Anthropogenic and climate driven ocean acidification: observations and modeling

Conventional wisdom has it that global warming and seawater acidification are different manifestations of increased atmospheric CO2. Although atmospheric CO2 rises at similar rates globally, the question of why the rates at which surface seawater pH decreases differ remains unanswered. Here we use data from six decadal time-series stations globally and thermodynamic calculations to provide an explanation on the inconsistency in rates of change in pH measured at 25°C (pH25) and at in situ temperature (pHinsitu). An anti-phase between rates of change in pHinsitu and pH25 vs. rates of temperature change is demonstrated for the first time. As the air-sea CO2 exchange lags behind changing seawater temperature, warming accelerates seawater acidification (-0.00722 簞C-1) at a rate 6.6-times that expected under only the air-sea CO2 equilibrium (-0.00109 簞C-1). These findings explain that inconsistencies in reported acidification rates worldwide are mainly due to different rates of temperature change and different types of pH use (pHinsitu or pH25). A surprising finding is that surface ocean pH can be written as a linear function of seawater temperature and time, and dissolved inorganic carbon (DIC) can be written as a linear function of seawater temperature, salinity and time. The modeled pH and DIC can further be used to model the seawater pCO2 and saturation state of calcium carbonate.

During 1994-2012, Nanwan Bay, southern Taiwan, suffered 1.5-times the expected acidification rate due to the increased atmospheric CO2, however, the rate of DIC increase is only about 25% of that expected due to the increased atmospheric CO2. This was initially due to increased intrusions of the West Philippines Sea (WPS) water during strong El Nino periods, and was then due to enhanced upwelling since 2005. The low DIC increasing rate is because the decrease in salinity dampened the increase in DIC under air-sea CO2 exchange and upwelling. Combining intrusions of WPS water and upwelling at Nanwan Bay, seawater suffered 0.06 pH decrement within just two years (1/2003~10/2005). Such a magnitude of change is equivalent to 35 years of acidification due to increased atmospheric CO2.

The results above provide an insight on how future climate change can alter the ocean acidification. Implications are that marine ecosystems suffering from ocean acidification could happen earlier than expected when ocean climate changes are considered along with increasing atmospheric CO2.

Lui H.-K., 2012. Anthropogenic and climate driven ocean acidification: observations and modeling. PhD thesis, National Sun Yat-Sen University, 135 p. Thesis (access restricted).

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