It is now generally thought that the aberrations occurred because of the 5 April – 15 April 1815 volcanic eruptions of Mount Tambora[4] on the island of Sumbawa in the Dutch East Indies (in today's Indonesia) which ejected immense amounts of volcanic dust into the upper atmosphere.

From Wiki: Year without Summer

And on the topic of GD: how about here, Real Climate April 2006 admitting that GD makes the GW model "complex"?

Or here, the NOVA documentary?

Or here, wiki page on GD?

Greenland has, in the past, warmed by as much as 7-8 degrees centigrade in under 50 years, and yet Greenland has also shown shifts towards ice age temperature ranges that have occurred in less than 5 years.

This is all basic climate science. Rapid climate change in both directions is the admission that climate scientists have had to make to themselves mostly from the early 1990s forward although a number of pioneers have explored this idea even earlier while the dominant thinking still restricted itself to climate changes occurring only over thousands of years. The truth is that climate can change drastically in a few decades to levels either hot or cold not experienced by homo sapiens since the advent of agriculture and modern civilization. A significant shift in either direction would be a catastrophe for the current global civilization and yet we go on playing with GHGs as if we are exempt from the consequences of that action. In reality, it may not be our children or our grandchildren who suffer these consequences but ourselves. And frankly, that would be most fitting, wouldn't it?

P.S. Rises in GHGs and temperature are correlated for the ends of prior ice ages although which is cause and which is effect remains unclear. They also happen to be correlated for the ends of several prior warm periods as well. While I personally still believe that, barring other factors, current GHG emissions will lead to inevitable global warming, it remains possible that this is simply prelude to yet another ice age. One thing is dramatically clear - no current model has been accurate about the extent and rate of warming to date. All models have consistently underestimated what is happening.

We've had a number of volcano belches (El Chichon, Pinatubo, Krakatoa, Tambora) and none plunged us into an ice age.

Not recently, but Toba was a success, ice ages are "good for you" just as Global Warming : A mini ice age may have warmed human relationships.

Do not despair, Yellowstone is promising.