Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean
Key finding. Ocean general circulation model simulations give global surface ocean pH reductions of 0.3 to 0.5 units by 2100 under the IPCC SRES A1, A2, B1, and B2 emission pathways; cumulative atmospheric emission of 5,000 petagrams of carbon produces aragonite undersaturation in most of the surface ocean and a surface pH reduction of 0.8 units by 2300, and 20,000 petagrams produces a reduction of 1.4 units.
What question did this research address?
Ocean acidification had been established in principle, but marine biologists needed the chemistry expressed as it would actually unfold — per emission pathway, per region, and per mineral — to know which experiments correspond to which futures.
Deep-sea CO2 injection was also being proposed as an alternative to atmospheric release. That trade needed to be stated chemically: less impact where, in exchange for more impact where.
What did we find?
Under the SRES A1, A2, B1, and B2 pathways, and the stabilization pathways holding CO2 at 650 ppm or above, parts of the Southern Ocean become undersaturated with respect to aragonite — the mineral form used by pteropods and corals.
Cumulative emission of 5,000 petagrams of carbon makes most of the surface ocean undersaturated with respect to aragonite; 10,000 petagrams also makes it undersaturated with respect to the more stable calcite.
Stabilizing atmospheric CO2 at 450 ppm — a low target — still produces both calcite and aragonite undersaturation through most of the deep ocean.
Surface pH falls 0.3 to 0.5 units by 2100 under the SRES pathways; by 2300 the same reduction is reached by the 650 and 1000 ppm stabilization scenarios and by a 1,250 petagram emission scenario, while 5,000 and 20,000 petagrams give 0.8 and 1.4 units.
Deep ocean injection as an alternative to atmospheric release buys less impact on the surface ocean and climate at the price of greater chemical impact on the deep ocean. The paper states this as a trade rather than a solution.
Changes of the magnitude shown are likely to be biologically significant, which is the point of tabulating them scenario by scenario.
Why does it matter?
It supplied the numbers the ocean acidification literature has used ever since to connect laboratory carbonate chemistry to specific emission futures, so a perturbation experiment can be labelled with the scenario and year it represents.
Showing that even 450 ppm stabilization leaves most of the deep ocean corrosive establishes that acidification is not avoided by moderate mitigation — only its severity is.
Framing deep-sea injection as relocating the chemical impact rather than removing it set the terms on which ocean carbon disposal has been argued since.
Citation
Ken Caldeira and Michael E. Wickett (2005). Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean. Journal of Geophysical Research - Oceans 110, C09S04.
Related
- What does adding carbon dioxide to the ocean do to it?
- How long does a carbon dioxide emission go on warming the planet?
- Anthropogenic carbon and ocean pH (Caldeira and Wickett, 2003)
- Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications (Caldeira and Rau, 2000)
- Depth, radiocarbon, and the effectiveness of direct CO2 injection as an ocean carbon sequestration strategy (Caldeira et al., 2002)