Major volcanic eruptions can have a temporary cooling effect on Planet Earth. The eruptive power of a big one is strong enough to propel ash plumes directly into the stratosphere, where sulfur dioxide aerosols (liquid droplets)and volcanic gases can linger for months, deflecting sunlight, and lowering world temperatures for a short time.
In 1991, the eruption of Mt. Pinatubo in the Philippines sent an ash plume 20 miles (34km) high, where upper atmosphere winds circulated the aerosols and gases around the world, dropping temperatures by 0.9°F (0.5°C). The eruption of Indonesia’s Mt. Tambora in 1815 produced what was called The Year Without Summer, when lack of sunlight led to widespread crop failures and a spike in lung disease. The Krakatoa eruption in 1883 lowered earth’s temperature by as much as 2.2°F (1.2°C), and disrupted world climate for 5 years.
It was assumed that only the biggest eruptions were capable of sending volcanic aerosols into the stratosphere. Now a new study indicates that some smaller eruptions can do the same thing. The study, led by Adam Bourassa of the University of Saskatchewan, found that sulfur dioxide from a small eruption at Mt. Nabro in northeast Africa “hitchhiked” its way into the stratosphere. Warm air rising from the seasonal Asian Monsoon lifted Nabro’s aerosol from the lower atmosphere into the stratosphere, where it was detected by the Canadian Space Agency’s OSIRIS, an instrument specifically designed to measure atmospheric aerosols. Even though the sulfur dioxide came from a small eruption, the concentration of it from Mt. Nabro was the largest load of stratospheric SO2 aerosol ever recorded by OSIRIS in its 10 years of operation.
The average annual number of volcanic eruptions worldwide has remained constant at 50 to 70 for many years. Most are minor. About half are explosive, ejecting some material into the atmosphere, and half produce lava flow only. Most of the gases and aerosols produced by the smaller eruptions go no farther than the lower atmosphere, where they quickly dissipate. A small percentage may encounter the right combinations of winds and updrafts to be carried, like the emissions from Mt. Nabro, into the stratosphere. Whether enough of it collects there to make a difference is still under study.
The idea of cooling the planet by injecting volcano-like aerosols into the stratosphere artificially has been under study by the scientific community for many years, but so far no practical means of accomplishing this has been worked out. The field is called geo engineering. Besides the engineering challenges of such an undertaking, other potential problems that have been blocking progress include the fear of harmful side effects and other unintended consequences.
Author’s note: Using geo engineering to cool the planet is one of the major themes in my novel Red Hot Sky.