In the ongoing culture war among climate scientists, climate scientologists and climate deniers, few things stands out like the effect of volcanoes. 

Volcanoes are well-known for cooling the climate but how much has been unclear, leading to radically differing claims and interpretations. Atmospheric chemists from the Tokyo Institute of Technology and the University of Copenhagen say that patterns of isotopes found in ancient volcanic sulfur trapped in ice cores, and patterns due to stratospheric photochemistry, are a way to say for sure which historic episodes of global cooling were caused by volcanic eruptions. 

 Powerful volcanoes can shoot gases through the atmosphere and high into the stratosphere where it can affect climate globally for a year or more. Less powerful eruptions can also have powerful impacts, but only locally, and for shorter times. And that's the trick. High plumes spend longer in the harsh sunlight of the stratosphere, and that changes the chemical signature of the sulfur in the plume. The balance of various isotopes is changed according to very precise rules, they say. 

Matthew Johnson, associate professor in chemistry at  the University of Copenhagen, studies chemical mechanisms in the atmosphere, and wanted to provide a more precise tool to historians studying cold spells. "Historical records are not always so accurate. Some may have been written down long after the fact, or when a different calendar was in use by a different culture. But the chemistry does not lie.

"Using our method we can determine whether a given eruption was powerful enough for the plume to enter the stratosphere affecting global climate. If we can find material from ancient eruptions it can now be used to give an accurate record of global volcanic events extending many hundreds of thousands of years back in time."

Clue to fires found in ice

The best place to look for traces of the fiery events is in ice. Tracking climate history is performed on cores drilled from the ice shields of Greenland and Antarctica. Much like tree rings, the snows of each year is compacted into a layer representing that year. As you go further down in the borehole, you descend deeper into history.

If volcanic material shows up in a layer, you know there was an eruption in that year. Using the method developed by Johnson and his colleagues it is now possible to analyze exactly how powerful a given eruption was.

"With the sulfur isotope method, we now have a way to prove whether a given eruption was so explosive that it entered the stratosphere, affecting global climate and civilizations, or, whether a given eruption was confined to the troposphere and local in its effects" says Johnson. "There are many controversial eruptions. The Mediterranean island of Santorini blew apart and caused the end of the Minoan culture. But there is a huge debate about when exactly this occurred. 1601 was the 'year without a summer' - but nobody knows where the volcano was that erupted. There's debate over whether there was an eruption on Iceland in 527, or 535, or 541. The sulfur isotope trick is a definite method to solve debates like this and get the most information out of the ice core records."

Denmark has no volcanoes so collaborating with Japan made sense. "The Tokyo Institute of Technology specializes in analysis of the patterns of sulfur isotopes found in samples in nature, and was able to synthesize the isotopically labeled samples. The University of Copenhagen has a strong group in atmospheric chemistry and spectroscopy; the laboratory measurements were carried out in Copenhagen. Together we were able to do the experiments and build the atmospheric chemical model that demonstrated the stratospheric photo-excitation mechanism."


Published in PNAS