A changing ocean seen with clarity

The Hawaiian archipelago, the most remote group of islands on Earth, has long been associated with the world's most recognizable image of global change. The Mauna Loa atmospheric CO₂ record, begun in March 1958 by Charles David Keeling, shows with startling clarity the saw-tooth pattern of the seasonal changes of land vegetation, and the still astonishing, dominating, rise forced by fossil fuel burning which is rapidly changing our world. Within perhaps only 5 years the peak in the annual signal atop Mauna Loa will touch the 400 ppm by volume mark, which would have been inconceivable to scientists of the first half of the twentieth century. But there is one huge and environmentally critical signal that is not easily seen in the “Keeling curve,” and that is the oceanic uptake of fossil fuel CO₂. In this issue of PNAS, Dore et al. (1) document with great clarity the changes in ocean CO₂ chemistry and pH occurring in the ocean in the waters off Hawaii from fossil fuel CO₂ invasion.

Background

The changes in pCO₂ (partial pressure of CO₂) in the atmosphere are exactly paralleled in the ocean, but the consequences are very different. CO₂ has no atmospheric chemistry and is simply mixed. But increasing CO₂ in sea water induces changes in pH, and Dore et al. (1) have measured these changes with remarkable accuracy and precision. They thereby forcefully link air and sea and provide unmistakable evidence of ocean acidification and the complex and still poorly understood consequences of this. And they go beyond the simple surface expression to explore the changes taking place at depth.

Large-scale uptake of atmospheric fossil fuel CO₂ has long been recognized (2) as a fundamental consequence of the acid–base balance of slightly alkaline ocean surface waters, poised at about pH 8.2, exposed to an atmosphere of steadily increasing CO₂ (3). The quantities involved are huge. Ocean uptake of fossil fuel CO₂ is now proceeding at about 1 million metric tons of CO₂ per hour, and the accumulated burden of fossil fuel CO₂ in ocean waters is now well over 530 billion tons.

But the direct measurement of the pH changes brought about by this CO₂ uptake has challenged ocean scientists for decades. Dore et al. (1) have taken modern accurate spectrophotometric ratio techniques (4) and applied these with extraordinary care to obtain an almost 20-year record of pH changes at the now legendary station ALOHA off Hawaii. The dedication is extraordinary, and the results are unassailable. They show that the change in surface ocean CO₂ properties produces a “long-term decreasing trend in surface layer pH that is indistinguishable from the rate of acidification expected from equilibration with the atmosphere.”

Why should this finding be important? The annual changes in pH in surface waters off Hawaii are in milli-pH units, but the downward trend is clear. Why have such changes not been routinely documented for approximately the same time as the atmospheric CO₂ record? Is a slavish response to the atmosphere all that will occur? Will the deep ocean waters respond to change in the same manner as the ocean surface? Are there impacts that will be felt by marine life, or perturbations of carefully poised biogeochemical cycles?

Ocean uptake of fossil fuel CO₂ is now proceeding at about 1 million metric tons of CO₂ per hour.

The seemingly simple matter of accurate measurement of oceanic CO₂ system properties, and in particular pH, has engaged scientists for a long time. Early investigators routinely made very large numbers of electrode-based pH measurements around the world. But in the words of Keeling (5) “These investigators, in their optimism for having found a simple measuring routine, failed to note that the new method was scarcely capable of detecting the small changes in pH of surface ocean water that reflect significant changes in pCO₂.” These measurements were therefore of little practical use for tracking the changes taking place.

A major improvement was initiated in 1967 with the call by the influential chemist Lars Gunnar Sillen (6) for pressure–temperature-independent data, and for accurate measurement of the mass properties of total CO₂ and alkalinity. These can be fundamentally calibrated and are required data for incorporating the CO₂ system in ocean circulation models. But the methods recommended still relied on glass electrodes and on a complex array of thermodynamic constants. It was inevitable that trouble would follow (7) with a decade of inconsistent data from the world-wide Geosecs expedition that took heroic efforts to untangle (8). New nonelectrode techniques were developed, standards were created, and internal consistency was found.

Geochemists were then happier with the state of the art, and rapid progress was made. As a result, time series stations were established at Bermuda (9) and Hawaii (10) with the purpose of detailed tracking of oceanic biogeochemical cycles through time. These stations and the record that flows from them are now part of the crown jewels of US global change science. From these and other data ocean chemists could uncover the massive imprint of the fossil fuel CO₂ signal (11).

