North American Carbon Sources and Sinks: Magnitude, Attribution and Uncertainty

1. North American Carbon Sources and Sinks: Magnitude, Attribution and Uncertainty Anthony King Daniel Hayes Deborah Huntzinger Tristram West Wilfred Post 4th NACP All Investigators Meeting February 4, 2013 Albuquerque, New Mexico

2. The First State of the Carbon Cycle Report. SOCCR was Released as U.S. CCSP Synthesis and Assessment Product 2.2 The First State of the Carbon Cycle Report (SOCCR): North American Carbon Budget and Implications for the Global Carbon Cycle November 13, 2007 SOCCR CCSP SAP 2.2 State of the Carbon Cycle Report

3. National Climate Assessment • Currently in public review • Scheduled for publication in 2013 • 15. Interactions of Climate Change and Biogeochemical Cycles King, AW, DJ Hayes, DN Huntzinger, TO West, WM Post. Front Ecol Environ 2012; 10(10): 512–519, doi:10.1890/120066

4. Balance of sources and sinks Human activities have significantly altered the balance of sources that add CO2 to the atmosphere and sinks that remove it. The result is a buildup of CO2 in the atmosphere. Understanding the carbon cycle and human influence on sources and sinks is crucial to any effort to mitigate potential climate change by stabilizing atmospheric CO2 concentrations.

5. SOCCR summary: The sources • North America is currently a net source of CO2 to the atmosphere. • The combustion of fossil fuels in North America released nearly two billion metric tons of carbon to the atmosphere in 2003. • Fossil-fuel emissions are dominated by emissions from the United States (85% in 2003, Canada 9% and Mexico 6%). • Combustion of fossil fuel to produce energy commodities (primarily electricity) is the largest contributor (42% in 2003), transportation the second (31%). • Including the emissions from generating electricity used in buildings, the buildings sector of North America in 2003 was responsible for 37% of total North American carbon dioxide emissions and 10% of global emissions. • United States buildings alone were responsible for more carbon dioxide emissions than total carbon dioxide emissions of any other country in the world, except China

6. Fossil fuel emissions --- the source

7. SOCCR summary: The sinks (ca 2003) • A terrestrial sink of 500 (50%) Mt C per year removed the equivalent of nearly 30% of North American fossil fuel emissions in 2003. • The terrestrial sink is primarily associated with regrowing forests in the United States ( 50% of the sink). • About 10% of the forest sink is in urban forests, with the urban forest sink equivalent to 1 to 3% of the North American fossil-fuel emissions in 2003. • The second largest sink, woody encroachment, is also the least well known.

8. Terrestrial sink (2000-2005): meta-synthesis of NACP regional interimsynthesis

9. SOCCR sources and sinks:

10. Production and Consumption Sources and Sinks

11. Updated sources and sinks

12. In a nutshell… • North America is currently a net source of atmospheric CO2; in the first 5 years of the 21st century, continental ecosystems annually absorbed the equivalent of only ~35% of the CO2 from 2010 North American fossil-fuel emissions, a source-to-sink ratio of approximately 3:1. • Regrowing forests account for between 30% and 70% of the North American terrestrial sink for atmospheric CO2. • Uncertainties in estimates of the size of the North American CO2 sink remain high (about ± 50–80%); much of this uncertainty is associated with ecosystems (eg shrublands) that are excluded from forest and cropland inventories and with some model results. • Despite the high uncertainty associated with individual approaches, synthesis across alternatives yields more robust estimates of uncertainty (about ± 25%).