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Nearly everyone accepts the data from ice cores that show a long period—8000 years or so—of relatively stable concentrations of carbon dioxide, methane and nitrous oxide in Earth’s atmosphere, followed by a rapid increase to today’s elevated values beginning about 150 years ago. For CO2 it is clear that fossil fuel combustion and deforestation have led to the rise in concentration. For nitrous oxide, the rise is most likely due to the use of nitrogen fertilizer in agricultural systems worldwide. Attribution of the rise in methane is less clear, but a variety of human activities have played a role.
Unfortunately, there is no ice-core record of the change in global temperature and the widespread use of thermometers to record temperature began only about 200 years ago. We must derive a record of changes in Earth’s temperature from indirect sources—what climate scientists call proxies. A variety of these are available, including tree rings, measures of glacial retreat, and anecdotal observations, such as the “ice-out” time in rivers and lakes.
About 20 years ago, Michael Mann, now at Penn State University, compiled a record of the past 1000 years of Earth’s temperature from tree rings. It, too, shows a long period of relatively stable (even slightly declining) temperature, followed by a rapid increase beginning about 150 years ago. While correlation does not prove causation, the resemblance of Mann’s temperature record to the record of changing concentrations of “greenhouse” gases was striking.
But, with its publication, Mann’s temperature record drew a rancorous response from conservative, climate-change denying folks. Even now, whenever, I show a picture from Mann’s publication—which looks like a hockey stick—I can expect a flurry of criticism from climate change deniers in the audience.
This continuing disbelief that the Earth’s temperature has risen sharply from relatively stable levels during most of the past 1000 years is surprising. In 2006, The National Academy of Science brought together a panel of experts, including a well known climate-change denier John Christy, to evaluate Mann’s work. Their conclusion: “it can be said with a high degree of confidence that global mean surface temperature was higher during the last few decades of the 20th century than during any comparable period during the preceding four centuries.” In other words, Mann was right.
The unprecedented rise in temperature during the past 150 years is consistent with a variety of proxy measures published since the National Academy report, including the timing of bird migration and the springtime blooming of plants. The Academy put less confidence on the reconstructed record of temperature before 1600, but none of the proxy data extending from 1000 to 1600 A.D. changed the basic conclusions about recent, rapid global warming, as derived from tree rings.
It is time to silence the critics of the Hockey Stick. Certainly, tree rings are not perfect proxies of past temperature, but when the tree-ring records agree with a variety of other records, we can have confidence in what they say. The disruption of thoughtful evaluations of Earth’s past temperature by those who wish to deny the reality of the ongoing climate change serves no useful purpose, any more than a denial of satellite pictures of our planet will make the Earth flat.
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