Acidification, how serious is it really?

Cyril Roger Brossard

Well-known member
Aug 30, 2012
In addition to what was discussed in this thread: Environmentally (and socially) responsible pearl farming? .

As seen in post#31
As CO2 levels increase in the atmosphere, so too do they in the sea. Although direct effects of moderately elevated CO2 in sea water may be of little consequence, indirect effects may be profound. For example, lowered pH and calcium carbonate saturation states may influence both deposition and dissolution rates of mineralized skeletons in many marine organisms. The relative impact of elevated CO2 on deposition and dissolution rates are not known for many large-bodied organisms. We therefore tested the effects of increased CO2 levels—those forecast to occur in roughly 100 and 200 years—on both shell deposition rate and shell dissolution rate in a rocky intertidal snail, Nucella lamellosa. Shell weight gain per day in live snails decreased linearly with increasing CO2 levels. However, this trend was paralleled by shell weight loss per day in empty shells, suggesting that these declines in shell weight gain observed in live snails were due to increased dissolution of existing shell material, rather than reduced production of new shell material. Ocean acidification may therefore have a greater effect on shell dissolution than on shell deposition, at least in temperate marine molluscs.
Written by Sarah Nienhuis, A. Richard Palmer and Christopher D. G. Harley -Bamfield Marine Sciences Center.

Allow me to add the two articles on the subject.
As mentioned before acidification is more a problem of dissolution than absence or reduction of nacre (shell) creation, hence "it melts faster than it deposits."

Article 1.

The shells of some sea snails in the Southern Ocean are already dissolving as a result of ocean acidification, according to a new study. In an analysis of free-swimming pteperods collected from Antarctic waters in 2008, scientists found that the outer layers of the animals’ shells showed signs of unusual corrosion, potential evidence that ocean acidification caused by excess carbon dioxide in the atmosphere may already be disturbing vulnerable marine species. Laboratory tests have shown that acidic water threatens many invertebrate marine species, such as clams and corals, since it hinders their ability to grow shells and exoskeletons. The most vulnerable species are those, like pteropods, that build their shells from aragonite, a form of calcium carbonate that is sensitive to increased acidity. “The corrosive properties of the water caused shells of live animals to be severely dissolved and this demonstrates how vulnerable pteropods are,” Nina Bednaršek, a scientist at the U.S. National Oceanic and Atmospheric Administration (NOAA) and lead author of the study, published in the journal Nature Geoscience, told Reuters. According to scientists, pH levels in the oceans are dropping faster than during any period over the last 300 million years.

read also this.
An Ominous Warning on the
Effects of Ocean Acidification
A new study says the seas are acidifying ten times faster today than 55 million years ago when a mass extinction of marine species occurred. And, the study concludes, current changes in ocean chemistry due to the burning of fossil fuels may portend a new wave of die-offs.
by carl zimmer

The JOIDES Resolution looks like a bizarre hybrid of an oil rig and a cargo ship. It is, in fact, a research vessel that ocean scientists use to dig up sediment from the sea floor. In 2003, on a voyage to the southeastern Atlantic, scientists aboard the JOIDES Resolution brought up a particularly striking haul.

They had drilled down into sediment that had formed on the sea floor over the course of millions of years. The oldest sediment in the drill was white. It had been formed by the calcium carbonate shells of single-celled organisms — the same kind of material that makes up the White Cliffs of Dover. But when the scientists examined the sediment that had formed 55 million years ago, the color changed in a geological blink of an eye.

“In the middle of this white sediment, there’s this big plug of red clay,” says Andy Ridgwell, an earth scientist at the University of Bristol.

In other words, the vast clouds of shelled creatures in the deep oceans had virtually disappeared. Many scientists now agree that this change was caused by a drastic drop of the ocean’s pH level. The seawater became so corrosive that it ate away at the shells, along with other species with calcium carbonate in their bodies. It took hundreds of thousands of years for the oceans to recover from this crisis, and for the sea floor to turn from red back to white.

The clay that the crew of the JOIDES Resolution dredged up may be an ominous warning of what the future has in store. By spewing carbon dioxide into the air, we are now once again making the oceans more acidic.

Today, Ridgwell and Daniela Schmidt, also of the University of Bristol, are publishing a study in the journal Natural Geoscience, comparing what happened in the oceans 55 million years ago to what the oceans are Storing CO2 in the oceans comes at a steep cost: It changes the chemistry of seawater.experiencing today. Their research supports what other researchers have long suspected: The acidification of the ocean today is bigger and faster than anything geologists can find in the fossil record over the past 65 million years. Indeed, its speed and strength — Ridgwell estimate that current ocean acidification is taking place at ten times the rate that preceded the mass extinction 55 million years ago — may spell doom for many marine species, particularly ones that live in the deep ocean.

“This is an almost unprecedented geological event,” says Ridgwell.

