It’s a volcano in Iceland which erupted in 2010. Please don’t ask me to pronounce the name.
In comments to the last post, there was speculation (and even evidence) that the CO2 emitted by the eruption was less than the CO2 which was not emitted because of the curtailing of air traffic which it caused. What piqued my interest is that one particular comment suggested that the principal effect of the eruption on atmospheric CO2 might have been the iron content of the volcanic ash, which may have drawn down atmospheric CO2 by “iron fertilization” of the oceans:
The eruption [of Eyjafjallajökull] may have affected atmospheric carbon dioxide levels by fertilizing oceans with iron. According to the Nordic Volcanological Center at the University of Iceland ash samples contained 8 to 12% iron oxide. Observations at the Mauna Loa Observatory show increased carbon dioxide absorption for each of the three months following the eruption compared to the 30 year mean for the same months. Over May, June and July 2010 atmospheric carbon dioxide decreased by a total of 2.40 ppm. The thirty year mean for the same months is 1.66 ppm with a standard deviation of 0.52ppm. The probability of a chance result is less than 8%.”
I agree that it’s not likely to be due only to random fluctuation (but it could be). Still, that doesn’t mean it’s due to iron fertilization, or even to the volcanic eruption, because many things can influence the April-to-July difference in atmospheric CO2.
CO2 concentration generally declines from April to July, because that’s the time during which northern hemisphere land plants grow rapidly, absorbing CO2 to build their tissues during spring and summer. In fall and winter, the decay of land plants returns that CO2 to the air, all part of a notable annual cycle in atmospheric carbon dioxide concentration:
However, the annual cycle itself has not remained constant over time. In fact the size the of the cycle (its amplitude) has increased, mainly during the 1970s. Here’s the semi-amplitude (which is just half the amplitude) estimated from wavelet analysis:
In addition to the overall increase (mostly in the 1970s), there’s also fluctuation in the cycle’s amplitude.
Yet another factor which affects the April-July difference is the timing of the annual cycle. Here’s the time of the annual peak, also estimated by wavelet analysis:
It turns out the the annual cycle in CO2 has been peaking earlier in the year, having migrated about 10 days since we’ve been monitoring it at the Mauna Loa atmospheric observatory.
The overall increase in CO2 (the trend, not the annual cycle) has not been linear — its increase is faster now than when the Mauna Loa data begin. But since about 1995, the increase has been at least approximately linear. Therefore I fit, to the data since 1995, a straight line to model the trend, and a 4th-order Fourier fit to model the annual cycle:
That leaves these residuals (click the graph for a larger, clearer view):
There is indeed a decline (even in the residuals) from April to July 2010. But that seems to be because the April 2010 value was higher than usual, not because the July 2010 value was lower than usual.
I also computed the difference from April to July for the data since 1995:
The latest value (from 2010) is one of the lowest, but is not the lowest, and frankly, it doesn’t strike me as that unusual.
Therefore it’s my opinion that the not-so-extreme decline from April to July 2010, coupled with the higher-than-usual April value, coupled with the changes (both trend and fluctuation) in both the size of and the timing of the annual cycle, are such that there’s insufficient evidence to conclude that the Eyjafjallajökull eruption caused a noticeable change in atmospheric CO2, whether by emissions from the eruption, the lack of emissions from air traffic, or iron fertilization of the oceans. Just my opinion.