The North American Carbon Budget

Most published information on carbon cycling across North America is focused on the United States and Canada; thus, there is greater uncertainty about carbon dynamics for Mexico (Vargas et al., 2012). Data from SOCCR1 (CCSP 2007) suggested a large uncertainty in lands with woody encroachment and wetlands, so resolving whether these places acted as persistent carbon sources or sinks across North America was not possible at the time. SOCCR2 assessments suggest that the main uncertainties are in grasslands, wetlands, inland waters, and the Arctic. Importantly, because woody encroachment is considered implicitly in this report to be within grasslands and forests, it contributes to the uncertainty of these two sectors. Fossil fuel emissions continue to be the largest source of carbon to the atmosphere, and current estimates are consistent with those from SOCCR1. Attempts to quantify the coastal ocean component of the continental carbon budget has contributed a substantial amount of uncertainty in these assessments. Although SOCCR1 considered the coastal ocean a net source of carbon, new and better information from advances in measurement and modeling approaches now suggests it represents a net carbon uptake (see Ch. 16: Coastal Ocean and Continental Shelves). The Arctic and boreal regions continue to be areas of uncertainty with large carbon stocks in permafrost and freshwater wetlands and with unknown land-atmosphere fluxes of CO₂ and CH₄ (McGuire et al., 2012; Petrescu et al., 2010; Schuur et al., 2015). Expanding research capabilities across different regions of North America will contribute to reducing uncertainty in key areas such as grasslands, wetlands, boreal and Arctic ecosystems, and tropical to subtropical regions.

For the ca. 2003 time frame, SOCCR1 estimated that about 30% of the combined fossil fuel emissions from the three North American countries were offset by CO₂ uptake in their ecosystems (Pacala et al., 2007). Based entirely on inventory estimates, carbon sinks in that analysis were attributed mostly to the forest sector, including tree growth, vegetation regeneration after agricultural land abandonment, fire suppression, and storage in wood products (Pacala et al., 2007). Estimates for fossil fuel emissions from 2000 to 2014 average approximately 1.8 ± 0.5 Pg C per year, with about 40% being offset by the land carbon sink (see Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane). Several studies support forests remaining as the key sector with a persistent sink globally (Pan et al., 2011) and across the United States (Woodall et al., 2015) and Canada (Kurz et al., 2013; Stinson et al., 2011). The SOCCR2 assessment presented here suggests that forests across North America offset fossil fuel emissions by about 12%, with U.S. forests accounting for most of that sink (i.e., 11%; see Table 2.2). When these estimates are divided by fossil fuel emissions per country, the country-specific offset by forests suggests a slightly higher potential for Mexico (i.e., offsetting approximately 25% of in-country emissions), followed by the United States (about 13%). However, Canada’s forests act as an additional source (about 11%) on top of the country’s fossil fuel emissions. There is additional uncertainty surrounding boreal forests and tundra ecosystems in the northern high latitudes of North America (see Ch. 11: Arctic and Boreal Carbon), particularly since these remote areas of unmanaged land in Canada and Alaska are not included in either of their country’s formal carbon inventories and reporting programs (Kurz et al., 2009). In studies based on time series, optical satellite data have shown both “greening” in Arctic tundra and “browning” in boreal forests (e.g., Beck and Goetz 2011), suggesting regional variability in vegetation photosynthetic dynamics that could lead to carbon gains and losses, respectively (e.g., Epstein et al., 2012). Large carbon stocks stored in the frozen soils of North American landscapes underlain by permafrost are vulnerable to thaw under a warming climate, leading to carbon decomposition and subsequent release to the atmosphere as CO₂ or CH₄ (Hayes et al., 2014; Schuur et al., 2015). The increasing frequency and severity of disturbances in these regions, particularly wildfire, have the potential to impact vegetation and soil carbon stocks and fluxes in complicated feedback mechanisms (e.g., Abbott et al., 2016).

