Wetlands and Climate Change

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This book excerpt discusses the significant, yet underappreciated role of wetlands in the global carbon cycle. By Dr. William J. Mitsch. Editor's Note: Excerpted ...
National Wetlands Newsletter, Vol. 38, No. 1, Copyright © 2016 Environmental Law Institute ® Washington, D.C., USA

Wetlands and Climate Change Wetlands emit 20 to 25 percent of global methane emissions to Earth’s atmosphere, yet they also have the best capacity of any ecosystem to retain carbon through permanent burial (sequestration). Both processes have implications for climate change. Of the total storage of organic carbon in Earth’s soils, 20 to 30 percent or more is stored in wetlands, and that storage is vulnerable to loss back to the atmosphere if the climate warms or becomes drier. This book excerpt discusses the significant, yet underappreciated role of wetlands in the global carbon cycle. By Dr. William J. Mitsch

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here is little doubt that something significant is happening to our climate. According to the consensus of hundreds of scientists who have been involved in the Intergovernmental Panel on Climate Change (IPCC), some major findings should concern anyone interested in our planet and its future. The IPCC was established by the World Meteorological Organization and the United Nations Environmental Programme to assess scientific, technical, and socioeconomic information relevant for understanding climate change, its potential impacts, and options for adaptation and mitigation. Some of the dominant conclusions of the panel, drafted in its most recent multivolume reports and summaries (IPCC 2013, 2014a, b; The Royal Society & The National Academy of Sciences 2014) are summarized here: • The global average surface temperature has increased over the period 1880 to 2012 by about 0.85 degrees Celsius (∘C). The temperature increase was about 0.25∘C more than that estimated by the IPCC (2001) for the 20th century (0.6∘C). This temperature increase in the twentieth century had also been determined to be the largest increase in the last 1,000 years. • Each of the last three decades has been successively warmer at Earth’s surface than any preceding decade since 1850. In the Northern Hemisphere, 1983 to 2012 was likely the warmest 30-year period of the last 1,400 years. • The surface 75 meters (m) of oceans warmed by 0.11∘C per decade over the period 1992 to 2005. It is also likely that regions of high salinity, where evapotranspiration (ET)>> precipitation (P), have become more saline while humid regions, where P>ET, have become fresher since the 1950s.

• The average rate of ice loss from glaciers around the world, excluding glaciers on the periphery of the ice sheets, was very likely 226 gigatons per year (Gt yr−1) over the period 1971 to 2009 and very likely 275 Gt yr−1 over the period 1993 to 2009. Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers continue to shrink worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease. • Sea level has risen globally about 1.7 millimeters per year (mm yr−1), 19 centimeters (cm) total, between 1901 and 2010 and at a much greater rate of 3.2 mm yr−1 from 1993 to 2010. Glacial mass loss and ocean thermal expansion explains 75 percent of this observed global sea-level rise.

Causes of Climate Change The cause of climate change is the increasing concentration of the so-called greenhouse gases in the atmosphere, mostly caused by anthropocentric emissions. These gases adsorb several wavelengths of long-wave radiation, causing Earth to be a little warmer if the gas concentrations increase. The primary greenhouse gas is carbon dioxide (CO2), which is released through the burning of fossil fuels and also by cement production. Atmospheric CO2 is estimated to have increased by over 30 percent since the mid-18th century. The longest record of continuous monitoring of CO2 in the atmosphere is at Mauna Loa, Hawaii, started by C. David Keeling of the Scripps Institution of Oceanography in March 1958, at a National Oceanic and Atmospheric Administration facility. CO2 increased at a rate of 2.2 parts per million per year (ppm yr−1) over 2009 to 2013, more than double the rate at which it was increas-

Editor’s Note: Excerpted with permission from the publisher, Wiley, from Wetlands, Fifth Edition by William J. Mitsch and James G. Gosselink. Copyright 2015. This excerpt has been edited for style and space considerations.

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National Wetlands Newsletter, Vol. 38, No. 1, Copyright © 2016 Environmental Law Institute ® Washington, D.C., USA

and especially denitrification. While N2O is a normal product of denitrification, it is usually a small percentage of denitrification products, with most of the nitrates converted to dinitrogen (N2) gas. N2O has increased by about 20 percent in the atmosphere since preindustrial times.

