Trouble at sea: what carbon dioxide emmisions are doing to our oceans
This article first appeared in the New Humanist, Winter 2017 issue.
Somewhere in the cold, dark waters of the Arctic Ocean, there is a Greenland shark who has patrolled the waters for over 400 years. A member of the world’s longest-living vertebrate species, she was born a few centuries before the end of the Holocene, the geological era defined by the 12,000 years of stable climate since the end of the last ice age. She was a pup during the reign of the first Queen Elizabeth; during her youth Galileo built telescopes and confirmed that the Earth orbits the Sun. For most of her life, great and sometimes cataclysmic human events have passed her by.
Until, at the beginning of the 21st century, and in her old age, she is at last noticing the effects the human race is having on her environment. The carbon dioxide that we produce in such colossal quantities is heating the planet and changing the climate. The ice above her is melting. But there is another effect that this gas is having on the globe: one that is just as disturbing as the changes being wrought on the atmosphere. The carbon dioxide is also altering the very chemistry of our oceans, turning it more acidic and upsetting a balance that has been stable for hundreds of millennia. The shark may even be able to taste and smell the difference, but the effects on marine life stretch far beyond just one ancient beast.
The gas that causes these environmental changes was discovered by a young Scot, Joseph Black, in 1754 while studying for his MD thesis, on a subject related to the treatment of renal stones. He heated limestone and collected the gas that was liberated. Denser than air, it extinguished fire and could not sustain animal life. He named the gas, which would later be called carbon dioxide, “fixed air”. Unbeknownst to Black and the scientific community of the time, the reaction he observed was a small part of a natural cycle that blends geological, chemical and biological forces to continuously recycle rock, gas and life. Limestone is largely calcium carbonate, formed from the deposits of skeletal fragments of marine organisms. Over millions of years the shells of micro-organisms, molluscs and corals accumulate and are compacted, creating the stone that forms most of the oceans’ bedrock. The marine organisms produce their shells by precipitating calcium and carbonate ions extracted from sea water. In turn much of the carbonate is derived from atmospheric carbon dioxide dissolved from the atmosphere and into the oceans. This carbon dioxide had been violently expelled from deep within the planet, at mid-ocean ridges and volcanoes. And the source of that subterranean expulsion was limestone (just as Black had shown in his Scottish laboratory), heated in the hot interior of the planet after the ocean floor had been dragged down at subduction points where one tectonic plate slides beneath another.
For hundreds of thousands of years this finely balanced carbon cycle has been stable. But the cycle has a weak point. That vulnerability lies within the chemistry of the atmosphere and in turn that of the oceans. These, too, had been stable for aeons, until the dawn of the industrial revolution when humans started to produce significant amounts of carbon dioxide. Two hundred years later and now nearly 35 billion metric tonnes of CO2 is produced by humanity per annum; as a consequence the concentration of the gas in the atmosphere has increased 50% on pre-industrial levels (concentrations of CO2 made up 0.027% of the atmosphere in the late 18th century, now it is over 0.04%).
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The impact on the climate should not have been a surprise, since we have had fair warning of the consequence of burning fossil fuels. In the last years of the 19th century a renowned Swedish chemist, Svante Arrhenius, took note of the CO2 emissions produced by humanity. At the time they were approaching 500 million metric tonnes per year. Arrhenius, even at these relatively low levels, was concerned that humanity’s actions might have an impact on global temperatures. In his 1896 paper “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground” he coined the term “greenhouse effect” and predicted that doubling CO2 would result in a 5°C rise in global temperatures. Things aren’t that bad yet, and modern predictions suggest that Arrhenius’s calculations overestimate the warming effect of carbon dioxide. Instead we are contending with a 1°C rise since his time. Nevertheless, this is enough to have caused the melting of ice sheets and noticeable climate change.
We have also been saved from the full impact of our emissions by the two thirds of the planet that is covered in water. The oceans act as a sink, absorbing some 20–30% of CO2 emissions (while a similar amount is taken up by plants), so limiting the effect the gas can have on the atmosphere and climate. Well into the last century this was seen by climate scientists as an unmitigated blessing. Somehow they overlooked the effects that tens of billions of tonnes of carbon dioxide would have on the ocean’s chemistry.
Acids are continually finding their way into the oceans: CO2 from the atmosphere becomes carbonic acid, while hydrochloric and sulfuric acid leak from the interior of the Earth. Countering this are alkalis dumped into the mix from weathered rock and the fixing of CO2 into carbonate shells and exoskeletons. Together, these opposing forces buffer the ocean’s level of acidity, keeping the surface stable at between pH 8.1 and 8.3 for the last million years or so. Of course measuring historical ocean pH is tricky — it can’t be checked directly, so proxies are used instead. Ocean pH subtly affects a host of chemical processes, from the isotopic ratios of carbon and boron to the uptake of trace metals into marine shells.
So by dating sedimentary limestone (which is, as you remember, made from layers of shells) and then inspecting these chemical markers, it is possible to determine the historical ocean pH going back hundreds of millions of years. For more recent data, ice cores dating back 800,000 years contain trapped pockets of air, and the CO2 content correlates strongly with ice ages as well as ocean pH. All of which shows that, in the last million years, surface pH has rarely dropped below the current level of pH 8.1. Fluctuations do occur but they happen very slowly: over the last 65 million years the ocean pH drifts at a rate of about 0.002 units per 100 years.
