Coral reefPhoto: Yuriy via PicasaDive into the NRDC’s new documentary Acid Test and you’re immediately immersed in a beautiful undersea world complete with vibrant coral reefs, graceful kelp beds, and rhythmic schools of fish.

But Acid Test is no Blue Planet, thanks to heavy use of green-screen technology. And what’s in front of those screens is a lot less pleasant than the fish porn projected onto them. (No offense to the scientists, commercial fisherfolk, and other experts who are doing the talking, of course — it’s more about what they’re saying.)

The 30 minute film, part of Discovery Planet Green’s “Blue August” month of online and onscreen ocean coverage, is about the threat of ocean acidification, the gradual chemical changes in our waters linked to increased levels of carbon dioxide. Just how much CO2? Turns out that since the Industrial Revolution, the ocean has absorbed about one quarter of the carbon dioxide produced by burning fossil fuels.

But don’t go celebrating all the sequestered CO2 that’s been kept from contributing to global warming, because it’s beginning to cause more problems than it’s solving, increasing the acidity of the water by 30 percent. And that acidity is starting to dissolve seashells in areas as close to home as the California coast, meaning tragic consequences for many organisms — and the millions more who count on them for food, including us.

It’s a scary phenomenon that scientists are only just coming to understand, and it’s only going to get worse — leaving us with “an urgent choice,” as narrator Sigourney Weaver puts it, “to move beyond fossil fuels or to risk turning the ocean into a sea of weeds.”

As you watch Acid Test, keep an eye on the beauty projected onto the green screen and the choice seems pretty obvious.

Acid Test premieres tonight on Planet Green and continues to air throughout the month. Catch the trailer below: