Ocean acidification is wreaking havoc on the ocean’s tiniest inhabitants, and the entire ocean is likely feeling the effects.

As a geology student, many things can be unusual. You start to think about time in millions of years, which is completely counterintuitive. You start to realize just how intricate (and beautiful) the processes that shape our planet are, and you also start to understand that there are firm physical laws governing how our planet looks like. There’s a reason why mountains on Earth don’t grow forever and why the continents move about the way they do — the laws of physics constrain geology, and they constrain nature.

What does this have to do with plankton, one might ask, and the oceans in general?

Well, many of the ocean’s inhabitants have soft bodies protected by hard shells. Clams, oysters, and sea snails have them, as do multiple other types of mollusks and plankton. These seashells are almost always made of calcium carbonate — which, under most conditions, is fine. The ocean water is well-suited to support calcium carbonate under normal conditions. But here, too, there is a physical rule that allows this.

Seawater is slightly basic (meaning pH > 7). When we increase the amount of carbon dioxide (CO2) we emit, not all of it goes into the atmosphere. Much of it, in fact, is absorbed by the oceans. As oceans absorb CO2, their chemistry starts to change, and they become more acidic.

When the acidity reaches a certain threshold, animals can no longer build and maintain seashells, and they can’t survive anymore.

This is already happening, a new study shows.

Plankton old, plankton new

The new study started in a museum.

When it comes to comparing our current environment with that of the past, museums can provide a trove of information. The museum is not just what you see when you visit it — museums have additional storage rooms, where they sometimes keep thousands upon thousands of samples gathered by researchers. In this case, Lyndsey Fox, a researcher from Kingston University in London, analyzed plankton fossils gathered by the 1872–76 expedition of the HMS Challenger.

Studying micro-fossils is never an easy task. Analyzing how thick their shells are and then using a tomography scanner to create 3D models of their shells (which are less than 1 millimeter in diameter) is an even trickier job. But Fox and colleagues succeeded, and built stunning reconstructions of this century-old plankton. They then did the same thing for plankton gathered from a 2011 expedition to the eastern equatorial Pacific Ocean called Tara.

The results were striking.

All modern plankton had much thinner shells — up to 76% thinner. In some cases, the shells were so thin that the team wasn’t even able to image them.

It’s a shocking result. Researchers were well aware that ocean acidification was taking a toll, but the extent to which this was observed is concerning.

Stress on all sides

Some species seem to handle it better than others, presumably due to biological differences among species (though researchers did not attempt to explain this).

“Whilst all specimens analyzed showed some reduction in shell thickness, the degree to which different species responded varied greatly,” the authors of the study write.

There are plenty of old samples in museums, and researchers want to look at more species from different areas of the ocean and study the differences and peculiarities — but the elephant in the room is clear. As we pump more and more carbon dioxide, much of it will end up in our oceans, with long-lasting consequences for the entire ecosystem. We are reaching a point where some organisms are already struggling to maintain their shells, the study highlights.

“Oceanic carbonate ion concentrations decrease as a consequence of increased atmospheric CO₂ levels, which, in turn, has a negative effect on the capacity for calcifying organisms (such as molluscs, crustaceans, corals, and foraminifera) to form their essential skeletal or shell material out of calcium carbonate,” the study continues.

It’s not just microscopic creatures, either. A recent study found that ocean acidification is also destroying the shells of crabs, and while some creatures might take it better than others, no creature is spared from its effects. When the plankton suffers, the entire food chain on Earth suffers.

The evil twin of global warming, as ocean acidification is often referred to, is even more insidious than its sibling. We don’t see when plankton is being dissolved in the ocean. We hardly know how many creatures are unable to maintain their shells due to it. We may not know the full scale of the problem, but we know the cause, and we know that if we want to address it, reducing our emissions is key.

To make matters even worse, ocean acidification doesn’t act in a vacuum. The oceans are getting warmer, and as the oceans gather more carbon, they have less available oxygen — which creatures also need. This is a one-two punch which, many creatures are struggling to withstand.

“Ocean acidification is not the only stressor faced by the world’s oceans in the coming decades and over the time period studied here. Rising temperatures and deoxygenation are also likely to have a substantial impact on marine ecosystems, and eastern boundary upwelling systems are likely to be strongly affected by all three stressors,” the study concludes.

We might not see it, but it just goes to show how insidious the effects of global warming really are.

The study was published in Scientific Reports.