15 Feb 2010: Analysis

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.

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.

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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.

POSTED ON 15 Feb 2010 IN Business & Innovation Climate Climate Energy Oceans Science & Technology 


The author of this article gives a poor explanation of the saturation depth. In the ocean there is a constant rain of organic material moving downward. Oxidation of this organic debris produces increasing CO2 levels with depth. The CO2 acidifies the water. The saturation depth is the depth where the reaction to form calcium carbonate solid from calcium and carbonate dissolved in the water is exactly balanced by the dissolution reaction. Below the saturation depth the acidity of the water dissolves calcium carbonate shells.

Posted by George B on 15 Feb 2010

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

Was she being ironic? If not, that statement seems wildly optimistic.

Most of the oxygen on earth comes from life in the sea. Between killing that off, and killing the trees and other vegetation that produce the rest with our toxic emissions from burning fossil and biofuels, I'm not quite sure what people in the not to distant future will be breathing.

photos and links to scientific research at

Posted by Gail on 16 Feb 2010

In order to understand what this REALLY means -- which isn't what the article suggests -- let's apply some simple math to the findings reported in this article.

We have two computer models, in which acidification happens 10 times as fast in the "modern" model as it does in the "historical" model.

10 times as fast. Let's remember that.

Now, that was the difference in "output". What then, exactly, was the difference in "input"?

Model A, "historical":
-> "6.8 trillion tons of carbon over 10,000 years"

Model B, "modern":
-> "2.1 trillion tons of carbon over just a few centuries"

To make these two inputs comparable, let's assume that "just a few" means two. So, we have:

A: 6.8 trillion tons over 100 centuries
B: 2.1 trillion tons over 2 centuries

So, WRT "input" we have that the "modern" model "A" receives input in an order of magnitude approximately

(6.6 / 2.1 = 3.2) x (100 / 2 = 50) = 160

160 times FASTER than the "historical" model "B".

WRT output,we have that

B = 10 x A

So, for 160 times more input, you only get 10 times more output.

Which means, again, that the "modern" model (our current world) is 16 times more robust to ocean acidification than the historical one.

I wouldn't call that cause for alarm. Quite the opposite. This is good news.

Posted by John Smith on 16 Feb 2010

John Smith--
Please correct me if I am misunderstanding completely, but I'm pretty sure all of your math is convoluting the only pertinent piece of information: acidification happens 10 times faster in the modern model as in the historical model. So, regardless of the input to output ratio necessary for this to be the case, the fact is that our oceans are changing very rapidly, with potentially devastating effects. And while the mathematical analysis you provide would be comforting if we could simply limit our inputs, the entire problem stems from our inability to do this at the moment. I'm not sure why you're quite so optimistic.

Posted by Hmmm... on 16 Feb 2010

More WWF/Greenpeace/IPCC inspired speculation. Long on histrionics, short on facts.

Let's interject some common sense into this "debate"... if all creatures in the ocean were as sensitive to miniscule changes in pH as the author claims, life would never have evolved.

Just another silly pneumocephalic politically slanted article.

Posted by Shoshin on 17 Feb 2010


Evolution is not linear. It has generated various levels of biodiversity and will continue to do so. Sometime it becomes more rich, sometimes it loses a lot of its diversity and those losses can be (and sometime are) the result of human activities.

We have already eliminated many species and will continue to do so.

Posted by William J on 17 Feb 2010

I don't know how those researchers came up with 2.1 trillion tonnes being added to the air over 'a few' centuries. Current world CO2 output is about 30 billion tonnes per year. Adding 2.1 trillion tonnes to the air would take 700 years, assuming it all stayed in the atmosphere. But then, if it all stayed in the atmosphere it wouldn't contribute to ocean acidification. In fact, CO2 has a half life in the atmosphere of somewhere between 50-150 years. Eventually it winds up in many different deposits. So we're really talking about something well over 1,000 years in the future - and it's almost certain that we will stop burning fossil fuels long before then.

Also, if ocean acidification is happening much faster than previously thought, that would imply that the lifespan of CO2 in the atmosphere is shorter than previously thought, which would lower the overall impact of global warming.

Posted by Dan on 17 Feb 2010


Your math is off by a factor of 10. Using your numbers, it would take 70 years (30 * 70 = 2,100).
Posted by Cyrus on 17 Feb 2010

This study contributes to mounting evidence that ocean acidification is already happening and when it happened to a similar (even lesser) extent in the distant past the result was extinction of entire undersea ecosystems, most notably coral reefs... not once but several times. So, you can fiddle while Rome burns - or you can get on with it and adopt policy and practices that promote a carbon-free economy.