When we humans burn fossil fuels, we pump carbon dioxide into the atmosphere, where the gas traps heat. But much of that carbon dioxide does not stay in the air. Instead, it gets sucked into the oceans. If not for the oceans, climate scientists believe that the planet would be much warmer than it is today. Even with the oceans’ massive uptake of CO2, the past decade was still the warmest since modern record-keeping began. But storing carbon dioxide in the oceans may come at a steep cost: It changes the chemistry of seawater.

At the ocean’s surface, seawater typically has a pH of about 8 to 8.3 pH units. For comparison, the pH of pure water is 7, and stomach acid is around 2. The pH level of a liquid is determined by how many positively charged hydrogen atoms are floating around in it. The more hydrogen ions, the lower the pH. When carbon dioxide enters the ocean, it lowers the pH by reacting with water.

The carbon dioxide we have put into the atmosphere since the Industrial Revolution has lowered the ocean pH level by .1. That may seem tiny, but it’s not. The pH scale is logarithmic, meaning that there are 10 times more hydrogen ions in a pH 5 liquid than one at pH 6, and 100 times more than pH 7. As a result, a drop of just .1 pH units means that the concentration of hydrogen ions in the ocean has gone up by about 30 percent in the past two centuries.

To see how ocean acidification is going to affect life in the ocean, scientists have run laboratory experiments in which they rear organisms at different pH levels. The results have been worrying — particularly for species that build skeletons out of calcium carbonate, such as corals and amoeba-like organisms called foraminifera. The extra hydrogen in low-pH seawater reacts with calcium carbonate, turning it into other compounds that animals can’t use to build their shells.

These results are worrisome, not just for the particular species the scientists study, but for the ecosystems in which they live. Some of these vulnerable species are crucial for entire ecosystems in the ocean. Small shell-building organisms are food for invertebrates, such as mollusks and small fish, which in turn are food for larger predators. Coral reefs create an underwater rain forest, cradling a quarter of the ocean’s biodiversity.

But on their own, lab experiments lasting for a few days or weeks may not tell scientists how ocean acidification will affect the entire planet. “It’s not obvious what these mean in the real world,” says Ridgwell.

One way to get more information is to look at the history of the oceans themselves, which is what Ridgwell and Schmidt have done in their new study. At first glance, that history might suggest we have nothing to worry about. A hundred million years ago, there was over five times more carbon dioxide in the atmosphere and the ocean was .8 pH units lower. Yet there was plenty of calcium carbonate for foraminifera and other species. It was during this period, in fact, that shell-building marine organisms produced the limestone formations that would eventually become the White Cliffs of Dover.

But there’s a crucial difference between the Earth 100 million years ago and today. Back then, carbon dioxide concentrations changed very slowly over millions of years. Those slow changes triggered other slow changes in the Earth’s chemistry. For example, as the planet warmed from more carbon dioxide, the increased rainfall carried more minerals from the mountains into the ocean, where they could alter the chemistry of the sea water. Even at low pH, the ocean contains enough dissolved calcium carbonate for corals and other species to survive.

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...Today, however, we are flooding the atmosphere with carbon dioxide at a rate rarely seen in the history of our planet. The planet’s weathering feedbacks won’t be able to compensate for the sudden drop in pH for hundreds of thousands of years.

Scientists have been scouring the fossil record for periods of history that might offer clues to how the planet will respond to the current carbon jolt. They’ve found that 55 million years ago, the Earth went through a similar change. Lee Kump of Penn State and his colleagues have estimated that roughly 6.8 trillion tons of carbon entered the Earth’s atmosphere over about 10,000 years.

Nobody can say for sure what unleashed all that carbon, but it appeared to have had a drastic effect on the climate. Temperatures rose between 5 and 9 degrees Celsius (9 to 16 Fahrenheit). Many deep-water species became extinct, possibly as the pH of the deep ocean became too low for them to survive.

But this ancient catastrophe (known as the Paleocene-Eocene thermal maximum, or PETM) was not a perfect prequel to what’s happening on Earth today. The temperature was warmer before the carbon bomb went off, and the pH of the oceans was lower. The arrangement of the continents was also different. The winds blew in different patterns as a result, driving the oceans in different directions. All these factors make a big difference on the effect of ocean acidification. For example, the effect that low pH has on skeleton-building organisms depends on the pressure and temperature of the ocean. Below a certain depth in the ocean, the water becomes so cold and the pressure so high that there’s no calcium carbonate left for shell-building organisms. That threshold is known as the saturation horizon.

To make a meaningful comparison between the PETM and today, Ridgwell and Schmidt built large-scale simulations of the ocean at both points of Our carbon-fueled civilization is affecting life everywhere on Earth — even deep underwater. time. They created a virtual version of the Earth 55 million years ago and let the simulation run until it reached a stable state. The pH level of their simulated ocean fell within the range of estimates of the pH of the actual ocean 55 millions years ago. They then built a version of the modern Earth, with today’s arrangements of continents, average temperature, and other variables. They let the modern world reach a stable state and then checked the pH of the ocean. Once again, it matched the real pH found in the oceans today.

