A little over 13,000 years ago the world was emerging from the last Ice Age. The warming was started by orbital cycles, and amplified by greenhouse gases. However, a set of sedimentary records has shown a burst of volcanic activity accelerated the process by depositing ash on the great ice sheets of northern Europe. The finding makes sense out of some other recent discoveries, while also reinforcing concerns about our own emissions.
Recent evidence has shown temperature rises as the Earth left the last Ice Age were more jagged than once thought. Sea levels, rather than looking like a smooth ramp during the transition, resembled a set of stairs, it was revealed only last week, probably because ice sheets melted very fast in between periods of relative stability.
In part this can be attributed to volcanic activity under the West Antarctic Ice Sheet, injecting halogens into the atmosphere and messing with the southern hemisphere's atmospheric circulation. However, most of the ice melt was in the northern hemisphere, and there Dr Francesco Muschitiello of Columbia University has found a simpler volcanic explanation.
Muschitiello is first author of a paper in Nature Communications exploring a very precise 1,257-year-long record revealed by sediments from south-eastern Sweden, running from roughly 13,200 to 12,000 years ago. What was then the Baltic Ice Lake was filled each year by meltwater from the ice sheets of the era, which carried silt in summer. Over winter a layer of clay formed to neatly distinguish one year's runoff from another.
Melting was far from even through this period, and peaked at times that match high latitude volcanic eruptions – mostly from Iceland. We have a record of the timing of these from the ash deposited on Greenland's ice sheets. Muschitiello concluded that it was this ash that caused the increased melting.
Clean ice reflects sunlight. Ash absorbs it, warming the ice beneath so more of it melts, so the effect is certainly to be expected. The important thing about Muschitiello's work is it gives an idea of how important the volcanic ash was.
Eruptions increased while the world was leaving the Ice Age, probably because the reduction in ice, and accompanying erosion, released pressure above volcanic chambers. Consequently, it appears there was a positive feedback mechanism, where initial melting triggered an increase in eruptions, which lead to more melting. This worked in partnership with the better known positive feedback from raised carbon dioxide levels to extract the world from its prison of ice.
Although volcanic ash is not particularly problematic at the moment, black carbon and soot from factories is known to be hastening the melting of ice sheets today, and this work could help us understand the process.
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