate Assessment published Tuesday unequivocally laid global warming and its resulting environmental impacts at the feet of human civilization, stating ominously that “climate change, once considered an issue for a distant future, has moved firmly into the present.”
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