When a house is on fire, some people might prefer to sit around and argue about the original source of the conflagration. Perfectly ok, but don't expect others to waste precious time sitting around on their duffs.
Posted by George Sranko on 18 Feb 2010


You misplaced a decimal point: 1.5 trillion / 30 billion = 70 years, not 700!

I think you are misreading the meaning of "adding" 2.1/6.8 trillion tons of CO2 to the atmosphere; 2.1 trillion tons is the gross amount, not the net amount, so it already includes any that is removed from the atmosphere by ocean acidification or photosynthesis. And keep in mind that 520 billion tons of carbon have already been added to the atmosphere.
Posted by Lauren on 18 Feb 2010

William J:

The issue is not gradualism vs punctuated
equilibrium, and suggesting so is irrelevant and

The issue is that the article contains a flawed
but very popular eco-radical premise; that all
life hangs by a single fraying thread and that
man is ultimately responsible for the breaking
of that thread.

Life is hardy. It began long before man evolved
and it will continue long after we, like Elvis,
have left the building.

Only through increasingly doom and gloom
reports can the eco-radicals gather support and

Now, if you want to talk about curbing pollution
in China or India, child poverty, AIDS, malaria,
clean water or a host of real problems, you
have my support.

But using "dire warnings" "could", "should",
"might", "may" as words to overstate and divert
needed resources to a morally bankrupt and
defrocked eco-radical movement, well you lost
my attention.

Posted by Shoshin on 18 Feb 2010

More computer models, more untested claims,
more headline grabbing predictions.

No-one has ever verified that the oceans today
have dealkalinesed by Ph 0.1. That too is
supposition based on very limited models, but
has been seized on, as in this research, as a
fundamentally established. But it is not.

For these researchers to create a predictive
model and then claim that this "shows" the
oceans are "acidifying" faster than normal shows
a fundamental confusion about what constitutes
evidence in the real world.
Posted by Shim Shimuzu on 18 Feb 2010

Dan, check your math. To get to 2.1 trillion tons, you only need 70 years at 30 billion tons/year.

Everyone, have you considered that these carbonate ions are the same as what is in the sodas people drink? As a soda goes flat, the acidic taste goes away; the carbonic acid has come out of solution as carbon dioxide, and the solution left behind approaches a neutral pH of 7. Consider the world ocean as a giant soda, dissolving lots of carbon dioxide to form carbonic acid.

Two problems not yet being discussed: First, carbonic acid is not interchangeable with carbon dioxide in biological pathways; it is not accessible to to algae for photosynthesis. (It is important to remember that algae in the oceans do over 80% of all photosynthesis on Earth, and land plants are physically incapable of making up the difference if the algae suffers catastrophic environmental change.)

Second, Earth is warming. Some people have made much of a graph purporting to show that rises in carbon dioxide lag behind changes in temperature. (The graph shows no such thing but, for the sake of argument . . . ) What happens when a soda gets too warm?

At some critical temperature, the oceans are likely to outgas much of the carbon dioxide they have absorbed from our industrial processes. If you thought 30 billion tons per year was bad, imagine all of it coming back to bite us - at an even higher rate, with no absorption by the oceans to offset it.

Posted by Daniel on 18 Feb 2010

Dan, I think you've made a math error. Adding 2.1 trillion tons to the air would only take 70 years at a rate of 30 billion tons per year, not 700 as you claim. Also, the article doesn't say that 2.1 trillion would be added to the air, but to the entire model (or in other words, to the environment as a whole). There model is actually being pretty lenient since a projection of only 2.1 trillion tons over the next few centuries implies that our rate of carbon emissions is actually going to decline under the model (otherwise the emissions they talk about would
only take about 70 years to spew into the environment).

Posted by Math. on 18 Feb 2010

A solution has a pH level that is a measure of the acidity or alkalinity. The pH scale is from 0 to 14 and a measure of 7 is neutral. The scale is created relative to standard solutions and agreed on internationally. Above 7 the solution is more alkaline and below 7 it is more acid. The oceans are considered to have a pH of 8.2 with a variance of 0.3, so it is an alkaline solution.
Using the proper word alkaline is not as scary as the scary misleading word " ACID " another boggy man tactic.

Another Myth Busted before it could take off and cause world wide panic. OceanGate?

