Tree rings’ lack of volcanic signature confuses climate calculations

Call it the curious incident of the trees in the volcano-time.

Like the dog in the night-time, which Sherlock Holmes realized did not bark during a horse theft, so too are these trees mute on a big happening right under their nose. Their bark doesn’t reveal evidence for the largest volcanic eruption of the last millennium. And that could be bad news for scientists trying to understand how natural factors affect Earth’s climate.

Powerful eruptions can spew sulfur particles kilometers high, where they spread out and act like an umbrella to block the sun’s rays. The last time this happened in a big way, when Mount Pinatubo erupted in the Philippines in 1991, it cooled the planet up to half a degree Celsius for several years — noticeable enough for even Dr. Watson to spot.

Imagine, then, what a truly colossal eruption could do to global climate. The 1815 eruption of Tambora, in Indonesia, caused the famous “year without a summer” in much of the Northern Hemi-sphere. Snow fell in New England in June, crops failed across much of Europe, and Mary Shelley, huddling in a cabin out of the chilly rain, wrote Frankenstein. Now imagine doubling the amount of sulfur that Tambora spewed out.

Scientists know that such an eruption happened in the year 1258 — probably somewhere in the tropics, because sulfur and ash from it appear in well-dated ice cores from both the Arctic and Antarctic. Weather records from Europe suggest the mystery volcano probably went off in the first half of that year. So where, then, is evidence for a global chill from this eruption, the world’s biggest since the Crusades?

Not in tree rings, says a team led by Michael Mann of Pennsylvania State University. In a paper published online February 5 in Nature Geoscience, the researchers try to explain why the colossal 1258 eruption is essentially missing from one of the best-known records of past climate.

Like an accountant faithfully inscribing tax returns every April 15, trees mark each year by growing layers of new cells just beneath their bark. The tissue grows fastest in spring and summer, tapering off as winter approaches. When times get tough, like during a drought, trees tighten their belts and conserve resources by growing as little as possible. The result: a narrow ring for that year.

Dendrochronologists, the scientists who study tree rings, have devised intricate methods for wringing out information about past environments. Mann and his colleagues wanted to see how this record meshed with computer simulations of Northern Hemisphere climate over the last 1,000 years. The researchers ran two such simulations, which included natural factors such as changes in the sun’s radiation and volcanic eruptions along with humankind’s fingerprint in rising levels of heat-trapping greenhouse gases.

The climate simulations matched the tree ring record well, except for a few big eruptions — the one in 1258, along with (less noticeably) one in 1452 — and a double shot of 1809 plus 1815’s Tambora. For the 1258 event, the simulations predicted a 2-degree cooling; tree rings show about half a degree. To explain this discrepancy, Mann’s team argues that things cooled so much that tree rings may simply not have formed in environments limited by cold, such as near the boreal tree line. In other words, the rings are missing.

Them’s fightin’ words to dendrochronologists, some of whom have already come out swinging against the new paper. Trees do grow rings at near-Arctic temperatures, those experts point out, and estimating how much a particular eruption changes the climate is a pretty uncertain business. Other factors in 1258, such as a potentially strong El Ni±o, may have also dampened the climate response.

For his part, Mann says he wasn’t trying to tar all of dendrochronology with the same brush. If the paper inspires a better explanation for the missing cooling, he wrote on RealClimate.org, he’ll be happy.

No matter what happened in 1258, the controversy underscores the strengths of science. Like a detective sifting through clues, looking for the one that will identify the horse thief, researchers must sort through a pile of data to understand the workings of the natural world. Some information will prove crucial; some will be red herrings. Lots of times the data will seem to contradict one another.

But in the end, the planet’s past holds the information scientists need to unlock the future. Knowing how the climate once responded to volcanoes will help researchers better understand what it might do as greenhouse gases continue to accumulate. And that’s a puzzle all the brainpower in the world, even Sherlock if he could, should tackle.

SN Prime March 5, 2012 | Vol. 2, No. 9