A group of researchers led by climatologist Michael E. Mann of the Earth System Science Center of Pennsylvania State University says the dendrochronological record does not always offer the best temperature reconstruction. Apparently some trees find it hard to distinguish between cool, and cooler.

These large-scale tropical volcanic eruptions occurred somewhere in the 13^th century. The single-largest eruption of the past millennium started in 1258 to be precise, and lasted a year. The associated aerosols emissions were ‘several times higher’ than the Mount Pinatubo eruption of 1991, the largest in recent history.

This medieval volcanology also reached science news about a month ago, when a group of American scientists reported in Geophysical Research Letters that they think the eruptions were the primary trigger for the Little Ice Age.

As the LIA lasted until 1850* just for the fun of it here at Bitsofscience.org we came up with an opposing hypothesis. These volcanic eruptions did not start the LIA, they ended the Medieval Warm Period [for which we conveniently offer no explanation, but when you look at the temperature graph it does look a bit spikier than the LIA, doesn’t it?]. That subsequent ‘Little Ice Age’ is to be considered late Holocene baseline – and that indeed was breached from somewhere around 1850 by anthropogenic forces, as atmospheric carbon concentration were then already on the rise.

Do let us know what you think.

[*) Aerosols – click and scroll down a bit to see why – can indeed float around in the atmosphere for a long time, but let’s say most are down after a year or so after the eruption ended – this means the LIA hypothesis leans heavily on climate feedback mechanisms, like Arctic albedo.]

The new research is not about the LIA. It is simply about the temperature response of historic volcanic eruptions – but from that perspective it does offer some additional support for late-medieval cooling.

For that period not all paleoclimatological temperature reconstructions correspond and evidence for the global cooling effect of the 13^th century volcanoes is absent from tree-ring-based temperature reconstructions.

The researchers used a tree-growth model and simulated temperature variations, which produced a discrepancy between expected and reconstructed temperatures.

And that discrepancy, they write in their publication in Nature Geoscience, could be explained because trees near the tree line [don’t underestimate such trees] respond differently to cooling – and because the dendrochronological record could be missing single tree rings, and because volcanic eruptions can have other (climatological) impacts on tree ring  growth, which may distort the temperature response.

“Our findings suggest that the evidence from tree rings is consistent with a substantial climate impact of volcanic eruptions in past centuries that is greater than that estimated by tree-ring-based temperature reconstructions.”

So don’t look at the tree rings. Look at the tree ring models.

© Rolf Schuttenhelm | www.bitsofscience.org