Pond Scum: The Movie

This sounds like the premise of a bad sci-fi movie:

Driven by fluctuations in oil prices, and seduced by the prospect of easing climate change, experts are ramping up efforts to squeeze fuel out of a promising new organism: pond scum.
As it turns out, algae -- slimy, fast-growing and full of fat -- is gaining ground as a potential renewable energy source.
[….]
University labs and start-up companies across the country are getting involved. Over the summer, the first mega-corporation joined in, when ExxonMobil said it would sink 600 million dollars into algae research in a partnership with a California biotechnology company.

Mimicking I Am Legend, this basic premise is explained in the first few minutes, while the rest of the movie revolves around the story of genetically engineered algae monsters who hate humans and, as it turns out at the end, work for an ExxonMobil subsidiary. 

The main character will be a Greenpeace activist from our time who went into a coma when the anti-whaling ship he was on rammed into a whaling ship, and he doesn't wake up until some time in the indeterminate future.  (And yes, this is the sort of coma in which you don't age, though he does wake up with a long beard, ala Rip Van Winkle.)  As he discovers, the world was depleted of carbon dioxide because of our efforts to stop Global Warming, which created global cooling and brought about another ice age. And that’s the reason why the algae monsters hate humans, because humans ruined the climate when they overreacted on the whole global warming thing.

In the final scene, they prove to him that global warming was a myth and how the effort to stop it doomed the planet. The hero eventually acknowledges man’s error and watches in sadness as all the algae monsters die from the cold and lack of carbon dioxide. He weeps and vows to return warmth and carbon emissions to the earth again in order to save the last algae monster, a young girl who he befriended and who saved his life several times, but who is now in a coma; thus setting up the sequel. The end.