Model the ocean’s future

Previously, we’ve looked at how the Paleocene-Eocene Thermal Maximum provides us an analogue to the future of our oceans, which projects a similar or even worse marine mass extinction in the future. This week, I’m going to explore what do climate and oceanic models say about our ocean’s destiny in the next 100 years.

Ocean-carbon cycle model projections

Orr et al. (2005) have used 13 coupled ocean-carbon cycle models to simulate changes in surface ocean pH, carbonate ion concentration [CO₃^2-] and calcium carbonate saturation under the Intergovernmental Panel on Climate Change (IPCC) IS92a ‘business-as-usual’ scenario over the 21^st century, which are adopted by the IPCC 4^th Assessment Report (IPCC AR4) Chapter 10.4.1-2. Their multi-model projections indicated large decrease in oceanic pH and carbonate ion concentrations across the world’s ocean during the 21^st Century, driven by direct geochemical effect of increasing anthropogenic CO₂ emissions only (Figure 1).  Currently, surface ocean pH is already 0.1 unit lower than pre-industrial values. It is further projected for another 0.3-0.4 units decrease by the end of this century under the IPCC IS92a scenario, which is equivalent to a 100-150% increase in [H⁺] (Orr et al., 2005). Their results also highlighted similar latitudinal variability pattern; surface water pH and [CO₃^2-] are higher in high latitudes and decrease significantly towards mid and low latitudes. However, according to their model projections, latitudinal differences in [CO₃^2-] between mid and high latitudes will be reduced towards 2100 under the IS92a scenario (Figure 1c). Carbonate ion concentrations i.e. aragonite saturation state will begin to decline and the undersaturation of aragonite is projected to extent throughout the entire Southern Ocean and into the subarctic Pacific Ocean by 2100 (Figure 2).

Figure 1 Projected (a) atmospheric CO2, (b) ocean surface pH and (c) [CO₃^2-] during 21st Century (Orr et al., 2005)



Figure 2 Projected levels of aragonite saturation (%) over the 21st Century (IPCC, 2007)

The authors also found that this aragonite undersaturation in the Southern Ocean could threaten high-latitude ecosystems within decades, not centuries as previous studies suggested; it is likely to be first detected during winter. A more recent paper by McNeil and Matear (2008) further estimated that this Southern Ocean wintertime aragonite undersaturation will occur by the year 2030 and no later than 2038 with atmospheric CO2 of ~450ppmv under the IPCC IS92a scenario.

Coupled atmosphere ocean-climate model projections

In order to simulate the effects of climate variability and climate change (changes in temperature and ocean circulation etc.) on ocean carbonate ion concentrations on top of the direct geochemical effects, Orr et al. (2005) have also analysed three coupled atmosphere ocean-climate models. Surprisingly, their analysis suggested that physical climate change alone will not alter high-latitude surface [CO₃^2-] substantially during the 21^st Century. This argument is also reaffirmed by the coupled climate/ocean-carbon cycle model results generated by Cao and Caldeira (2008). It is largely due to the air-sea CO2 exchange mostly compensates for changes in surface dissolved inorganic carbon (DIC) caused by changes in marine productivity and circulation.

However, high-latitude subsurface [CO₃^2-] is projected to be decreased significantly during the 21^st Century based on all the three coupled model results, with small uncertainties in changes in temperature, ocean stratification and marine biological production and re-mineralization (IPCC, 2007; Orr et al., 2005). Orr et al. (2005) further suggested that the biggest uncertainty comes from atmospheric CO2 trajectories, which is the only means to limit further decline in ocean [CO₃^2-].  Figure 3a showed the atmospheric CO₂ projections under the six IPCC emission scenarios, which exhibits high levels of uncertainty, especially towards Year 2100. As surface ocean [CO₃^2-] is highly dependent upon atmospheric CO₂, high levels of uncertainty in atmospheric CO₂ trajectories have a significant impact on predicting global pH changes as well as the timing and extent of aragonite undersaturation (Figure 3b and c).

Figure 3 Model projections of  (a) Atmospheric CO2, (b) Global Ocean pH and (c) Southern Ocean Saturation under six IPCC emission scenarios (IPCC, 2007)

It is concluded that the increased anthropogenic CO₂ emissions has a significant impact on ocean carbonate chemistry, i.e. ocean acidification, based on both ocean-carbon cycle models and coupled climate/ocean-carbon cycle models. Despite of the large uncertainty in atmospheric CO₂ level projections, the changes in oceanic carbonate ion concentrations is closely linked with atmospheric CO₂. Although more research is required to provide more reliable projections of future atmospheric CO₂, it is clear that the impact of ocean acidification will be more severe in the future under ‘business-as-usual’ scenario. In the following weeks, I will be moving on to discuss ways to mitigate this ocean’s acid test challenge. Stay Tuned!