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Scientists extend and straighten iconic climate “hockey stick”

Enlarge / The ice age climate (left) gave way to one that slowly warmed until industrial times. (credit: Matthew Osman)

The climate “hockey stick” refers to a reconstruction of temperatures over the past 1,000 years. The data shows flattish temperatures over the last millennium, like the handle of a Hockey stick, ending in a “blade” of rapidly rising temperatures since the industrial revolution. The idea first appeared in a paper by Michael Mann and Raymond Bradley of the University of Massachusetts and Malcolm Hughes of the University of Arizona. The work became famous after appearing in a UN climate report, after which it was the focus of climate denial, hacking, defamation, and disinformation, all of which was dramatized in a recent BBC TV drama called “The Trick.”

Today, in a paper published by Nature, scientists show that the “handle” of the “hockey stick” extends back 9,500 years, while its “blade” is taller—the last decade was 1.5° C hotter than the average temperature over the last 11,700 years. “Human-caused global temperature change during the last century was likely faster than any changes during the last 24,000 years,” said lead author Dr. Matt Osman of the University of Arizona.

An animation showing the warming that ended the last ice age. (credit: Matthew Osman)

Taking the temperature of times before thermometers

To measure temperatures at times long before the invention of thermometers, scientists must use indirect proxies. For the new study, scientists carefully vetted over 500 proxy records from oceans around the world; the data shows the fossilized remains of plankton and microbes in sediments where the age is known from radiocarbon dating.

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