My apologies for the long and convoluted title; but this post will tie ocean acidification (Chicken Little of the Sea), the Pacific Decadal Oscillation (PDO), seawater salinity and the little-known mid-20th Century decline in atmospheric CO2 all into one hopefully coherent blog post.
Chicken Little of the Sea, with a side order of PDO
According to Byrn et al., 2010, “Global ocean acidification is a prominent, inexorable change associated with rising levels of atmospheric CO2…”
Declining pH has been “associated with rising levels of atmospheric CO2” since about 1990. Prior to 1990, rising and falling pH levels weren’t associated with rising or falling levels of CO2.
However, the rising and falling pH was and still appears to be inversely correlated with the Pacific Decadal Oscillation (PDO)…
That’s really funny… Because the PDO supposedly can’t drive anything; and it’s an index of North Pacific sea surface temperatures and Flinders Reef is in the Coral Sea. The PDO and Flinders Reef are on opposite sides of the equator. I’m not the only one to notice this correlation.
Pelejero et al., 2005 also found a cyclical correlation between Flinders Reef pH and the PDO…
Yes… I know that the pH of reef water should be more variable than open ocean water. But, most of the paleo-pH data are derived from corals. That’s why I included he Station ALOHA data from the Hawaii Ocean Time-series.
Hold the Salt
Ocean acidification can only occur if Dissolved Inorganic Carbon (DIC) is rising faster than Total Alkalinity (TA). This nomogram (Zeebe and Wolf-Gladrow) demonstrates the relationship of TA & DIC to pH…
DIC and TA “are conservative quantities, i.e. their concentrations measured in gravimetric units (mol kg−1) are unaffected by changes in pressure or temperature, for instance, and they obey the linear mixing law. Therefore they are the preferred tracer variables in numerical models of the ocean’s carbon cycle.” (Zeebe and Wolf-Gladrow). However, DIC and TA are affected by changes in salinity. 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:
So, it appears to me that salinity changes can have a large impact on DIC and TA. Since DIC has a steeper slope than TA, increasing salinity will raise the pH of seawater. And I think that explains both the inverse correlation of the PDO to pH and the lack of correlation of CO2 to pH at Flinders Reef. The phase of the PDO appears to be related to the phase of the ENSO. A La Niña-dominated ENSO tends to be associated with a generally negative phase PDO and with a rise in pH at Flinders Reef. The PDO has been correlated to salinity variations in the tropical Pacific, including the late 20th Century “freshening” (Overland et al., 1999, Nurhati et al., 2010).
Despite the lack of a correlation between CO2 and pH and good correlation of PDO to pH in his Flinders Reef study, Pelejero et al., 2010 assert a correlation between CO2 and pH variations across Pleistocene glacial-interglacial cycles.
Rising sea levels are associated with decreasing salinity; conversely falling sea levels are associated with increasing salinity. So, it seems far more likely that the Pleistocene pH variations are more associated with salinity variations than with CO2. One of the arguments against the highly variable atmospheric CO2 concentrations indicated by plant stomata reconstructions is the lack of evidence of preindustrial Holocene ocean acidification. The argument is that the increased oceanic uptake of CO2 during prior Holocene cooling episodes should be supported by evidence of cyclical acidification, therefore the Antarctic ice cores are right: Preindustrial Holocene CO2 levels were relatively stable.
There’s just one slight problem with this argument.
The Mid-20th Century Decline in Atmospheric CO2
Plant stomata reconstructions (Kouwenberg et al., 2005, Finsinger and Wagner-Cremer, 2009) and contemporary chemical analyses (Beck, 2007) indicate that CO2 levels in the 1930′s to early 1940′s were in the 340 to 400 ppmv range and then declined sharply in the 1950’s. These findings have been rejected by the so-called scientific consensus because they run contrary to the Antarctic ice cores and the lack of a pH signature due to the requisite oceanic uptake.
Somebody (MacFarling Meure et al., 2006) went and found evidence of that mid-20th Century CO2 decline in the DE08 ice core…
The stabilization of atmospheric CO2 concentration during the 1940s and 1950s is a notable feature in the ice core record. The new high density measurements confirm this result and show that CO2 concentrations stabilized at 310–312 ppm from ~1940–1955. The CH4 and N2O growth rates also decreased during this period, although the N2O variation is comparable to the measurement uncertainty. Smoothing due to enclosure of air in the ice (about 10 years at DE08) removes high frequency variations from the record, so the true atmospheric variation may have been larger than represented in the ice core air record. Even a decrease in the atmospheric CO2 concentration during the mid-1940s is consistent with the Law Dome record and the air enclosure smoothing, suggesting a large additional sink of ~3.0 PgC yr-1 [Trudinger et al., 2002a]. The d13CO2 record during this time suggests that this additional sink was mostly oceanic and not caused by lower fossil emissions or the terrestrial biosphere [Etheridge et al., 1996; Trudinger et al., 2002a]. The processes that could cause this response are still unknown.
 The CO2 stabilization occurred during a shift from persistent El Niño to La Niña conditions [Allan and D’Arrigo, 1999]. This coincided with a warm-cool phase change of the Pacific Decadal Oscillation [Mantua et al., 1997], cooling temperatures [Moberg et al., 2005] and progressively weakening North Atlantic thermohaline circulation [Latif et al., 2004]. The combined effect of these factors on the trace gas budgets is not presently well understood. They may be significant for the atmospheric CO2 concentration if fluxes in areas of carbon uptake, such as the North Pacific Ocean, are enhanced, or if efflux from the tropics is suppressed.
If oceanic uptake of CO2 caused ocean acidification, shouldn’t we see some evidence of it? Shouldn’t “a large additional sink of ~3.0 PgC yr-1” (or more) from ~1940–1955 have left a mark somewhere in the oceans?
Well, it may have left a mark… The Great Barrier Reef seems to have been busy “eating” some of that extra ~3.0 PgC yr-1 from ~1940-1955. The average calcification rate was actually a bit higher than the long-term trend from 1940-1975…