Ridgwell and Schmidt then jolted both of these simulated oceans with massive injections of carbon dioxide. They added 6.8 trillion tons of carbon over 10,000 years to their PETM world. Using conservative projections of future carbon emissions, they added 2.1 trillion tons of carbon over just a few centuries to their modern world. Ridgwell and Schmidt then used the model to estimate how easily carbonate would dissolve at different depths of the ocean.

The results were strikingly different. Ridgwell and Schmidt found that ocean acidification is happening about ten times faster today than it did 55 million years ago. And while the saturation horizon rose to 1,500 meters 55 million years ago, it will lurch up to 550 meters on average by 2150, according to the model.

The PETM was powerful enough to trigger widespread extinctions in the deep oceans. Today’s faster, bigger changes to the ocean may well bring a new wave of extinctions. Paleontologists haven’t found signs of major extinctions of corals or other carbonate-based species in surface waters around PETM. But since today’s ocean acidification is so much stronger, it may affect life in shallow water as well. “We can’t say things for sure about impacts on ecosystems, but there is a lot of cause for concern,” says Ridgwell.
Ellen Thomas, a paleoceanographer at Yale University, says that the new paper “is highly significant to our ideas on ocean acidification.” But she points out that life in the ocean was buffeted by more than just a falling pH. “I’m not convinced it’s the whole answer,” she says. The ocean’s temperature rose and oxygen levels dropped. Together, all these changes had complex effects on the ocean’s biology 55 million years ago. Scientists now have to determine what sort of combined effect they will have on the ocean in the future.

Our carbon-fueled civilization is affecting life everywhere on Earth, according to the work of scientists like Ridgwell — even life that dwells thousands of feet underwater. “The reach of our actions can really be quite global,” says Ridgwell. It’s entirely possible that the ocean sediments that form in the next few centuries will change from the white of calcium carbonate back to red clay, as ocean acidification wipes out deep-sea ecosystems.

“It will give people hundreds of millions of years from now something to identify our civilization by,” says Ridgwell.

Article 2.


Antarctic marine wildlife is under threat, study finds
25 November 2012
Marine snails in seas around Antarctica are being affected by ocean acidification, scientists have found.

An international team of researchers found that the snails' shells are being corroded.

Experts says the findings are significant for predicting the future impact of ocean acidification on marine life.

The results of the study are published in the journal Nature Geoscience.

The marine snails, called "pteropods", are an important link in the oceanic food chain as well as a good indicator of ecosystem health.


"They are a major grazer of phytoplankton and... a key prey item of a number of higher predators - larger plankton, fish, seabirds, whales," said Dr Geraint Tarling, Head of Ocean Ecosystems at the British Antarctic Survey (BAS) and co-author of the report.

The study was a combined project involving researchers from the BAS, the National Oceanic and Atmospheric Administration (NOAA), the US Woods Hole Oceanographic Institution and the University of East Anglia's school of Environmental Sciences.

Ocean acidification is a result of burning fossil fuels: some of the additional carbon dioxide in the atmosphere is absorbed into oceans.

This process alters the chemistry of the water, making it more acidic.

During a research cruise in the Southern Ocean in 2008, scientists assessed the corrosive effects of upwelled water on pteropod shells.

Upwelling occurs when winds push cold layers of deeper seawater from around 1,000m towards the surface layers.

Seawater from these depths is more corrosive to aragonite, the type of calcium carbonate that forms pteropod shells. The point at which this occurs is known as the "saturation horizon".


"Carbonates in shells dissolve more when temperatures are cold and pressure is high, which are the characteristic properties of the deep ocean," Dr Tarling explained.

Scientists found that the combined effect of increased ocean acidity and natural upwelling meant that in some areas of the Southern Ocean the saturation horizon was around just 200m - the upper layer of the ocean where pteropods live.

Dr Tarling explained the significance of these findings: "The snails do not necessarily die as a result of their shells dissolving, however it may increase their vulnerability to predation and infection, consequently having an impact to other parts of the food web."

He said that although upwelling sites are a natural phenomenon in the Southern Ocean, "instances where they bring the saturation horizon above 200m will become more frequent as ocean acidification intensifies in the coming years".

Interpreting the results
Dr Tarling said the study is "very much... a pilot study" and that it has provided an important body of work regarding "how pteropods will respond to future oceanic conditions".

To date there have been a number of laboratory studies predicting the effects of ocean acidification on marine organisms, but none assessing the impacts on live specimens in their natural environment.

"It took us several years even to develop a technique sensitive enough to look at the exterior of the shells under high-power scanning electron microscopes, since the shells are very thin and the dissolution pattern, subtle," commented Dr Tarling.

