Graph [not from new publication] by Berkeley PhD student Robert A. Rohde compiling 11 different temperature records showing both the Medieval Warm Period, the Little Ice Age, and the onset of the GHG-induced warming up to the year 2004. In such graphs the general pattern says more than the exact temperature differences – as the data are derived from different sources (for instance tree rings, ice cores) at different latitudes – which can explain amplitudal changes for different temperature sets.

When it comes to Earth’s climate system, not all volcanoes are equal. For them to exert a cooling effect, they need to ideally be pyroclastic (so carry an explosive punch) and be situated in the tropics.

[That means the only good thing the eruption of Eyjafjallajökull (a relatively harmless shield volcano) on Iceland did for the climate was to halt these European air travel CO2 emissions for a while.]

Such tropical stratovolcanoes can build up a lot of energy and when they erupt emit huge quantities of sulfur aerosols high up into the troposphere – from where they spread across much of the planet, reflecting and blocking solar radiation and thereby cooling the Earth.

We know megascale volcanic activity can lead to enormous climatic disruptions and even mass extinctions, as for instance seems to have happened at the Permian-Triassic and the Triassic-Jurassic.

‘Winter is coming’ – does Game of Thrones’ Stark family recall the LIA?

These events occurred however respectively some 250 and 200 million years ago. But lesser episodes of increased volcanic activity can still exert climatic influence – and we need only to go one step back in the paleoclimatic record to see their importance, an international team of geologists and atmospheric scientists says – to the Little Ice Age, a period roughly between the 1300/1550 and 1850 during which especially in Europe temperatures were a bit lower on average and winters were considerably colder.

Yesterday researchers from the Department of Geological Sciences at the University of Colorado in Boulder, the Institute of Earth Sciences at the University of Iceland and the US National Center for Atmospheric Research (NCAR) reported in Geophysical Research Letters that they discovered sedimentary evidence (sudden plant die-off close to expanding glaciers) of two episodes of high tropical volcanic activity – which they think should be linked to the onset of this Little Ice Age.

Four large sulfur-rich explosive eruptions, most likely in the tropics, would have occurred over a mere 50-year period just before the year 1300. Afterwards icecaps in Canada (Baffin Island in the Arctic) and on Iceland show growth, the researchers say.

The same thing would have happened somewhere around the year 1550.

Alternative cooling mechanisms

If these episodes of high volcanic activity were indeed the cause of the Little Ice Age, they point to the importance of positive climate feedbacks, most probably the albedo effect of increased Arctic sea ice – to maintain the lower temperatures for several centuries on end, as the direct cooling effect of the sulfur aerosols dies out within one or two years after the eruptions.

Thus far scientists have also looked at the Maunder Minimum – a semi permanent solar minimum – to explain the Little Ice Age. This hypothesis was challenged last year by another Geophysical Research Letters publication – which states even during extreme solar minima the minimal solar irradiance is still too high to explain any substantial cooling.

As much of the Little Ice Age seems to have been an Arctic and European phenomenon* though, there seems to be another solar route to explain the local temperature dip – and especially the colder winters: a more dominant negative phase in the Arctic Oscillation and the North Atlantic Oscillation – that’s at least what the 20th century climate record would suggest.

[*) Associated climatic variations have been observed on all continents – and at least Atlantic North America was notably colder during the Little Ice Age as well.]

And what was again that other thing that so clearly separated both the Artic and European climate from the rest of the Earth? Indeed, the thermohaline circulation – the Gulf Stream. There is some variation after all.

You’ve guessed it – we’re not convinced yet that it was merely down to these two volcanic episodes. And that is mostly because to our liking the really cold part of the Little Ice Age happened a bit too late (several centuries) after these sulfur explosions.

Can’t we settle for a combination theory? After all, if Arctic albedo is so self-sustaining after one temperature dip – why then did the LIA end after all? Or was that already our early 19th century climate forcing?

© Rolf Schuttenhelm | www.bitsofscience.org