Guest post by David Middleton
My never-ending search for actual observational data that support the hypothesis of catastrophic anthropogenic ocean acidification (Chicken Little of the Sea) has taken me to offshore Hawaii and Station ALOHA.
The alarmists claim that anthropogenic CO2 emissions have lowered the average pH of the world’s oceans from 8.2 to 8.1 over the last 250 years; and that future emissions will lower the pH by 0.3 to 0.4 over the remainder of this century (Dore et al., 2009). What is the basis of this hypothesis? Systematic measurements of oceanic pH don’t go back much before 1990. Actual pH measurements are few and far between. The oceans have probably absorbed at least half of the anthropogenic carbon emissions of the last couple of centuries. The assertion of CO2-driven ocean acidification appears to he almost entirely based on the rising atmospheric CO2 level. Is this the only basis for the dire warnings of catastrophic ocean acidification? All other things being equal, the addition of CO2 should lower the pH of seawater… However, all other things are seldom equal.
Station ALOHA and the Hawaii Ocean Time-series (HOT)
The Station ALOHA data from the Hawaii Ocean Time-series are very extensive and easily accessible. These data were used by Dore et al., 2009 to demonstrate a ~20-yr trend of apparently CO2-related acidification offshore Hawaii…
At first glance, this appears to be a slam-dunk. The seawater at Station ALOHA have been acidifying in concert with rising atmospheric CO2 for ~20 years. But, looks can be deceiving. One of the tricky things about oceanic pH is that it is very dependent on water temperature, pressure and salinity. Most studies derive pH from Dissolved Inorganic Carbon (DIC or ΣCO2) and Total Alkalinity (TA). DIC is the sum of free CO2 ([CO2(aq)] & [H2CO3]), carbonate ([CO3–]) and bicarbonate ([HCO3-]). TA is the sum of carbonate (2x[CO3–]), bicarbonate ([HCO3-]), tetrahydroxyborate ([B(OH)4-]), hydroxide ([OH-]), hydrogen ([H-]) and other minor compounds. DIC and TA are “conservative quantities” – They are unaffected by pressure and temperature. The ratio of TA:DIC is a very robust measure os the alkalinity/acidity of seawater.
Most of the pH values in Dore et al., 2009 are not measured; they are derived from DIC and TA. While the calculated pH trend appears to show a continuous acidification trend, a plot of TA vs. DIC tells a slightly different story. The alkalinity of the seawater at ALOHA Station actually rose from ~2000 to ~2005, despite steadily rising atmospheric CO2 levels…
Why did the seawater become more alkaline from 2000-2005?
The most likely answer is that the seawater concentration of [CO3–] (carbonate) increased while the concentration of [CO2] decreased during that time interval. This happened despite the fact that atmospheric CO2 levels continued their inexorable rise over that same time period. The ocean at Station ALOHA was actually a net source of atmospheric CO2 for about 5 years.
The reason for the carbonate concentration increase and [CO2] decrease from 2000-2005 is probably related to the slight rise in water temperature over that period and the elevated salinity. The change in water temperature and salinity was also the most likely reason that the 5-year trend of increasing alkalinity was not reflected in the calculated pH trend.
So, the data do support the notion that rising atmospheric CO2 levels can contribute to a lowering of oceanic pH; but that other processes can periodically raise the pH despite the rise in atmospheric CO2.
How Dangerous is Chicken Little of the Sea?
Ocean acidification can only occur if Dissolved Inorganic Carbon (DIC) is rising faster than Total Alkalinity (TA). This nomogram demonstrates the relationship of TA & DIC to pH…
According to Dore et al., 2009, “Over the past 250 years, the mean pH of the surface global ocean has decreased from ≈8.2 to 8.1… This acidification of the sea is driven by the rapidly increasing atmospheric CO2 concentration, which results from fossil fuel combustion, deforestation, and other human activities. Models predict that surface ocean pH may decline by an additional 0.3–0.4 during the 21st century”… A total pH decline of 0.4 to 0.5 (8.2 to 7.7 or 7.6).
I used a linear regression to estimate TA and DIC at ~275 and ~550 ppmv…
If I plot their in situ TA vs in situ DIC and extrapolated it as above (red curve), I get a very strong correlation (R^2=0.72); but I don’t get anything close to a 0.5 to 0.6 pH decline from a doubling of pre-industrial CO2 levels. I get a total decline of 0.16 (8.30 to 8.14) due to a doubling of pre-industrial atmospheric CO2 levels.
The only way I get a pH decline comparable to 0.4 to 0.5 is when I use the TA and DIC values that were normalized to a salinity of 35 (blue curve). This yields a pH decline of 0.44 (8.40 to 7.96); but it is a horrible correlation (R^2=0.05). TA and DIC are highly correlated to salinity(R^2=0.88, 0.74). DIC has a moderate correlation (R^2=0.39) and TA has a weak correlation (R^2=0.12) to atmospheric CO2…
The normalization of TA and DIC to a constant salinity subdues the buffering provided by salinity; while amplifying the acidification effect of increasing CO2. A realistic treatment of salinity, yields an insignificant lowering of pH from a doubling of pre-industrial CO2. Chicken Little of the Sea does not appear to be very dangerous.
Will Chicken Little of the Sea Wreak Havoc on Marine Calcifers?
The Hawaii Ocean Time-series included ~20 years worth aragonite saturation data for Station ALOHA; so we can estimate the effects of atmospheric CO2 changes on aragonite saturation. Ries et al., 2009 experimentally demonstrated the effects of aragonite saturation changes on marine calficers under several CO2 scenarios. So, we can combine Ries’ experimental results with the observational data from Station ALOHA.
I plotted Ries’ CO2 vs. aragonite saturation assumptions and found that a power function trend-line was the best fit. I then plotted the Station ALOHA aragonite saturation vs. MLO CO2 and applied a power function trend-line to extrapolate the aragonite saturation out to about 2900ppmv CO2.
The Station ALOHA data demonstrate a significant difference in the actual effects of CO2 on aragonite saturation relative to the Ries et al., 2009 scenarios…
The observational data show that rising atmospheric CO2 is less detrimental to aragonite saturation levels than was assumed in the Ries experiment.
The Station ALOHA data indicate that marine calcifers have a much higher threshold CO2 level at which the calcification rate drops below its current range than that indicated by Ries…
The observed relationship between atmospheric CO2 and aragonite saturation from Station ALOHA indicates that even the most vulnerable marine calcifer in the Ries study (soft clam) was not adversely affected by CO2 levels below 800ppmv.
Richard E. Zeebe and Dieter A. Wolf-Gladrow
CARBON DIOXIDE, DISSOLVED (OCEAN)
Hawaii Ocean Time-series (HOT)
Station ALOHA Surface Ocean Carbon Dioxide
Dore JE, Lukas R, Sadler DW, Church MJ, KarlDM (2009)
Physical and biogeochemical modulation of ocean acidification in the central North Pacific.
Proc Natl Acad Sci USA 106:12235–12240.
Ries, J.B., A.L. Cohen, D.C. McCorkle. Marine calcifers exhibit mixed responses to CO2-induced ocean acidification. Geology 2009 37: 1131-1134.