7.9.1: Introduction

Wetlands In a Changing Climate: The Science

The United Nations Framework Convention on Climate Change calls for the “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system” (UNFCCC 1992). The Paris Climate Agreement in 2015 (UNFCCC 2017) established a goal of keeping global average temperature increase substantially less than 2^oC above the preindustrial value, and making every attempt to keep it below 1.5^oC.

In order to have a two in three probability of keeping global average temperature from rising by more than 2^°C, it is essential to have “negative emissions" of GHGs; in other words, meeting the goal of the Paris Climate Agreement requires active removal and sequestration of atmospheric C (Sanderson et al. 2016). Sequestration is used here to refer to the photosynthetic removal of CO₂ from the atmosphere and its conversion into cellulose and other carbon compounds in plants, and its conversion from decaying plants into soil organic matter. Ricke and Caldeira (2014) have shown that peak warming occurs within about one decade after a pulse of CO₂ is added to the atmosphere. Hence the benefits of avoided CO₂ emissions will be manifested within the lifetimes of people who acted to avoid those emissions. Solomon et al. (2009) have shown that after peak warming is reached, effects will persist for 1000 years. IPCC estimates that depending upon the scenario, “about 15 to 40% of CO₂ emitted by 2100 will remain in the atmosphere longer than 1000 years” (Ciais et al. 2013) affecting 40 generations. Hence avoiding emissions of GHGs to the atmosphere is recommended to be a prime consideration that benefits both present and future generations.

For most types of wetlands, the bulk of sequestered carbon is in the soils rather than in the plant communities. Draining these wetlands to convert them to agriculture as has been done in many countries and regions including Indonesia, Malaysia, Russia, New Zealand, Florida Everglades and in Northern Europe, allows soil organic matter to be oxidized and release CO₂ into the atmosphere. When mangroves are removed for coastal development and for aquaculture, or forested wetlands are harvested, additional carbon is released from soils and harvest residues. In the Southeast United States, a major wood pellet fuel industry has developed where the carbon in the wood is released as CO₂ immediately upon combustion. The use of wood pellets to replace coal for electricity, on the mistaken assumption that it is carbon neutral, is expected to grow substantially by 2050 (IEA 2017), further degrading forested wetlands while adding large amounts of CO₂ to the atmosphere.

CO₂, added to the atmosphere by human activity, is the primary GHG responsible for climate change, followed by CH₄ and N₂O (Myhre et al 2014). These gases move among the natural reservoirs of terrestrial and marine plants, soils, oceans and the atmosphere. Human activity has reduced the size and capacity of these reservoirs while increasing GHG emissions (Ciais et al. 2013). Altering albedo (solar reflectivity from the earth’s surfaces) from land use change can increase or decrease global warming. Climate forcing (heat trapping) from black C (particulate matter from fossil fuel and biofuels combustion (Bond et al. 2013) is a significant contributor to global warming.

The average annual anthropogenic CO₂ emissions for the period 2006-2015 are estimated to be 10.3 PgCy^-1 (Petagrams C per year or 10¹⁵ grams C per year) with 9.3±0.5 PgCy^-1 from fossil fuels and industrial processes and 1.0±0.5 PgCy^-1 from land use change (Fig. \(\PageIndex{1}\), Le Quéré et al. 2016). The total CO₂ emissions from fossil fuels and industrial processes between 1750 and 2011 are estimated to be 375±30 PgC, and the total amount from land use change is estimated to be 180±80 PgC. Therefore, nearly one-third of CO₂ added to the atmosphere from human activity has come from deforestation and oxidation of disturbed soil organic matter (Ciais et al. 2013). By November 2017, CO₂ in the atmosphere had increased to 865 PgC or 406 ppm (NOAA 2018).

Figure \(\PageIndex{1}\): (Le Quéré et al. 2016). Note that approximately 10% of annual emissions are from land use change, and that the land sink removes an amount equal to about 33% of annual emissions. This value is calculated by difference, and wetlands are not counted separately from the total land sink. 1 Gigatonne (10⁹ tonnes) equals 1 Petagram (10¹⁵⁾ grams. Le Quéré et al. 2016

The net annual increase of CO₂ in the atmosphere each year is 4.5±0.5 PgCy^-1 or slightly less than half of annual emissions, and concentrations have increased by over 40% above preindustrial levels. The biosphere has been the major means for removing and sequestering atmospheric CO₂ for over 300 million years, but its potential to be a major resource for addressing climate change has been underappreciated in current policy discussions. Each year, 2.6±0.5 PgC equal to about 25% of annual emissions is removed by the ocean’s phytoplankton or is dissolved in the ocean’s waters. The difference between total emissions to the atmosphere and net removals by the oceans requires that an additional amount of CO₂ equivalent to 3.1±0.9 PgCy^-1 would need to be removed by terrestrial ecosystems to balance the carbon flows. This is nearly 30% of annual anthropogenic emissions from all sources. This analysis only reports estimates of the aggregate removal of CO₂ by the terrestrial biosphere (all plants and soils), and does not explicitly consider the specific role of wetlands as either a source or a sink (Fig. \(\PageIndex{1}\), Le Quéré et al. 2016).

