With the recent, rather large eruption of the Chilean volcano, Calbuco, it is important to understand the impact of volcanic eruptions on climate. Despite common belief, the greatest impact of an erupting volcano comes from ash fallout, not the lava flow. Explosions from volcanic eruptions can throw ash and debris miles into the atmosphere. This ash can even be picked up by the jet stream and carried across several continents.
Ash blown high into the atmosphere is concerning for our climate because it doesn't disperse easily. This is due to the airflow in the upper atmosphere (above the jet stream) being extremely slow, which allows the ash to linger for several years. Unfortunately, this airborne volcanic ash contains a high amount of sulfur dioxide and sulfuric acid, which essentially “blocks” sunshine (solar radiation) from entering the lower atmosphere in which we live. With less sunshine and thermal radiation reaching the surface, a cooling effect is produced, particularly under where the ash, sulfur dioxide, and sulfuric acid is “trapped” in the upper atmosphere.
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In North America, the most explosive eruptions of the past two million years have occurred with the Yellowstone Super Volcano. The Yellowstone eruptions of the past were 1 – 2 thousand times GREATER than Mt. St. Helens in 1980. If an eruption of that size were to happen again, the effects would be catastrophic. Several inches of ash would accumulate over the Great Plains, with layers of ash stretching all the way to the East Coast. The thick ash could kill crops and destroy machinery. The excessive amount of ash and sulfer dioxide thrown into the upper atmosphere would cool the entire globe substantially. Some models estimate the planet could cool down 6 to 10 degrees on average. The largest cooling effect would be felt across the northern hemisphere where average temperatures could drop over 10 degrees.
The exact impacts of a Yellowstone eruption are dependent on a few things, including the eruption size and time of year. The last two eruptions occurred 1.3 million and 640,000 years ago. Geologists are certain another massive erupiton will occur in the future. However, there is no need to panic! There are many detection and tracking systems spread throughout Yellowstone designed to monitor the volcano's activity. Although we are technically in the general timeframe for the next eruption (give or take several thousand years), experts believe the chances of another supereruption occurring within our lifetime is exceedingly low.
References:
http://www.yellowstonepark.com/wildlife/more-yellowstone-wildlife/
http://www.bbc.co.uk/sn/tvradio/programmes/supervolcano/article2.shtml
http://volcanoes.usgs.gov/observatories/yvo/
