The First State of the Carbon Cycle Report (SOCCR): The North American Carbon Budget and Implications for the Global Carbon Cycle

The combustion of fossil fuels has increased carbon dioxide in the atmosphere, and the oceans have annually absorbed an equivalent of 20-30% of the carbon dioxide in fossil-fuel emissions. The present annual uptake by the oceans of approximately 1.8 billion tons of carbon (26% of global fossil-fuel emissions in 2003) is well constrained, has slightly acidified the oceans and may ultimately affect ocean ecosystems in unpredictable ways. The carbon budgets of ocean margins (coastal regions) are not as well-characterized due to lack of observations coupled with complexity and highly localized geographic variability. Existing data are insufficient, for example, to estimate the amount of carbon derived from human activity stored in the coastal regions of North America or to predict future scenarios. New air-sea carbon flux observations reveal that on average, waters within about 100 km (60 miles) of the shores surrounding North America are neither a source nor a sink of carbon dioxide to the atmosphere. A small net source of carbon dioxide to the atmosphere of 19 million tons of carbon per year (with large uncertainty) is estimated mostly from waters around the Gulf of Mexico and the Caribbean Sea. This is equivalent to about 1% of the global ocean uptake. With the exception of one or two time-series sites, almost nothing is known about historical trends in airsea fluxes and the source-sink behavior of North America’s coastal oceans. The Great Lakes and estuarine systems of North America may be net sources of carbon dioxide where terrestrially-derived organic material is decomposing, while reservoir systems may be storing carbon through sediment transport and burial. Options for sequestering carbon in the ocean include iron fertilization in sunlit surface waters and injection of carbon dioxide in subsurface coastal waters. However, sequestration capacity and potential adverse effects on marine environments need to be investigated. Highly variable air-sea carbon dioxide fluxes in coastal areas may introduce errors in North American carbon dioxide fluxes calculated by atmospheric inversion methods. Reducing these errors and the uncertainties regarding the variability of carbon cycling in coastal oceans will require observation systems utilizing fixed and mobile platforms, novel instrumentation to measure critical stocks and fluxes, and coordinated national and international research programs. Experimental studies involving coastal carbon cycling should be encouraged. •