Single-cell life-forms thrive throughout the world's oceans—and have for hundreds of millions of years. Tiny varieties known as calcareous nanoplankton build exuberant, microscopic shells—resembling wagon wheels, fishlike scales, even overlapping oval shields decorated with craggy explosions at their centers—known as "coccoliths". The ability to form these shells rests on the amount of calcium carbonate (CaCO3) dissolved in the seawater—and that amount depends on the concentrations of atmospheric carbon dioxide (CO2).

CO2 is the ubiquitous greenhouse gas emitted by human activity, particularly fossil-fuel and forest burning. As levels rise in the atmosphere (currently at 390 parts per million and counting), the ocean's surface waters absorb more of the molecule. This water–CO2 mixture forms carbonic acid, which slightly lowers the ocean's overall pH (the lower the pH, the more acidic). More acidic ocean water means less calcium carbonate—and less material for shell-building plants and animals of all sizes, including the nannoplankton that constitute the base of the food chain.

Of course the present era is hardly the first time the planet has seen higher levels of CO2. In fact, roughly 121 million years ago—during an age known as the early Aptian—global CO2 levels were likely higher than 800 ppm (and possibly as high as 2,000 ppm) thanks to cataclysmic volcanic eruptions. Now new research published in Science July 23 shows how ancestors of today's nannoplankton fared in those acidic oceans of long ago.

It was a time of "severe global warming," paleobiologist Elisabetta Erba of the University of Milan and her colleagues wrote, after studying the carbon isotopes embedded in deep seabed cores drilled in the Pacific Ocean and locations in the ancient Tethys Ocean, which existed during the Mesozoic era. The records reveal that acidification proved a big problem for nannoplankton. "During the Aptian episode, marine calcifiers experienced a major crisis due to increasing CO2-induced acidification," Erba says.

But that crisis was not a major extinction event. The nannoplankton responded by doing less shell-forming—the heaviest shell-formers, known as nannoconids, largely disappeared from the fossil record (although they did not go extinct, the same species reappear after acidification dwindles)—and by diversifying into new, smaller species. In some cases species even increased in abundance but shrank in size—by as much as 60 percent. "Malformation is also ascertained for some [widespread] species," Erba notes.

It took at least 25,000 years for the new acidity levels reached in the surface waters to transfer to deeper waters, according to the research—and the ocean took 75,000 years to reach its peak acidity for that episode, as well as at least 160,000 years to recover. The length of this episode derives "most probably because several CO2 pulses [volcanic eruptions] contributed to ocean acidification," Erba says. Further, she plans to examine other high CO2 events in the geologic record to see "if the same causes—excess CO2, global warming, ocean acidification—trigger similar effects on marine calcifiers at different times."

But the 25,000-year time lag between acidification of the surface waters and deeper waters is mysterious, points out geoscientist Timothy Bralower of The Pennsylvania State University, who was not involved in this study. "In the modern ocean, a similar input of carbon would involve a lag on the order of centuries," he notes. "So something is very different." And the nannoconids begin to disappear even before the fossil record indicates lighter volcanic carbon isotopes—in other words, presumably before the actual acidification. Nevertheless, he says, "it provides the state of the art in terms of our understanding of the effects of the introduction of massive amounts of CO2 on surface ocean ecosystems."

That's probably bad news for modern nannoplankton—and other shell-building microscopic life, such as foraminifera. Foraminifera responded similarly to the Aptian event, Erba says, although "data are still sparse." Modern day experiments agree with the fossil record: High CO2 levels in lab tests prompt "selective coccolith malformation, dwarfism and decrease in calcification," Erba notes, whereas these results have been conflicting at times, Bralower adds.

Regardless, the shells of at least one modern foraminifera in the Southern Ocean are already smaller than those of their ancestors from a mere century ago. And the modern buildup of atmospheric CO2 is happening far faster than these ancient episodes. "The current rate of ocean acidification is about a hundred times faster than the most rapid events" in the geologic past, notes marine geologist William Howard of the Antarctic Climate and Ecosystems Cooperative Research Center in Hobart, Tasmania. Plus, the direct impacts of global warming may complicate the picture—just as modern coral suffer from increased bleaching thanks to warmer ocean temperatures as well as the reduced carbonate exoskeleton–building capacity brought on by ocean acidification. Bralower adds: "The big question is whether modern species will be able to adapt to what I expect will be much more rapid pH reduction in coming centuries."

This transformation of the tiny shell-forming creatures that are the basis of the food chain or, like corals, provide the very habitats that allow other species to thrive will unsettle the oceans. And, as this study suggests, recovery from ocean acidification is likely to take millennia—relying as it does on the steady, slow weathering of continental rock to flush more carbonate into the oceans. "If this and other studies' conclusions are correct," Howard notes, "the ocean's recovery will take hundreds of thousands of years."