by Jacqueline Sussman
Research Associate, Biodiversity for a Livable Climate
Solar panels on rooftops. Hybrid and electric vehicles. Meatless Mondays. What do all of these indicators of societal progress have in common? They are just some examples among the many widely attainable, lifestyle modifiers for reducing energy consumption in our fossil fuel-addicted world. But while replacing SUVs with hybrid cars and changing lifestyle habits to reduce individual carbon footprints is important, it simply isn’t enough to reverse climate change. We have long surpassed the point where phasing down fossil fuel emissions alone will arrange for a biologically-diverse and livable climate.
The rapid rise of atmospheric CO2 to levels recently exceeding 400 parts per million (ppm) is an unprecedented climatic event in the past several centuries of modern human evolution (Royal Society 2014). The speed and scale of current climate change trends is truly remarkable, making adaptation by both humans and the natural world a complex and unique challenge. In addressing this challenge, many politicians, climate activists, and policymakers often point to reducing fossil fuel and greenhouse gas (GHG) emissions as the sole answer. Science is telling a different story, however, and it is time for us to listen.
Even in the best case scenario for GHG emissions reduction–a zero sum emissions scenario–global temperatures will not decrease fast enough to reverse the current trends of climate change. Climate models indicate that even with a zero emissions scenario, atmospheric temperature will continue to rise and, at best, stabilize or slowly decrease over decades to even centuries before hitting safe levels (Zickfeld et al. 2013, MacDougall et al. 2012). To reduce temperatures there must be a net removal of carbon dioxide from the atmosphere, for which there are multiple viable strategies that actuate the CO2 capture and storage potential of the biosphere. This truth is well known among a minority of climate scientists and activists, but continues to be omitted from conventional dialogues about the climate change crisis.
Natural carbon sinks exist in biomes and ecosystems around the world. Biomes differ in the extent of their carbon sequestration capacity, but there is massive carbon-storing potential in natural resources around the globe for long-term climate stability. By working with natural systems to sequester carbon from the atmosphere and store it in the world’s trees, plants, and soils we can effectively reverse climate change and lay a foundation for sustainable global vitality.
Forests contain the highest reserves of soil carbon worldwide (about 35% of the global total) and have a proportionately high ratio of carbon storage to spatial coverage (SWS 2013). The biophysical influences of forests on atmospheric temperatures depend on multiple factors that affect carbon sequestration, such as the regional climate system (Jackson et al. 2008). Tropical forests promote the evaporation of water from land to air, contributing to cloud formation that increases the amount of sunlight reflected back to space. Boreal forests have a much more limited capacity for carbon storage due to limited sunlight, cooler temperatures, the long-term presence of snow and ice throughout each year, and other factors. Replacing snow covered land with trees that absorb more sunlight in northern countries can actually accelerate warming regionally, creating a positive feedback loop that leads to further loss of highly-reflective snow and ice (DellaSala et al. 2003).
The biophysical influence of temperate forests is complex and highly variable on a region-to-region basis (Field et al. 2007). Some studies show that replacing temperate forests with grasslands will cool surrounding surface areas, while other studies show the opposite. While temperate forests can store between 150 and 230 tons of carbon per hectare (Kemper et al. 2009), these forests can contribute to a warmer atmosphere because they are darker and absorb more sunlight than grasslands and croplands. As the dynamic of temperate forests and atmospheric temperatures varies greatly, increasing temperate forest coverage might not be the best solution for carbon sequestration in all regions of the globe (Jackson et al. 2008). Thus, ecosystem assessments are crucial for determining whether increasing temperate forest coverage will benefit atmospheric CO2 concentrations on a case-by-case basis.
Climate researchers often suggest that the greatest potential for carbon sequestration in forests lies in the tropics; avoided deforestation, forest restoration and afforestation in these regions can play a significant role in mitigating global climate change (Jackson et al. 2008; Shukla et al. 1990; Dickinson 1992). In other regions of the world, forest projects and policies should be crafted to seek a balance between protecting ecosystem services and stabilizing local and global temperatures. In dryland ecosystems, forestry projects that consume scarce water resources are not favorable to other types of ecological restoration projects, such as grassland restoration (Jackson et al. 2008; Hurteau et al. 2008).
