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March 18, 2004

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Global reach of volcanic gases

When we think of the negative impact that volcanoes have on people, the local geographic area of the eruption generally comes to mind. Ash, lava, and mudflows are all destructive and hazardous for people who live close to volcanoes.

A less well-known product of eruptions, however, sometimes has a more widespread effect. The gases emitted by volcanoes can, under certain circumstances, affect global climate and even cause mass extinctions of flora and fauna.

Sulfur dioxide (SO2) is the main gas released by volcanoes that can affect climate in the short term. Chemical reactions that occur when SO2 reaches the atmosphere produce tiny sulfuric acid droplets called ?aerosol.? Very energetic eruptions push the aerosol up into the stratosphere, the layer of the atmosphere from 10 to 50 km altitude (around 32,000 to 164,000 ft), where it inhibits the sun?s energy from reaching and warming the earth?s surface.

Once in the stratospheric jet stream, the aerosol quickly encircles the globe. The microscopic droplets tend to remain aloft for months to years, promoting global cooling.

In the longer term, huge volcanic eruptions can have another effect. The release of greenhouse gases, such as carbon dioxide, into the atmosphere can cause warming rather than cooling.

Notable eruptions in recent years appear to have affected climate. One example is the 1991 eruption of Mount Pinatubo in the Philippines, which injected nearly 20 million tons of SO2 into the stratosphere that became dispersed around the globe in about 3 weeks. The recorded effect was a 0.5 degrees C (0.9 degrees F) drop in temperature for the following two years.

Remember, this small-sounding temperature decrease is a global average. Even small temperature changes can affect weather systems. The year after the eruption, the U.S. experienced its third coldest and wettest summer in 77 years, and major flooding of the Mississippi River occurred. These observations are consistent with predictions made by climate modelers of Pinatubo?s effect.

The much larger eruption of Tambora, Indonesia, in 1815 produced the greatest volcanic effects on climate in recorded history, with as much as 1 degree C (1.8 degrees F) global temperature decrease, causing crop failures in Europe the following year, termed ?the year without a summer.?

Two types of eruptions have never occurred during human history but have the potential to affect climate even more severely. Supervolcanoes are those responsible for the earth?s largest-ever explosive eruptions. They release thousands of times more energy than any eruptions witnessed by humans. Supervolcano eruptions are thought to be able to cause potential global temperature decreases of as much as 5 degrees C (9 degrees F). Such a decrease is enough to have a major negative impact on climate, weather, and ecosystems.

The other type of volcanic event yet unwitnessed by humans, but with potentially disastrous climatic effects, is a flood-basalt eruption, which discharges huge volumes of very fluid basaltic lava onto the earth?s surface. Flood-basalt events are unlikely to have the explosive power of a supervolcano, but they release far more SO2 and CO2 than do more common eruptions such as that at Kilauea today.

Recent evidence links the most devastating extinction event ever - estimated to have killed 95 percent of life on earth at the time - with the eruptions that produced the Siberian Traps, one of the biggest flood-basalt fields, some 245 million years ago. The volume of lava erupted is thought to have been at least 1 million cubic kilometers (240,000 cubic miles), a huge amount compared to the relatively meager 2.6 cubic kilometers (0.6 cubic mile) erupted over the past two decades by the ongoing Pu`u `O`o?Kupaianaha eruption. Most of the species were probably killed as a result of greenhouse warming caused by CO2 release. The warming lasted longer than any initial cooling caused by SO2.

As we ponder the localized effects of Kilauea?s gas emissions and vog, such as the acrid, milky appearance of the sky downwind of the eruption and the decrease in rainfall in South Kona since the eruption began, we can be thankful that this eruption has been relatively kind to the planet as a whole.

Activity update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued during the past week. Most lava flows have been at the lower and upper ends of the rootless shield complex along the Mother?s Day lava tube south of Pu`u `O`o. The flows have been small and short-lived. Vents within the crater of Pu`u `O`o remain incandescent and are sometimes visible from Mountain View and Glenwood. No active flows have reached Pulama pali or the coastal flat below Paliuli, and no lava is entering the ocean.

Two earthquakes were reported felt on the island during the past week. The largest, at 01:26 March 13, had a magnitude of 3.4 and was felt at Kailua, Kona Palisades, and Waikoloa. It was located about 25 km (16 miles) west-southwest of Waikoloa at a depth of 49 km (30 miles). The second earthquake was felt at HVO and the Kilauea summit region at 09:06 March 18. It had a magnitude of 3.3 and occurred at a shallow depth of 3 km (2 miles) only 5 km (3 miles) south of Volcano village.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with no earthquakes located in the summit area during the past week.

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Updated: March 24, 2004 (pnf)