Learn how plants respond to higher atmospheric CO2 concentrations

How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Ocean Acidification: The "Evil Twin" of Global Warming
Volume 13, Number 26: 30 June 2010

In introducing their new study of the ocean acidification phenomenon, Carles Pelejero, Eva Calvo and Ove Hoegh-Guldberg (2010) write that "the surface waters of the oceans have already acidified by an average of 0.1 pH unit from pre-industrial levels," and that "by the end of the twenty-first century, projections based on different scenarios indicate that ocean pH will have decreased by 0.3 to 0.4 pH unit." This "steady acidification of the oceans (nicknamed the 'evil twin' of global warming)," as they describe it, is said by them to be yet another "insidious consequence of rising levels of atmospheric CO2," and that "evidence gathered over the last years suggests that ocean acidification could represent an equal (or perhaps even greater) threat to the biology of our planet." But is this really so?

Consider the absolute pH values pertinent to these claims. The preindustrial-to-present 0.1-pH-unit drop reported to have occurred by the three researchers represents the decline from a mean value of about 8.16 to a value of 8.06, as best we can discern from their graphical representation of the decline. However, another of their graphs depicts interannual pH variations in the North Atlantic Ocean near Bermuda ranging from a high of approximately 8.18 to a low of about 8.03 at various times over the period 1984 to 2007 (Bates, 2007), which demonstrates that even larger pH variations are occurring in some ocean basins as a result of seasonal seawater variability.

Even greater natural pH variability is evident on both shorter and longer time scales in still other of Pelejero et al.'s graphs. Over a mere two days in July 2001 on a Molokai (Hawaii) Reef flat, for example, seawater pH ranged from a high of 8.29 to a low of 7.79 (Yates and Halley, 2006); while over a period of about a decade in the mid-20th-century, the pH at Arlington Reef in Australia's Great Barrier Reef system ranged from a high of approximately 8.25 to a low of about 7.71 (Wei et al., 2009). And both of these natural and recurring pH declines (0.50 and 0.54) are greater than the 0.3 to 0.4 projected decline that Pelejero et al. expect to occur between now and the end of the century. What is more, the calcifying organisms in these regions have fared just fine, as they have faced and successfully adapted to each one of the worse-than-"evil" pH declines.

In conclusion, and on top of everything else, the analysis of Tans (2009) indicates that the pH decline expected by Pelejero et al. between now and the end of the century is fully twice as great as what is likely to occur in reality, and that by AD 2500 the spatial and vertical pH distributions within the world's oceans will have likely returned to almost the same sets of values that are characteristic of today, which is not exactly what we would call an "evil" phenomenon.

Sherwood, Keith and Craig Idso

Bates, N.R. 2007. Interannual variability of the oceanic CO2 sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades. Journal of Geophysical Research 112: 10.1029/2006JC003759.

Pelejero, C., Calvo, E. and Hoegh-Guldberg, O. 2010. Paleo-perspectives on ocean acidification. Trends in Ecology and Evolution 25: 332-344.

Tans, P. 2009. An accounting of the observed increase in oceanic and atmospheric CO2 and an outlook for the future. Oceanography 22: 26-35.

Wei, G., McCulloch, M. T., Mortimer, G., Deng, W. and Xie, L. 2009. Evidence for ocean acidification in the Great Barrier Reef of Australia. Geochimica et Cosmochimica Acta 73: 2332-2346.

Yates, K.K. and Halley, R.B. 2006. CO32 concentration and pCO2 thresholds for calcification and dissolution on the Molokai reef flat, Hawaii. Biogeosciences 3: 357-369.