He went on: "We are now undertaking a much more comprehensive programme completely focussed on the effects of ocean acidification, not just on pteropods but to a wider range of organisms."
As seen here:
here is the article:
Washington State Declares War on Ocean Acidification

The state, a leading U.S. producer of farmed shellfish, has launched a $3.3-million, science-based plan to address this growing problem for the region and the globe

By Virginia Gewin and Nature magazine

Washington state, the leading US producer of farmed shellfish, today launched a 42-step plan to reduce ocean acidification. The initiative — detailed in a report by a governor-appointed panel of scientists, policy-makers and shellfish industry representatives — marks the first US state-funded effort to tackle ocean acidification, a growing problem for both the region and the globe.

The state governor Christine Gregoire, says she will allocate $3.3 million to back the panel's priority recommendations.

“Washington is clearly in the lead with respect to ocean acidification,” says Jane Lubchenco, administrator of the US National Oceanic and Atmospheric Administration (NOAA).

As growing carbon dioxide gas emissions have dissolved into the world’s oceans, the average acidity of the waters has increased by 30% since 1750. Washington, which produces farmed oysters, clams and mussels, is particularly vulnerable to acidification, for two reasons: seasonal, wind-driven upwelling events bring low-pH waters from the deep ocean towards the shore, and land-based nutrient runoff from farming fuels algal growth, which also lowers pH.

As a result, the region is already experiencing levels of acidity three-fold greater than the global ocean average, with devastating impacts on the state’s US$270-million shellfish industry. Acidic waters are corrosive to many larval shellfish, and they reduce the amount of available carbonate, which some marine organisms need to form calcium carbonate shells or skeletons.

Sea-grass soak
The panel recommends creating of an "acidity" budget to account for natural and human-influenced sources of acidity; improved methods of forecasting corrosive conditions; and finding ways to use sea grasses to soak up carbon dioxide in shellfish hatcheries.

Improved monitoring will be crucial for better understanding acidification trends, its contributing factors and the biological responses of marine organisms. “We lack real-time, high resolution data — which is key because conditions are so variable,” says panel member George Waldbusser, a marine chemist at Oregon State University in Corvallis.

Sensors to measure pH and carbon dioxide abundance have so far been added to 17 existing observing systems nationally, says Dick Feely, a panel member and marine chemist with the NOAA Pacific Marine Environmental Laboratory in Seattle, Washington. The NOAA plans to have 60 such monitoring sites nationally in the next few decades. “It’s an early warning system we’re trying to put together to help industry adapt to changing, sometimes severe, conditions,” says Feely.

The effort to find comprehensive, creative solutions regionally is impressive, says Scott Doney, a marine chemist at the Woods Hole Oceanographic Institution in Massachusetts, who was not involved with the panel. “It’s interesting how they tailored applied science to help aquaculture with a broader research agenda to understand the factors affecting the shellfish industry,” he says.

Despite the report’s regional focus, the authors clarify that the most urgent need is to reduce global carbon dioxide emissions. “Reducing carbon emissions is crucial, but it’s not a problem that Washington alone can solve,” says panel co-chair Jay Manning, an environmental lawyer at Cascadia Law Group in Olympia, Washington, and former director of the Washington Department of Ecology.

Lubchenco emphasizes that the report highlights the need to formulate adaptation strategies to ocean acidification as well as the urgency to create a stronger momentum to reduce global carbon emissions. "We don't have the luxury of doing one or the other," she says. "We have to do both."
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Those who love pearls need to be informed about ocean acidification. That is a measurable trait whether one believes in global warming- or not. Like the ugly results of acid rains a couple of decades ago, unbalanced pH messes with everything- even in the ocean. Pearls and sea shells dissolve in mildly acidic water and I think we are already moving toward that in some parts of the world...

Allow me to add to the previously posted articles.
I believe this one will bring into perspective what is stated above.
Although a bit old, it holds very informative writings.