CH₄ has a 100-year Global Warming Potential more than 28 times that of CO₂ (Myhre et al. 2013). It is estimated that between 1750 and 2011 human activity has increased atmospheric CH₄ by a factor of 2.5 from 1984 to 4954 Tg CH₄ y^-1 (722 ppb to1803 ppb) (1 Teragram CH₄ is 10¹² grams CH₄) (Ciais et al. 2013). Currently the major sources of emissions arise from fossil fuel usage (85-105 Tg CH₄ y^-1), ruminant livestock (87-94 Tg CH₄ y^-1), landfills and waste (67-90 Tg CH₄ y^-1), and rice production (33-40 Tg CH₄ y^-1). Average annual anthropogenic emissions of CH₄ from all these sources between 2000 and 2009 total between 272-329 Tg CH₄ y^-1 CH₄ is removed from the atmosphere at a rate of 492-785 Tg CH₄ y^-1 mostly by atmospheric chemistry with small contributions from soil oxidation (Fig. \(\PageIndex{2}\)) (Ciais et al. 2013). CH₄ emissions from wetlands are between 177 and 284 Tg CH₄ y^-1, with an additional 8-73 Tg CH₄ y^-1 emitted from freshwater sources.

Figure \(\PageIndex{2}\): Global methane (CH₄) budget. Average annual anthropogenic emissions of methane from fossil fuels, ruminants, landfills and waste and rice cultivation between 2000 and 2009 total between 272-329 Tgy^-1. For comparison, natural wetlands and freshwaters are estimated to release between 185-357 Tgy^-1. CH₄ is removed at a rate of 492-785 Tgy^-1 mostly by atmospheric chemistry with small contributions by soil oxidation. Black arrows represent natural emissions, red arrows represent anthropogenic emissions since 1750, and the brown arrow represents emissions from both natural fires and anthropogenic biomass burning. 1 Teragram equals10¹² grams. Ciais et al. 2013

Nitrous oxide (N₂O) has a radiative forcing ~300 times that of CO₂. It is a byproduct of both nitrification (under aerobic conditions) and denitrification (under anaerobic conditions), and thus can be produced in wetland soils (Megonigal et al. 2004). However, freshwater and saltwater wetland soils are a source of N₂O only if they receive excessive levels of reactive nitrogen – otherwise they may be a sink for this potent GHG (e.g. Auget et al 2014, Chmura et al. 2016).

While natural solutions have focused on the role of forests to remove and sequester CO₂, there is substantially more C sequestered in soils than in vegetation. The range of estimates for carbon sequestered in vegetation is 450-650 PgC, while the estimate for C stored in soils is 1500-2400 PgC with an additional 1700 PgC estimated to be in permafrost (Ciais et al. 2013). The large amount of carbon sequestered in wetlands is discussed in subsequent sections. As soils warm, and as permafrost thaws, these soils release their stored C as CO₂ or CH₄ resulting from microbial decomposition of soil organic carbon (SOC). These feedback emissions trap additional heat and warm the planet further. A first priority is to avoid disturbing wetlands and keep temperatures from rising as much as possible. As the subsequent sections illustrate, wetlands can play a significant role in addressing climate change by sequestering C, and by providing climate resiliency and adaptation while providing additional ecosystem services.

To limit excessive warming, it is necessary to stabilize CO₂, CH_4, N₂O and other GHG concentrations in the atmosphere at an appropriate level, by decreasing emission rates and increasing removal rates. There are three basic strategies for accomplishing this goal.

1. Reduce the addition of GHGs into the atmosphere from fossil fuels, biofuels, industry, agriculture and other sources to near zero.
2. Prevent the climate and land-use mediated release of additional GHGs (CO_2, CH₄, N₂O) from wetlands, including wetlands underlain by permafrost, from deforestation and forest degradation, and from all soils including degraded grassland and agricultural soils.
3. Increase the capacity of natural systems including wetlands to actively remove CO₂ from the atmosphere and sequester the C for the long-term.

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Excerpted from:

Moomaw, W.R., Chmura, G.L., Davies, G.T. et al. Wetlands In a Changing Climate: Science, Policy and Management. Wetlands 38, 183–205 (2018). Accessed December 2023 https://doi.org/10.1007/s13157-018-1023-8 CC-BY