CO2, Acid Oceans, and Coral Reef Loss

I assume everyone reading this blog has heard of global warming. You know the story. Burning fossil fuel releases greenhouse gases like carbon dioxide (CO2), which build up in the atmosphere and make the planet hotter. Before the year 1850 the CO2 concentration was about 285 parts per million. Now it's 390 ppm, and forecast to be 760 or more by 2100 if we keep burning fossil fuel! That's a huge change!

This graph shows how the CO2 level has changed since the 1950s. The little squiggles show yearly ups and downs caused by plants growing in summer (which sucks CO2 out of the atmosphere), and decaying in winter, which gives off CO2. But beneath that natural oscillation is a long-term increase that's definitely caused by humans.



Global warming isn't the only bad repercussion of the rise in CO2. There's also ocean acidification. Here's how that happens:

1. Pollution puts more CO2 into the atmosphere.
2. Some CO2 from the atmosphere dissolves into the ocean.
3. The dissolved CO2 reacts with H20 (water).
4. The reaction releases H+ ions, the cause of acidity.

Acidity is measured as "pH", and the more acid something is, the LOWER its pH is. In the 1950s the ocean water pH was 8.20. Now it's down to 8.07. That might not seem like much, but it actually means there are 25% more H+ ions in the water than there used to be. I.e., the ocean is 25% more acid than before! If the pH drops to 7.8, as is predicted for the future CO2 increase, the ocean will be 250% more acid!

That's not enough to burn your flesh or anything, but it's definitely enough to mess up the body chemistry of animals and plants that live in the ocean. Particularly, it makes it hard for them to form shells and skeletons out of calcium carbonate (CaCO3), because the CaCO3 disintegrates in acidic water. It's the same idea as when you put a tooth (also made of calcium carbonate) in a glass of coke.



Coral skeletons are made out of CaCO3, too, and the continued existence of coral reefs is dependent on the rate of accretion of new coral skeleton exceeding the rate of breakdown of old coral skeleton. Acid in the ocean is a double whammy for reefs because it slows down the growth of new coral AND increases the dissolution of old coral.

Even if we weren't doing anything else that was bad for reefs, the CO2 acidity increase could completely destroy them within a few decades. Add to that several other negative impacts humans are having on reefs, such as nutrient pollution, overfishing, and climate change, and the corals are basically screwed. It's not like a "this might happen in the future" thing, either. It's already well under way. The average coverage of live coral on Caribbean reefs used to be 50-60%. Now it's less than 10%, with most of the space taken up by dead, dissolving coral overgrown with algae, bacteria, and weedy sponges. Diving in the Dry Tortugas last month I saw some of the best-protected reefs in the Caribbean... and even there the live corals were few and far between.

The situation is definitely bad. In an excellent article for the public, reknowned coral reef expert and conservationist Dr. Nancy Knowlton put it this way: "If people don’t change the way they’re doing things, reefs as we know them will be gone by the year 2050. It’s actually really depressingly unbelievable." Yikes! Time to get way more serious about curbing emissions, and reducing nutrient pollution and overfishing. No bull.

FYI: Much of the information for this post came from a seminar given at the Smithsonian Marine Station by Dr. Chris Langdon, a coral reef expert from the University of Miami's Rosentiel School of Marine and Atmospheric Science.