Jorge Sarmiento and colleagues are working on a variety of projects to evaluate the future of ocean carbon uptake through both natural and engineered processes. These studies have shown that understanding the circulation and biology in the Southern Ocean is critical to forecasting the potential of ocean carbon sinks.


Carbon mitigation with engineered sinks

One project completed early in the grant was a study of the potential of deep-sea injection as a sink for CO2. Results from a general circulation model indicated that at least 70% and up to 93% of the carbon injected below 3000 meters water depth remains in the oceans after 500 years. However the sequestration efficiency is a strong function of ocean circulation in the model, particularly in the Southern Ocean, and possible environmental impacts raised by other researchers need to be addressed before deep-sea injection could be implemented.

Another area of investigation has been the possibility of fertilizing the ocean to enhance phytoplankton uptake of CO2 and the delivery of this carbon to the deep ocean via an increased flux of organic matter. Early model simulations suggest that only 2-10% of additional carbon flux to the deep ocean would come from the atmosphere, and that fertilization might eventually decrease biological production and impact fisheries at lower latitudes. New work testing iron fertilization with the latest models of iron chemistry and biology show much higher efficiency, an issue which will be addressed in future work.

Recent modeling work shows that the impacts of the nutrient depletion that might occur in response to iron fertilization are highly dependent on where the fertilization is carried out. An important finding has been that nutrient depletion in the Subantarctic zone would have a relatively small impact on atmospheric CO2 levels, but could dramatically decrease productivity at low latitudes. The group’s work shows that about three-quarters of the biological production outside of the Southern Ocean results from nutrients supplied by Subantarctic Mode Water (SAMW). The response of this water mass to climate change is thus likely to have a significant impact on global biological production.

In contrast, the researchers have found that nutrient depletion in the deep water formation regions of the Antarctic zone would substantially draw down atmospheric CO2, and have a much smaller impact on low-latitude biological productivity. The findings suggest that surface nutrient concentrations in the deepwater formation regions of the Antarctic zone of the Southern Ocean control the ocean-atmosphere exchange of CO2, and that past large changes in atmospheric CO2 were likely linked to Antarctic processes.


Ocean impacts of higher CO2 and global warming

The ocean carbon models have also been used to examine the potential impact of ocean carbon chemistry changes and global warming on ocean biology. One early study on long-term ocean chemistry suggested that the ocean sink of carbon dioxide will shrink in the future, as the carbonate buffer that now allows the ocean to absorb large quantities of atmospheric carbon becomes saturated in a few centuries. Although this timescale is shorter than the planned lifetime of many CO2 storage projects, the ocean’s reduced capacity had not commonly been considered in specifying leakage limits for underground storage.

This year Princeton researchers participated in a study of near-term ocean chemistry that demonstrated that CO2-induced ocean pH changes may lead to calcium carbonate undersaturation at the surface of the ocean, particular in the Antarctic region, much earlier than had previously been expected. The change in ocean chemistry would likely endanger high-latitude organisms such as sea urchins, cold-water corals, coralline algae, and plankton known as pteropods that form their shells from susceptible carbonate minerals.

In addition to changes in ocean chemistry, the group has modeled changes in biological productivity with climatic warming. If productivity were to increase, an accompanying export of organic carbon into the deep ocean might lower atmospheric carbon dioxide levels. However, the researchers’ results indicate that the total warming of the climate between the beginning of the Industrial Revolution to 2050 would be accompanied by only a 0.7 to 8 % increase in primary productivity. Even in the most extreme case considered, this change would have only a modest effect on export production and atmospheric CO2 levels through the first half of this century.