Ocean acidity increases with atmospheric CO2 concentration. Since the turn of the 21st century, ocean acidity has caused a decrease of carbon concentration in the exoskeletons of the primary productivity organisms of the oceans. This means, not only are this fundamentally important part of the ocean system at risk, but the oceans are not absorbing as much CO2 as they should be.
In the last 100 years, the exoskeletons of the primary productivity creatures have decreased in thickness by 30 to 35%. Scientists in Tasmania "trapped" these one celled primary productivity organisms and compared their weights to those that died and sank to the ocean floor over the last 50,000 years – how do you trap an algae?! :) The bodies of these little creatures are made out of limestone basically (calcium carbonate), which is a carbon product created from the dissolved carbon dioxide in ocean waters.
The problem is, more CO2 dissolved in the oceans makes the oceans more acidic. The more acidic waters start to dissolve the limestone shells, or don’t let the shells grow as much, or as thick as they would otherwise. Primary productivity is a major player on our planet’s environment. Worldwide it is responsible for maybe up to half of our planet’s natural carbon dioxide sequestration and about the same amount of production of oxygen.
As the oceans become more acidic, these little creatures will start to become extinct, decades to generations ahead of schedule (see here). The scientist see in their research that the algae with the smallest and thinnest of shells are becoming less abundant in their traps. They also see a worsening trend in recent (last 7 years) shell thicknesses. But the trends, as they say in academia, are not robust. They are not clear enough to say with absolute certainty "that they are occurring", with absolute certainty. So like in the 80s, we wait until their is enough data, for the industry of science to be satisfied. In the meantime, thresholds are passed. There is no know technique for increasing the ocean’s acidity other than by decreasing atmospheric CO2, then – it’ll take generations to centuries to fill in this huge hole we have dug ourselves with our "innocent" CO2 emissions.
The bottom line is that these little one celled animals of the ocean are the most important of the mega abundant life forms on the planet (bacteria, viruses, nematodes and other microbes) of the food chain. Losing their participation in our environment could turn out to be the single most important aspect of climate change. It’s all about the little guy. Nothing happens without the little guy.
And of course, this reduction of CO2 sequestration was also not taken into consideration in climate models. It wasn’t even forecast to begin occurring for another 40 to 70 years (here).
Another astonishing discovery – The rate of ocean acidification today is unparalleled in the last 65 million years. Since the great extinction event at the end of the Cretaceous Period – the end of the age of the dinosaurs, there has not been an increase in ocean acidity that is comparable to what is happening today because of increasing carbon dioxide emissions.
Moy, Howard, et. al., Reduced calcification in modern Southern Ocean planktonic foraminifera, Nature Geoscience, March 2009.