But communicating the consequences of these changes to the broader community, and to physiologists concerned with the inner workings of marine animals, proved hard. By reporting on mass properties, and assuming that the pH changes were understood, ocean scientists did not get their message out; the required language simply was not there. In a 2004 lecture to a fisheries meeting I remarked: “So complex is the full accounting of this process that the message has often been blurred. The use of a confusing set of apparent thermodynamic constants, the existence of several pH scales, the arcane distinctions between pCO2 and fCO2, the strictures on careful measurement, and the use of these systems in dynamic models have all deterred the non-specialist.”

Public Awareness

Breakthroughs in public awareness occurred in a strange way. Direct disposal of fossil fuel CO₂ in the ocean as a means of climate control had been advocated as early as 1977 (12), but when even very-small-scale experiments took place (13) the images obtained aroused environmental concern as no graph or table ever could. A typical reaction was “My God! You are going to change pH.” This new medium conveyed the message. With a formal assessment of the engineering issues and biological consequences by the Intergovernmental Panel on Climate Change (IPCC) (14) the door was opened. A critical meeting held in Paris in 2004 (15) shaped the rapidly emerging field of ocean acidification studies (16), and a cascade of important papers followed.

The first concern was for changes in calcification, with impacts on coral reefs and pelagic organisms with calcareous shells (17). The 20-year record in surface waters off Hawaii shows only a −0.03 pH unit change. But extrapolating backwards to the preindustrial era indicates that a modern change of −0.1 pH has already occurred. And projecting forward using well-known IPCC scenarios shows that a change of about −0.3 pH should occur by mid-century. The Dore et al. (1) data set shows that we are right on this track.

Consequences

The consequences for coral reefs (18) arouse concern because lowered carbonate ion concentration directly affects the ability of organisms to precipitate aragonite—the most common isomorph of calcium carbonate in coralline animals and the basic building block of coral reefs. Schneider and Erez (19) report a reduction in calcification rate of 55% for Acropora eurystoma under a doubling of atmospheric CO₂ (≈pH − 0.3). There is strong evidence that this is not controlled by an external surface equilibrium process. Rather the coralline animal actually engulfs sea water into an internal vacuole and works to form the skeletal material from the enclosed fluid. Although an increase in temperature could extend the latitudinal range of some corals, the net result of the combined effects of warming and acidification is likely to be strongly negative.

Hawaii is surrounded by vast expanses of coral reefs, but interestingly Dore et al. (1) do not directly address this issue. Instead they step tentatively onto new ground. They report on the vertical profiles of changing pH in the ocean water column and note that the rate of change “is elevated within distinct subsurface strata.” The deeper strata are colder waters formed at higher latitudes, but they take time to transit from their point of equilibration with the atmosphere and thus “saw” an earlier, lower-CO₂, atmosphere. Why then should the rate of change of pH be higher?

The generally quoted change of pH of ≈0.3 for a doubling of atmospheric CO₂ applies only to well-buffered surface waters; it will be greater at depth. The huge capacity of the ocean for CO₂ uptake is related to the well-known Revelle factor expressed approximately as the ratio of a fractional change in pCO₂ to the fractional change in total CO₂ (TCO₂), or

For surface waters this typically has a value of ≈10. But colder, deep waters in which pH and carbonate ion have already been much reduced by the addition of respiratory CO₂ have far less buffer capacity. Thus the changes in both pCO₂ and pH created at depth as the CO₂ invasion moves into abyssal waters will far exceed the surface changes now widely discussed in the ocean acidification literature. The relatively well-oxygenated deep waters off Hawaii measured by Dore et al. (1) hint at this process.

For an extreme case Brewer and Peltzer (20) have recently investigated the changes at an eastern Pacific station where O₂ levels are very strongly depleted, and CO₂ highly enriched, at only 500-m depth. There surface waters labeled by a doubling of atmospheric CO₂ and translated to depth will raise the pCO₂ from about 1,000 ppm to 2,000 ppm and more. The equivalent changes in pH will occur. These extraordinary numbers pose a challenge not simply for calcareous organisms but when combined with very low O₂ values pose a respiratory limit to all aerobic life. The end result is disturbing. There is already clear evidence of expansion of the low-oxygen regions of the oceans (21), and when these are combined with rising CO₂ levels we will surely see true dead zones created.

The remote Hawaiian waters are still apparently unaffected by the trend in declining O₂, but the rise in CO₂ and decline in pH that are observed, although still small, indicate that these ocean processes and worrisome trends are universal.