Paper 1797. Reston, Virginia: U.S. Geological Survey.
9 Zhu, Z., & Reed, B. C. (Eds.) (2014). Baseline and Projected Future Carbon Storage and Greenhouse‐Gas Fluxes in Ecosystems of the Eastern United States. Professional Paper 1804. Reston, Virginia: U.S. Geological Survey.
10 Zhu, Z., & McGuire, A. D. (Eds.) (2016) Baseline and Projected Future Carbon Storage and Greenhouse‐Gas Fluxes in Ecosystems of Alaska. Professional Paper 1826. Reston, Virginia, USA: U.S. Geological Survey.
1 A Review of Global Wetland Carbon Stocks and Management Challenges
Benjamin Poulter1, Etienne Fluet-Chouinard2, Gustaf Hugelius3, Charlie Koven4, Lola Fatoyinbo1, Susan E. Page5, Judith A. Rosentreter6, Lindsey S. Smart7, Paul J. Taillie8, Nathan Thomas1,9, Zhen Zhang10, and Lahiru S. Wijedasa11,12
1 Biospheric Sciences Laboratory, Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
2 Department of Earth System Science, Stanford University, Stanford, California, USA
3 Department of Physical Geography, Stockholm University, Stockholm, Sweden; and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
4 Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
5 School of Geography, Geology & the Environment, University of Leicester, Leicester, UK
6 Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW, Australia
7 Center for Geospatial Analytics, North Carolina State University, Raleigh, North Carolina, USA
8 Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, Florida, USA
9 Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA
10 Department of Geographical Sciences, University of Maryland, College Park, Maryland, USA
1 Integrated Tropical Peat Research Program, NUS Environmental Research Institute, T-Labs, National University of Singapore, Singapore
12 ConservationLinks, Singapore
ABSTRACT
Wetlands have unique soil, vegetation, and biogeochemistry that arises from their landscape position and wetland hydrology, which creates low oxygen levels in the soil. With reduced oxygen availability, plants develop adaptations to survive, such as aerenchyma, that allow transport of atmospheric oxygen to their roots, and soil microbial communities become dominated by anaerobic respiration processes that are less efficient in oxidizing carbon. Combined, the above‐ and belowground carbon stocks of wetlands play a key role in the global carbon cycle at varying time scales. This chapter provides a comprehensive assessment of wetland carbon stocks, research methodologies, and their historical and future trajectories. We estimate wetland carbon stocks range between 520–710 PgC (and 1792 to 1882 PgC with permafrost carbon) globally.
1.1. INTRODUCTION
1.1.1. Wetlands in the Global Carbon Cycle
Wetlands play an important role in the global carbon cycle because of the large amounts of organic carbon they store in vegetation and soils. The accumulation of carbon is partly due to the high productivity of some wetlands, such as in tropical salt marsh and mangrove ecosystems. But over long timescales, large wetland carbon stocks are found belowground, mainly because of how water‐saturated soils slow rates of organic matter decomposition. Because of the large amount of carbon stored in wetlands, these ecosystems are considered to be particularly vulnerable to climate change and may act as a positive feedback to atmospheric carbon dioxide and methane concentrations as wetlands become drier or warmer, or as permafrost thaws. Estimates of global wetland carbon stocks remain uncertain due to a combination of challenges in field sampling, the scaling of site level and in‐situ observations to regions, definitions and inclusivity of wetland types in different assessments, and due to year‐to‐year or decadal variability in wetland extent caused by human management, climate variability, and climate change. This chapter aims to provide a comprehensive assessment of global wetland carbon stock estimates, taking into account these sources of uncertainty. The chapter presents a brief overview of methods that are commonly used to estimate wetland carbon stocks, then individual sections provide estimates of stocks for key wetland types found in tropical, boreal, and temperate regions, and including those associated with “blue carbon” or coastal systems. Two additional sections on historical losses of wetlands and future trends in wetland carbon stocks are presented to provide a temporal context for carbon accounting.
According to the Intergovernmental Panel on Climate Change Fifth Assessment Report (Ciais et al., 2013), for the period 2000–2009, global carbon stocks are distributed across the major components of the Earth system in the following reservoirs (where 1 Petagram = 1 PgC = 1015 gC): the atmosphere (829 PgC); the oceans (38,858 PgC, including surface, intermediate, deep sea, and dissolved organic carbon and marine biota); ocean sediments (1750 PgC); vegetation (420–620 PgC), permafrost (~1700 PgC, includes yedoma deposits); and soils (1500–2400 PgC, including litter), and with fossil fuel reserves ranging from 637–1575 PgC. Wetland soil carbon was estimated to account for 300–700 PgC (Bridgham et al., 2006), and when combined with permafrost (although with some double counting), the total wetland soil carbon stocks range from 2000–2400 PgC. While the oceans are the largest pool of carbon, most of this is not available to be exchanged with the atmosphere on decadal to centennial timescales and thus the carbon stored in vegetation and soil is more relevant when considering anthropogenic carbon‐climate feedbacks. The observed 40% increase in atmospheric carbon (~240 PgC increase) from fossil fuel and land‐use change activities since 1850, has led to an almost 1 °C change in global mean surface temperature, and represents a smaller order of magnitude of carbon than the combined vegetation and soil carbon pools. This means that understanding the distribution and processes responsible for global wetland carbon accumulation and oxidation is directly relevant for the climate system.
1.1.2. Wetland Definitions
Wetlands are defined by having unique hydric soils, vegetation, and hydrology due to their topographic position. Wetlands can be saltwater, freshwater, or brackish, develop carbon rich histosol soils, and host diverse aquatic adapted flora and fauna. There are many ways that the soil, vegetation, and hydrology properties can intersect with one another and this has led to a large range of wetland types and an extensive and complex nomenclature that includes more familiar categories such as “swamps,” “bogs,” and “marshes” to less familiar categories including “morass,” “muskeg,” and “carr.” Here, we loosely follow the comprehensive wetland classification system established by Cowardin et al. (1979), used by many State and Federal agencies in the United States and by international treaties such as the Convention on Wetlands of International Importance (RAMSAR). The Cowardin system groups wetlands into five major systems: Marine, Estuarine, Riverine, Lacustrine, and Palustrine. In addition, the Cowardin system includes permafrost as wetlands, meaning that almost the entire Arctic region is treated as a wetland. Arguably, permafrost does not fulfil the criteria for the Cowardin wetland definition, i.e., hydrology or soil type or vegetation. Here, we distinguish between permafrost soils and high‐latitude wetlands (both organic soil and mineral) aligned more closely with the classification system developed by the Canadian National Wetlands Working Group (1997). We do not provide a detailed carbon