Acid Attack: the Marine Meltdown

             Coral Collapse: Could our world’s most wondrous marine environments be about to melt away?

We have all heard about the dangers of man made climate change: how the planet will warm, causing extreme weather that could have profound impacts for all terrestrial life, including humans. But whilst these widely anticipated changes on land are only slowly becoming a reality, the oceans have responded much more rapidly to our greenhouse gas emissions. An equally important consequence of increased carbon dioxide in the atmosphere is acidification of the seas, and in that regard the storm is already raging, with little sign of calmer waters ahead.

Carbon dioxide is soluble in water. When its concentration in the atmosphere increases, more of the gas becomes available to be dissolved in the surface waters of the ocean. For this reason, around a third of all the greenhouse gasses humans have emitted since the industrial revolution have already been absorbed by the oceans; were we to stop emitting now, the rest of our post-industrial emissions would probably be absorbed within about a thousand years. But whilst this absorption is good news from the perspective of the temperature of the planet, it’s not good news for life on Earth. Whether ocean-dwelling or not, all sorts of species are likely to suffer dire repercussions.

Once it crosses from atmosphere to oceans, carbon dioxide rarely remains intact. Through a series of chemical reactions, ninety-nine per cent of the gas is converted into singly-negative bicarbonate or doubly-negative carbonate ions, and it’s in these forms that the majority of the carbon ‘locked up’ in the oceans is to be found. When more carbon dioxide is added, a chemical equilibrium shifts to increase the concentration of protons (H⁺ ions), which is what makes the ocean more acidic. But the change in acidity itself isn’t the biggest problem; on the pH scale the ocean’s acidity has been increasing by about 0.02 per decade since the 1980s, but this only means moving slightly closer to ‘pure’ water (pH 7.0) from its currently alkaline state (pH 8.1). Most ocean life should be able to adapt to this slight change if the rest of ocean chemistry remained undisturbed.

But, more importantly, increasing carbon dioxide in the oceans shifts the chemical equilibrium to favour the production of the bicarbonate ion at the expense of the carbonate. For life-forms such as coccolithophores and other plankton that build their shells out of calcium carbonate, that change can have devastating consequences. At the moment, most of the oceans are ‘super-saturated’ in calcium carbonate, which means that there is plenty available for organisms to draw out of the water and grow. But as acidification increases and the carbonate concentration decreases, this growth becomes harder and harder. If the concentration drops so low that the oceans become under-saturated, calcium carbonate shells will begin to dissolve, meaning that, rather than growing over time, they’ll begin to break down. But this is exactly what’s predicted to happen – except in the warmest and shallowest waters – under more extreme carbon dioxide emissions scenarios.

‘Calcifying’ animals have evolved to live in alkaline oceans with plenty of readily-available carbonate to form shells. Without them, they’ll be left vulnerable to predators, less able to float and less capable of competing for resources; the ultimate result is that their populations are likely to plummet if current emissions trends continue. And they won’t be alone in their suffering. As with all life-forms on Earth, calcifying organisms represent just one part of a complex food web that supports a plethora of larger species – from tiny mackerel to cod and whales and coastal human societies – that rely, either directly or indirectly, upon calcifying species to provide a large proportion of their food.

The most easily visible manifestation of ocean chemistry change is the decline of corals. In their healthy state, these animals provide a rich, dynamic environment in which fish and plant life can flourish. In carbonate-depleted or acidic seas, meanwhile, corals become stressed, and in under-saturated waters are subject to the same shell dissolution that plagues plankton. In either case, they are likely to enter a ‘naked’ state, stripped of their carbonate shells and no longer capable of supporting vibrant biological communities. Very rapidly, ocean oasis can be transformed into lifeless desert, often inhabited only by invasive species such as sea grass. One of the world’s most wondrous environments simply melts away.

Of course, in the long term life will not be put off by any such change in ocean chemistry. New species will eventually evolve and no doubt flourish in forms as diverse as those we run the risk of losing today. Corals have existed for two-hundred million years, and will almost certainly weather the anthropogenic storm somehow. Life always bounces back from extinction to regain its former glory. But this will take tens of thousands or millions of years, and provides little consolation to the billions of life-forms that will suffer in our own time and over the coming centuries and millennia from the floral and faunal famine we are beginning to induce. A life deprived of natural habitat, food or protection from the elements will be one of agony for the many animals that find the oceans they are adapted to living in or alongside changing at unprecedented rates. A world devoid of its bountiful natural beauty, such a wonderful free gift to us all that through thoughtless greed is being so rapidly eroded away, will be one not nearly so pleasant for its future human inhabitants. For the sake of countless animals – humans included – living today and tomorrow we urgently need to wake up to the reality of what we’re doing to our oceans and take steps to reverse the trend. Climate change isn’t about a few more summer downpours and a slightly more Mediterranean climate for southern England. It’s causing real suffering. And it’s happening now.