Background
The authors write that "natural variability in pH is seldom considered when effects of ocean acidification are considered," and they suggest that this omission is disturbing, because "natural variability may occur at rates much higher than the rate at which carbon dioxide is decreasing ocean pH," which is about 0.0017 pH unit per year according to Dore et al. (2009) and Byrne et al. (2010). And because of this fact, they contend that "ambient fluctuation in pH may have a large impact on the development of resilience in marine populations," noting that "heterogeneity in the environment with regard to pH and pCO2 exposure may result in populations that are acclimatized to variable pH or extremes in pH."

What was done
To further explore this possibility, Hofmann et al. recorded continuous high-resolution time series of upper-ocean patterns of pH variability with autonomous sensors deployed at fifteen different locations stretching from 40.7303°N to 77.8000°S latitude and from 0 to 166.6712°E longitude and 0 to 162.1218°W longitude, over a variety of ecosystems ranging from polar to tropical, open-ocean to coastal, and kelp forest to coral reef.

What was learned
The eighteen researchers report that their measurements revealed "a continuum of month-long pH variability with standard deviations from 0.004 to 0.277 and ranges spanning 0.024 to 1.430 pH units," which variability was "highly site-dependent, with characteristic diel, semi-diurnal, and stochastic patterns of varying amplitudes."

What it means
Directly quoting Hofmann et al., "these biome-specific pH signatures disclose current levels of exposure to both high and low dissolved CO2, often demonstrating that resident organisms are already experiencing pH regimes that are not predicted until 2100." And these facts suggest that the current real-world heterogeneity of the world's oceans with regard to pH and pCO2 exposure may indeed "result in populations that are acclimatized to variable pH or extremes in pH," such as those that have been predicted to be the new norm in 2100.

References
Byrne, R.H., Mecking, S., Feely, R.A. and Liu, X. 2010. Direct observations of basin-wide acidification of the North Pacific Ocean. Geophysical Research Letters 37: 10.1029/2009GL040999.

Dore, J.E., Lukas, R., Sadler, D.W., Church, M.J. and Karl, D.M. 2009. Physical and biogeochemical modulation of ocean acidification in the central North Pacific. Proceedings of the National Academy of Sciences USA 106: 12,235-12,240.