"The year without a summer" that occurred in 1816 near the end of the Little Ice Age .  The cooling was apparently caused largely by the 1815 volcanic eruption of Mt. Tambora, one of the largest eruptions in recorded history.


The reason why strong volcanic eruptions (such as the fairly recent 1991 Mt Pinatubo eruption in the Philippines) cause a short time change in climate (short term cooling) is shown in the figure above.  A strong eruption is capable of sending SO2  gas into the stable stratosphere (in a weaker eruption the SO2 would remain in the troposphere where it could be removed).  The SO2 can form sulphate particles (SO4) or small drops of sulfuric acid (H2SO4).  These particles and drops scatter and reflect incoming sunlight.  A decrease in the amount of sunlight reaching the ground causes cooling.  Because the stratosphere is stable the particles can remain for a few years. 

A nuclear war or an asteroid impact on the earth could have a similar effect (see nuclear winter).  In that case dark particles of dust and soot would be sent up into the stratosphere where they would absorb sunlight.  This would prevent the sunlight from reaching the ground and cause a drop in surface temperatures.  The absorption of sunlight would also warm the stratosphere creating very stable atmospheric conditions and making it harder to remove the particles from the atmosphere.