"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. 

Ocean acidification is a second global impact of rising atmospheric CO2 concentrations (in addition to climate change).  Approximately one third of all anthropogenic (man-made) CO2 emissions is absorbed by the oceans.  This CO2 immediately becomes an acid in water, and increases the acidity of the oceans.  Since the industrial revolution, average acidity of ocean water has increased by 30%.  This change in ocean chemistry makes it more difficult for shelled organisms (e.g., crabs, lobsters, oysters, sea urchins, corals and some phytoplankton) to build their shells.  If emissions continue on a business as usual trajectory, ocean acidity will approach triple the pre-industrial levels by the end of the century.  At these levels, many species or organisms - particularly shelled organisms - will struggle to survive.

The argument in the Monaco Statement is built on six undisputed observations and one highly likely prediction (#4):

1. Ocean acidification is underway.

2. Ocean acidification is already detectable.

3. Ocean acidification is accelerating and severe damages are imminent.

4. Ocean acidification will have socioeconomic impacts.

5. Ocean acidification is rapid, but recovery will be slow.

6. Ocean acidification can be controlled only by limiting future atmospheric CO2 levels.

7. There is still time to act if serious and sustained actions are initiated without further delay.

Two points are worth emphasizing: 'severe damages are imminent' and 'recovery will be slow'. 

At current CO2 emissions rates, large regions of the polar oceans will become corrosive to shells of key organisms (base of the food chain) in the next 20-30 years and they will no longer be able to survive in these regions.  When atmospheric CO2 concentrations reach double the pre-industrial level (560 ppm) - which could very well happen by mid-century - the erosion of coral reefs will out-pace their growth and they will likely go extinct globally (or certainly in the reef-building form that we know them).  This prediction is based on empirical observations of coral calcification rates.  The timing of the prediction might vary by decades but the phenomenon and outcome is not in dispute.  Tipping points for other calcifying organisms exist but they remain unknown.

If carbon emissions are not stabilized during this century, it will require thousands of years for the oceans' chemistry to return to current conditions.  And it will be hundreds of thousands to millions of years for coral reefs to return.

The phenomenon of ocean acidification is simple.  It is undeniably caused by human activities, and there is relatively little variation in model predictions compared to climate change.  The Monaco Declaration reveals a growing concern and strong consensus among ocean experts: we are now at the cusp of severe and essentially permanent damage to marine ecosystems and we must act now.

The site of the meeting is the oceanographic museum built by Prince Albert I in 1910 which stands as an utterly breathtaking symbol of Monaco's enduring commitment to the marine environment.

They have joined 250 scientists from around the world to learn the most up-to-date knowledge on the rapidly emerging global threat of ocean acidification and to discuss necessary policy actions.

Only four years after the very first meeting on the topic - during which scientists widely adopted the term 'ocean acidification' and embarked on a world-wide cooperative research effort - scientists have made impressive advances in our knowledge of the topic. 

Over the past four years researchers have:

• generated clear predictions of what pH changes will occur in the world's oceans given different carbon emissions scenarios (In other words, we now have a pretty clear idea of how much, where, and when ocean pH will change, given certain fossil fuel emission scenarios.)
• identified which ecosystems are going to experience changes first and most dramatically (Antarctica, the Arctic, and eastern boundary upwelling systems like the west coast of the United States)
• identified which ecosystems and communities are likely to be most vulnerable (coral reefs - both tropical and deep water)
• established that organisms around the globe are already experiencing the stress of increased acidity
• improved our understanding that there will be 'winners' and 'losers' in the changing oceans and who they may be ..... (For example, in addition to corals, coralline algae which help to 'glue' coral reefs together will likely suffer dramatically, as will many types of echinoderms such as starfish, sea urchins, and sea cucumbers.  On the other hand, there appears to be variation in the response of ecologically important phytoplankton species, such as coccolithophores, (i.e., some showing signs of stress and some not) suggesting that perhaps some species will be able to adapt - or that some species-rich communities may avoid collapse by shifting their composition.)

The scientific accomplishments are impressive.  However much remains to be done - not the least of which is the establishment of chemical and biological monitoring in key vulnerable and rapidly changing areas.  Little can be done without a 'Keeling' curve for the changing chemistry of the seas.

Two aspects of science and policy were evident from this meeting. 

First, when scientists coalesce around a set of questions and work cooperatively, a lot can get done.  A handful of talented and dedicated researchers have successfully elevated this global issue to the international policy arena (as evidenced by the mention of ocean acidification in the most recent IPCC report and some discussion of the issue in meetings leading up to COP15).  Policy makers need to sustain the momentum on this topic.

Second, when a major research initiative is supported by government, as was done by the European Union with EPOCA, scientific progress is substantially faster. 

The United States has yet to allocate dedicated funds to the study of ocean acidification (note:  a bill addressing this concern (HR 4174), introduced by Rep. Thomas Allen, passed in the House in July 2008 but the Senate companion bill, introduced by Senator Frank Lautenberg (S1581) has not yet passed) and this is evident by a comparatively smaller research effort in the U.S.  A major U.S. commitment is needed to ensure that this critical research field is brought out of its infancy.

The proceedings from the Second International Symposium in a High CO2 World will be published in a special issue of the journal Biogeosciences.