Posted by wilbert robichaud on 19 Feb 2010

By strange coincidence one anti-science poster on a local newspaper (who refuses to identify himself) posts this:

“Coral reefs dying because of climate change? Yes, they are dying from climate change………..from the adverse affect of cooling water, not CO2, most recently in Florida. CO2 is necessary for corals and their symbiotic algae to precipitate calcite and aragonite (calcium carbonate compounds). The myth of CO2 acidification of ocean water and causing harm to corals is easily debunked. Only those that fail to study and understand environmental chemistry can believe such ridiculous claims.

As for media coverage, most mainstream media outlets (ABC, CBS, MSNBC, CNN, NYT) favor the carbon hysteria side of things. On can learn nothing from those outlets of the recent Phil Jones (CRU, IPCC) admissions of error and lack of warming for example.

As for peer review claims………given the repeated scandals, those claims appear no more valid now than one burglar vouching for the whereabouts of another burglar.”

And also this:

“Marine life killed by cold:


I suspect this individual is actually from an ultra (uber?) conservative think tank who is being fed this information in order to appear to be a “scientist”.

Posted by jia on 12 Mar 2010

Carl Zimmer is one of our very best science writers, but even he doesn't get everything right. The largest danger of ocean acidification to shelled animals is not that their fully-formed shells will spontaneously dissolve, but that embryonic animals will not be able to incorporate calcium and build new shells as they grow.

Posted by chenjia on 13 Mar 2010

I’ve been saying for some time that ocean acidification deserves much more attention than it gets, simply because it has such an immense potential for human impacts via the food supply. The truly nasty part of OA (aside from its insanely low profile) is how hard it is to address via geoengineering. All the orbital mirrors and albedo fixes don’t do diddly for this, since they don’t pull CO2 out of the atmosphere and away from the oceans.

By the way–when are we going to get over this habit of giving really big, scary problems overly scientific and bland names? “Ocean acidification” sounds like a chapter in your high school chemistry book that you ignored. How about “ocean poisoning”?

Posted by Tom on 06 Apr 2010

Horrifying material indeed, but I’m glad that laboratory scientists are working so hard to do the tests and simulations.

This is going to create feedbacks, too. The ocean currently emits 332 Gt of CO2 and sequesters 338 Gt. Rapid acidification is going to cause impoverishment of the ocean food chain, and there is not going to be enough time for species to adapt and evolve. This is unprecedented, and can’t be simulated or tested very accurately. The likely result will nevertheless be the oceans turning into net sources of CO2, in addition to the starvation of fisheries-dependent human populations.

I hope this message gets out, since the basic fact of rapid ocean acidification is impossible to dispute, and tech fixes won’t work. I still predict that the deniers will give it their best shot anyway.

Posted by Shiny on 27 Apr 2010

What is a wisdom-compassion lesson to learn from all this?

For me, if not sure about the possible consequences of the current warming, I reduce my own eco-footprint and then encourage others to do that too - just in case, before the signs of climate change get any worse on the land and in the oceans.

Perhaps like when you or someone you love is showing signs of a new illness with increasing temperature ... what do you do?


Posted by Dana Murty on 06 May 2010

The childish logic of 'cooling' oceans killing sea life or recent cool winters is ignorance of something anyone learned in grade-school earth science class. Ice cubes placed in boiling water will cool the water but only temporarily if the pot is still left on the fire.

The same effect from melting icecaps and glacier waters flowing into the oceans and seas. The homeschooling and undercutting of science education is to blame. Now anyone who thinks they opened a book once on a topic is a valid expert and researcher.

Posted by megan on 18 May 2010

John Smith,

You deduced "good news" because you bungled the math. Even assuming "2" centuries is correct (you pulled the number "2" out of your hat), the ratio you are interested in is 2.1/6.8*100/2 = 15. If you had pulled "3" out of your hat instead of "2", the ratio would have been 10. A factor of 10 at the input leads to a factor of 10 at the output.

Posted by Sailesh Rao on 22 Feb 2011

The weight of the earth is what keeps us and other planets in orbit, what does it mean if we keep reducing the weight, by taking it from the ground?

Posted by Bill Realph on 26 May 2011

Comments have been closed on this feature.
Carl Zimmer writes about science for The New York Times and a number of magazines. A 2007 winner of the National Academies of Science Communication Award, Zimmer is the author of six books, including Microcosm: E. coli and the New Science of Life. In previous articles for Yale Environment 360, he has written about the prospect of a warming world causing an evolutionary explosion and about using assisted migration to save species threatened by climate change.



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