Coral reefs aren't just pretty places for scuba divers (although they do bring in billions of tourist dollars). These rich ecosystems supply the inhabitants of coral reef countries with the fish that they depend on as their main source of protein. Coral reefs, like rainforests, are also treasure troves of biodiversity that may hold the keys to fighting diseases like cancer and arthritis. Human wellbeing is tightly bound to the health of coral reefs.

Unfortunately, coral reefs are in trouble, and climate change plays a major role.

This week, marine biologists, reef managers, fishermen and divers are gathered in Fort Lauderdale to share the latest information on coral reef biology and conservation. On the opening day of the symposium, NOAA scientists released a report on the state of coral reef ecosystems in the U.S. The dense 569-page document is sobering. Some key findings:

• Branching corals that once formed "vast stands" across the Atlantic and Caribbean have declined as much as 90 percent at some sites.
• Populations of reef fish are "largely depleted" in the Atlantic and in "poor condition" in the Pacific.
• The overall condition of reefs has declined over the past 25 years, and threats are increasing.

What's behind these problems? The scientists and managers surveyed identified various local threats to individual reefs, but climate change emerged as a widespread concern. More than two-thirds of U.S. reef jurisdictions reported that threats from climate change had increased over the past 10-25 years. The report's authors highlighted growing worry about ocean acidification (caused by increased CO₂ in the atmosphere), which could "prevent future reef growth altogether."

To minimize the effects of warming and acidification on coral reefs, we must decrease our emissions of CO₂ and other greenhouse gases. But coral reefs face other threats, as well, such as disease, coastal development, tourism and recreation, fishing, and invasive species. Protecting the world's remaining coral reefs will require action on many fronts.

The situation is grim, but all is not lost. We can help coral reefs withstand the stresses of climate change by reducing other direct threats. Corals are sensitive but also can be very resilient – if we can reduce the stresses that they face.

The concept is simple: phytoplankton (tiny one-celled algae) take up carbon dioxide (CO₂) during photosynthesis. Fertilizing the ocean encourages growth of phytoplankton, and increases the rate at which CO₂ is consumed – presumably leading to less CO₂ in the atmosphere. Since ocean photosynthesis is often limited by lack of iron, the idea is to dump iron into the ocean and watch the phytoplankton bloom. Planktos sees this as an economic opportunity: Increase CO₂ uptake in the ocean, and sell it as an offset to carbon emitters. (I talked more about how offsets work in a previous post on land-based offsets.)

Ocean fertilization may sound like a good idea, but it has some very serious problems. Here's why.

Scientists have been looking at ocean fertilization for a while, and so far the results have been very mixed, and often discouraging. After reviewing many studies, the latest IPCC report [PDF] concluded that "Geo-engineering options, such as ocean fertilization to remove CO₂ directly from the atmosphere… remain largely speculative and unproven, and with the risk of unknown side-effects."

Ocean fertilization is dangerous due to the threat of unintended consequences (see my geo-engineering post). Not all phytoplankton are alike; some need iron more than others. So when you add iron to the ocean, you are favoring one species over another. That can have profound consequences, since phytoplankton are at the bottom of the food chain.

• One possibility is that ocean fertilization will encourage the spread of so-called "de-nitrifiers" – critters that produce nitrous oxide. Many of us know of nitrous oxide as laughing gas, but when it gets into the atmosphere it's no laughing matter. Nitrous oxide is 120 times more potent as a greenhouse gas than CO₂. If this happened, ocean fertilization could turn out to be a net global warmer rather than a global cooler.

There are also problems with ocean fertilization as an offset. For a carbon offset to be credible, the quantity of CO₂ that has been removed by the project must be accurately quantified. This is a major hurdle for an ocean fertilization project.

• Just because phytoplankton consume CO₂ during photosynthesis doesn't mean that CO₂ has been removed from the climate system. The vast majority of phytoplankton are eaten by other critters in the surface ocean that, through respiration, release the CO₂ back into the system. As much as 10 percent of the phytoplankton die and sink into the deep ocean before they are consumed. Only the carbon that accompanies that small fraction of phytoplankton qualifies as an offset, because that's the only carbon that's permanently removed from the system.
• Now imagine how difficult it would be to measure the amount of carbon that sinks into the deep ocean. You'd have to contend with crosscurrents bringing unfertilized waters into your project area while moving your fertilized waters away. And there are also vertical eddies, bringing deep water up to your area, that may or may not contain some of your recently fertilized but now dead phytoplankton. Upwelling can bring carbon up from the deep ocean, releasing more CO₂ into the atmosphere than what's taken out. In short, it's a mess.

And that is why ocean fertilization isn't cool in my book. Let me know if you hear of any other too-good-to-be-true fixes to climate change, and we can take a look at them.

1. More Acidic Oceans
2. Drinking Water and Disease
3. Shifts in Lifecycle Timing
4. Drought and Violence
5. Melting of the North Pole

Everyone knows that carbon dioxide (CO₂) warms the globe. But many people don't know about its other dangerous effect. The build-up of CO₂ is undermining ocean life through "ocean acidification". I'll start by explaining why our oceans are becoming more acidic, and then illustrate why this is so dangerous to ocean life and our entire food chain.

When CO₂ in the atmosphere dissolves into the ocean, as naturally happens, it forms carbonic acid. As CO₂ concentrations increase from the burning of fossil fuels, more CO₂ is dissolved into the ocean. In one sense this is a good thing because the dissolved CO₂ can't act as a greenhouse gas to warm the planet. But it's also a bad thing, because it changes the pH balance of the ocean – the degree to which the ocean is acidic or alkaline ("basic").

The natural state of the ocean is a weak base (the opposite of acid). This environment is great for the many forms of ocean life that use calcium carbonate to form their skeletons or shells. These range from familiar species such as corals and shellfish, to less well-known creatures like the tiny one-celled algae called coccolithophores. Ocean acidification makes it harder for these "calcifying organisms" to maintain themselves.

You can see why yourself with a simple experiment. Calcium carbonate comes in many forms. In addition to comprising the shells of sea creatures, it's the primary component in chalk, lime, and marble. Take a piece of chalk and put it in a glass of water. It will just sit there – a wet piece of chalk. Now slowly pour in some vinegar – an acid. The water will start to bubble, emitting CO₂, and the chalk will dissolve.

This is an exaggerated example of what's happening in the ocean. The ocean isn't as acidic as a glass of water with vinegar, so calcifying organisms aren't actually dissolving in front of our eyes. But the ocean's increased acidity makes it harder for them to form healthy shells and skeletons.

As the ocean’s acidity increases from the build-up of CO₂, corals and shellfish lose calcium. This isn't just a problem of aesthetics, that scuba divers won't have pretty coral reefs to look at. Coral reefs provide habitat for many of the world’s fish, and a billion or more people depend upon fish as their main source of protein.

There is also the possibility of a climate feedback. Coccolithophores are covered by calcium carbonate plates. Because the plates are white and the coccolithophores hang out near the ocean surface, these creatures act as tiny mirrors reflecting sunlight away from the Earth's surface. It's possible that ocean acidification, by decreasing the abundance of coccolithophores, could decrease the reflectivity of the Earth and thus accelerate global warming.

In this NASA photo, what looks like clouds in the water
is actually the reflected light from billions of coccolithophores.

Ocean acidification is one of the most serious consequences of increased CO₂ in the atmosphere, yet no one talks about it. And this is not the only under-reported consequence. Next week I’ll tell you how global warming can spread disease by fouling our drinking water.

Left: Healthy corals (iStockphoto). Right: bleached corals (Ray Berkelsman, CRCReef, Townsville).