Tenth Year Annual Report: Carbon Science
The Science Group, composed of the Pacala, Sarmiento, Morel and Bender teams, uses both observational data and models to improve understanding of carbon sinks and predict the impact of climate change on the carbon cycle.
Earth System Model
Over the course of the grant a powerful new Earth System Model (ESM) has been developed to detect and attribute changes in carbon sources and sinks and predict the consequences of global warming. Incorporating GFDL's state-of-the-art ocean and atmosphere models, the ESM also includes a dynamic global land model developed by the Pacala group in collaboration with GFDL and USGS scientists and a complex ocean carbon cycle model developed at GFDL with input from the Sarmiento Group. The ESM puts CMI at the forefront of carbon and climate modeling and is providing important insights into the nature of the earth's carbon sinks.
Narrowing Uncertainty in the Ocean Sink
Understanding the fate of anthropogenic carbon requires knowing the magnitude of carbon sources, the growth rate of carbon in the atmosphere, and distribution of carbon uptake between the land and ocean. The first two elements of the equation are well-known. The Sarmiento Group has determined the third by combining observations and modeling to constrain the size of the ocean sink with an unprecedented degree of certainty (2.0 ± 0.2 GtC/yr) for the 1990s, leaving about 1.15 GtC/yr uptake by the land biosphere, which is much smaller than previously thought. The narrowed estimate for the 1990s is independently corroborated by oxygen measurements made by the Bender Group, and Sarmiento and colleagues are now working to predict the ocean sink's subsequent evolution.
Iron Fertilization as a Mitigation Strategy
CMI has played a role in exposing the critical flaws in iron fertilization as a carbon mitigation option. Simulations of iron fertilization by the Sarmiento Group have demonstrated the difficulties of verifying the amount carbon sequestered and retained by the ocean, as well as the negative ecological consequences.
Understanding the "Missing Sink"
Although the magnitude of the terrestrial sink has been estimated, pinpointing the location of this terrestrial carbon uptake or "missing sink," remains one of the great challenges in climate science. By accounting for both land use changes and nitrogen cycling in the LM3 land model, the Pacala Group has now shown that part of the sink likely comes from CO2 fertilization of lowlatitude forests. Their complementary analyses of forestry inventories indicate that the effect of CO2 fertilization on Northern Hemisphere is small to absent, and that the remainder of the sink may come from Northern Hemisphere land use changes.
Continuous Observations of Carbon Sinks and Ocean Productivity
Since the inception of the grant, the Bender Group has been developing and deploying instruments to better measure the interannual variability of carbon sinks and the spatial and temporal variability of biological productivity. Early in the grant the researchers built multiple automated sampling devices that have significantly increased the accuracy of atmospheric argon and oxygen records around the world, creating increasingly reliable estimates of the size and variability of the land and ocean carbon sinks. This year an instrument for continuously measuring dissolved inorganic carbon in ocean surface water has been constructed that can serve as a component of the global seagoing CO2 observing system. The instrument will soon be deployed to provide the most detailed information available on Southern Ocean productivity and carbon cycling.
Causes of Glacial/Interglacial Change
The work of the Sigman Group has been influential in moving the search for the cause of ice ages from low to high latitudes. Since CMI's inception, Daniel Sigman and colleagues have developed new techniques for analyzing minute amounts of organic nitrogen in preserved in microfossils from deep sea sediments to reconstruct the chemistry of ancient surface waters. With cores now collected and analyzed from the Southern Ocean and North Pacific, their data have revealed that surface waters in ancient polar oceans were stratified, "locking" CO2 in the deep ocean.
Impacts of Ocean Acidification
Rising CO2 in the atmosphere impacts not only global temperature, but also the pH of the surface oceans. Recent investigations by the Morel Group have shown that although increased CO2 might be expected to increase phytoplankton productivity, increase in efficiency are not always seen in experiments and may depend on the composition of the phytoplankton community. In addition, the group's experiments and field observations have shown that the lowering of pH can decrease the bioavailability of essential trace metals and the activity of enzymes involved in nutrient uptake, possibly counteracting any CO2 fertilization effect.