(Reuters)

It’s a question marine experts have been racing to get a handle on in recent years. Here’s what they do know: As humans keep burning fossil fuels, the oceans are absorbing more and more carbon-dioxide. That staves off (some) global warming, but it also makes the seas more acidic — acidity levels have risen 30 percent since the Industrial Revolution.

There’s reason for alarm here: Studies have found that acidifying seawater can chew away at coral reefs and kill oysters by making it harder to form protective shells. The process can also interfere with the food supply for key species like Alaska’s salmon.

But it’s not fully clear what this all adds up to. What happens if the oceans keepacidifying and water temperatures keep rising as a result of global warming? Are those stresses going to wipe out coral reefs and fisheries around the globe, costing us trillions (as one paper suggested)? Or is there a chance that some ecosystems might remain surprisingly resilient?

That’s one of the big outstanding questions on climate change. “We understand the physics of simple things like how oceans become acidic,” said Richard Norris, a paleobiologist at Scripps Institution of Oceanography, UC San Diego. “But when it comes to how ecosystems might react, that’s big and complex and messy, with all these interactions going on, both physiological and how organisms interact with each other.”

How to study acidification’s effects

Broadly speaking, there are two ways that scientists can try to study the effects of ocean acidification — and both have limitations. First, they can examine various species in laboratories or in the field and see how corals and molluscs and fish respond to changes in ocean pH levels. The downside is that it’s hard to see how these species might adapt (or not) over the longer run.

Another approach is to examine the fossil record. There have been multiple periods in the Earth’s history where atmospheric carbon dioxide levels rose sharply (for natural reasons) and the oceans became warmer and more acidic. That includes the Paleocene–Eocene Thermal Maximum, an era 55 million years ago with greenhouse-gas concentrations in the atmosphere roughly comparable to what the Earth could soon face.

In a recent paper for Science, Norris and his co-authors found that this ancient world had few coral reefs, a poorly oxygenated ocean, and drastically different food chains that had difficulty sustaining large predators like sharks and whales. On the flip side, “the extinction of species was remarkably light, other than a mass extinction in the rapidly warming deep ocean.” So that’s one possible glimpse into our future:

Yet even the fossil record isn’t a perfect analogue. For one, the rate of today’s acidification — driven by human activity — is at least 10 times as fast. What’s more, modern-day oceans are facing all sorts of additional pressures, from fishing to pollution runoff, that could place additional stress on various species.

Getting a more precise estimate of the damage

So, recently, biologists at the Alfred Wegener Institute in Germany decided to try to combine the two approaches. They tallied up much of the field research that’s been done to date and compared it against the fossil record, in an attempt to get a broad overview of the effects of acidification on corals, molluscs, echinoderms, crustaceans and fishes.

The results, published this week in Nature Climate Change, aren’t exactly encouraging: “Our analysis demonstrates that all considered groups are impacted negatively, albeit differentially, even by moderate ocean acidification.”

Here’s what that looks like in chart form — the red bars show that the number of “negative effects” on the fitness of species tend to go up as the amount of carbon in the ocean increases:

The picture is also complicated. A good many corals and molluscs (say, oysters or mussels or squid) turn out to be extremely sensitive to changes in pH levels and are likely to have trouble adapting to the level of acidity currently projected for the decades ahead.

That doesn’t bode well for areas like the Great Barrier Reef, which has already lost roughly 50 percent of its live coral over the past 27 years, thanks to a combination of tropical cyclones, an invasive sea star, and “bleaching” caused by higher ocean temperatures. Rising acidification is likely to add to that stress. It’s also bad news for, say, oyster hatcheries in the U.S. Pacific Northwest, which have already been decimated by rising acidity levels.

There’s more hope for other species, though. For instance, even though field research suggests that fish larvae are highly sensitive to acidification, the fossil record suggests that fish can adapt decently well to changes in acidity over time. “That was surprising to us,” says Hans-Otto Pörtner, one of the study’s co-authors. But the big caveat, he adds, is that the ocean is now changing much faster than it ever has in the past.

“There’s a note of precaution there,” says Pörtner. “Those changes in the past were slower than they are today. What this tells me is that we’re bringing these species much sooner to the edge of their capacity to compensate.”

So how much could this all cost humanity?

That’s the million- or billion- or maybe even trillion-dollar question. So far, there have been remarkably few studies published on the social and economic costs of acidification and other ongoing changes to the ocean.

One study published last year in Climatic Change suggested that the loss of mollusks — one of the easier-to-forecast effects of acidification — could cost the world around $100 billion per year by the end of the century. The main variable here is how much China and other fast-growing countries are likely to depend on these species for food in the future.

Beyond that, though, there’s a fairly wide range of possible impacts: It’s obvious that seafood is hugely important, with fish representing about 6 percent of the protein people eat. And the world’s coral reefs provide another $20 billion to $30 billion in economic value each year — not just by supporting tourism and fisheries, but also by protecting coasts against storms (see chart). There’s certainly potential for costly damage.

Recently, the National Research Council suggested that U.S. efforts to address acidification need to get a much better handle how these changes might affect people and the economy. If emissions keep rising at their current rates, the report noted, ocean acidity levels are likely to rise as much as 150 percent by the end of the century. Figuring out what exactly that might mean, the report argued, is pretty urgent.

Brad Plumer covers energy and environmental issues, which ends up including just about everything from climate change to agriculture to urban policy and transportation.