On one hand, it sounds crazy. On the other, he is far and away the strongest Democratic candidate if Hillary doesn't run.
It would be an interesting argument.
Gore has name recognition, and a reputation on the left for leadership on a crucial issue. And he could split an important part of Hillary's primary coalition while being the most prominent alternative for the ABC crowd.
But I think he'd realize the same problem that Kerry had. His flaws in 2000 led to George W Bush being President, and there's more interest in the first female President than in what he represents.
I don't know if he'd be that effective in a General Election. A campaign on environmentalism allows Republicans to note the economic costs when resources are not allowed to be tapped.
Not just climate change, also the Internet.
Also, too bad that we didn't have an expert in anti-terrorist aviation security in the White House in 2001.