Abstract
Abstract. The attribution of spatial and temporal variations in terrestrial methane (CH4) flux is essential for assessing and mitigating CH4 emission from terrestrial ecosystems. In this study, we used a process-based model, the Dynamic Land Ecosystem Model (DLEM), in conjunction with spatial data of six major environmental factors to attribute the spatial and temporal variations in the terrestrial methane (CH4) flux over North America from 1979 to 2008 to six individual driving factors and their interaction. Over the past three decades, our simulations indicate that global change factors accumulatively contributed 23.51 ± 9.61 T g CH4-C (1 Tg = 1012 g) emission over North America, among which ozone (O3) pollution led to a reduced CH4 emission by 2.30 ± 0.49 T g CH4-C. All other factors including climate variability, nitrogen (N) deposition, elevated atmospheric carbon dioxide (CO2), N fertilizer application, and land conversion enhanced terrestrial CH4 emissions by 19.80 ± 12.42 T g CH4-C, 0.09 ± 0.02 T g CH4-C, 6.80 ± 0.86 T g CH4-C, 0.01 ± 0.001 T g CH4-C, and 3.95 ± 0.38 T g CH4-C, respectively, and interaction between/among these global change factors led to a decline of CH4 emission by 4.84 ± 7.74 T g CH4-C. Climate variability and O3 pollution suppressed, while other factors stimulated CH4 emission over the USA; climate variability significantly enhanced, while all the other factors exerted minor effects, positive or negative, on CH4 emission in Canada; Mexico functioned as a sink for atmospheric CH4 with a major contribution from climate change. Climatic variability dominated the inter-annual variations in terrestrial CH4 flux at both continental and country levels. Precipitation played an important role in the climate-induced changes in terrestrial CH4 flux at both continental and country-levels. The relative importance of each environmental factor in determining the magnitude of CH4 flux showed substantially spatial variation across North America. This factorial attribution of CH4 flux in North America might benefit policy makers who would like to curb climate warming by reducing CH4 emission.
Highlights
Following carbon dioxide (CO2), methane (CH4) is the second most radiatively important anthropogenic greenhouse gas which contributes approximately 15% (Rodhe, 1990), or even higher (Shindell et al, 2005), to the increases in radiative forcing caused by anthropogenic release of greenhouse gases to the atmosphere (Lelieveld and Crutzen, 1992; Forster et al, 2007)
The severely O3-polluted area over North America locates in western part of North America such as the northwestern United States of America (USA) which could be as high as more than 5000 ppb-hr, while the other areas, especially northern end of continental North America, feature low O3 pollution (Fig. 2b)
It should be noted that the changes in CH4 flux result from net changes in CH4 production and consumption; for example the increases in CH4 emission might result from either increases in CH4 production or decreases in CH4 consumption; the increases in CH4 uptake might result from either increases in CH4 oxidation or decreases in CH4 production; Liu and Greaver’s study solely reported production or uptake (2009), while this study reported the net flux from production, oxidation, and transport (Materials and methods section)
Summary
Following carbon dioxide (CO2), methane (CH4) is the second most radiatively important anthropogenic greenhouse gas which contributes approximately 15% (Rodhe, 1990), or even higher (Shindell et al, 2005), to the increases in radiative forcing caused by anthropogenic release of greenhouse gases to the atmosphere (Lelieveld and Crutzen, 1992; Forster et al, 2007). In the globally changing environment, a number of factors may change these substrates and/or environmental factors and further alter CH4 production and consumption; for instance, elevated atmospheric CO2 may enhance CH4 emission by stimulating CH4 production (Hutchin et al, 1995) or reduce CH4 oxidation in soils (Phillips et al, 2001); O3 pollution might suppress CH4 emission (Morsky et al, 2008); climate change may increase or decrease CH4 emission (Cao et al, 1998; Frolking and Crill, 1994; Martikainen et al, 1993); N input (Ding et al, 2004b) including N deposition (Steudler et al, 1989) and N fertilization (Zou et al, 2005) might increase (Borjesson and Nohrstedt, 1998; Bodelier et al, 2000) or decrease (Mer and Roger, 2001; Liu and Greaver, 2009; Steudler et al, 1989) CH4 oxidation; and changes in land cover types may increase or decrease CH4 flux, depending on the direction of land conversion (Willison et al, 1995; Huang et al, 2010; Jiang et al, 2009)
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