Ocean Acidification

By Elizabeth Kolbert
Photograph by David Liittschwager

Published: April 2011
The Acid Sea
The carbon dioxide we pump into the air is seeping into the oceans and slowly acidifying them. One hundred years from now, will oysters, mussels, and coral reefs survive?
By Elizabeth Kolbert
Photograph by David Liittschwager
Castello Aragonese is a tiny island that rises straight out of the Tyrrhenian Sea like a tower. Seventeen miles west of Naples, it can be reached from the somewhat larger island of Ischia via a long, narrow stone bridge. The tourists who visit Castello Aragonese come to see what life was like in the past. They climb—or better yet, take the elevator—up to a massive castle, which houses a display of medieval torture instruments. The scientists who visit the island, by contrast, come to see what life will be like in the future.
Owing to a quirk of geology, the sea around Castello Aragonese provides a window onto the oceans of 2050 and beyond. Bubbles of CO2 rise from volcanic vents on the seafloor and dissolve to form carbonic acid. Carbonic acid is relatively weak; people drink it all the time in carbonated beverages. But if enough of it forms, it makes seawater corrosive. "When you get to the extremely high CO2, almost nothing can tolerate that," Jason Hall-Spencer, a marine biologist from Britain's University of Plymouth, explains. Castello Aragonese offers a natural analogue for an unnatural process: The acidification that has taken place off its shore is occurring more gradually across the world's oceans, as they absorb more and more of the carbon dioxide that's coming from tailpipes and smokestacks.
Hall-Spencer has been studying the sea around the island for the past eight years, carefully measuring the properties of the water and tracking the fish and corals and mollusks that live and, in some cases, dissolve there. On a chilly winter's day I went swimming with him and with Maria Cristina Buia, a scientist at Italy's Anton Dohrn Zoological Station, to see the effects of acidification up close. We anchored our boat about 50 yards from the southern shore of Castello Aragonese. Even before we got into the water, some impacts were evident. Clumps of barnacles formed a whitish band at the base of the island's wave-battered cliffs. "Barnacles are really tough," Hall-Spencer observed. In the areas where the water was most acidified, though, they were missing.
We all dived in. Buia was carrying a knife. She pried some unlucky limpets from a rock. Searching for food, they had wandered into water that was too caustic for them. Their shells were so thin they were almost transparent. Bubbles of carbon dioxide streamed up from the seafloor like beads of quicksilver. We swam on. Beds of sea grass waved beneath us. The grass was a vivid green; the tiny organisms that usually coat the blades, dulling their color, were all missing. Sea urchins, commonplace away from the vents, were also absent; they can't tolerate even moderately acidified water. Swarms of nearly transparent jellyfish floated by. "Watch out," Hall-Spencer warned. "They sting."
Jellyfish, sea grass, and algae—not much else lives near the densest concentration of vents at Castello Aragonese. Even a few hundred yards away, many native species can't survive. The water there is about as acidified as the oceans as a whole are forecast to be by 2100. "Normally in a polluted harbor you've got just a few species that are weedlike and able to cope with widely fluctuating conditions," Hall-Spencer said once we were back on the boat. "Well, it's like that when you ramp up CO2."
Since the start of the industrial revolution, enough fossil fuels—coal, oil, and natural gas—have been burned and enough forests cut down to emit more than 500 billion tons of CO2. As is well known, the atmosphere has a higher concentration of CO2today than at any point in the past 800,000 years and probably a lot longer.
What is less well known is how carbon emissions are changing the oceans too. The air and the water constantly exchange gases, so a portion of anything emitted into the atmosphere eventually ends up in the sea. Winds quickly mix it into the top few hundred feet, and over centuries currents spread it through the ocean depths. In the 1990s an international team of scientists undertook a massive research project that involved collecting and analyzing more than 77,000 seawater samples from different depths and locations around the world. The work took 15 years. It showed that the oceans have absorbed 30 percent of the CO2 released by humans over the past two centuries. They continue to absorb roughly a million tons every hour.
For life on land this process is a boon; every ton of CO2 the oceans remove from the atmosphere is a ton that's not contributing to global warming. But for life in the sea the picture looks different. The head of the National Oceanic and Atmospheric Administration, Jane Lubchenco, a marine ecologist, has called ocean acidification global warming's "equally evil twin."
The pH scale, which measures acidity in terms of the concentration of hydrogen ions, runs from zero to 14. At the low end of the scale are strong acids, such as hydrochloric acid, that release hydrogen readily (more readily than carbonic acid does). At the high end are strong bases such as lye. Pure, distilled water has a pH of 7, which is neutral. Seawater should be slightly basic, with a pH around 8.2 near the sea surface. So far CO2 emissions have reduced the pH there by about 0.1. Like the Richter scale, the pH scale is logarithmic, so even small numerical changes represent large effects. A pH drop of 0.1 means the water has become 30 percent more acidic. If present trends continue, surface pH will drop to around 7.8 by 2100. At that point the water will be 150 percent more acidic than it was in 1800.
The acidification that has occurred so far is probably irreversible. Although in theory it's possible to add chemicals to the sea to counter the effects of the extra CO2, as a practical matter, the volumes involved would be staggering; it would take at least two tons of lime, for example, to offset a single ton of carbon dioxide, and the world now emits more than 30 billion tons of CO2 each year. Meanwhile, natural processes that could counter acidification—such as the weathering of rocks on land—operate far too slowly to make a difference on a human time-scale. Even if CO2 emissions were somehow to cease today, it would take tens of thousands of years for ocean chemistry to return to its pre-industrial condition.
Acidification has myriad effects. By favoring some marine microbes over others, it is likely to alter the availability of key nutrients like iron and nitrogen. For similar reasons it may let more sunlight penetrate the sea surface. By changing the basic chemistry of seawater, acidification is also expected to reduce the water's ability to absorb and muffle low-frequency sound by up to 40 percent, making some parts of the ocean noisier. Finally, acidification interferes with reproduction in some species and with the ability of others—the so-called calcifiers—to form shells and stony skeletons of calcium carbonate. These last effects are the best documented ones, but whether they will prove the most significant in the long run is unclear.
In 2008 a group of more than 150 leading researchers issued a declaration stating that they were "deeply concerned by recent, rapid changes in ocean chemistry," which could within decades "severely affect marine organisms, food webs, biodiversity, and fisheries." Warm-water coral reefs are the prime worry. But because carbon dioxide dissolves more readily in cold water, the impact may actually show up first closer to the Poles. Scientists have already documented significant effects on pteropods—tiny swimming snails that are an important food for fish, whales, and birds in both the Arctic and the Antarctic. Experiments show that pteropod shells grow more slowly in acidified seawater.
Will organisms be able to adapt to the new ocean chemistry? The evidence from Castello Aragonese is not encouraging. The volcanic vents have been pouring CO2 into the water for at least a thousand years, Hall-Spencer told me when I visited. But the area where the pH is 7.8—the level that may be reached oceanwide by the end of the century—is missing nearly a third of the species that live nearby, outside the vent system. Those species have had "generations on generations to adapt to these conditions," Hall-Spencer said, "yet they're not there.
"Because it's so important, we humans put a lot of energy into making sure that the pH of our blood is constant," he went on. "But some of these lower organisms, they don't have the physiology to do that. They've just got to tolerate what's happening outside. And so they get pushed beyond their limits."

