Al Gore literally has a world of good publicity now, with the Nobel Committe granting him the closest thing humanity has created to a secular Sainthood. The calculation he has to make is whether he wants to trade in his now-substantial run of good fortune (which began with his remarkable concession speech) for a bitter political fight to wrest the Democratic nomination, and likely the Presidency itself, away from Hillary Clinton (who, incidentially, had very nice things to say about Gore when asked about his Nobel win). In the end, it is a mission that doesn't make much sense.
Running for President is about a nakedly political move anyone can make, a fact which runs counter to Gore's assertion that global warming is (or should be) a moral problem. So why would he ruin all of his good will by taking on Hillary, particularly since her likely win will only help his cause? On the other hand, holding the purse strings of government allows for a more direct power over issues than even the most amibitious nongovernmental mission, a fact which Bono has long recognized in getting people to act to persuade their goverments.
The answer is simple. If Bill Clinton-Al Gore was a compelling ticket in 1992, isn't Hillary Clinton-Al Gore more compelling now? As Veep, Gore can back his global mission with the full authority of the US government behind him. He also can lend his influence more directly to resolving Iraq, an issue he was right about both times it has arisen. And in running for Veep, rather than the Presidency itself, he would impliticty tuck his political ambitions behind those of, say, a Rudy Guiliani (who should look ludicrious for running already and would more so), or for that matter, Hillary. Think of him like Robert Duvall, rarely the guy with his name in bold above the title of the movie, but an Oscar winner who is likely to be a better reason to see the picture.
If you are the Democrat who wins the nomination, you would be crazy not to float this idea to Gore. Does any Democrat (other than Bill Clinton) have a more compelling resume than Al Gore right now? Would any candidate better bring out the base? For that matter, would any other possibility make as good a President should duty call?
And speaking of the Presidency, in 2016, Al Gore will only be 68.