Question

What effect can a very large volcanic eruption have on earth’s climate?

Answer 1

Volcanic eruptions release a lot of greenhouses gasses into the atmosphere.

For example, lately (see paper in last week's Nature) a group related release of aerosols to the Atlantic Multidecadal Oscillation, which is a ~60 year cycle in barometric pressure over the North Atlantic, which in turn drives temperature and precipitation cycles in North Africa, North America and Europe.

Answer 2

A very large eruption could cause a nuclear winter which can cool down everything, kill crops, and destroy society. So, let's say Yellowstone blew, it would cause a nuclear winter. Yellowstone is a supervalcano. You don't need to worry since, Yellowstone isn't going to blow in the near future.
Source(s):
http://en.wikipedia.org/wiki/Nuclear_win …