Redoubt volcano unlikely to have a major climate impact
Alaska's Redoubt Volcano continues to erupt, with the latest blast coming just after midnight Eastern time (7:41pm AKDT). The latest eruption threw ash 50,000 feet into the air, but the ash has settled to the ground and the ashfall advisory for cities to the north and northwest of Anchorage such as Talkeetna has expired. Redoubt is located about 100 miles southwest of Alaska's most populous city, Anchorage. The prevailing southerly winds deposited a swath of ash about 200 miles long to the north of the volcano (Figure 1). Redoubt last erupted between December 1989 - April 1990, and its ash clouds presented a major hazard to aviation. On December 16, 1989, Redoubt's eruption spewed ash into the air to a height of 14,000 m (45,000 ft) catching KLM Royal Dutch Airlines flight 867, a Boeing 747 aircraft, in the plume. All four engines stalled and the aircraft plummeted 13,000 feet before the pilot was able to restart the engines and land safely in Anchorage. The total costs to the aviation industry from the 1989 - 1990 eruption were about $100 million. Eighty percent of these costs were due to damaged equipment. For more information on the Redoubt eruption, check out the Alaska Volcano Observatory home page.
Figure 1. Ash on the snow to the north of Alaska's Mt. Redoubt crater in this true color image from NASA's Terra satellite. Image taken 21:49 GMT March 23, 2009. Image credit: Johnathan Dehn, Geographic Information Network of Alaska..
Redoubt's effect on the climate should be minimal
Many historic volcanic eruptions have had a major cooling impact on Earth's climate. However, Redoubt is very unlikely to be one of them. To see why this is, let's examine recent volcanic eruptions that have had a significant cooling effect on the climate. In the past 200 years, Mt. Pinatubo in the Philippines (June 1991), El Chichon (Mexico, 1982), Mt. Agung (Indonesia, 1963), Santa Maria (Guatemala, 1902) Krakatoa (Indonesia, 1883), and Tambora (1815) all created noticeable cooling. As one can see from a plot of the solar radiation reaching Mauna Loa in Hawaii (Figure 2), the Mt. Pinatubo and El Chichon eruptions caused a greater than 10% drop in sunlight reaching the surface. The eruption of Tambora in 1815 had an even greater impact, triggering the famed Year Without a Summer in 1816. Killing frosts and snow storms in May and June 1816 in Eastern Canada and New England caused widespread crop failures, and lake and river ice were observed as far south as Pennsylvania in July and August. Volcanic eruptions cause this kind of climate cooling by throwing large amounts of sulfur dioxide gas into the stratosphere. This gas reacts with water to form sulphuric acid droplets (aerosol particles), which are highly reflective, and reduce the amount of incoming sunlight.
Figure 2. Reduced solar radiation due to volcanic aerosols as measured at Mauna Loa Observatory, Hawaii. Image credit: NOAA/ESRL.
You'll notice from the list of eruptions above that all of these climate-cooling events were from volcanoes in the tropics. Above the tropics, the stratosphere's circulation features rising air, which pulls the sulfur-containing volcanic aerosols high into the stratosphere. Upper-level winds in the stratosphere tend to flow from the Equator to the poles, so sulfur aerosols from equatorial eruptions get spread out over both hemispheres. These aerosol particles take a year or two to settle back down to earth, since there is no rain in the stratosphere to help remove them. However, if a major volcanic eruption occurs in the mid-latitudes or polar regions, the circulation of the stratosphere in those regions generally features pole-ward-flowing, sinking air, and the volcanic aerosol particles are not able to penetrate high in the stratosphere or get spread out around the entire globe. Redoubt is located near 59° north latitude, far from the tropics, and thus is unlikely to be able to inject significant amounts of sulfur aerosols into the stratosphere. Furthermore, the previous 1989 - 1990 eruption of Redoubt (Figure 3) put only about 1/100 of the amount of sulfur into the air that the 1991 eruption of Mt. Pinatubo did, according to the TOMS Volcanic Emissions Group. We can expect the current eruption of Redoubt to be similar in sulfur emissions to the 1989 - 1990 eruption, and have an insignificant impact on global climate.
Figure 3. Amount of sulfur gases put into the air by recent volcanic eruptions. Note that the 1989 eruption of Redoubt put only 1/100 the amount of sulfur dioxide (SO2) into the air that the 1991 eruption of Mt. Pinatubo did. Image credit: TOMS Volcanic Emissions Group.
For more information
Realclimate.org has a nice article that goes into the volcano-climate connection in greater detail. One interesting quote from the article: There can be some exceptions to the tropics-only rule, and at least one high latitude volcano appears to have had significant climate effects; Laki (Iceland, 1783-1784). The crucial factor was that the eruption was almost continuous for over 8 months which lead to significantly elevated sulphate concentrations for that whole time over much of the Atlantic and European regions, even though stratospheric concentrations were likely not particularly exceptional.
scienceblog.com has an interesting article about the largest volcanic eruption of the 20th century--the 1912 eruption of Alaska's Mt. Novarupta, located in the same chain of volcanoes as Mt. Redoubt. According to a NASA computer model, Novarupta's climate-cooling aerosols stayed north of 30°N latitude, and did not cause global cooling. However, the model indicates that the eruption may have indirectly weakened India's summer monsoon, producing an abnormally warm and dry summer over northern India.