Читать книгу Wetland Carbon and Environmental Management - Группа авторов - Страница 17

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.