In June of 1783, the Laki volcano in Iceland started an 8-month-long eruption sequence, including 10 explosive eruptions and continuous emission into the lower atmosphere.
The event had far-reaching consequences: Air pollution and acid rain decimated crops in Iceland, contributing to a famine responsible for the deaths of 60% of the island’s livestock and 20% of the human population within a year. In Europe, a layer of hazy “dry fog” settled over the continental mainland, leading to respiratory problems, headaches, and other health problems for millions.
Although these consequences of the eruption are well documented, scientists have been conflicted as to whether the global climate anomalies in the following year were caused by the eruption or merely the result of normal year-to-year variability.
Following a large volcanic eruption, scientists typically expect to see net cooling effects due to the release of sulfur dioxide, which travels to the stratosphere and forms sulfuric acid aerosols. These aerosols reflect incoming light from the Sun, effectively shading the planet. But eruptions can also exert a warming influence on the climate as they release large amounts of greenhouse gases.
Following the Laki eruption, the summer of 1783 was anomalously warm in Europe, leading some experts to question whether a magnified greenhouse effect was responsible for the heat wave. Similarly, the winter of 1783–1784 was particularly cold in Europe, and scientists have wondered whether the negative radiative forcing from sulfuric acid aerosols could be to blame.
In a new study, Zambri et al. used the Community Earth System Model from the National Center for Atmospheric Research to simulate the Laki eruption in conjunction with other sources of natural climate variability from the time period.
Their analysis indicated that the abnormally warm European summer of 1783 was, in fact, not attributable to the Laki eruption. Instead, the researchers found that the high temperatures could be explained by atmospheric blocking, in which a high-pressure system to the north of the continent impeded the southerly flow of cold polar air, keeping Europe warmer than average. In fact, the authors suggest that without the negative radiative forcing from the eruption, Europe may have been even hotter still.
As for the cold winter that followed, the researchers concluded that the Laki eruption was not the cause of the negative North Atlantic Oscillation (NAO) and thus not responsible for the anomaly—at least not for the months of December, January, and February. For the later winter and into the spring, the researchers report that they did find a “robust” negative NAO response as well as an El Niño–Southern Oscillation response to the Laki eruption, suggesting, again, that the primary climate impact from the event was cooling.
The results, the scientists say, show the limitations in forecasting how future eruptions may shape the planet’s climate in the short term and help to paint a clearer picture of how various climate forces interact. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1029/2018JD029554, 2019)
—David Shultz, Freelance Writer