Wetlands

The cited comparison from UNEP specifically concerns peatlands. Peatlands are a particular type of wetland formed from partially decomposed organic matter, which cover only around 3% of the world’s land surface but store approximately twice as much carbon as all the world’s forests. Many other wetlands - including mangroves, salt marshes and seagrass meadows - also store substantial amounts of carbon. Because peatlands form part of this wider wetland category, the comparison provides a conservative basis for the statement to apply to all wetlands collectively. However, it does not mean that every type of wetland stores a proportionally large amount of carbon relative to the world’s forests.

Estimates of global wetland loss vary substantially because studies use different definitions of wetlands, historical baselines and methods, and may measure either net loss of wetland area or wider degradation. [3] estimated that 64-71% of wetlands had been lost since 1900 and suggested losses could have reached 87% since 1700; [4] summarizes the earlier literature as approximately 50% since 1900 and potentially 87% since 1700, while [5] repeats the 87% estimate. However, a newer global reconstruction estimated that approximately 21% of inland wetland area had been lost since 1700 ^A. The “nearly 90%” figure should therefore be understood as an upper-end historical estimate rather than a settled global total. [6] provides further discussion of the uncertainties surrounding global wetland extent and trends.

^A Extensive global wetland loss over the past three centuries | Nature

indicates that wetlands cover 12.1 × 10⁶ km². This is approximately 2.3% of the Earth’s surface.

states wetlands cover 1.5-3% of the Earth’s surface globally. It is a press release, written by Giuseppe Dodaro, the Wetland Restoration Expert at MedWet (The Mediterranean Wetlands Initiative), on the publication of the Restoring Mediterranean Wetlands Policy Playbook (2022) ^B, to which MedWet contributes. While the figure of 1.5-3% is not mentioned specifically in this report, it is widely quoted in adjacent press releases by other organizations that contributed to the report.

indicates that wetlands cover 6% of Earth’s land surface. Since land accounts for 29% of Earth’s total surface ^C, 6% of land area = 1.74% of total area. [12] stated wetlands account for 5-8% of Earth’s land surface, as cited in [14], reinforcing figures from [10] and [11].

^B Restoring Mediterranean Wetlands Policy - Playbook 2023 | Wetland Based Solutions Initiative

^C 1.1 Overview of the Oceans - Introduction to Oceanography | Roger Williams University

indicates coastal wetlands alone cover 54.6 million hectares, which is approximately 1% of Earth’s surface. This source also states that of the 54.6 million hectares of coastal wetlands, 11.48 million hectares are protected (8.04 million hectares of strict and non-strict protection categories + 3.44 million hectares Indigenous People’s Lands (IPLs)). Subsequently, this means approximately 79% of coastal wetlands are unprotected. Inland wetlands have not been included in this estimate.

indicates that 89% of inland wetlands are unprotected (i.e. excluding coastal). [35] also states that 92.8% of continental wetland area is inland and only 7.2% is coastal. Combining this with information from [34] noted above, it follows: (79% of 7.2%) + (89% of 92.8%) = 88% total wetlands are unprotected.

Wetland ecosystems are globally significant reservoirs of carbon, but their degradation and destruction due to land-use change and other anthropogenic activities have substantial implications for greenhouse gas emissions. The statement, “Scientists estimate that, combined, the emissions released from the destruction of wetlands equate to over 1 billion metric tons of carbon dioxide every year,” is derived from a synthesis of multiple sources and studies. Below is a breakdown and clarification of the scientific findings supporting this statement:

A global meta-analysis conducted by Lishan Tan et al. (2019) reported an annual emission of at least 0.96 ± 0.22 gigatons (Gt) of CO₂-equivalent (CO₂e) from land-use and land-cover change (LULCC) across all wetlands. This figure incorporates the cumulative impacts of CO₂, methane (CH₄), and nitrous oxide (N₂O), adjusted by their relative warming impacts.

Natural wetlands are known to be significant sources of methane, and their conversion often leads to an overall reduction in CH₄ emissions due to the drainage and aeration of soils, which inhibit methanogenesis. Wetlands contribute to natural cycles and levels of atmospheric methane, but climate change threatens to disrupt this process, whereby a warmer climate is increasing rates of anaerobic decomposition and methanogenesis, outpacing methane produced by global use of fossil fuels (see additional links below). Nevertheless, destruction of wetlands unleashes substantial carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions resulting from the decomposition of organic matter and the application of fertilizers in agricultural settings, which far outweigh reductions in methane emissions.

Net reductions in natural methane emissions is therefore likely to have a significant negative effect on overall CO₂e emissions estimate, while release of N₂O is likely to have a significant positive effect. [53] expands on this paper through further analysis of soil nitrogen and carbon dynamics.

Joosten et al. (2016) estimated that the oxidation of degraded peatlands alone contributes approximately 1.15 Gt of CO₂ annually, excluding emissions from peat fires. When fires are included, this figure rises to approximately 2 Gt CO₂.

Pendleton et al. (2012) estimated emissions from the destruction of coastal wetlands to be around 450 million tons (0.45 Gt) of CO₂ per year. However, other sources suggest this figure could be as high as 1.02 Gt CO₂, depending on assumptions about the scale and nature of degradation.

A more recent study published in Earth System Science Data (2023) estimated that gross emissions from LULCC in wetlands, modeled across multiple wetland types, amount to 0.75 Pg of carbon per year. This equates to approximately 2.75 Gt CO₂ annually when converted from carbon to CO₂ using a factor of 3.67. This estimate is inclusive of peat oxidation and decomposition but may or may not fully account for peat burning, as detailed in the study’s methodology.

Regional Contributions: For the tropics, [56] uses data from Randerson et al. (2013) and (2018) on emissions from burning peatlands and Hooijer et al. (2010) on draining of peatlands, extrapolated to the present. Outside of the tropics (Northern Hemisphere), they use estimates of carbon emissions from Qiu et al. (2021) [full references in paper). Hooijer et al. (2010) and other studies on Southeast Asia have highlighted that tropical peatland degradation, particularly in Indonesia and Malaysia, is a disproportionately large source of wetland emissions, accounting for significant portions of both global peat oxidation and fire-related emissions.

Scientific Context and Considerations:

• The variation in reported figures arises from differences in methodologies (e.g., meta-analyses, model-based estimates, empirical measurements), definitions of wetland types, and the specific greenhouse gases included.
• Wetland degradation, including peat drainage and oxidation, is considered functional destruction of wetlands due to the irreversible or permanent loss of carbon storage and sequestration capacity, even without complete land conversion or clearance. Additional impacts include soil loss, compaction, nutrient depletion, water table mismanagement and pollution.
• The synthesis of these data sources supports the statement that emissions from wetland destruction exceed 1 Gt CO₂ annually, particularly when considering the combined effects of peatland degradation, peat fires, and coastal wetland conversion.
• Uncertainties remain in exact figures due to regional disparities in data availability, varying assumptions in global models, and the variable inclusion/exclusion of certain emissions processes (e.g. methane from anaerobic decomposition).

Additional Links - Methane Emissions:

Recent intensification of wetland methane feedback | Nature Climate Change (In: ‘Exceptional’ surge in methane emissions from wetlands worries scientists | Carbon Brief)

Rapid shift in methane carbon isotopes suggests microbial emissions drove record high atmospheric methane growth in 2020-2022 | PNAS

Large Methane Emission Fluxes Observed From Tropical Wetlands in Zambia | Global Biogeochemical Cycles (Press release: Tropical Wetlands Emit More Methane Than Previously Thought | Eos)