"It's ignorant to think the time frame of manned space exploration in the next few decades is relevant to the danger from impact-induced extinction."

This is true. The only possible impact-induced extinction could from a very large comet. As far as I'm aware we never seen a comet large enough to do this. We could hit by comet big enough to kill millions to billions of people within next few decades, but this is very low odds.
The highest possibility in terms of being hit by an impactor within next few decades is to be hit by something smaller than 200 meters. Being hit by 100-200 meter diameter rock will be around the biggest thing the modern humans have ever witnessed in terms of destructive force, but it won't cause extinction.

Hmm, now have to check on the sizes of biggest comets:
NEAT:
"The actual comet itself, the chunk of rock and ice, is only a few miles across," he said. "What we see in the [SOHO] images is the cloud of gas surrounding the nucleus, evaporated off the surface by the heat of the Sun. That cloud is huge, but the comet itself is tiny."
http://www.space.com/scienceastronomy/comet_conspiracy_030228.html

So, a mile or two in diameter is fairly common.
Comet Hale-Bopp:
"Analysis indicated later that its nucleus was 60±20 kilometres in diameter, approximately six times the size of Halley. "
http://en.wikipedia.org/wiki/Comet_Hale-Bopp
Ok, so I take that back, if Hale-Bopp was 80 km in diameter and it hit earth, it could be close to an extinction level event. Apparently we can't seem to measure the size nucleus very well- for some reason.
But even a 40 km diameter the comet [with it's very high velocity] would kill more than most of us.
Anyhow, the chances such a large comet impacting earth within next few decades is very tiny. I think the chance Al Gore is 90% correct is far greater. And either are about as likely as sperm whale and a potted petunia materializing above the planet in the next few decades.

"Or even more laughably, but still argued by some, the need to get off-planet before the Sun brightens enough to boil the oceans.

(Speaking as a fellow American, btw.) "

But some people are worried about a century or so into the future. When you think in terms of centuries- which you must do if you imagine most of Greenland's ice melting or sea level rising by more than a few meters, small impactors are a bigger real threat and more likely threat.