Attribution of spatial and temporal variations in terrestrial methane flux over North America (original) (raw)
Related papers
2013
Effects of various spatial scales of water table dynamics on the land-atmospheric methane (CH 4) exchange have not yet been assessed for large regions. Here we used a coupled hydrology-biogeochemistry model to quantify daily CH 4 exchange over the pan-Arctic from 1993 to 2004 at two spatial scales (100 km and 5 km). The effects of sub-grid spatial variability of the water table depth (WTD) on CH 4 emissions were examined with a TOPMODEL-based parameterization scheme for northern high latitudes regions. Our results indicate that 5 km CH 4 emissions (38.1-55.4 Tg CH 4 yr −1 , considering the spatial heterogeneity of WTD) were 42 % larger than 100 km CH 4 emissions (using grid-cell-mean WTD) and the differences in annual CH 4 emissions were due to increased emitting area and enhanced flux density after WTD redistribution. Further, the inclusion of sub-grid WTD spatial heterogeneity also influences the inter-annual variability of CH 4 emissions. Soil temperature plays a more important role in the 100 km estimates, while the 5 km estimates are more influenced by WTD. This study suggests that previous macro-scale biogeochemical models using grid-cell-mean WTD might have underestimated the regional CH 4 budget. The spatial scale-dependent effects of WTD should be considered in future quantifications of regional CH 4 emissions.
2010
Continental-scale estimations of terrestrial methane (CH₄) and nitrous oxide (N₂O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH₄ and N₂O fluxes in North America’s terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a¯¹ (1 T g = 10¹² g) of CH₄, and 1.94 ± 0.1 T g N a¯¹ of N₂O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH₄ sources to the atmosphere, but Mexico mainly oxidized and consumed CH₄ from the atmosphere. Wetlands in North America contributed predominantly to the regional CH₄ source, while all other ecosystems acted as sinks for atmospheric CH₄, of which forests accounted for 36.8%. Regarding N₂O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N₂O emission. The inter-annual variations of CH₄ and N₂O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH₄ flux, both mean annual temperature and annual precipitation were significantly correlated to annual N₂O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH₄ and N₂O fluxes in North America generated in this study provide useful information for global change research and policy making.
Global Biogeochemical Cycles, 2004
The MIT Joint Program on the Science and Policy of Global Change is an organization for research, independent policy analysis, and public education in global environmental change. It seeks to provide leadership in understanding scientific, economic, and ecological aspects of this difficult issue, and combining them into policy assessments that serve the needs of ongoing national and international discussions. To this end, the Program brings together an interdisciplinary group from two established research centers at MIT: the Center for Global Change Science (CGCS) and the Center for Energy and Environmental Policy Research (CEEPR). These two centers bridge many key areas of the needed intellectual work, and additional essential areas are covered by other MIT departments, by collaboration with the Ecosystems Center of the Marine Biology Laboratory (MBL) at Woods Hole, and by short-and long-term visitors to the Program. The Program involves sponsorship and active participation by industry, government, and non-profit organizations. To inform processes of policy development and implementation, climate change research needs to focus on improving the prediction of those variables that are most relevant to economic, social, and environmental effects. In turn, the greenhouse gas and atmospheric aerosol assumptions underlying climate analysis need to be related to the economic, technological, and political forces that drive emissions, and to the results of international agreements and mitigation. Further, assessments of possible societal and ecosystem impacts, and analysis of mitigation strategies, need to be based on realistic evaluation of the uncertainties of climate science. This report is one of a series intended to communicate research results and improve public understanding of climate issues, thereby contributing to informed debate about the climate issue, the uncertainties, and the economic and social implications of policy alternatives. Titles in the Report Series to date are listed on the inside back cover.
The MIT Joint Program on the Science and Policy of Global Change is an organization for research, independent policy analysis, and public education in global environmental change. It seeks to provide leadership in understanding scientific, economic, and ecological aspects of this difficult issue, and combining them into policy assessments that serve the needs of ongoing national and international discussions. To this end, the Program brings together an interdisciplinary group from two established research centers at MIT: the Center for Global Change Science (CGCS) and the Center for Energy and Environmental Policy Research (CEEPR). These two centers bridge many key areas of the needed intellectual work, and additional essential areas are covered by other MIT departments, by collaboration with the Ecosystems Center of the Marine Biology Laboratory (MBL) at Woods Hole, and by short-and long-term visitors to the Program. The Program involves sponsorship and active participation by industry, government, and non-profit organizations.
