skip to main content

→ Top Stories:
Clean Power plan
Safe Chemicals

Lisa Suatoni’s Blog

Why Scientists Agree Ocean Acidification is Undeniably Caused by Humans

Lisa Suatoni

Posted October 16, 2009

Share | | |

A past comment on this blog raised an excellent question.  Captain Crutcher writes:

"How can we be sure that this cycle we are observing with our highly tuned skills is caused solely by humans ... has this happened before, prior to our human ability [to] influence marine ecosystems?"

So, how do scientists know that the observed decrease in average ocean pH - or rising acidity - over the past two hundred years is the direct result of human activities?

The short answer is because theory (i.e., the laws of physics and chemical thermodynamics) predicts it, and observations confirm it.

There are three well established processes in the phenomenon of ocean acidification:

  1. Atmospheric CO2 concentrations are rising, primarily from the combustion of fossil fuels (see 'Keeling curve' below).
  2. The ocean absorbs a large amount of this CO2.  It has to.  There's no known mechanism that would prevent this.
  3. When CO2 gas dissolves into water, it becomes an acid.

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-. (H2CO3 is carbonic acid which immediately dissociates and releases a hydrogen ion, H+, which is the 'acid').

It is important to note that these processes are not complex; they are known to be true and are observable and measurable.

The theoretical underpinnings of chemistry allow scientists to predict the expected change in pH (or 'rise in acidity') in surface waters, given the influx of a known amount CO2 into the ocean.  These predictions are verified by what we measure happening in the ocean. 

The figure below, showing field measurements of CO2 in the air and the ocean at Hawaii, illustrates this point well.  The red diamonds show rising atmospheric CO2 concentrations as a result - primarily - of fossil fuel combustion (this is the famous 'Keeling curve').  The dark blue diamonds show the concomitant rise in CO2 in the ocean.  Although more variable (due to local variations in weather, temperature, biology, and mixing) the rise in oceanic CO2, clearly tracks the rise in atmospheric CO2.  Finally, the light blue diamonds show ocean pH declining by the expected amount given the measured influx of CO2.  Other long-term pH sampling stations around the world (e.g., Bermuda and the Canaries) tell similar stories.


Time series of: (a) atmospheric CO2 at Mauna Loa (in parts per million volume, ppmv) (red ), surface ocean pH (cyan), and pCO2 (μatm) (tan) at Ocean Station ALOHA in the subtropical North Pacific Ocean (From Doney et al., Annual Review of Marine Science, 2009 1: 169-92)

In other words, we see a declining trend in ocean pH, and we can attribute that trend quantitatively to the rise in atmospheric CO2 due to fossil fuels.  The concurrence between theory and observation - as well as the absence of good alternative explanations - gives scientists high confidence that carbon dioxide pollution is causing ocean acidification. 

It's really not much different from predicting that a cup of vinegar added to a gallon of distilled water will drive the acidity of that water up by a given amount - adding the vinegar - and then observing that the acidity did, indeed, go up by the expected amount.  The logical, and most parsimonious, explanation is that the added vinegar caused the rise in acidity.  To conclude otherwise would require an explanation for 1) what unknown process(es) neutralized the added acidity of the vinegar and 2) what alternative, unseen constituent(s), alternatively, caused the observed rise in acidity.

To return to the question, "Is nature causing the recent observed decline in global ocean pH?"  - The answer is, extremely unlikely.  Currently, there are no known natural explanations for the observed decline in GLOBAL AVERAGE ocean pH, and, there is one, clear human-caused explanation (note the global bit is important - there is natural local variation in pH but we are concerned with global shifts). 

Captain, if your ship were to suddenly take on water and you could identify a clear leak in the hull (one the size that would explain the amount of incoming water) - wouldn't it be negligent to ignore that leak in favor of finding an unseen one?

On Natural Variation of Global Surface Ocean pH -

Direct historical measures of ocean pH (on the scale of millions of years) do not exist.  However, scientists have used proxies (e.g., boron isotopes from ancient plankton shells) to formulate reasonable estimates of historic ocean pH. 

Pearson and Palmer (2000 Nature) use this technique, and show that global average surface ocean pH has varied over time (though not necessarily cyclically), but that it has been relatively stable over the past 24 million years, ranging from 8.3-8.1.  Most importantly, changes in average surface pH appear to be gradual, on the scale of tens of thousands to millions of years.  What concerns scientists most about the recent observed, and predicted, changes in ocean pH is that it is extremely - unprecedentedly - rapid (check out Stanford's tutorial of ocean acidification - slide 4 -  for a nice visual of this).  Scientists predict a change in average surface ocean pH from 8.10 to as low as 7.8 in 100 years, if we continue to pollute as we are now.  It is this rapid rate of change that is most threatening to biology because evolution might not be able to keep up with the environmental change.

