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Our Dying Oceans

Acidification Threatens the Entire Marine Ecosystem

Change in sea surface pH caused by anthropogenic CO2 emissions between the 1700s and 1990s. SOURCE: Wikimedia Commons A growing body of research demonstrates that global waters are absorbing massive amounts of carbon dioxide, threatening species at the bottom of the food chain. So why are we still paying so little attention to climate change’s elephant in the room? Above: Change in sea surface pH caused by anthropogenic CO2 emissions between the 1700s and 1990s.

Extinctions. Droughts. Melting glaciers. Even for those of us not steeped in the nitty gritty of climate change, it’s been almost impossible to avoid the ongoing news coverage of scientists’ increasingly gloomy prognostications about our planet’s future. Look past the blaring headlines, however, and many will tell you that far too little attention is still being paid to the real elephant in the room: ocean acidification.

The unprecedented influx of anthropogenic CO2 emissions since the 1800s has fundamentally altered the equation.

Starting in the late 1950s with the groundbreaking research of Roger Revelle and Charles Keeling, scientists have long been aware of the essential role played by the ocean in mitigating the impact of elevated atmospheric carbon dioxide (CO2) levels. Ice core record measurements of carbon dioxide taken mid-century showed that atmospheric concentrations had remained about constant for several thousand years until the rapid onset of industrialization during the 1800s, after which they began their meteoric rise. Revelle’s work was instrumental in demonstrating that a large fraction of the gas remained in the atmosphere. At the same time, it also suggested that a significant amount was being absorbed by the ocean—a realization that would lead him to conclude that, over the long term, it would permanently change the chemistry of seawater. A number of oceanographer-led global surveys completed in 2004 determined that the ocean had absorbed nearly half of all carbon emitted since the start of the Industrial Revolution. Other studies have found that around a third of fossil fuel-derived CO2 is currently taken up by the ocean [1].

Upon entering the ocean, a portion of CO2 reacts with water to form carbonic acid, a weak acid; the other portion stays in dissolved form. Some fraction of the acid will then release hydrogen ions into solution, yielding either bicarbonate or carbonate ions, while a smaller fraction will remain as carbonic acid. The relative proportion of these three forms of dissolved inorganic carbon—carbon dioxide, bicarbonate ions and carbonate ions—acts as a natural buffer, called the “carbonate buffer,” by absorbing small pH changes induced by the increase in hydrogen ion concentration. The pH scale, which ranges from 0 to 14, is used by scientists to measure a solution’s acidity or basicity—the lower the value, the more acidic the solution. The scale is logarithmic, so a one-pH unit drop corresponds to a ten-fold increase in the hydrogen ion concentration, making seawater more acidic. With an average pH of 8.1, seawater is considered slightly basic, or alkaline.

This buffering system has helped keep the ocean’s pH in check for thousands of years. However, the unprecedented influx of anthropogenic CO2 emissions since the 1800s has fundamentally altered the equation, threatening to overwhelm the delicate balance maintained by this system and tipping the ocean into a period of prolonged acidification. The problem is simple: as increasing amounts of atmospheric CO2 are absorbed by surface waters, more hydrogen ions are formed—which leads to an overall decrease in seawater pH. Many of these hydrogen ions will combine with carbonate ions, forming bicarbonate ions and reducing the concentration of carbonate ions. The net effect is to weaken the carbonate buffer, rendering it less effective at keeping slight pH variations in check.

By some estimates, all of the planet’s corals could disappear by century’s end if present trends continue.

Researchers believe this process lowered the oceans’ average pH by 0.1 since the pre-industrial era—equivalent to a 30 percent increase in the ocean’s average hydrogen ion concentration [2]. A recent analysis postulated that pH levels might fall by as much as 0.5 units by 2100, which would be equivalent to a three-fold increase in the hydrogen ion concentration since pre-industrial times [3]. The impacts of ocean acidification are already being felt closer to home: a report published just this past month in Science showed evidence for the upwelling of “acidified” water onto the Pacific continental shelf between central Canada and northern Mexico. Seasonal upwelling, which brings nutrient-rich deep waters up to the surface, is a natural phenomenon in this region and one that is critical for many developing marine organisms.

“So what?” you may ask. Why should I care about this when other climate-induced phenomena like heat waves and droughts seem much more urgent? Diminishing the ocean’s capacity to absorb CO2 is no small problem in itself, because without the ocean serving as a carbon sink, more carbon dioxide will have no where to go but into the atmosphere. But aside from that, what worries scientists most about ocean acidification is that it will inhibit certain organisms’ ability to produce calcium carbonate shells—to the extent that they would have great difficulty growing. And not just any organisms: those, like phytoplankton, which support entire food webs by acting as the ocean’s primary producers (like plants in terrestrial ecosystems). Without them—or with their numbers greatly reduced—many populations and ecosystems could simply collapse. Moreover, oceanographers are deeply concerned about the potential impact of acidification on corals. These tiny organisms, which secrete calcium carbonate skeletons that, over time, accumulate to form large reef assemblages, could become more prone to so-called “bleaching” episodes—in which algae that form symbiotic associations with the corals (and give them their colors) are expelled, depriving the latter of a critical source of nutrients. Worse, the precipitous drop in carbonate ion concentration could make many regions of the ocean acidic enough to dissolve calcium carbonate structures [4]. Corals, phytoplankton and other calcifying organisms would be unable to survive under such “undersaturated” conditions. By some estimates, all of the planet’s corals could disappear by century’s end if present trends continue. The continued uptake of carbon dioxide from the atmosphere will cause these areas to expand until only a sliver of the ocean’s surface layer remains inhabitable.

That’s not to say that certain species won’t also benefit. Indeed, a few recent studies have demonstrated that some phytoplankton species may thrive under conditions of elevated CO2 concentrations. Larger organisms, like seagrasses, use dissolved carbon dioxide directly and could therefore also experience gains. While the current state of research may be ambiguous in some areas, it is clear that the overall picture is decidedly grim. Though more studies are needed, scientists are concerned that acidification is taking place at such speed that we—let alone marine species—will have little time to adapt.

Jeremy Jacquot is a graduate student in marine environmental biology at the University of Southern California and is the Los Angeles correspondent for TreeHugger.com.

Notes

[1] Doney, S.C. 2006. The dangers of ocean acidification. Scientific American: 58 – 65.

[2] Brewer, P.G. 1997. Ocean chemistry of the fossil fuel CO2 signal: the haline signal of “business as usual”. Geophys. Res. Lett. 24: 1367 – 1369.

[3] Caldeira, K. and Wickett, M.E. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.

[4] The Royal Society. 2005. Ocean acidification due to increasing atmospheric carbon dioxide.

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