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Fifty miles off the coast of Australia and half a world away from Castello Aragonese lies the equally tiny One Tree Island. One Tree, which actually has several hundred trees, is shaped like a boomerang, with two arms that stretch out into the Coral Sea. In the crook of the boomerang there's a small research station run by the University of Sydney. As it happened, just as I arrived one spectacular summer afternoon, an enormous loggerhead turtle heaved herself up onto the beach in front of the lab buildings. The island's entire human population—11 people, not including me—gathered around to watch.
One Tree Island is part of the Great Barrier Reef, the world's largest reef complex, which stretches for more than 1,400 miles. The entire island is composed of bits of coral rubble, ranging from marble to basketball size, that began piling up after a peculiarly violent storm about 4,000 years ago. Even today, the island has nothing that could really be called dirt. The trees seem to rise up directly out of the rubble like flagpoles.
When scientists first started visiting the island in the 1960s, they posed questions like, How do reefs grow? Nowadays the questions are more urgent. "Something like 25 percent of all species in the oceans spend at least part of their life in coral reef systems," Ken Caldeira, an expert on ocean acidification at the Carnegie Institution, said one evening before heading out to collect water samples on the reef. "Corals build the architecture of the ecosystem, and it's pretty clear if they go, the whole ecosystem goes."
Coral reefs are already threatened by a wide array of forces. Rising water temperatures are producing more frequent "bleaching" events, when corals turn a stark white and often die. Overfishing removes grazers that keep reefs from being overgrown with algae. Agricultural runoff fertilizes algae, further upsetting reef ecology. In the Caribbean some formerly abundant coral species have been devastated by an infection that leaves behind a white band of dead tissue. Probably owing to all these factors, coral cover in the Caribbean declined by around 80 percent between 1977 and 2001.
Ocean acidification adds yet another threat, one that may be less immediate but ultimately more devastating to hard, reef-building corals. It undermines their basic, ancient structure—the stony skeleton that's secreted by millions upon millions of coral polyps over thousands of years.
Coral polyps are tiny animals that form a thin layer of living tissue on the surface of a reef. They're shaped a bit like flowers, with six or more tentacles that capture food and feed it to a central mouth. (Many corals actually get most of their food from algae that live and photosynthesize inside them; when corals bleach, it's because stress has prompted the polyps to expel those dark symbionts.) Each polyp surrounds itself with a protective, cup-shaped exoskeleton of calcium carbonate that contributes to the collective skeleton of the whole colony.
To make calcium carbonate, corals need two ingredients: calcium ions and carbonate ions. Acids react with carbonate ions, in effect tying them up. So as atmospheric CO2 levels rise, carbonate ions become scarcer in the water, and corals have to expend more energy to collect them. Under lab conditions coral skeleton growth has been shown to decline pretty much linearly as the carbonate concentration drops off.
Slow growth may not matter much in the lab. Out in the ocean, though, reefs are constantly being picked at by other organisms, both large and small. (When I went snorkeling off One Tree Island, I could hear parrotfish chomping away at the reef.) "A reef is like a city," said Ove Hoegh-Guldberg, who used to direct the One Tree Island Research Station and now heads the Global Change Institute at Australia's University of Queensland. "You've got construction firms and you've got demolition firms. By restricting the building materials that go to the construction firms, you tip the balance toward destruction, which is going on all the time, even on a healthy reef. In the end you wind up with a city that destroys itself."
By comparing measurements made in the 1970s with those taken more recently, Caldeira's team found that at one location on the northern tip of the reef, calcification had declined by 40 percent. (The team was at One Tree to repeat this study at the southern tip of the reef.) A different team using a different method has found that the growth of Porites corals, which form massive, boulderlike clumps, declined 14 percent on the Great Barrier Reef between 1990 and 2005.
Ocean acidification seems to affect corals' ability to produce new colonies as well. Corals can, in effect, clone themselves, and an entire colony is likely to be made up of genetically identical polyps. But once a year, in summer, many species of coral also engage in "mass spawning," a kind of synchronized group sex. Each polyp produces a beadlike pink sac that contains both eggs and sperm. On the night of the spawning all the polyps release their sacs into the water. So many sacs are bobbing around that the waves seem to be covered in a veil of mauve.
Selina Ward, a researcher at the University of Queensland, has been studying coral reproduction on Heron Island, about ten miles west of One Tree, for the past 16 years. I met up with her just a few hours before the annual spawning event. She was keeping tabs on a dozen tanks of gravid corals, like an obstetrician making the rounds of a maternity ward. As soon as the corals released their pink sacs, she was planning to scoop them up and subject them to different levels of acidification. Her results so far suggest that lower pH leads to declines in fertilization, in larval development, and also in settlement—the stage at which the coral larvae drop out of the water column, attach themselves to something solid, and start producing new colonies. "And if any of those steps doesn't work, you're not going to get replacement corals coming into your system," Ward said.
The reefs that corals maintain are crucial to an incredible diversity of organisms. Somewhere between one and nine million marine species live on or around coral reefs. These include not just the fancifully colored fish and enormous turtles that people visit reefs to see, but also sea squirts and shrimps, anemones and clams, sea cucumbers and worms—the list goes on and on. The nooks and crevices on a reef provide homes for many species, which in turn provide resources for many others.
Once a reef can no longer grow fast enough to keep up with erosion, this community will crumble. "Coral reefs will lose their ecological functionality," Jack Silverman, a member of Caldeira's team at One Tree, told me. "They won't be able to maintain their framework. And if you don't have a building, where are the tenants going to live?" That moment could come by 2050. Under the business-as-usual emissions scenario, CO2 concentrations in the atmosphere will be roughly double what they were in preindustrial times. Many experiments suggest that coral reefs will then start to disintegrate.
"Under business as usual, by mid-century things are looking rather grim," Caldeira said. He paused for a moment. "I mean, they're looking grim already."
Corals, of course, are just one kind of calcifier. There are thousands of others. Crustaceans like barnacles are calcifiers, and so are echinoderms like sea stars and sea urchins and mollusks like clams and oysters. Coralline algae—minute organisms that produce what looks like a coating of pink or lilac paint—are also calcifiers. Their calcium carbonate secretions help cement coral reefs together, but they're also found elsewhere—on sea grass at Castello Aragonese, for instance. It was their absence from the grass near the volcanic vents that made it look so green.

