Given the Earth's atmosphere and its oceans are a closely interlinked natural complex, steeply rising levels of atmospheric carbon dioxide are having dramatic impacts on the seas as well.
A marine emergency, to match the climate emergency, is now in the making. Massive and irretrievable damage to the ocean environment, scientists predict, could occur within decades rather than centuries as previously thought.
The seas of the future will be warmer, and their chemistry sharply different. The biological changes cannot be predicted so precisely, but it seems certain they will be drastic. Higher temperatures will sharply alter the populations of numerous marine creatures.
Oxygen levels will change, in some cases falling to the point where local ecosystems cannot survive. The oceans will also become increasingly acidic, as carbon dioxide from the atmosphere reacts with water. This could have devastating effects on numerous organisms that rely on the current ocean chemistry to allow them to form hard protective shells.
Among these creatures are various species of plankton — the drifting organisms, ranging from microscopic to fingernail-sized, that form the basis of marine food chains.
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Even present-day carbon dioxide levels threaten to set off a cascade of destruction. Factor in rapid "business as usual" emissions increases, and the results for the oceans could include mass species extinctions and a collapse of biological productivity — all within the lifetimes of people now among us.
Of the extra heat rising greenhouse emissions have trapped in the Earth's atmosphere over the past two centuries, more than 80% has gone to warm the oceans. Average temperatures of the upper oceans have shown a strong rising trend since at least 1955, but the rises are very unevenly distributed.
The greatest increases have been in polar latitudes, with temperatures in the Arctic Ocean in recent years as much as 4.3° Celsius above the long-term average. Sea temperatures east of Tasmania have risen by more than 2°C since the 1940s.
Such changes are not good news for marine creatures adapted to life within narrow temperature bands.
Some species may survive by shifting their distribution toward the poles — various seaweeds, for example, have been observed spreading southward along Australia's east coast. But in many cases, geography will preclude this.
At worst, temperature rises threaten to wipe out whole marine ecosystems. In many places throughout the tropical world, warm water coral reefs have been observed in drastic decline, hit by warming-induced mass bleaching events. Warming of as little as a degree or two above normal maximums can cause the coral to expel the colourful algae that provide the coral polyps with food.
Researchers estimate that some 80% of the reef corals in the Caribbean have been lost during the past three decades, partly because of higher temperatures.
As reported in the November 1 Weekend Australian, University of Queensland marine biologist Ove Hoegh-Guldberg predicts that sea temperatures around Australia's Great Barrier Reef will rise by 2°C over the next three decades, with lethal results for the coral.
Some of the worst effects of ocean warming are associated with declines in oxygen content as temperatures rise.
The May 1 New Scientist reported large volumes of ocean previously rich in oxygen have become "oxygen minimum zones", in which finned fish, squid, crustaceans and other sea creatures have begun to suffocate and die.
A study published earlier this year by researchers at Kiel University in Germany describes how low-oxygen layers of water in the tropical Atlantic to the west of Africa have expanded from a thickness of 370 metres in 1960 to 690 metres in 2006.
Part of the reason for oxygen depletion is that oxygen is simply less soluble in warmer water. At 0°C a kilogram of seawater can hold about 10 millilitres of dissolved oxygen, while at 25°C this figure is only four millilitres. The astonishing fertility of polar seas is partly due to their high oxygen content.
Still more important than solubility for creating "oxygen minimum zones" is the fact that warm seas rarely undergo much vertical mixing. While cold seas are characteristically murky, tropical waters are usually a clear, translucent blue. The reason is that most tropical seas are biological deserts.
Warm water is less dense than cold and, in tropical seas, generally forms a stable surface layer. Nutrients from deeper layers are not carried to the surface waters, while oxygen absorbed from the atmosphere cannot reach waters lower down.
Much of the oxygen in the depths of tropical and subtropical oceans has been carried there by subsurface currents, after surface water has cooled and sunk in subpolar regions. But with sea surface temperatures near the poles growing steadily warmer, less water sinks, and the ocean currents are believed to be slowing down.
In places like the west coast of North America, oxygen-poor water is now often present at shallow enough depths to spill onto the continental shelf, normally home to a rich variety of marine life. Scientists are now finding seasonal "dead zones" in what once were highly productive fishing grounds.
Of the carbon dioxide that human activity has emitted to the atmosphere since 1800, about half is still there. Somewhere close to a quarter has been captured by land plants. The rest, an estimated 525 billion tonnes, has dissolved in rivers, lakes and oceans.
When it dissolves, carbon dioxide forms a weak acid. Seawater is normally slightly basic; just before the industrial revolution the average pH of the upper oceans was 8.16, compared with a neutral value of 7. The figure now is about 8.05. Because pH is an exponential and not a linear scale, the fall in pH of about 0.1 points represents an increase of some 30% in the concentration of hydrogen ions.
More than seven billion tonnes of carbon dioxide now enter the oceans each year. In a "business as usual" scenario, one that sees atmospheric carbon dioxide increase from the present level of about 387 parts per million to 800ppm at the end of the century, the average pH of the upper oceans could fall as low as 7.6.
In a more modest projection, which sees atmospheric carbon dioxide in the year 2100 at 560ppm — essentially the level envisaged by Australian government climate change adviser Ross Garnaut when he called recently for cuts of 5-10% in Australian emissions by 2020 — ocean surface pH at the end of the century is predicted to be about 7.9.