Figure 1: (a) Concentration of CO2 in atmosphere at Mauna Loa Observatory in Hawaii for 1958 through mid-2014. (b) Details of last four years of seasonal CO2 fluctuations. Peaks are at the beginning of the Northern Hemisphere growing season, after which photosynthesis reduces the CO2 until the end of the growing season. Data from Scripps Institution of Oceanography and NOAA Earth System Research Laboratory.

ing in the 1960s. Monthly average concentrations reached 400 ppm during the spring of 2014 (see Figure 1). There has been much discussion about sources of CO2 besides fossil fuel burning, such as tropical forest deforestation and burning. IPCC (2013) estimates that fossil fuel combustion and cement production combined have released 375 petagrams (Pg) Pg (= petagram= Gt (gigaton) = 1015 g) of CO2 as carbon to the atmosphere while deforestation and other land-use changes are estimated to have released 180 Pg to the atmosphere. Fossil fuel consumption continues to rise, from 6.7 Pg yr-1 in the mid-2000s to 10 Pg yr-1 in 2013. The second most important greenhouse gas is actually water vapor, but it is not known to have any trend or change. It is one of the most abundant gases in the troposphere. When water vapor and other aerosols condense, they have a net negative radiative forcing on the atmosphere, offsetting a major portion of the global mean radiative forcing from other greenhouse gases (IPCC 2013). The third most important greenhouse gas is methane (CH4), which has been estimated to have more than doubled in concentration, from about 720 parts per billion (ppb) in preindustrial times to about 1,803 ppb in 2011. Before about 1980, CH4 was assumed to be a stable concentration in the atmosphere, but it increased by 13 percent between 1978 and 1999 alone (Whalen 2005). What should be clear is that if one argues that Earth has lost half of the world’s wetlands as a result of human activity over the last 100 years when CH4 concentrations are increasing, there is a disconnect. If wetlands were the major source of CH4, we would have seen a decrease in CH4 in the atmosphere over the last 100 years. A fourth important greenhouse gas, nitrous oxide (N2O), also comes from wetlands as a result of nitrification

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Wetlands in the Global Carbon Cycle Although soil carbon in wetland soils is recognized as an important component of global carbon budgets and future climate change scenarios, very little work has been done to consider the role of wetlands, particularly those in temperate and tropical regions of the world, in the global carbon cycle. A carbon budget for the world, with wetlands included to show their relative contributions, is shown in Figure 2. Following, we discuss the role of wetlands in this carbon budget in terms of carbon storage in peat, carbon sequestration through peat and organic soil development, and CH4 emissions. This budget is a major revision from the budget published in the fourth edition of Wetlands (Mitsch & Gosselink 2007). The major changes are a significantly higher carbon sequestration of 1 Pg yr−1 (=1,000 Tg (teragram) yr−1 = 1015 g yr−1) estimated for the world’s wetlands, based on new data from several wetlands around the world (Mitsch et al. 2013), and a continual increase in carbon emissions from fossil fuel combustion from a mid-2000s estimate of 6.3 Pg yr−1 to the current rate of 10 Pg yr −1, a 60 percent increase in emissions between two editions of this textbook (Mitsch & Gosselink 2007, 2015).

Figure 2: Global carbon budget with estimated role of wetlands in the carbon cycle. Fluxes are in Pg/yr; storages are in Pg. Pg = 1015 g. Data from CH4 emissions from wetlands and rice paddies from Bloom et al., 2010; terrestrial ecosystem and fossil fuel inputs to CO2 from IPCC, 2013; carbon sequestration by wetlands from Mitsch et al., 2013.

