The continued eruption of the Indonesian volcano Merapi has forced hundreds of thousands of people from homes, and the ash clouds has caused suspension of airline flights to and from the region. Yesterday, President Obama had to cut short his visit to Indonesia because of the threat. Historically, major volcanic eruptions in the tropics have been followed by atmospheric cooling. The eruption of Tambora in 1815 played a role in causing the "year without a summer" in New England in 1816. Krakatoa, another Indonesian volcano, produce cooling following its eruption in 1883. More recently, the greatest cooling measured after a volcanic eruption in the last half-century followed Pinatubo's outburst in the Philippines in 1991.

Tropical volcanoes can have a greater effect on the weather than similar eruptions at higher latitudes. In the tropics, where there tends to be upward air motion in the stratosphere, any materials that reach that height are pulled higher and tend to remain there for an extended period. When material does not extend that high, it can be quickly and easily washed out of the air by precipitation.

Dust and ash can restrict the amount of sunlight that reaches the ground, and thus cause cooling. The tell-tale sign that such material is present is the sight of vivid and colorful sunrises and sunsets. Volcanoes also spew out large amounts of sulfur dioxide, which is converted to sulfates in the atmosphere. This ingredient fosters atmospheric cooling, the opposite of the effect of increased carbon dioxide.

Some climate scientists have suggested that the relative cooling that was observed between the 1940s and the 1960s occurred because of the high amounts of sulfates in the atmosphere at that time (caused by human activity). Since then, various initiatives to clean the air have been successful, and so it is believed that with the decrease in sulfates, the warming influence of carbon dioxide has been greater than it would have been had the air remained so polluted.

The key to how much effect the current eruption could have will be determined how much of its byproducts reach the stratosphere. Eye witness reports on the ground have suggested the ash cloud has not reached that high, but accuweather.com meteorologist Henry Margusity showed me a seven-day movie from a European satellite that shows how sulfur dioxide has been spreading. He has also seen some satellite photos that show some of the ash reaching all the way up and past some towering thunderstorms. If an important level of sulfates does get into the stratosphere, it could disrupt the course of events we would normally expect with the La Nina event now taking place. While there is a scientific basis for expecting some effect that can be measured, what effects there are and how important they would be are still subjects of speculation. Certainly the last chapter on this issue has yet to be written.

The following video explores forecast issues for the Northeast and Great Lakes.

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