Global Warming Questions

One of my concerns about the global warming debate is the failure of those who say it exists to address arguments that undermine their views. For example, I just read this:

I also asked Dr. [Ian Howat of the University of Washington] about the argument that, since Greenland went through decades of relatively warm weather in the first half of the 20th century without catastrophic consequences, it’s unlikely that the glaciers are suddenly going to plunge into the ocean because of the current warming. His response:

Greenland was about as warm or warmer in the 1930’s and 40’s, and many of the glaciers were smaller than they are now. This was a period of rapid glacier shrinkage world-wide, followed by at least partial re-expansion during a colder period from the 1950’s to the 1980’s. Of course, we don’t know very much about how the glacier dynamics changed then because we didn’t have satellites to observe it. However, it does suggest that large variations in ice sheet dynamics can occur from natural climate variability. The problem arises in thepossibility that, due to anthropogenic warming, warm phases will become longer and more severe, so that each time the glaciers go through a period of retreat like this, they won’t fully grow back and they will retreat farther the next time.

So why was it so warm in the 1930s and 1940s? Does that tell us anything about why it is warm today? The global warming experts' answer to these questions, to the best I can tell, seems to be, "I don't know." Which isn't very satisfying, and leaves some reasonable doubt in relation to the global warming debate.