Another Volcano, Another Cold Snapby Bill Chameides | November 13th, 2009
posted by Erica Rowell (Editor)
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In the aftermath of the 1991 Mount Pinatubo explosion, global temperatures plummeted, providing direct evidence volcanic explosions can be a driver of global cooling. (USGS / J.N. Marso, July 1991)
Analysis of sulfur isotopes in ice cores fingers a volcano in the 19th-century dip in global temperatures.
Studying the climate is kind of like putting together a jigsaw puzzle. We’ve got enough puzzle pieces in place to know the basic picture — for example that greenhouse gases have contributed to the recent global warming trend. But there are still parts of the puzzle that remain fuzzy, and scientists continue to debate which puzzle pieces belong there.
We know that global temperature fluctuations can be caused by a variety of factors. In addition to greenhouse gases, solar variations and oscillations in the Southern Ocean between El Nino and La Nina are important. On longer time scales, variations in the Earth’s orbit about the sun can also be a factor.
Volcanoes, Sulfur Dioxide, and Global Cooling
Large volcanic eruptions that inject huge amounts of material into the stratosphere also drive climate change — but unlike greenhouse gases, they cause global cooling. You can chalk that up to the presence of sulfur dioxide (SO2) in the volcanic output.
Once in the stratosphere, that SO2 is spread around the globe and oxidized into sulfate particles, which typically remain there for a few years. The sulfate particles scatter and reflect sunlight and in so doing cause a cooling of global temperatures. (Another effect of those sulfate particles in the stratosphere is spectacular sunsets.)
A prime example of volcanic cooling is the 1991 Mount Pinatubo eruption. Inspection of the global temperature record clearly shows a precipitous drop following the explosion and then a slow recovery over several years. Such events serve as great experiments for climate scientists who can use them to quantify how the climate responds to other perturbations such, as greenhouse gas warming.
The 19th-Century Puzzle Piece
One of the coldest decades of the last few centuries occurred between 1810 and roughly 1819. Particularly extreme was 1816 — the “year without a summer.”
Scientists have debated for decades over the cause of those cool temperatures. Was it volcanoes ? Was it a decrease in solar activity? Was it both?
The evidence for solar activity is significant. The decade was midway through the period known as the Dalton Minimum, during which solar activity was anomalously low. The Little Ice Age, which lasted about 400-600 years, had peaked during the Maunder Minimum (somewhere between 1645 and 1715), another period of extremely low solar activity.
Yet the evidence in favor of a role for volcanoes is also strong. One of the largest eruptions in recent history was observed from Indonesia’s Mount Tambora in 1815, just before the year without a summer. But what about those anomalously cool temperatures between 1810 and 1815?
A new paper in press in Geophysical Research Letters by Jihong Cole-Dai of South Dakota State University and colleagues provides new evidence pointing to an earlier volcanic eruption as the culprit.
Variations in the relative concentrations of two sulfur isotopes (S-34 and S-33) in ice cores recovered from Greenland and Antarctica suggest that the sulfur in those ice cores came from a volcanic eruption. The simultaneous appearance of the sulfur signature in ice cores from both poles (i.e., Greenland and Antarctica) suggests that the sulfur was globally distributed and thus must have come from the stratosphere. In addition, the timing of the sulfate deposition in cores from both locations suggests that the as-yet-identified eruption occurred in 1809 just before the cool decade began.
This is cool stuff, and pretty compelling, but I suspect the debate is not over and we can look forward to seeing the solar-variation camp and volcano camp joust over this puzzle piece for many years to come.filed under: Antarctica, El Nino-Southern Oscillation, faculty, global warming, La Nina, Planetary Watch
and: El Nino-Southern Oscillation, global cooling, Greenland, La Nina, Mount Pinatubo, Mount Tambora, solar cycle, solar variation, sulfur dioxide, volcanoes