Biogeosciences, 2010
Continental-scale estimations of terrestrial methane (CH 4 ) and nitrous oxide (N 2 O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH 4 and N 2 O fluxes in North America's terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a −1 (1 T g = 10 12 g) of CH 4 , and 1.94 ± 0.1 T g N a −1 of N 2 O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH 4 sources to the atmosphere, but Mexico mainly oxidized and consumed CH 4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH 4 source, while all other ecosystems acted as sinks for atmospheric CH 4 , of which forests accounted for 36.8%. Regarding N 2 O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N 2 O emission. The inter-annual variations of CH 4 and N 2 O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH 4 flux, both mean annual temperature and annual precipitation were significantly correlated to annual Correspondence to: H. Tian (tianhan@auburn.edu) N 2 O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH 4 and N 2 O fluxes in North America generated in this study provide useful information for global change research and policy making.
Interannual variability of methane exchange over a temperate-boreal lowland and wetland forest
Biosphere-atmosphere exchange of methane was estimated for a temperate/boreal lowland and wetland forest ecosystem in northern Wisconsin for 1997-1999 using the Modified Bowen Ratio (MBR) method. Gradients of CH 4 and CO 2 and CO 2 flux were measured on the WLEF-TV tower as part of the Chequamegon Ecosystem-Atmosphere Study (ChEAS). No systematic diurnal variability was observed in CH 4 flux. In all three years, emissions reached maximum values during June-August (1 ± 0.6 mg m-2 h-1) and tended to increase one to two weeks following precipitation events. In 1997 and 1998 the onset of emission was coincident with ground temperatures increasing above zero. The onset of emission in 1999 lagged 100 days behind the 1997 and 1998 onsets, and was likely related to post-drought recovery of the regional water table to typical levels. Thus, we suggest the water table exerts strong control on CH 4 emissions at the ChEAS site. The net annual emissions were 2950, 3100, and 2135 mg m-2 for 1997, 1998, and 1999, respectively. Annual emissions for wetland regions within the source area (28% of the land area) were 13.2, 13.8, and 10.3 g m-2 , assuming moderate rates of oxidation of CH 4 in upland regions in 1997, 1998, and 1999, respectively. Differences in mean annual temperature did not affect the maximum magnitude of CH 4 emissions obtained during each year; however, reduced precipitation and water table levels suppressed CH 4 emission during 1999, suggesting long-term climatic changes that reduce the water table will likely transform this landscape to a reduced source or possibly a sink for atmospheric CH 4 .
Source attribution of the changes in atmospheric methane for 2006–2008
Atmospheric Chemistry and Physics, 2011
The recent increase of atmospheric methane is investigated by using two atmospheric inversions to quantify the distribution of sources and sinks for the 2006-2008 period, and a process-based model of methane emissions by natural wetland ecosystems. Methane emissions derived from the two inversions are consistent at a global scale: emissions are decreased in 2006 (−7 Tg) and increased in 2007 (+21 Tg) and 2008 (+18 Tg), as compared to the 1999-2006 period. The agreement on the latitudinal partition of the flux anomalies for the two inversions is fair in 2006, good in 2007, and not good in 2008. In 2007, a positive anomaly of tropical emissions is found to be the main contributor to the global emission anomalies (∼60-80%) for both inversions, with a dominant share attributed to natural wetlands (∼2/3), and a significant contribution from high latitudes (∼25%). The wetland ecosystem model produces smaller and more balanced positive emission anomalies between the tropics and the high latitudes for 2006, 2007 and 2008, mainly due to precipitation changes during these years. At a global scale, the agreement between the ecosystem model and the inversions is good in 2008 but not satisfying in 2006 and 2007. Tropical South America and Boreal Eurasia appear to be major contributors to variations in methane emissions consistently in the inversions and the ecosystem model. Finally, changes in OH radicals during 2006-2008 are found to be less than 1% in inversions, with only a small impact on the inferred methane emissions.
