There is the tendency in our fast-paced world for lots and lots of articles to get written about science before anyone beyond the researchers and the reviewers actually sees the science. This is mostly thanks to the fact that press releases come out before the actual study - and who has time to read a study when there is a handy press release with all the bits? Yesterday saw an example of just this - a whole lot of "news" without a lot of assessment of the study itself. The paper itself is called "Abrupt Onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks" by Gifford Miller and a host of coauthors (mostly climatologists) in the Geophysical Research Letters. After seeing a post about it on Dot Earth, I knew that the media would eat this up and wouldn't you know it, within hours there were dozens of articles mostly telling us what the initial press release already said ... and not much else. It took a while for the PDF of the article to appear on the GRL website, but after it did, I sat down with it to see what the "smoking guns" were that they identified. I'm not going to discuss the climate models or interpretation - more or less, they sampled moss and lake sediment in Canada and Iceland to constrain the dates of the onset of the Little Ice Age. Then, they used climate models and data about volcanic atmospheric sulfur (from Gao et al., 2008, more on this paper in a bit) to model how the atmosphere and oceans would respond and if it correlated with their ages. The long and short is they found that a large sulfur loading in the atmosphere could trigger increased sea ice that would prompt cooler global climate, thus the Little Ice Age. What I was interested was their assertion that it was volcanism, specifically with what they call "two of the most volcanically perturbed half-centuries of the past millennium", specifically 1275-1300 AD and 1430-1455 AD. Were they really that perturbed and are there, as they claim (from data out of Gao et al., 2008), at least 4 large explosive eruptions between 1275-1300 AD and one massive eruption in 1452 AD? Interestingly (and frustratingly), they never actually mention a single volcano by name, so off to the Global Volcanism Program "Largest Holocene Eruptions" database I went. Before I get there, I feel I should discuss this Gao et al. (2008) paper. It looked at ice cores in the northern and southern hemispheres to determine sulfur loading in the atmosphere from volcanoes over the last 1,500 years. They found a number of spikes (see Fig. 2 from the paper below). However, I look at that spectrum and don't see what Miller et al. (2011) claim - sure, there is a big spike during the late 1200s/early 1300s (more on this later) and a single spike around 1450. However, what about the late 1700s/early 1800s? I'd say it is more "volcanically perturbed" than the 1430-1455 period they identify. Overall, you could even make an argument for lower total but sustained larger eruptions around 1600 as well.
In looking at the "Largest Holocene Eruptions" database, I wanted to try to identify the volcanoes in questions - likely eruptions that were at least VEI5-6 to allow for a large plume and maximum dispersal of volcanic aerosols like sulfur dioxide. However, some eruptions do release more sulfur dioxide than their size might imply - so we need to keep that in mind. Potential culprits for 4 large explosive eruptions between 1275-1300 AD:
Quilotoa, Ecuador (VEI 6 in 1280)
And that's it. So, unless Gao et al. (2008) or Miller et al. (2010) know something we don't, there are a number of mystery eruptions in that period. Interestingly Katla and Hekla both had VEI 4 eruptions in 1262 and 1300 respectively, so these could have influenced the northern hemisphere more strongly, but so far, it seems that Quilotoa is the only monster here. So, what about 1430-1455 AD? Culprits might include:
Pinatubo, Philippines (VEI 5 but dated at 1450 ± 50 years)
And that's it. However, between the 1450 eruption of Pinatubo and 1482, there might have been four VEI 5+ eruptions - Pinatubo, Sakurajima, Japan (1471), Bardarbunga, Iceland (VEI 6 in 1477) and two VEI 5 from St. Helens in 1480 and 1482. That seems to me to be much more "volcanically perturbed" than 1430-1455. Now, the question becomes: whose dating method do we believe? Maybe these four big eruptions really did happen between 1430-1455 instead of 1450-1482. Maybe it was the other way around? Well, that is what future research might solve. I'm not going after the Miller et al. (2011) paper with any agenda. However, when I see headlines in the news that unequivocally say "Little Ice Age was caused by Volcanoes", well, I'd like to see the evidence. Sure, we have a lot of correlations - evidence of cold climate with evidence of higher sulfur loads in the atmosphere. However, right now we don't have eruptions to match with the big sulfur loads. This is not to say they might not have been identified yet - you'd be surprised how easy it might be to hide the evidence of a large eruption in some remote place in the world (Kamchatka? Aleutians? High Andes?) However, there is also a chance that the sulfur loads might be more local events spread across the world rather than a big single eruption (or string). What the Miller et al. (2011) study does show is that, with their model, you can generate a period of global cooling if you have a string of especially explosive, sulfur-rich eruptions in succession - however, that is not the case today.
Image 1: Quilotoa caldera in Ecuador. Image by Eric Schmuttenmaer/Flickr.
Image 2: Figure 2 from Gao et al. (2008), Geophysical Research Letters
