Study: Avoiding Damage from Ocean Acidification May Require Deeper Cuts in CO2 Emissions Than to Mitigate Climate Change
|The white contour lines illustrate the expected maximum pH decrease of average surface ocean waters in the future (in pH units) as a function of total anthropogenic CO2 emissions (in petagrams of carbon) and release time (in years). Click to enlarge|
Several studies conducted previously explored the detrimental impact of increasing levels of atmospheric carbon dioxide on ocean chemistry by causing a drop in pH—called “ocean acidification”. (Earlier post.)
Writing in the 4 July issue of the journal Science, a team of researchers led by Richard Zeebe of the University of Hawai’i at Manoa warns that the ecological and economic consequences of ocean acidification are difficult to predict but possibly calamitous, and that halting the changes already underway will likely require even steeper cuts in carbon emissions than those currently proposed to curb climate change.
Projected changes in ocean carbonate chemistry should serve as a guideline for policy protocols that identify CO2 emission targets to reduce the effects of human-made ocean acidification. For example, to avoid a surface ocean pH decline by more than 0.2 units, total emission targets would have to range from ~700 Pg [700 billion metric tons] C over 200 years to ~1200 Pg C [1,200 billion metric tons] over 1000 years...Such scenarios would be difficult to achieve, however, because they require immediate reductions in global emissions. If emissions can be reduced after the year 2050 and capped at 1500 Pg C, surface ocean pH would decline by ~0.35 units relative to preindustrial levels. The aragonite saturation state in the warm surface ocean would drop from ~3.5 to ~2.1 under this scenario...Substantial reductions in coral calcification have been reported over this range.
...Ocean chemistry changes, and not only climate effects, should be taken into consideration when determining CO2 emission targets; such consideration is likely to weigh in favor of lower emission targets.—“Carbon Emissions and Acidification,” Zeebe, et. al.
Unrelated to climate change, ocean acidification is an issue of basic chemistry: atmospheric CO2 is absorbed by and reacts with seawater to form carbonic acid (H2CO3).
Increasing the amount of CO2 dissolved in the ocean lowers the pH, decreases the availability of carbonate (CO32-) ions, and lowers the saturation state of the major shell-forming carbonate minerals. Carbonate ions are building blocks for the calcium carbonate that many marine organisms use to grow their skeletons and create coral reef structures.
Researchers have determined that with emissions of anthropogenic carbon dioxide continuing to rise, the partial pressure of CO2 (pCO2) dissolved in the surface ocean is likely to double its pre-industrial value within the next 50 years. Oceans are naturally alkaline, and they are expected to remain so, but the interaction with carbon dioxide is making them less alkaline and more acidic.
Zeebe; Ken Caldeira of the Carnegie Institution’s Department of Global Ecology; James Zachos, of the University of California Santa Cruz; and Toby Tyrrell of Southampton University (UK) note that the oceans have absorbed about 40% of the carbon dioxide (CO2) emitted by humans over the past two centuries—roughly 500 billion metric tons (500 Pg) of carbon dioxide.
This has slowed global warming, but at a cost: the extra carbon dioxide has caused the ocean’s average surface pH to drop by about 0.1 unit from pre-industrial levels—a 25% increase in hydrogen-ion concentration. Small changes in the pH value can make a big difference because pH is measured on a logarithmic scale (analogous to the Richter scale). For example, a drop by one pH unit means a ten-fold increase in acidity .Depending on the rate and magnitude of future emissions, the ocean’s pH could drop by as much as 0.35 units by the mid-21st century.
In contrast to climate model predictions, such future ocean chemistry projections are largely model-independent on a time scale of a few centuries mainly because the chemistry of CO2 in seawater is well known and changes in surface ocean carbonate chemistry closely track changes in atmospheric CO2.—Zeebe, et. al.
Experiments have shown that changes of as little as 0.2-0.3 units can hamper the ability of key marine organisms such as corals and some plankton to calcify their skeletons, which are built from pH-sensitive carbonate minerals. Large areas of the ocean are in danger of exceeding these levels of pH change by mid-century, including reef habitats such as Australia’s Great Barrier Reef.
Most marine organisms live in the ocean’s sunlit surface waters, which are also the waters most vulnerable to CO2-induced acidification over the next century as emissions continue. To prevent the pH of surface waters from declining more than 0.2 units, the current limit set by the US Environmental Protection Agency in 1976, carbon dioxide emissions would have to be reduced immediately.
Although the ocean’s chemical response to higher carbon dioxide levels is relatively predictable, the biological response is more uncertain. The ocean’s pH and carbonate chemistry has been remarkably stable for millions of years—much more stable than temperature.
If we continue with business as usual and don’t cut carbon dioxide emissions, carbonate reefs will ultimately start to dissolve. This is basic chemistry. The biology is a bit trickier. Most lab and field experiments show that calcifying organisms struggle under high-CO2 conditions but it’s very difficult to predict their long-term reaction, let alone responses of entire marine ecosystems.—Richard Zeebe
Reduced calcification will surely hurt shellfish such as oysters and mussels, with big effects on commercial fisheries. Other organisms may flourish in the new conditions, but this may include undesirable “weedy” species or disease organisms.
We need to consider ocean chemistry effects, and not just the climate effects, of CO2 emissions. That means we need to work much harder to decrease CO2 emissions. While a doubling of atmospheric CO2 may seem a realistic target for climate goals, such a level may mean the end of coral reefs and other valuable marine resources.—Ken Caldeira
Richard E. Zeebe, James C. Zachos, Ken Caldeira, and Toby Tyrrell, “Carbon Emissions and Acidification”, Science 4 July 2008 doi: 10.1126/science.1159124