(The author is a Reuters columnist. The opinions expressed are his own)
By Gerard Wynn
LONDON, Sept 6 (Reuters) - The world's oceans will become dangerously acidic for corals and shellfish this century if carbon dioxide (CO2) levels continue to rise at current rates, adding to the urgency to reduce manmade CO2 emissions, according to an emerging science.
Evidence for ocean acidity may also prove less controversial than the science of global warming, as it seems likely there will be less doubt surrounding the chemical and biological processes involved.
That could add to the impetus to cut manmade carbon emissions, particularly if signs emerge of a threat to the world's fisheries.
In the past 200 years, people have released more than 2 trillion tonnes of additional CO2 into the atmosphere by fossil fuel burning, cement production and changes in land use such as deforestation.
That has in turn added more than 520 billion tonnes of CO2 to the world's seas, scientists estimate, or a quarter of all emissions.
CO2 dissolves in and reacts with water to form carbonic acid (H2CO3) which disassociates to form hydrogen and bicarbonate ions. The excess hydrogen ions combine with carbonate ions producing more bicarbonate.
Adding more CO2 to sea water therefore increases aqueous CO2, bicarbonate and hydrogen ions, the latter lowering sea water pH (a measure of acidity or alkalinity) and carbonate ions.
One effect has been a fall in ocean surface pH of 0.1 units compared with pre-industrial levels, to an average of 8.1.
That is equivalent to a 30 percent increase in average acidity of surface ocean waters worldwide.
Faster falls in pH have been observed in the Arctic (as colder water absorbs more CO2), with pH dropping by about 0.02 units per decade since the late 1960s in the Iceland and Barnet's Seas, according to the "Arctic Ocean Acidification Assessment 2013", published by the Oslo-based Arctic Monitoring and Assessment Programme.
A second, related effect has been a drop in the level of calcium carbonate, vital for the shells and skeletons of many marine organisms.
As with atmospheric CO2, there is a lag effect where it will take tens of thousands of years for the world's ocean chemistry to recover from carbon emissions now, even if the latter were to halt today, meaning the effects that do emerge will be felt at least for centuries.
One way to project the possible effects of ocean acidification is to extrapolate from events in the Earth's geological past, especially more recently.
For example, the authors of an article published last year in the journal Science, "The Geological Record of Ocean Acidification", analysed the end of the last glacial period, 20,000 years ago, when CO2 levels rose by a third over thousands of years.
They estimated that ocean surface pH dropped 50 times more slowly than at present (0.002 units per century), and that the shell weights of tiny creatures called foraminifera fell by 40 to 50 percent.
Analysis is more limited regarding the more distant past.
For example, the authors were unsure how far ocean pH fell 56 million years ago, during a period of warming, when CO2 was up to five times present levels.
A major extinction of tiny creatures with carbonate shells may have been caused by higher ocean CO2, falling ocean oxygen, higher temperatures, or a combination of all three.
Others studies have estimated that coral reef volumes dropped by 99 percent during the same period, called the Palaeocene Eocene Thermal Maximum.
An alternative technique is laboratory analysis of existing shellfish species in an aquarium with modified chemistry.
A study published last week in the journal Nature Climate Change, "Sensitivities of extant animal taxa to ocean acidification", reviewed research on such animal impacts.
It analysed 167 studies reporting effects from higher ocean CO2 on the performance of 153 species.
The authors concluded that more active creatures with less calcified shells (less calcium carbonate) and the ability to regulate their own body fluid pH survived better.
That favoured crustaceans (crabs, lobsters, crayfish and shrimp) and fish, and counted against corals, echinoderms (starfish and sea urchins) and molluscs (mussels, clams and oysters).
One contrary effect is for higher ocean CO2 levels to stimulate photosynthesis (where CO2 is a raw material), potentially increasing the productivity of microscopic plants called phytoplankton.
At CO2 levels that could occur by the end of this century assuming annual emissions continue to rise through 2100 (a very pessimistic outlook), most investigated echinoderm species and about half of researched molluscs would be negatively affected, the study concluded.
There are several gaps and difficulties facing this emerging science.
First, the combined effect of other pressures on sea life, including warmer water, lower oxygen levels and over-fishing, complicates the task of unravelling and projecting their individual effects.
Second, sea creatures do not exist in isolation but in food chains, or ecosystems, where the disappearance of one, as a result of ocean acidification, could affect many more; such wider analysis has hardly begun.
And third, there is a large uncertainty over how far countries will control carbon emissions this century, with some ocean acidification studies to date assuming high levels of emissions which may ultimately be avoided.
An edition of the scientific journal "Philosophical Transactions of the Royal Society" dedicated to ocean acidification, published last week, summed it up: "Whilst much has been learned in the past decade about the potential implications of climate change on marine organisms and ecosystems, substantial knowledge gaps still exist."
Something to agree on is that ocean acidification, like climate change, poses a serious threat, and adds to the impression that carbon emissions must be cut sooner than later. (Editing by Mark Potter)