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National Wetlands Newsletter, Vol. 38, No. 1, Copyright © 2016 Environmental Law Institute ® Washington, D.C., USA

of 56 g-C m−2 yr−1 over a 24,000-year period and a carbon sequestration rate of 94 g-C m−2 yr−1 for the past 500 years in the upper meter of the core. The accumulation of peat in tropical wetlands may be due more to the slow decomposition of recalcitrant lignin in roots and woody material under constant high water rather than to high productivity of these systems (Chimner & Ewel 2005). The lower rates of carbon sequestration in seasonally flooded tropical wetlands are probably due to the high temperatures year-round, especially in the dry season, when some of the carbon is oxidized, or in some cases the presence of fire. Temperate freshwater wetlands showed some of the Carbon Sequestration highest rates of carbon sequestration of any of the three Many studies have now estimated carbon sequestration climates investigated by Mitsch et al. (2013). Carbon in wetlands in a variety of temperate and tropical settings sequestration in temperate-zone wetlands ranges from 230 to augment the frequent estimates that already existed to 320 to g-C m−2 yr−1. Brix et al. (2001) estimated a high for northern peatlands (see Mitsch & Gosselink 2015 rate of more than 500 g-C m−2 yr−1 in a productive Phragfor detailed list). For northern mites marsh in Denmark. peatlands, the vertical accuCreated and restored wetmulation rate of between 20 lands might be the best oppor“These peat deposits, if and 200 cm/1,000 yr usually tunity for carbon sequestration. disturbed, however, could results in carbon accumulation A carbon sequestration rate of in the range of 10 to 50 grams contribute significantly to 180 to 190 g-C m−2 yr−1 for of carbon per square meter worldwide atmospheric CO 2 two created wetland basins of wetland per year (g-C m−2 in Ohio (Anderson &Mitsch levels, depending on the yr−1). This is a typical range of 2006) 10 years after the wetcarbon accumulation in peatlands were created increased to balance between draining lands. A reasonable average of 220 to 270 g-C m−2 yr−1 by the and oxidation of the peat −2 −1 29 g-C m yr was found in time the wetlands were 15 years deposits and their formation old (Bernal & Mitsch 2013a). a review of the literature for eight recent peatlands around About one-fourth of that carin active wetlands.” the world where carbon sequesbon sequestration was as inortration was measured. ganic carbon, precipitated as Most of the rates for carbon sequestration in tropical/ calcite/calcium carbonate (CaCO3) due to high producsubtropical wetlands and for coastal mangroves and salt tivities in the water column. Euliss et al. (2006) compared marshes are in the range of 150 to 250 g-C m−2 yr−1. Carbon the carbon sequestration in several wetlands that had been sequestration by coastal wetlands (salt marshes, mangroves, restored for more than a decade in the prairie pothole wetsea grasses) now has enormous international support and lands of North America and found 305 g-C m−2 yr−1. This recognition, partially because it is referred to in the litera- is not surprising, because restoration in these cases meant ture and popular press as blue carbon (Mcleod et al. 2011; reflooding agricultural land, allowing organic carbon to Vaidyanathan 2011; World Wildlife Fund 2012; also see once again build up in the soil. For comparison, Euliss http:// thebluecarboninitiative.org/). The tropical wetlands et al. (2006) estimated an accumulation rate in reference included some high rates of carbon sequestration such as (natural) marshes in the region of 83 g-C m−2 yr−1 based on seen for Cyperus wetlands in Uganda (Saunders et al. 2007), average sedimentation rates of 2 mm/yr. but also relatively low rates of carbon sequestration in seasonally flooded wetlands in Costa Rica and Botswana (Ber- Methane Emissions nal & Mitsch 2013b). In a study of long-term accumulation Wetlands are estimated to emit about 20 to 25 percent of in the tropics, Page et al. (2004) investigated a 9.5-m core current global CH4 emissions or about 115 to 170 teraof peat from a tropical peatland in Kalimantan, Indone- grams of methane per year (Tg-CH4 yr−1). Thus, in climate sia, and found an average carbon sequestration of the core change discussions concerning wetlands, these “natural Peat Storage and a Global Carbon Budget Peat deposits in the world’s wetlands, particularly in boreal and tropical regions, are substantial storages of carbon (C) in the lithosphere. Of the total storage of C in Earth soils of 1,400 to 2,500 Pg-C (Pg = 1015 g), anywhere from 20 to 30 percent is stored in wetlands (Mitsch & Wu 1995; Roulet 2000; Hadi et al. 2005; Lal 2008). These peat deposits, if disturbed, however, could contribute significantly to worldwide atmospheric CO2 levels, depending on the balance between draining and oxidation of the peat deposits and their formation in active wetlands.