Excluding Greenland and Antarctica, grassland biomes constitute approximately 40% of the global land surface area and possess massive potential for sequestering carbon in arid and semi-arid areas of the world (Safriel et al. 2005; Savory 2013; Teague et al. 2011; Tainton et al. 1999). Healthy grasslands provide habitat for a biologically diverse range of plant and animal species, and also serve as a large reserve for soil carbon (Safriel et al. 2005; Savory 2013). While temperate forests can store carbon in above-ground biomass, grasslands have higher soil organic carbon (SOC) capacity, storing carbon in stable complex biomolecules for hundreds to even thousands of years (Scurlock and Hall 1998; Savory 2013). Studies estimate that grassland SOC carbon stocks comprise up to 30% of the total soil carbon worldwide (Anderson 1991; Scurlock and Hall 1998).
Grasslands that sustain abundant stores of SOC have higher rates of water infiltration and retention and are less susceptible to environmental degradation such as soil erosion and drought. These “non-forest” biomes, primarily located in the terrestrial regions of the Northern Hemisphere, constitute a significant fraction of the “missing carbon sink” in the global carbon budget (Scurlock and Hall 1998). Widespread mismanagement of grassland ecosystems has degraded their primary production functioning and SOC storage abilities, leading to a “desertification” crisis worldwide, characterized by processes such as vegetation degradation, water and wind erosion, salinization, soil compaction and soil nutrient depletion (Safriel et al. 2005). Although grasslands are currently facing desertification around the globe, proper land management can restore vegetation cover to protect soils from erosion and enhance the carbon sequestration capacity of these vital ecosystems (FAO 2004; Savory 2013).
While forests and dryland ecosystems contain the most soil carbon globally, the carbon density of wetland ecosystems is three and six times that of forests and grasslands/shrublands, respectively (Society of Wetland Scientists (SWS) 2013). The deep organic-rich soils of coastal wetlands contain carbon reserves that have been sequestered over millennia (Emmiett-Mattox et al. 2011). These tidal ecosystems are able to store large quantities of carbon for two primary reasons: high-nutrient conditions that promote the rapid growth of carbon-sequestering plant biomass (primary production), and anaerobic soils that suppress microbial decomposition in the water-logged environment, allowing for long-term carbon storage (Chmura et al. 2003; SWS 2013; Mitsch and Bernal 2013). Tidal saline wetland ecosystems, including salt marshes and mangrove swamps, exhibit remarkable rates of belowground biomass production and enormous capacity as carbon sinks (Chmura et al. 2003).
While wetlands do emit carbon dioxide and methane (CH4), studies with analysis on model carbon fluxes over a 100-year period indicate that most wetlands are net carbon sinks (SWS 2013). In addition to their carbon storage capacity, wetlands provide a multitude of environmental benefits including habitat for a diverse range of species, water filtration mechanisms, and protection against floods and storms (Mitsch and Bernal 2013). Wetlands cover roughly 6% of the earth’s land surface and contain over 10% of the “global carbon pool” (IPCC 1996; Erwin 2009), making the health of wetland ecology an important segment of the global climate picture.
Biochar is a stable, carbon-dense form of charcoal created from the burning of plant material under low-oxygen or zero-oxygen conditions, called pyrolysis. Biochar is primarily produced by wildfires and can exist in some soils for hundreds of years (Lehmann et al. 2009). Controlled fire produces biochar that has the ability to simultaneously enrich soil health, sequester carbon, and promote additional growth of carbon-absorbing plants (Rostad and Rutherford 2011; Schwartz 2013). While biochar production currently remains an experimental strategy for global carbon sequestration, when applied responsibly it appears to have potential for aiding in efforts to enhance the long-term carbon storage of the biosphere.
A focus on fossil fuel emissions reduction as the primary solution to reversing climate change will continue to be ineffective. Arbitrary goals to reduce emissions by 10, 30, 50 or even 80 percent are not going to prevent future warming. Basic scientific principles dictate that atmospheric CO2 concentrations will not decline without a significant rise in the carbon sequestration mechanisms of the biosphere. The good news is that there is enormous carbon sequestering potential in multiple ecological systems around the globe. The only question now is whether we will work to enhance the carbon storage capacities of nature or effectively continue to weaken them.
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