Volcanic Ash and Global Cooling

AP

As Iceland's Grimsvotn volcano erupts, sending ash high into the atmosphere, my thoughts turn again to global cooling. I remember in particular when Mount Pinatubo erupted. Here in Southern California summer never came. We often have a "June gloom" pattern here where there is a local swirling of air bringing the marine layer clouds inland for the morning hours, causing afternoons to be sunny and cool. This pattern usually ends promptly in July followed by 3 months of unrelenting heat and sun. That summer the clouds came, did not burn off, and continued through the summer.
Last year USA Today pointed out:

The potential eruption of Iceland's volcano Katla would likely send the world, including the USA, into an extended deep freeze.

"When Katla went off in the 1700s, the USA suffered a very cold winter," says Gary Hufford, a scientist with the Alaska Region of the National Weather Service. "To the point, the Mississippi River froze just north of New Orleans and the East Coast, especially New England, had an extremely cold winter.

NASA says Mount Pinatubo cooled the Earth measurably for 2 years:

On June 15, 200 Mount Pinatubo in the Philippines erupted with a tremendous force, ejecting vast amounts of ash and gas high into the atmosphere; so high that the volcano's plume penetrated into the stratosphere. The stratosphere is the layer of atmosphere extending from about 10 km to 50 km (6-30 miles) in altitude. Pinatubo injected about 15 million tons of sulfur dioxide into the stratosphere, where it reacted with water to form a hazy layer of aerosol particles composed primarily of sulfuric acid droplets. Over the course of the next two years strong stratospheric winds spread these aerosol particles around the globe.
Unlike the lower atmosphere (or troposphere, which extends from the surface to roughly 10 km), the stratosphere does not have rain clouds as a mechanism to quickly wash out pollutants. Therefore, a heavy influx of aerosol pollutants, like the plume from Mount Pinatubo, will remain in the stratosphere for years until the processes of chemical reactions and atmospheric circulation can filter them out. In the case of Mount Pinatubo, the result was a measurable cooling of the Earth's surface for a period of almost two years.

All the information you never knew you needed to know.  I might need to pick up a couple more spring cardigans.

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