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National Wetlands Newsletter, Vol. 38, No. 1, Copyright © 2016 Environmental Law Institute ® Washington, D.C., USA

electron acceptors oxygen (O2), nitrates (NO3 −), and sulfates (SO 4 =) have been reduced. Sources Megonigal et al. (2004) Whalen (2005) Bloom et al. (2010) Conversely, nonflooded upland soils (e.g., forests, grasslands, ara115 145 170 Natural Wetlands ble land) are regarded as the major 65 Tropics biological sink of atmospheric CH4 40 Northern latitude (the major sink overall is tropo10 spheric photochemistry). Obligate Others aerobic methanotrophic bacteria 45 45 Other Natural Sources ii use molecular oxygen to oxidize Anthropogenic CH4 to CO2 and cellular carbon. 60 80 57 Rice Paddies The consumption of atmospheric CH4 is the result of two physiolog315 330 Other iii ically distinct microbial groups: 535 600 Total Sources (1)  the methanotrophs, which i. Tg = 10 g. have a membrane-bound enzyme ii. Other natural sources include termites, ocean, freshwater, and geological sources. system, and (2) an autotrophic iii. Other anthropogenic sources include fossil fuels, landfills, domestic wastewater treatment, animal waste, enteric fermentation (ruminants), and biomass burning. nitrifier community. Methanotrophs are estimated to consume emissions” often receive the most attention. Rice pad- about 30 Tg-CH4 yr−1 (Whalen 2005). dies, which are essentially domestic wetlands, account CH4 production is much higher in the freshwater wetfor another 60 to 80 Tg-CH4 yr−1. Other anthropogenic lands than from saltwater wetlands. A major reason for low sources account for most of the rest. CH4 emissions are a CH4 emissions from saltwater wetlands is the high conconcern because CH4 is estimated to be 25 (now 28) times centration of sulfates in seawater relative to freshwater that more effective as a greenhouse gas on a molecular basis competes with carbon for oxidizable substrate. CH4 emissions from studies of various freshwater wetlands around the than is CO2 after 100 years (see Table 1). Tropical wetlands have been described recently as more world show have a considerable range and measurements at important than originally thought for methane emissions a given wetland are rarely normally distributed. Ebullition (IPCC 2013). Bloom (2010) suggests that 58 percent (132 is frequent yet hard to measure with enough frequency. In Tg-CH4 yr−1) of the total methane emissions from wetlands a word, it is extraordinarily difficult to obtain accurate and and rice paddies (227 Tg- CH4 yr−1) comes from the trop- repeatable CH4 emission measurements from wetlands. ics. Sjögersten et al. (2014) used a web analysis of current Most early CH4 emission studies were done in northern −1 literature to estimate 90 ± 77 Tg-CH4 yr of methane emis- peatlands (bogs and fens) in cold climates. Moore and Rousions from tropical wetlands. They suggest that the methane let (1995) suggested that most annual CH4 emission flux emissions in the tropics are greater from mineral soil wet- measurements in Canada are less than 10 g-CH4 m−2 yr−1 with the primary controlling mechanisms being soil temlands than organic soil wetlands. CH4 emissions are actually the result of two competing perature, water table position, or a combination of both. processes going on at the same time by microbial com- We estimate from recent studies using modern field and munities. The degradation of organic matter by aerobic laboratory methods that the general range of CH4 emisrespiration is fairly efficient in terms of energy transfer. sions from boreal wetlands is from 15 to 25 g-C m−2 yr−1. Because of the anoxic nature of wetland soils, anaero- An early estimate of CH4 emissions by Gorham (1991) that bic processes, which are less efficient in terms of energy has been used for determining the global contributions of transfer, occur in close proximity to aerobic processes. northern peatlands is 28 g-C m−2 yr−1. In general, CH4 emisMethanogenesis occurs when microbes called methano- sions from bogs are much lower than those from the more gens use CO2 as an electron acceptor for the production mineral-rich fens. Aselmann and Crutzen (1989) assumed of gaseous CH4 or, alternatively, use a low-weight organic rates of CH4 emissions in the order of increasing emissions is compound, such as one from a methyl group. CH4 pro- bogs