An analysis by King et al. (2015) demonstrates an 11% increase in the total magnitude of average annual continental emissions during 2000 to 2010 compared with 1990 to 2000. Since inventory data first became available in the 1960s, there has been a mostly uninterrupted increasing trend in overall fossil fuel emissions (Pacala et al., 2007). However, over the last decade, the combined fossil fuel emissions from Canada, the United States, and Mexico have been flat or declining. Combined annual emissions ranged from 1.7 to 1.8 Pg C between 2008 and 2013 and have not exceeded the approximately 1.9 Pg C peaks during 2005 and 2007 (see Figure 2.2). The lower emissions total resulted from the 2007 to 2009 global economic recession and subsequent decline in energy consumption by the industrial and transportation sectors (see Ch. 3: Energy Systems). From 2000 to 2009, annual per capita emissions were an estimated 20 tons (t) CO₂ in the United States, 18 t CO₂ in Canada, and 4 t CO₂ in Mexico. These estimates compare with a substantial decrease in per capita emissions by 2015 for the United States and Canada (about 17 t CO₂ and 16 t CO₂, respectively) and a stabilization in emissions for Mexico (about 4 t CO₂ per person; Le Quéré et al., 2016).

The trends in CH₄ emissions have been variable in recent decades, showing a renewed growth rate in global atmospheric concentrations since 2007 following a period of stabilization (Nisbet et al., 2016). However, the most recent budget by Saunois et al. (2016) compares CH₄ emissions from two decades: 2000 to 2009 and 2003 to 2012. This study found no significant increase in total natural and anthropogenic emissions for boreal North America (20 Tg CH₄ per year) and central North America (11 Tg CH₄ per year), and even a slight decrease for the conterminous United States (from 43 to 41 Tg CH₄ per year). Although shortwave infrared measurements of CH₄ from the Greenhouse Gases Observing Satellite (GOSAT) indicate a 30% increase from 2002 to 2014 in central United States, the U.S. Environmental Protection Agency’s (EPA) GHG inventory shows no such increase in anthropogenic emissions, despite a 20% increase in oil and gas production (Turner et al., 2016a). Changes in CH₄ emissions from high-latitude regions thus far appear to be fairly insensitive to warming (Sweeney et al., 2016), suggesting that changes in agriculture and livestock management are the key drivers in the recent increase in global CH₄ emissions (Schaefer et al., 2016). Using a one-box isotopic model, Schaefer et al. (2016) suggest that, outside the Arctic, activities related to food production are most likely responsible for the increasing CH₄ concentration in the atmosphere since 2007. Some research also considered a decrease in the hydroxyl sink for CH₄ as a driver of the renewed growth rate (Rigby et al., 2008); however, more recent multitracer assessments do not support this theory (Nisbet et al., 2016).

Monitoring networks suggest that the coastal margins of North America currently act as a net CO₂ sink, where the net uptake of CO₂ from the atmosphere is driven by high-latitude regions; however, the net flux from coastal margins is not ­well constrained (see Figure 2.4 and Ch. 16: Coastal Ocean and Continental Shelves). Ocean acidification trends are difficult to identify in coastal waters because highly variable carbonate chemistry is influenced by seawater temperature and transport, primary production, respiration, and inputs from land, in addition to the uptake of anthropogenic CO₂ from the atmosphere. In coastal ocean areas, major concerns for marine organisms, particularly calcifiers, are the increasing partial pressure of CO₂ (pCO₂) in seawater and reductions in pH that reflect greater acidity associated with increasing dissolved CO₂ concentrations in equilibrium with rising atmospheric CO₂—processes that could trigger ecosystem-scale effects. Ocean acidification also affects commercial shellfish stocks (mainly in the northwestern United States) and other environmental services (e.g., coastal protection by reefs) that ultimately may affect the carbon storage capacity of coastal ocean areas (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide).

SOCCR2 assessments provide high confidence that human activities (e.g., urban emissions, land management, and land-use change) will continue to be important drivers of carbon cycle changes across North America into the future. Current land use and land-use change result in net CO₂ emissions for Canada and Mexico, but future land use and land-use management potentially could result in net carbon sequestration (e.g., 661 to 1,090 Tg of CO₂ equivalent¹ by 2030; see Ch. 19: Future of the North American Carbon Cycle). However, there are large uncertainties in predicting future land-use trajectories. In addition, fossil fuel emissions from the energy sector may continue to be a large source of carbon, but future projections are uncertain because of changes in technologies (see Ch. 1: Overview of the Global Carbon Cycle and Ch. 3: Energy Systems) and efforts to reduce fossil fuel emissions. By 2040, estimates project that North American fossil fuel emissions will range from 1.6 to 1.9 Pg C per year, representing either a 9% decrease or a 6% increase in absolute emissions compared to 2015 levels (see Ch. 19).