Humans need water for drinking, cooking, bathing and the occasional swim. If it's a little dirty, we can boil it, filter it, or simply buy a bottle shipped from somewhere else. But when it comes to sea creatures, from the tiniest plankton to the biggest whale, water is the whole world, and changes in the chemical composition of the ocean can have dramatic effects.

There's evidence that these drastic changes have occurred in the distant past. While working in the South Atlantic in 2003, a group of Earth scientists discovered a sudden shift in the geological makeup of samples of ancient sediment drilled from the ocean floor. Deepest down, the sediment was white, made up of calcium from the dissolved shells of millions of years' worth of tiny animals. But just above that layer, the sediment suddenly turned red. Scientists believe that at the time this sediment was laid down, the ocean became so acidic that these creatures couldn't form their calcium shells. Once the water returned to normal, the sea life did as well, and the strata of white sediment resumed [source: Zimmer].

Scientists now fear we are headed toward another crisis. According to an April 2011 National Geographic report, human behavior has been responsible for putting more than 500 billion tons (454 metric tons) of carbon dioxide into the atmosphere since the beginning of the Industrial Revolution. CO2 in itself isn't the problem -- it's a necessary part of life. We produce it when we exhale. Plants consume it and replace it with oxygen. We even drink it in our fizzy sodas. The difficulties begin when we overload the atmosphere and waters with more carbon dioxide than the natural systems can process. Excess CO2 in the atmosphere reacts with sea water to produce acidic molecules, which accumulate in the oceans. Over the last century and a half, the acid level in sea water has jumped 30 percent [source: Schiermeier]. Scientists predict that animals with shells -- snails and mussels, for example -- will be endangered within a few decades. A chemical assault on small life forms like plankton would certainly cause problems up the food chain, and in fact, fish are not themselves immune from the effects of excess CO2. Researchers have demonstrated that excessive carbon dioxide interferes with the sense of smell of the juvenile clownfish, a fish that finds the safe haven of a sea anemone by its scent [source: Schiermeier]. Some undersea plants may actually grow faster as a result of having more CO2 available [source: USGS].

"We're going to be looking at growth rates of organisms over a long time …" says marine biologist Eric Pane of the Monterey Bay Aquarium Research Institute in California. "I think the least we can say is there's going to be profound changes to ocean ecosystems. From there, where we go and the judgments we make about that is an issue for further on" [source: Harris].