“Restoring the United States’ lands and coastal wetlands could have a much bigger role in reducing global warming than previously thought, according to the most comprehensive national assessment to date of how greenhouse gas emissions can be reduced and stored in forests, farmland, grasslands and wetlands.
The peer-reviewed study in Science Advances from The Nature Conservancy and 21 institutional partners found that nature’s contribution could equal 21% of the nation’s current net annual emissions, by adjusting 21 natural management practices to increase carbon storage and avoid greenhouse emissions. The study is the first to include the climate benefits of coastal wetlands and grasslands in a comprehensive mix along with forests and agriculture.”
Authors: Joseph E. Fargione1,*, Steven Bassett2, Timothy Boucher3, Scott D. Bridgham4, Richard T. Conant5, Susan C. Cook-Patton3,6, Peter W. Ellis3, Alessandra Falcucci7, James W. Fourqurean8, Trisha Gopalakrishna3, Huan Gu9, Benjamin Henderson10, Matthew D. Hurteau11, Kevin D. Kroeger12, Timm Kroeger3, Tyler J. Lark13, Sara M. Leavitt3, Guy Lomax14, Robert I. McDonald3, J. Patrick Megonigal6, Daniela A. Miteva15, Curtis J. Richardson16, Jonathan Sanderman17, David Shoch18, Seth A. Spawn13, Joseph W. Veldman19, Christopher A. Williams9, Peter B. Woodbury20, Chris Zganjar3, Marci Baranski21, Patricia Elias3, Richard A. Houghton17, Emily Landis3, Emily McGlynn22, William H. Schlesinger23, Juha V. Siikamaki24, Ariana E. Sutton-Grier25,26 and Bronson W. Griscom3″
In this study, scientists quantify the maximum potential for NCS in the United States and the portion of this maximum that could be achieved at several price points. They consider 21 distinct NCS to provide a consistent and comprehensive exploration of the mitigation potential of conservation, restoration, and improved management in forests, grasslands, agricultural lands, and wetlands (Fig. 1), carefully defined to avoid double counting (details in the Supplementary Materials). THey estimate the potential for NCS in the year 2025, which is the target year for the United States’ Nationally Determined Contribution (NDC) under the Paris Agreement to reduce GHG emissions by 26 to 28% from 2005 levels. Their work refines a coarser-resolution global analysis (3) and updates and expands the range of options considered in previous analyses for the United States (5–8).
“Reforestation: Additional carbon sequestration in above- and belowground biomass and soils gained by converting nonforest (<25% tree cover) to forest [>25% tree cover (45)] in areas of the conterminous United States where forests are the native cover type. We excluded areas with intensive human development, including all major roads (46), impervious surfaces (47), and urban areas (48). To eliminate double counting with the peatland restoration pathway, we removed Histosol soils (49). To safeguard food production, we removed most cropland and pasture. We discounted the carbon sequestration mitigation benefit in conifer-dominated forests to account for albedo effects.
Natural forest management: Additional carbon sequestration in above- and belowground biomass gained through improved management in forests on private lands under nonintensive timber management. The maximum mitigation potential was quantified on the basis of a “harvest hiatus” scenario starting in 2025, in which natural forests are shifted to longer harvest rotations. This could be accomplished with less than 10% reduction in timber supply with new timber supply from thinning treatments for fuel risk reduction until new timber from reforestation is available in 2030.
Fire management: Use of prescribed fire to reduce the risk of high-intensity wildfire. We considered fire-prone forests in the western United States. We assume that treatment eliminates the risk of subsequent wildfire for 20 years, but only on the land that was directly treated. We assume that 5% of lands are treated each year, and we calculated the benefits that accrue over 20 years, finding that the initial increase in emissions associated with prescribed fire treatment is more than offset over time by the avoided impacts of wildfires. We report the average annual benefit across these 20 years. The impact of wildfires includes both direct emissions from combustion and suppression of net ecosystem productivity following wildfires.
Avoided forest conversion: Emissions of CO2 avoided by avoiding anthropogenic forest conversion. Most forest clearing is followed by forest regeneration rather than conversion to another land use. To estimate the rate of persistent conversion (i.e., to another land use), we first calculated forest clearing in the conterminous United States from 2000 to 2010 and then used the proportion of forest clearing that historically was converted to another land use to estimate conversion rates in 2000 to 2010. We used estimates of avoided carbon emissions from above- and belowground biomass that are specific to each region and forest type. We did not count forest loss due to fire to avoid double counting with the improved fire management opportunity. We did not count forest loss due to pests because it is unclear whether this loss can be avoided. We reduced the benefit of avoided conversion in conifer-dominated forests to account for their albedo effects.
Urban reforestation: Additional carbon sequestration in above- and belowground biomass gained by increasing urban tree cover. We considered the potential to increase urban tree cover in 3535 cities in the conterminous United States. We considered the potential for additional street trees, and for those cities not in deserts, we also considered the potential for park and yard tree plantings. The potential percent increase in tree cover was estimated on the basis of high-resolution analysis of 27 cities, which excluded sports fields, golf courses, and lawns (50).
Improved plantations: Additional carbon sequestration gained in above- and belowground tree biomass by extending rotation lengths for a limited time in even-aged, intensively managed wood production forests. Rotation lengths were extended from current economic optimal rotation length to a biological optimal rotation length in which harvest occurs when stands reach their maximum annual growth.
