What effect does volcanic ash have on our climate? Up to now, scientists assumed that eruptions with high proportions of sulfur dioxide and which reached high altitudes caused aerosol formation. These aerosol clouds can cool the earth. Another effect, the fertilization of ocean algae, was now investigated by researchers of the Cluster of Excellence "Integrated Climate Analysis and Prediction" (CliSAP) of the University of Hamburg. The first results will appear in "Atmospheric Chemistry and Physics" and "Journal of Geophysical Research".
"In extensive areas of the ocean, algal growth is limited by iron. Iron salts in volcanic ash can cause algal blooms, thus fixing large amounts of CO2", according to Dr. Bärbel Langmann of the Institute for Geophysics. The algae take up the greenhouse gas in their metabolism, removing it from the atmosphere.
Dr. Langmann, Dr. Klemen Zakšek and Prof. Dr. Matthias Hort of the Institute for Geophysics showed that this phenomenon occurred in connection with a massive algal bloom in the Gulf of Alaska in 2008 in connection with the eruption of the volcano Kasatochi. In less than 24 hours the volcano spewed up to 600 megatons of ash into the air. Calculations show that the amount of iron contained would be sufficient to account for the algal bloom observed. The fact that the ash column reached altitudes of up to 15 km was favorable for long-range transports.
In comparison: the Icelandic Eyjafjallajökull has been erupting ash for a month now, but the amount per unit time is less than a tenth of that from the Alaskan volcano. It is interesting for climate researchers that a measurement buoy off the Alaskan coast actually registered a decrease in the concentration of the greenhouse gas CO2 in seawater in 2008.
A model study by the scientists of the University of Hamburg showed the trajectory the ash followed in the atmosphere: "More than 90% of the ash had fallen into the ocean after a few days. Our analysis shows that this amount of ash and the iron contained in it are sufficient to cause a fertilization effect, even after the long trajectory" Dr. Langmann explains.
The degree to which an increased input of volcanic ash can influence the climate in general will be studied during further investigations at the University of Hamburg. Central questions are: What are the exact amounts of iron contained in the ash? Are there certain conditions conducive to the formation of iron salts during a volcanic eruption? At the same time, computer models help to couple previous results with further factors. For example, in winter more nutrients are generally available, but there is only limited light. A "fertilization" would have no climate effect in this case.
More information in:„Volcanic ash as fertilizer for the surface ocean”, B. Langmann, K. Zaksek, M. Hort and S. Duggen, erschienen in Atmospheric Chemistry and Physics
„Atmospheric distribution and removal of volcanic ash after the eruption of Kasatochi volcano: A regional model study”, Journal of Geophysical Research (in press, accepted 18 March 2010)
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