Some idea of what this might mean in practice can be seen in from a recent study, reported by a ScienceNOW posting in June, of waters near the island of Ischia off south-western Italy. Here, undersea volcanic vents release carbon dioxide into the water and drop pH values to 7.9 and beyond.
"The whole ecology of the area is radically altered", lead researcher Jason Hall-Spencer of the University of Plymouth in Britain is quoted as saying. Thirty percent fewer species were recorded in the more acidic waters. Corals and sea urchins were absent, while invasive algae thrived.
Higher acidity is believed to have damaging effects on the physiology of finfish in their larval and juvenile stages. The organisms most at risk, however, are corals, shellfish such as mussels, oysters and scallops, and various species of plankton.
These species all draw carbonate ions from the seawater to form shells or skeletons of calcium carbonate. Crabs, shrimps and lobsters form their shells from a different substance, chitin, and are considered to be in less danger.
As ocean waters become more acidic, hydrogen ions react with carbonate ions to form bicarbonate, leaving less carbonate available for shell formation. A 2005 study led by James Orr of the Laboratory for Science of the Climate and Environment in France suggested that, by 2100, the amount of carbonate available to marine organisms could drop by 60%; as early as 2050, organisms in near-surface waters in the Southern Ocean and subarctic North Pacific might be unable to form shells.
Publishing his group's findings in the journal Nature, Orr remarked:
"Basic chemistry tells us that many folks alive today will live to see the polar oceans becoming inhospitable to key organisms, and unlike climate predictions, the uncertainties here are small."
Summing up a 2008 study of the consequences for ocean chemistry of stabilising atmospheric carbon dioxide at various levels, Stanford University marine scientist Ken Caldeira observed:
"If current trends in CO2 emissions continue unabated, in the next few decades we will produce chemical conditions in the oceans that have not been seen for tens of millions of years …. Ecosystems like coral reefs that have been around for many millions of years just won't be able to cope with the change."
Caldeira's study predicted that even with stabilisation at present-day levels of atmospheric carbon dioxide, fewer than half of existing coral reefs would remain in waters with the kind of chemistry that has sustained corals in the past. With stabilisation at 450ppm, a figure that could be surpassed within a few decades, fewer than 10% of reefs would remain in waters with such "classic" chemistry.
Higher acidity does not mean lifeless oceans. Scientists have found that at least one widespread species among the coccolithophores, shell-building plankton that are among the most common ocean organisms, increases in numbers and weight as acidity rises.
But other abundant groups such as the pteropods, tiny sea snails, appear under serious threat. Also in danger are ecologically important shellfish that have a juvenile planktonic stage, when their survival depends on their ability to form shells from calcium carbonate.
Acidification seems certain to cause the extinction, at least on a local scale, of large numbers of species. Ecosystems that contain fewer species usually generate less biomass; the oceans of the future, this suggests, will be less productive.
With fewer species available to fill particular ecological niches, ecosystems with low biodiversity are also more vulnerable to collapse if they come under additional pressure.
One source of such pressure could be the joining of oxygen depletion and acidification in a lethal combined assault. The deep, oxygen-poor water that has been detected spilling onto continental shelves is also relatively acidic.
Low pH is a normal characteristic of deep ocean waters, caused by the decomposition of organic matter that "rains down" as organisms closer to the surface die and sink.
On May 22, the British Telegraph reported a study by a team led by Richard Feely of the US government's National Oceanic and Atmospheric Administration in Seattle. Feely and his collaborators tested waters from the central Pacific coast of Canada down to northern Mexico.
"The pH ranged from 8.1 to as low as 7.6", the Telegraph related, "when they expected the figures to be no lower than 8.0." The decline in pH of 0.4 points represents more than a doubling of hydrogen ion concentration.
This research suggests that cold water corals — already damaged by commercial trawling — may be under serious threat on continental shelves in many parts of the world. These corals are a crucial habitat for the juvenile forms of numerous marine creatures, including commercially important species of fish.
With the rupture of food chains through the destruction of links such as coral ecosystems, populations of countless ocean dwellers, whales and large fish among them, could rapidly become unsustainable. This is in addition to the dire effects of overfishing and, in some areas, chemical pollution.
In the remote past, changes on such a scale generally occurred much more slowly — over many thousands of years at their fastest — giving ecosystems a chance to evolve and cope. The main exceptions have been during the cataclysmic extinctions, five so far, that have marked the boundaries between various periods of the Earth's geological history.
In the greatest of these biological catastrophes, the Permian Extinction of 251 million years ago, as many as 95% of marine species ceased to exist.
If greenhouse emissions are now rushing the Earth toward a sixth great ocean extinction, will this matter to human beings? Hundreds of millions of people rely on the seas as a key source of food, but this resource is already close to exhaustion, and substitutes will in any case have to be found on land.
Is the ocean environment, in essence, solely of moral and aesthetic importance to humanity, and hence dispensable? It might be possible to answer "yes" if scientists could state with certainty that human impacts on the marine world will not impede the functioning of the oceans as one of the Earth's main carbon "sinks", soaking up carbon dioxide and slowing its rise in the atmosphere.
For people of real moral and aesthetic awareness, of course, ocean life is a good in itself, to be preserved for its own sake. Major damage, that will persist for thousands of years, has been done to it already.
If new horrors are not to transpire, "business as usual" must be halted, greenhouse emissions quickly wound down, and ways found to remove carbon dioxide that is already in the atmosphere. Necessarily, this will require social and political changes of fundamental scope.