SOCCR-2, Chapter 2: A Synthesis of the North American Carbon Budget

Scientific information quantifying and characterizing the continental-scale carbon budget is necessary for developing national and international policy on climate change. The North American continent (NA) has been considered to be a significant net source of carbon to the atmosphere, with fossil fuel emissions from the U.S., Canada and Mexico far outpacing uptake on land, inland waters and adjacent coastal oceans. As reported in the First State of the Carbon Cycle Report (SOCCR-1), the three countries combined to emit approximately 1800 MtC of carbon in 2003, or 27% of the global total fossil fuel inventory. Based on inventory data from various sectors, SOCCR-1 estimated a 500 MtC/yr natural sink that offset about 30% of emissions primarily through forest growth, storage in wood products and sequestration in agricultural soils. Here we present a synthesis of the NA carbon budget for the next report (SOCCR-2) based on updated inventory data and new research over the last decade. The North American continent— including its energy systems, land-base and coastal oceans—is very likely to have been a net source of carbon to the atmosphere over the 2004-2013 time period, having contributed on average approximately 1037 (+/- 25%) MtC/yr. At 1765 (+/-2%) MtC/yr, total fossil fuel emissions from Canada, the United States, and Mexico very likely contributed the largest source of carbon over the 2004-2013 time period - a level of magnitude similar to that reported for 2003 (1856 MtC/yr +/- 10%) in SOCCR-1. Between one-quarter and one half of the total fossil fuel emissions over the 2004 - 2013 time period were likely offset by natural sinks on North American land and adjacent coastal ocean. The strength of the natural sink was likely persistent over the 2004-2013 time period as compared to the ca. 2003 magnitude reported in SOCCR-1 (500 MtC/yr), maintained primarily by carbon uptake with forest growth and storage in wood products offsetting carbon losses from natural disturbance and land-use change. Considering the uncertainty ranges around the two approaches, the magnitude of the continental carbon sink over the last decade is not significantly different between the top-down (634 +/- 288 MtC/yr) and the synthesis of bottom-up (577 +/- 433 MtC/yr) estimates in this report.