Compare that to more recent times. Just 100 years ago the oceans were noticeably more alkali at pH 8.2. This may not seem like a large change, but the pH scale is logarithmic so 0.1 units corresponds to 30% more acidity. This change has been caused by human-generated CO2 emissions. The weathering of rock might correct this over time, but it will take a million years to push the ocean pH back to pre-industrial levels. More worryingly still, we aren’t finished yet: current models suggest that by the end of the 21st century ocean surface pH may drop to 7.7 (corresponding to a 150% increase in acidity), a level not seen for over 50 million years. It is sobering to think that our species can achieve the sort of changes in the ocean that would take normal geological forces many thousands of years.
What effects are the more acidic oceans likely to have on our societies in the immediate future? Put simply, we don’t know for sure. There has been precious little research into the socioeconomic impact of ocean acidification; its effect on food webs, of which we are a part, is extremely difficult to predict. As a 2010 report from the US National Research Council (“Ocean Acidification: A National Strategy to Meet the Challenges of a Changing Ocean”) puts it:
Although the ongoing changes in ocean chemistry are well understood, the biological consequences are just now being elucidated. The problem is complicated because acidification is only one of a collection of stressful changes occurring in the world’s oceans. It is also fundamentally difficult to understand how biological effects will cascade through food webs, and modify the structure and function of marine ecosystems.
Nevertheless, economically important natural resources are already being affected. Oyster hatcheries worth $270 million in the Pacific Northwest have reported major failures as a result of higher acidities. These are unlikely to be isolated cases. Seafood, from finfish to crustaceans and molluscs, has been identified as being at risk from ocean acidification. We should prepare for significant changes to the supply of the food from the oceans.
Meanwhile coral reefs that support tourist industries and serve as physical barriers, reducing the effect of storms on coastal areas, are noticeably affected by the combined stresses of rapidly changing ocean chemistry and temperature. Some estimates put the cost of coral loss, and its effect on coastal ecosystems and resorts, at a trillion dollars.
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How does a drop in pH have such a dramatic effect on marine life? The most obvious answer relates to that step of the carbon cycle where marine organisms, from corals to micro-organisms, use calcium carbonate to form solid structures. Lower pH reduces the amount of carbonate available, meaning it becomes more difficult to make shells and exoskeletons. Even more disturbing is that the more acidic oceans may actually dissolve shells. The first direct evidence for this was seen in 2014. Surveys of the west coast of the US showed that 50% of pteropods (a small free-swimming snail that is a significant food source for salmon, mackerel and herring) had shells damaged by the corrosive sea water. Since then similar effects have been observed on reef corals, mussels and lobster.
Damage to coral and shells is just the most obvious manifestation of ocean acidification. The changing ocean chemistry has many more subtle but no less disturbing implications. The very taste of the sea may also be changing.
We have all tasted sea water: it is salty, and nearly indistinguishable from common table salt added to a glass of water. But its chemical composition is far more complex; a trained tongue may be able to pick out the nuances of different salts, minerals, ions, acids and bases. Ocean life will pick out even more subtle tastes. The ocean is the chemical soup in which marine life is adapted and has evolved. It is also the medium in which they sense the smells and tastes of food, predators and shelter. Through the waters drift chemical signals that allow organisms to track mates, react to their developing broods or select sites to settle on.
The changing pH of the oceans is affecting how life responds to these chemical signals and cues; for marine life the smell of the ocean is literally changing. As a result, organisms from clownfish to snails and crabs start behaving oddly in the more acidic waters. The fish wander too far from the safety of their protective anemones. The snails, dislodged by rough seas, fail to reattach to rocks, and the crabs stop looking after their eggs. Why this happens is not totally clear. One possibility, for which there is mounting evidence, is that as the pH drops the charge on the molecules changes. Many molecules are adorned with different chemical groups that have a tendency to either recruit or lose protons (hydrogen ions). A group may start with no net charge but if it gains a proton it will become positive, while one that has lost a proton will be negative. pH is basically a measure of the concentration of hydrogen ions in water. As pH drops, the proton concentration increases. This means there are more protons available to bind to those chemical groups that have either lost or like to gain hydrogen ions.
This in turn may alter the shape of a chemical. Imagine a long flexible molecule that is positive at one end and negative at the other. The opposite charges attract, bending the molecule. Now if that same chemical is placed in more acidic conditions the negative charge might be lost and so the flexible molecule will stay in an elongated shape. Smell and taste are chemical senses, where molecules dock into receptors, and this docking process is very sensitive to the charges and shapes of molecules. So through acidification the flavour of the ocean may be shifting.
There is one event in the Earth’s history that mirrors the rapid changes to our atmosphere and oceans. It gives us a taste of how ecosystems might react. At the end of the Permian period, 252 million years ago, massive volcanic eruptions in Siberia ejected huge quantities of CO2 into the atmosphere. Over 10,000 years, in the blink of a geological eye, the ocean’s pH dropped by 0.6 pH units. These events coincided with a mass extinction known as the Great Dying: 90% of marine life and 70% of land plants and animals vanished. 10,000 years was far too short a time for most life to adapt to the changing environment. What chance does it stand if similar changes occur over just 100 years?
Within the lifetime of a single fish, our Greenland shark, humanity has melted the ice sheets under which she swims and altered the chemistry of the planet’s oceans. She may even be able to smell these changes we have wrought on her environment.
One shark that lived before humanity had even discovered carbon dioxide has witnessed the unprecedented changes that our species has inflicted on the planet. She has witnessed the end of one geological era: the Holocene has drawn to a close. She will die in a new epoch, the Anthropocene, the age in which human activity will be the dominant factor on our pale blue dot.