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Major Bali eruption could cool the Earth for years to come

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Scott Sutherland
Meteorologist/Science Writer

Wednesday, November 29, 2017, 3:13 PM - Bali's Mount Agung appears to be on the brink of a major eruption, and according to scientists, such an event has the potential to cool the Earth for years to come, and could diminish the impacts of global warming for more than a decade.

Mount Agung, a stratovolcano on the island of Bali, Indonesia, has already been spewing ash and smoke for days now. Authorities have set up an 8-10 km exclusion zone around the volcano, and have issued evacuation orders that cover roughly 100,000 local residents, in anticipation of a major eruption that they say could be imminent.

The immediate threat to life and property from such an eruption is very high. The latest estimate from Indonesia's disaster mitigation agency says that less than half of those under the evacuation orders have actually left, and until Wednesday, Bali's I Gusti Ngurah Rai International Airport was closed, with all flights grounded, due to the danger the volcano's ash cloud represented for any aircraft that may fly near or through it.

In addition, there is also the potential for a long-term impact due to a major eruption from Mount Agung, one that could affect the planet for more than a decade.

As shown in the graphs below, a major, explosive eruption from Mount Agung in 2017, similar to the 1963 eruption from this same volcano, could cool the planet by as much as 0.16oC over the next two years. Thus, by 2019, rather than having temperatures that place the year as 2nd warmest on record, after 2016, the year could come in at roughly the same as 2014 (currently 3rd warmest, and likely 4th warmest once 2017's temperature records are tallied).

This animation, from Climate Brief and posted to SkepticalScience.com, shows the projected global temperatures up until 2030 for two scenarios - no major eruption from Mount Agung (left) and a major eruption from Mount Agung (right). If such an eruption takes place this year, global temperatures could drop by more than a tenth of a degree by 2019, and the reduction in global temperatures could still be felt until at least 2030. Click the image to access an interactive version of the graph. Image Credit: Carbon Brief. Animation: Scott Sutherland

The reason for this cooling is not the plume of ash and smoke belching from the volcano. The ash and smoke can certainly block out sunlight on a local scale, however the global impact would come from an explosive eruption blasting millions of tonnes of sulfur dioxide up into the stratosphere. In the stratosphere, sulfur dioxide (SO2) goes through a series of chemical reactions, resulting in sulfuric acid aerosols, which are extremely good at reflecting sunlight back into space. With a reduced amount of sunlight reaching Earth's surface, less infrared radiation would be emitted by Earth, which, in turn, would mean less infrared radiation trapped by greenhouse gases in the atmosphere. This would result in cooler temperatures, overall, at least compared with the most recent years in the record books.

The effect would only be temporary, however. As the sulfur aerosols settled out of the atmosphere, the amount of sunlight reaching the surface would return to normal, and global warming would get right back on track. Despite the brief cooling period, once it was over, global temperatures would sore even higher than before, due to the presence of even greater concentrations of heat-trapping carbon dioxide in the atmosphere.

This kind of effect is something we have already witnessed, in the aftermath of several large volcanic eruptions over the years. The eruption of Mount Tambora, in 1815, caused the infamous 1816 "year without a summer", and in more recent years, we've seen similar cooling impacts due to eruptions from Mt. Agung in 1963, Mexico's El Chichon in 1982 and Mt. Pinatubo, in the Philippines, in 1991.

This diagram shows the effect of explosive volcanic eruptions, injecting sulfur dioxide and other compounds into the stratosphere, which can cause a global cooling effect for years after. Credit: United States Geological Survey (USGS)

This natural process works so well that scientists exploring the potential applications of solar geoengineering have contemplated artificially injecting SO2 into the stratosphere, to hold back the effects of global warming. While implementing this kind of plan could give us extra time to transition away from fossil fuels, it fails to address the growing problem of ocean acidification. Thus, along with potential impacts on Earth's ozone layer, as these sulfur aerosols are known to enhance the loss of stratospheric ozone, this plan has remained on the drawing board.

A global cooling effect from a Mount Agung eruption is not, by any means, a sure thing. First, it must be an explosive eruption, powerful enough to launch megatons of sulfur dioxide around 30 kilometres up into the sky. If that doesn't happen, the addition of SO2 into the troposphere from weaker eruptions would not produce the reflective aerosols in the stratosphere needed to reflect sunlight back into space. Second, even if the eruption is that substantial, the Earth's large climate regulators, such as El Niño, could diminish its long-term impact. If the La Niña pattern that is currently developing in the Pacific persists, or we go through multiple La Niñas over the next few years, while at the same time having a layer of sulphur aerosols in the stratosphere, the combined cooling impact will be significant. If, however, we see the development of an El Niño pattern, the warming effect across the Pacific would likely offset much of the cooling from the aerosols. 

The hope, right now, is that the volcano will not experience a major eruption. A few years of temporary cooling is simply not worth the destructive impacts such an event would unleash on the local population, the potential far-reaching effects if regions of the world suffered through another "year without summer", and the possible loss of any progress in the recovery of the Antarctic ozone hole.

Sources: SkepticalScience.com | USGS | With files from Reuters

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