There have been a few dramatic ocean acidification events in Earth's distant history.  These were caused by massive, natural input of carbon dioxide into the ocean (for example from volcanoes).  The best studied example is the Paleocene-Eocene Thermal Maximum (PETM) 55 million years ago.  As expected, this event was associated with extinction of numerous calcifying organisms.  However, because the world was a very different place back then (for example tropical coral reefs did not exist) and because ocean pH was not the only environmental factor that changed (temperature and oxygen availability changed as well), it is not clear what lessons can be learned from these past events. 

Share | | |


Canada GuyOct 20 2009 04:05 PM

Here's a summary of some of the environmental threats to our oceans. The way things are going, there could be no fish left in the oceans in as little as 40 years.

Mark CrutcherNov 21 2009 07:12 PM


Thank you for the remarks. I was looking for something a little more in depth...the ocean being such a complex system. To present a simple chemical reaction caused by man as the sole factor without regard to the multitude of independent variables that could affecet this was a little under whelming, but I understand this is probably not the forum for more complex explanations.

I do have a little problem with your sinking ship reference. I manage 18 vessels that support oil exploration and production all over the world...and if one of my ship officers were conducting damage assessment and control activities in response to a flooding situation by blaming it on the first hole they found and walking away without conducting a complete evaluation of the damge to the vessel I would dismiss them for incompetence.

What is the current state of other science in regards to this issue?

Your organization does great work regardless and it is simple to understand that inaction is the greater evil than action without a full understanding in this case.

Good Luck!!!

Mark Crutcher

Orran AndradeNov 30 2009 10:45 PM

Wait till the global warming denying industry conjurs up an alternative explanation.

Mark CrutcherDec 25 2009 11:46 PM

I guess Copenhagen pretty much summed it up>

T. BaroneJan 10 2010 05:45 PM

Ocean pH has varied widely in the geologic past. Current ocean pH is well within normal historical parameters:

And the oceans are NOT "acidifying." Ocean pH is alkaline. And it fluctuates, as always.

Now that the CO2 = catastrophic AGW hypothesis has been thoroughly falsified, and coral bleaching has been shown to be natural and reversible, some people have a need to find another reason for alarm. Ocean pH satisfies their need to panic.

Wake the rest of us when this scare, like all the others, turns out to be baseless.

By then there will be yet another scare, claiming that the human race - all of which can fit into a sphere one kilometer in diameter with room to spare - is dooming the planet.

Welcome to the Dark Ages of pseudo-science, and rejection of the Scientific Method - where conclusions are arrived at first, and selected data is used to support the conclusions.

ZachMay 27 2010 02:13 PM

This doesn't prove global warming. This proves that CO2, at least our CO 2, causes acidification of the ocean. People are dumb. good article, but where is the proof thatis specifically our CO2

ken billingtonOct 1 2010 09:51 AM

Pity that there's no ocean pH data before 1940. We can only speculate what happened before. Is it not possible that ocean pH was significantly higher at an earlier period of the earth's history?

Peter K. AndersonOct 8 2010 05:29 AM

pH has slowly been rising from since before ~1750 (its been surmised), there's seen in 2010 even a number of Volcanic Events which each released CO2 equal to Humanity's (from 1900) additions (some even 3 times such). Yet there's been no rapid increase of 'acidity' or even Temperature. Seriously, with even the 'CO2' Temperature Scale the alarmists implement made in fraudulent manner how long will it take the 'climate scare' to subside? I consider such will occur when the 'climate paranoid' begins to be likened is style seen within...
'What it really means to be Green' by Miranda Devine (Thursday, October 07, 2010 at 12:00am)

"Eco-fanatics are no different to totalitarian ideologues through the ages. Evils such as Nazism don’t arrive with devil’s horns; they come in disguise - “sweet camouflaged poison”. So we should thank Frannie and her 10:10 comrades for their vile ad. They naively removed the camouflage too soon."

...which then would be 'now. There is no problem with Climate, there's also been no CO2 warming consistent with alarmist propaganda either.

Comments are closed for this post.


Switchboard is the staff blog of the Natural Resources Defense Council, the nation’s most effective environmental group. For more about our work, including in-depth policy documents, action alerts and ways you can contribute, visit

Feeds: Lisa Suatoni’s blog

Feeds: Stay Plugged In