Volcanic eruptions can lead to temporary global cooling.  The Mt. Pinatubo eruption in 1991 led to slight global cooling for years.  With the Iceland volcano (smaller than Mt. Pinatubo) continuing to disrupt the atmosphere, and will we be in for some global cooling effect from the volcano?  Probably not, say most scientists– tentatively.  It depends on how long the volcano erupts, and whether another one will join it.  The effects of Pinatubo, according to NASA:

When Mount Pinatubo erupted in the Philippines June 15, 1991, an estimated 20 million tons of sulfur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere. . . . Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.

According to USAToday, Mount Pinatubo pumped ash for two days in 1991, and spewed it 70,000 feet into the stratosphere. This dropped temperatures worldwide about four degrees for about a year.

“When volcanic ash reaches the stratosphere, it remains for a long time,” reports Hufford. “The ash becomes a very effective block of the incoming solar radiation, thus cooling the atmosphere’s temperatures.”

Notice the temperatures dropping worldwide for only a year. That’s not nearly enough to offset global warming. So it’s hard to conceive of how much sulfur dioxide would need to be “sprayed” into the air via some global geoengineering method to slow down climate change for any appreciable length of time.

Global cooling could happen temporarily happens if the Iceland volcano Katla blows.

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.

“Depending on a new eruption, Katla could cause some serious weather changes.”

From Scientific American:

OSLO – A thaw of ice caps in coming decades caused by climate change may trigger more volcanic eruptions by removing a vast weight and freeing magma from deep below ground, research suggests.

While that’s not the case with Iceland’s Eyjafjallajokull glacier, which is too small and too light to affect local geology, other volcanoes on the island nation are seen as vulnerable.

“Our work suggests that eventually there will be either somewhat larger eruptions or more frequent eruptions in Iceland in coming [...]