Oceans, Ocean-Atmospheric Coupling, and Carbon Cycling

Understanding the circulation and CO2 biogeochemistry of the oceans is key to our ability to predict and assess the future evolution of climate. The ocean is important in the regulation of heat and moisture fluxes, and oceanic physical and biogeochemical processes are major regulators of natural atmospheric carbon dioxide (as well as being an important sink of fossil fuel CO2). The physics, chemistry and biology of the ocean are coupled since the oceanic CO2 cycle is controlled by the ocean circulation, the supply of sunlight, and major and trace nutrients delivered by the oceanic and atmospheric circulations.

Due to computational and conceptual limitations, however, most climate models do not incorporate realistic descriptions of the coupling between ocean circulation and climate and none include a realistic description of the biogeochemical cycle of CO2. If the future oceanic CO2 uptake rates cannot be predicted, then certainly the future levels of atmospheric CO2 cannot be predicted, let alone how they will affect global climate.

To develop an improved understanding of the interaction between ocean circulation and the movement and storage of carbon in the oceans, the CGCS draws on three strengths of MIT: the presence of ongoing research programs in ocean physics, ocean chemistry and paleoceanography, and ocean phytoplankton ecology. By integrating these programs into a coherent approach we are improving understanding of the ocean’s role in climate and the carbon cycle. Examples of specific projects follow.

Ocean Circulation and Carbon Dioxide Budgets

Inverse models that employ large-scale observations of the ocean, along with known oceanic physics and chemistry, have been developed over the past 15 to 20 years to produce quantitative estimates of oceanic fluxes of mass, heat, freshwater, nutrients, and other properties, including occasionally, carbon. The models are of widely varying dynamical sophistication, areal and temporal scope, and have very diverse spatial and temporal resolution varying from extremely crude box models of basin-scale and large general circulation numerical codes. The resulting estimates of the flow fields have been used to compute estimates of basin-scale fluxes of properties, and of the more climate-relevant flux divergences to the atmosphere.

Only recently has it become possible to consider making such calculations on a global basis and, for the basin scale, to begin using dynamical models that have the spatial resolution to depict all of the dominant oceanic transport mechanisms, i.e. the so-called eddy-resolving models. Thus CGCS researchers have begun exploring the use of global quasi-steady models for property fluxes, and a program for regional studies with the most sophisticated general circulation models forced to consistency with a variety of oceanic observations. The intention is to estimate the flux of carbon and the rates and divergences of relevant biogeochemical processes in the ocean. In parallel, an effort is underway to apply similar (although simpler) models to the ancient oceans (including the last glacial period) so that we can come to understand the circulations patterns that may have controlled the very different atmospheric carbon concentrations in previous climates.

Paleoceanographic Reconstruction of Ocean Circulation and the Carbon System

During the last great Ice Age, about 18,000 years ago, the Earth’s climate was radically different from that of today. The difference was larger even than is sometimes projected for future climate states under doubled CO2. Studies of bubbles in polar ice cores have shown that Ice Age atmospheric CO2 was much lower than recent pre-industrial levels, which supports a strong link between CO2 and climate.
Studies of the geochemical composition of sea floor fossils have shown that the circulation patterns of the ocean during the last glacial maximum may have been quite different from those in the modern ocean. Both theory and these observations strongly support the idea that changes in ocean chemistry and circulation can act to alter atmospheric CO2, and the complete climate state. Furthermore, there is evidence that 11,000 years ago, the ocean circulation may have switched back abruptly to a state much closer to that of the Ice Age ocean-returning much of the world to glacial conditions. This switch appears to have occurred over a period of less than 300 years. Hence, it is important to consider the possibility that future ocean circulation could change drastically on the decadal-to-centuries time scale relevant to our immediate future. The potential for rapid switches in the mode of ocean circulation has obvious implications for carbon uptake by the ocean and climate change.

Climate prediction models need to be consistent with knowledge of past climate changes. The relationship between global deep ocean circulation patterns and past climate changes provides evidence for alternative states of ocean circulation and rates of transition between states. While the paleoceanographic record has coarse resolution in time and space compared to modern observations, it is our only record of large-magnitude climate changes with durations exceeding decades. Improving the spatial and temporal resolution of paleoceanographic data is one of the objectives of CGCS researchers. Another goal is to gather the diverse paleoceanographic data into a database that includes all of the paleochemical constraints on past ocean circulation changes that are relevant to the ocean role in regulating CO2. A third objective is to use these diverse data in the development of improved models of the role of the ocean in regulating atmospheric CO2.

The Role of Biological Processes in Oceanic Carbon Storage

Deep ocean waters are roughly 200% supersaturated with CO2 compared to surface waters and the atmosphere. This difference is due to the biological flux of organic material from surface to deep waters where it is re-mineralized, i.e. the “biological pump.” In order to understand and model the coupling between the biological pump and the physics and chemistry of the oceans and atmosphere, the regulating basic processes must first be understood. Because the pump is in reality a complex food web, issues of both nutrient availability and food web structure must be addressed to understand its efficiency.

With the goal of understanding the role of limiting nutrients and food web structure in the efficiency of the biological CO2 transfer from surface to deep waters, CGCS researchers are studying the extent to which trace elements (such as iron) limit productivity over large ocean regions, and the role of limiting nutrients in dictating phytoplankton community structure. The results of these efforts are elucidating how biological processes interact with large-scale ocean and atmospheric circulation to control the CO2 cycle, and allow hindcasting of changes that might have occurred during past large-scale climate perturbations as reflected in the paleoceanographic record.