... last part.

The seas are filled with one-celled calcifying plants called coccolithophores, whose seasonal blooms turn thousands of square miles of ocean a milky hue. Many species of planktonic forami?nifera—also one-celled—are calcifiers; their dead shells drift down to the ocean floor in what's been described as a never ending rain. Calcifiers are so plentiful they've changed the Earth's geology. England's White Cliffs of Dover, for example, are the remains of countless ancient calcifiers that piled up during the Cretaceous period.
Acidification makes all calcifiers work harder, though some seem better able to cope. In experiments on 18 species belonging to different taxonomic groups, researchers at the Woods Hole Oceanographic Institution found that while a majority calcified less when CO2was high, some calcified more. One species—blue mussels—showed no change, no matter how acidified the water.
"Organisms make choices," explained Ulf Riebesell, a biological oceanographer at the Leibniz Institute of Marine Sciences in Kiel, Germany. "They sense the change in their environment, and some of them have the ability to compensate. They just have to invest more energy into calcification. They choose, 'OK, I'll invest less in reproduction' or 'I'll invest less in growth.'" What drives such choices, and whether they're viable over the long term, is not known; most studies so far have been performed on creatures living for a brief time in tanks, without other species that might compete with them. "If I invest less in growth or in reproduction," Riebesell went on, "does it mean that somebody else who does not have to make this choice, because they are not calcifying, will win out and take my spot?"
Meanwhile, scientists are just beginning to explore the way that ocean acidification will affect more-complex organisms such as fish and marine mammals. Changes at the bottom of the marine food web—to shell-forming pteropods, say, or coccolithophores—will inevitably affect the animals higher up. But altering oceanic pH is also likely to have a direct impact on their physiology. Researchers in Australia have found, for example, that young clownfish—the real-life versions of Nemo—can't find their way to suitable habitat when CO2 is elevated. Apparently the acidified water impairs their sense of smell.
During the long history of life on Earth, atmospheric carbon dioxide levels have often been higher than they are today. But only very rarely—if ever—have they risen as quickly as right now. For life in the oceans, it's probably the rate of change that matters.
To find a period analogous to the present, you have to go back at least 55 million years, to what's known as the Paleocene-Eocene Thermal Maximum or PETM. During the PETM huge quantities of carbon were released into the atmosphere, from where, no one is quite sure. Temperatures around the world soared by around ten degrees Fahrenheit, and marine chemistry changed dramatically. The ocean depths became so corrosive that in many places shells stopped piling up on the seafloor and simply dissolved. In sediment cores the period shows up as a layer of red clay sandwiched between two white layers of calcium carbonate. Many deepwater species of forami?nifera went extinct.
Surprisingly, though, most organisms that live near the sea surface seem to have come through the PETM just fine. Perhaps marine life is more resilient than the results from places like Castello Aragonese and One Tree Island seem to indicate. Or perhaps the PETM, while extreme, was not as extreme as what's happening today.
The sediment record doesn't reveal how fast the PETM carbon release occurred. But modeling studies suggest it took place over thousands of years—slow enough for the chemical effects to spread through the entire ocean to its depths. Today's rate of emissions seems to be roughly ten times as fast, and there's not enough time for the water layers to mix. In the coming century acidification will be concentrated near the surface, where most marine calcifiers and all tropical corals reside. "What we're doing now is quite geologically special," says climate scientist Andy Ridgwell of the University of Bristol, who has modeled the PETM ocean.
Just how special is up to us. It's still possible to avert the most extreme acidification scenarios. But the only way to do this, or at least the only way anyone has come up with so far, is to dramatically reduce CO2 emissions. At the moment, corals and pteropods are lined up against a global economy built on cheap fossil fuels. It's not a fair fight.
Those pictures are the property of David Liittschwager. You may see them here.