Cover crops: Additional soil carbon sequestration gained by growing a cover crop in the fallow season between main crops. We quantified the benefit of using cover crops on all of the five major crops in the United States (corn, soy, wheat, rice, and cotton) that are not already growing cover crops (27), using the mean sequestration rate quantified in a recent meta-analysis (51).
Avoided conversion of grassland: Emissions of CO2 avoided by avoiding conversion of grassland and shrubland to cropland. We quantified avoided emissions from soil and roots (for shrubs, we also considered aboveground biomass) based on the spatial pattern of conversion from 2008 to 2012. We used spatial information on location of recent conversion and variation in soil carbon and root biomass to estimate mean annual emission rate from historic conversion. We estimated a 28% loss of soil carbon down to 1 m (26). We modeled spatial variation in root biomass based on mean annual temperature and mean annual precipitation using data from (52).
Biochar: Increased soil carbon sequestration by amending agricultural soils with biochar, which converts nonrecalcitrant carbon (crop residue biomass) to recalcitrant carbon (charcoal) through pyrolysis. We limited the source of biochar production to crop residue that can be sustainably harvested. We assumed that 79.6% of biochar carbon persists on a time scale of >100 years (53, 54) and that there are no effects of biochar on emissions of N2O or CH4 (55, 56).
Alley cropping: Additional carbon sequestration gained by planting wide rows of trees with a companion crop grown in the alleyways between the rows. We estimated a maximum potential of alley cropping on 10% of U.S. cropland (15.4 Mha) (57).
Cropland nutrient management: Avoided N2O emissions due to more efficient use of nitrogen fertilizers and avoided upstream emissions from fertilizer manufacture. We considered four improved management practices: (i) reduced whole-field application rate, (ii) switching from anhydrous ammonia to urea, (iii) improved timing of fertilizer application, and (iv) variable application rate within field. We projected a 4.6% BAU growth in fertilizer use in the United States by 2025. On the basis of these four practices, we found a maximum potential of 22% reduction in nitrogen use, which leads to a 33% reduction in field emissions and a 29% reduction including upstream emissions.
Improved manure management: Avoided CH4 emissions from dairy and hog manure. We estimated the potential for emission reductions from improved manure management on dairy farms with over 300 cows and hog farms with over 825 hogs. Our calculations are based on improved management practices described by Pape et al. (8).
Windbreaks: Additional sequestration in above- and belowground biomass and soils from planting windbreaks adjacent to croplands that would benefit from reduced wind erosion. We estimated that windbreaks could be planted on 0.88 Mha, based on an estimated 17.6 Mha that would benefit from windbreaks, and that windbreaks would be planted on ~5% of that cropland (8).
Grazing optimization: Additional soil carbon sequestration due to grazing optimization on rangeland and planted pastures, derived directly from a recent study by Henderson et al. (58). Grazing optimization prescribes a decrease in stocking rates in areas that are overgrazed and an increase in stocking rates in areas that are undergrazed, but with the net result of increased forage offtake and livestock production.
Grassland restoration: Additional carbon sequestration in soils and root biomass gained by restoring 2.1 Mha of cropland to grassland, equivalent to returning to the 2007 peak in CRP enrollment. Grassland restoration does not include restoration of shrubland.
Legumes in pastures: Additional soil carbon sequestration due to sowing legumes in planted pastures, derived directly from a recent global study by Henderson et al. (58). Restricted to planted pastures and to where sowing legumes would result in net sequestration after taking into account potential increases in N2O emissions from the planted legumes.
Improved rice management: Avoided emissions of CH4 and N2O through improved practices in flooded rice cultivation. Practices including mid-season drainage, alternate wetting and drying, and residue removal can reduce these emissions. We used a U.S. Environmental Protection Agency (EPA) analysis that projects the potential for improvement across U.S. rice fields, in comparison with current agricultural practices (59).
Tidal wetland restoration: In the United States, 27% of tidal wetlands (salt marshes and mangroves) have limited tidal connection with the sea, causing their salinity to decline to the point where CH4 emissions increase (30). We estimated the potential for reconnecting these tidal wetlands to the ocean to increase salinity and reduce CH4 emissions.
Peatland restoration: Avoided carbon emissions from rewetting and restoring drained peatlands. To estimate the extent of restorable peatlands, we quantified the difference between historic peatland extent [based on the extent of Histosols in soil maps (60)] and current peatland extent. Our estimate of mitigation potential accounted for changes in soil carbon, biomass, and CH4 emissions, considering regional differences, the type of land use of the converted peatland, and whether the peatland was originally forested.
Avoided seagrass loss: Avoided CO2 emissions from avoiding seagrass loss. An estimated 1.5% of seagrass extent is lost every year (61). We assumed that half of the carbon contained in biomass and sediment from disappearing seagrass beds is lost to the atmosphere (62).
Seagrass restoration: Increased sequestration from restoring the estimated 29 to 52% of historic seagrass extent that has been lost and could be restored (61). We estimated the average carbon sequestration rate in the sediment of seagrass restorations based on data from six seagrass restoration sites in the United States (63).”
“Reforestation has the single largest maximum mitigation potential (307 Tg CO2e year−1). The majority of this potential occurs in the northeast (35%) and south central (31%) areas of the United States (fig. S1). This mitigation potential increases to 381 Tg CO2e year−1 if all pastures in historically forested areas are reforested.”