Modeling modern methane emissions from natural wetlands: 2. Interannual variations 1982–1993
Journal of Geophysical Research, 2001
Methane is an important greenhouse gas which contributes about 22% to the present greenhouse effect. Natural wetlands currently constitute the biggest methane source and were the major source in preindustrial times. Wetland emissions depend highly on the climate, i.e., on soil temperature and water table. To investigate the response of methane emissions from natural wetlands to climate variations, a process-based model that derives methane emissions from natural wetlands as a function of soil temperature, water table, and net primary productivity is used. For its application on the global scale, global data sets for all model parameters are generated. In addition, a simple hydrologic model is developed in order to simulate the position of the water table in wetlands. The hydrologic model is tested against data from different wetland sites, and the sensitivity of the hydrologic model to changes in precipitation is examined. The global methanehydrology model constitutes a tool to study temporal and spatial variations in methane emissions from natural wetlands. The model is applied using high-frequency atmospheric forcing fields from ECMWF reanalyses of the period from 1982 to 1993. We calculate global annual methane emissions from wetlands to be 260 Tg yr Ϫ1. Twenty-five percent of these methane emissions originate from wetlands north of 30ЊN. Only 60% of the produced methane is emitted, while the rest is reoxidized. A comparison of zonal integrals of simulated global wetland emissions and results obtained by an inverse modeling approach shows good agreement. In a test with data from two wetlands the seasonality of simulated and observed methane emissions agrees well.
Understanding the temporal and spatial variation of wetland methane emissions is essential to the estimation of the global methane budget. Our goal for this study is three-fold: (i) to evaluate the wetland methane fluxes simulated in two versions of the Community Land Model, the Carbon-Nitrogen (CN; i.e., CLM4.0) and the Biogeochemistry (BGC; i.e., CLM4.5) versions using the methane emission model CLM4Me so as to determine the sensitivity of the emissions to the underlying carbon model; (ii) to compare the simulated atmospheric methane concentrations to observations, including latitudinal gradients and interannual variability so as to determine the extent to which the atmospheric observations constrain the emissions; (iii) to understand the drivers of seasonal and interannual variability in atmospheric methane concentrations. Simulations of the transport and removal of methane use the Community Atmosphere Model with chemistry (CAM-chem) model in conjunction with CLM4Me methane emissions from both CN and BGC simulations and other methane emission sources from literature. In each case we compare model-simulated atmospheric methane concentration with observations. In addition, we simulate the atmospheric concentrations based on the TransCom wetland and rice paddy emissions derived from a different terrestrial ecosystem model, Vegetation Integrative Simulator for Trace gases (VISIT). Our analysis indicates CN wetland methane emissions are higher in the tropics and lower at high latitudes than emissions from BGC. In CN, methane emissions decrease from 1993 to 2004 while this trend does not appear in the BGC version. In the CN version, methane emission variations follow satellite-derived inundation wetlands closely. However, they are dissimilar in BGC due to its different carbon cycle. CAM-chem simulations with CLM4Me methane emissions suggest that both prescribed anthropogenic and predicted wetlands methane emissions contribute substantially to seasonal and interannual variability in atmospheric methane concentration. Simulated atmospheric CH 4 concentrations in CAM-chem are highly correlated with observations at most of the 14 measurement stations evaluated with an average correlation between 0.71 and 0.80 depending on the simulation (for the period of 1993-2004 for most stations based on data availability). Our results suggest that different spatial patterns of wetland emissions can have significant impacts on Northern and Southern hemisphere (N-S) atmospheric CH 4 concentration gradients and growth rates. This study suggests that both anthropogenic and wetland emissions have significant contributions to seasonal and interannual variations in atmospheric CH 4 concentrations. However, our analysis also indicates the existence of large uncertainties in terms of spatial patterns and magnitude of global wetland methane budgets, and that substantial uncertainty comes from the carbon model underlying the methane flux modules.
Global Biogeochemical Cycles, 2004
Methane has exhibited significant interannual variability with a slowdown in its growth rate beginning in the 1980s. We use a 3-D chemical transport model accounting for interannually varying emissions, transport, and sinks to analyze trends in CH 4 from 1988 to 1997. Variations in CH 4 sources were based on meteorological and country-level socioeconomic data. An inverse method was used to optimize the strengths of sources and sinks for a base year, 1994. We present a best-guess budget along with sensitivity tests. The analysis suggests that the sum of emissions from animals, fossil fuels, landfills, and waste water estimated using the IPCC default methodology is too high. Recent bottom-up estimates of the source from rice paddies appear to be too low. Previous top-down estimates of emissions from wetlands may be a factor of two higher than bottom-up estimates because of possible overestimates of OH. The model captures the general decrease in the CH 4 growth rate observed from 1988 to 1997 and the anomalously low growth rates during 1992-1993. The slowdown in the growth rate is attributed to a combination of slower growth of sources and increases in OH. The economic downturn in the former Soviet Union and Eastern Europe made a significant contribution to the decrease in the growth rate of emissions. The 1992-1993 anomaly can be explained by fluctuations in wetland emissions and OH after the eruption of Mt. Pinatubo. The results suggest that the recent slowdown of CH 4 may be temporary.