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At Castello Aragonese, a volcanic island off Naples, Italy, healthy seafloor looks like this: a lumpy quilt of red sponges, white barnacles, lilac coralline algae, sea urchins, and (near the center of the photograph) one well-camouflaged fish. It's a tompot blenny.

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A few hundred yards from the preceding scene, CO₂ bubbling from seafloor vents acidifies the water to levels that might one day prevail all over the oceans. Dull mats of algae replace the colorful diversity?"fair warning," says biologist Jason Hall-Spencer.

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A blade of sea grass at Castello Aragonese is a microcosm. Coralline algae cover the blade, a snail grazes on the algae, tube worms colonize the snail. All three make calcium carbonate. Near the CO₂ vents, however, the grass is green, stripped of its companions?because acidification has stripped the water of carbonate.

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A sea star raised in normal water in Kiel, Germany, provides contrast with a sea star raised in conditions that could occur in the Baltic Sea by 2100 (next photo).

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Initially identical to the sea star in the previous photo, this star weighs only a fifth as much as its peer. Ocean acidification is amplified in some coastal waters by pollution from the land, which fertilizes blooms of microbes that take oxygen out of the water and put in more CO₂. Read also ?Effects of elevated temperatures and elevated CO2 on seastar growth? (Gooding, R.A., C.D.G. Harley, and E. Tang. 2009)

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Three weeks old. Healthy sea urchin larvae look like crystal spaceships, with long rods of calcium carbonate that form the larval skeleton.

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In contrast, urchin larvae raised in acidified water are stunted (like the one above) and more vulnerable to predators. Read also ?Response of sea urchin pluteus larvae (Echinodermata: Echinoidea) to reduced seawater pH: a comparison among a tropical, temperate, and a polar species? (Dana Clark ? Miles Lamare ? Mike Barker, 2009) and ?The perils of reduced pH on sea urchin development? (Havenhand JN, Buttler F‐R., Thorndyke MC, Williamson JE. 2008. Near‐future levels of ocean acidification reduce fertilization success in a sea urchin. Current Biol. 18: R651‐R652. )

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Barnacles and tube worms vie for space with red and green algae and orange bryozoans on a typical snail shell near Castello Aragonese.

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A snail found near the CO₂ vents provides contrast: The acidified water has denuded it, and its shell's outer layer has been corroded at the center, leaving a pearly sheen.

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A brittle star in the lab can handle water acidified by high CO₂. It can handle water spiked with triclosan, an antimicrobial found in soaps and skin creams?more and more of which is reaching the oceans through sewage discharges.

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When a brittle star is hit with high CO₂ and triclosan at once?mimicking the "stress cocktail" of the future?it sheds its arms. Read also ?A tale of two brittle stars: A comparison of the effects of hypercapnia, decreased pH and increased temperature on the respiration rates of Hemipholis cordifera and Amphipholis gracillima (Echinodermata, Ophiuroidea)? (Radivojevich-Cross, Kristina, M.S., LAMAR UNIVERSITY - BEAUMONT, 2012)

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This sea butterfly is a kind of pteropod, a swimming snail just a half inch across. It's the dominant zooplankton in polar waters, nicknamed the "potato chip of the sea" because fish and other predators eat so many. Within five years parts of the Arctic Ocean will be corrosive to its shell. Read also ?Effects of ocean acidification on overwintering juvenile Arctic pteropods Limacina helicina? (CNRS. S. Comeau, S. Alliouane, J.-P. Gattuso. 2012)