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Understanding, quantifying, and tracking atmospheric methane and emissions is essential for addressing concerns and informing decisions that affect the climate, economy, and human health and safety. Atmospheric methane is a potent greenhouse gas (GHG) that contributes to global warming. While carbon dioxide is by far the dominant cause of the rise in global average temperatures, methane also plays a significant role because it absorbs more energy per unit mass than carbon dioxide does, giving it a disproportionately large effect on global radiative forcing. In addition to contributing to climate change, methane also affects human health as a precursor to ozone pollution in the lower atmosphere. Improving Characterization of Anthropogenic Methane Emissions in the United States summarizes the current state of understanding of methane emissions sources and the measurement approaches and evaluates opportunities for methodological and inventory development improvements. This report will inform future research agendas of various U.S. agencies, including NOAA, the EPA, the DOE, NASA, the U.S. Department of Agriculture (USDA), and the National Science Foundation (NSF).
Methane is a significant greenhouse gas with 25-32 times the global warming potential of carbon dioxide. Global sources and sinks of methane are understood to be 550 ± 60 Tg a-1. The possible causes of changing decadal trends in atmospheric methane concentrations since the 1990's is not well understood, since this requires a precision in global emissions quantification better than 20 Tg a-1. Atmospheric observations at the local, regional, or national scale can provide "top-down" constraints on emissions to verify "bottom-up" emissions that may not be well characterized. Cavity ring down spectroscopy (CRDS) instruments deliver highly precise in-situ measurements of methane, with 1 Hz precision better than 2 ppb. A comprehensive aircraft campaign in the Athabasca Oil Sands Region of Alberta (AOSR) in summer 2013, led by Environment and Climate Change Canada (ECCC), deployed a CRDS alongside a suite of instrumentation to measure atmospheric pollutants and meteorological parameters. These observations allowed for the comprehensive identification and quantification of methane emissions from unconventional oil extraction. Emissions estimates were 48% higher than those reported in the national greenhouse gas inventory. A series of lower cost follow up campaigns in 2014 and 2017 using a CRDS instrument mobilized with a vehicle allowed for cold season monitoring of emissions and select quantification where atmospheric parameters were favorable, showing continued discrepancies with inventory reporting. To estimate emissions across Canada at the national scale, methane measurements from ECCC long-term monitoring stations over 2010-2015 were utilized in conjunction with satellite remote sensing observations from the Greenhouse Gas Observing Satellite (GOSAT) operated by the Japanese Aerospace Agency (JAXA). These atmospheric observations were assimilated in the GEOS-Chem chemical transport model to constrain emissions using a Bayesian inverse modelling methodology. Results showed 42% higher emissions from anthropogenic sources and 21% lower emissions from natural sources, which are mostly wetlands, when compared to the prior estimate. Through the combinations of all studies presented herein, approximately 2-4 Tg a-1 of methane emissions in Canada were reallocated for the year of 2013, where 1-3 Tg a-1 was added to anthropogenic sources and 2-4 Tg a-1 was deducted from natural sources, which is substantial relative to the anthropogenic inventory in Canada which is 4-5 Tg a-1. This reallocation is 0.4-0.8% of the entire global budget of 550 Tg a-1, where only a ~3% change in the source-sink balance can cause the observed trends in atmospheric methane. These results show that atmospheric observations from surface, aircraft and satellites are critical for constraining the methane budget in Canada, and improvements are necessary to these types of atmospheric observations over the world to constrain the methane cycle within the precision needed to understand decadal trends.
Methane is an important greenhouse gas that can cause global warming. The present concentrations of methane are nearly three times higher than several hundred years ago. Today, more than 60% of the atmospheric methane comes from human activities, including rice agriculture, coal mining, natural gas usage, biomass burning, and raising of cattle. Methane affects the stratospheric ozone layer and the oxidizing capacity of the atmosphere, which in turn control the concentrations of many man-made and natural gases in the atmosphere. This book brings together our knowledge of the trends and the causes behind the increased levels of methane. Based on the scientific information on the sources and sinks, and the role of methane in global warming, strategies to limit emissions can be designed as part of a program to control future global warming.
Methane (CH4) is the second most important greenhouse gas. Unlike CO2 whose rate of growth in the atmosphere has remained positive and increased in recent decades, the behavior of atmospheric methane is considerably more complex and is much less understood on account of the spatiotemporal variability of its emissions which include biogenic (e.g., wetlands, ruminants, rice agriculture), thermogenic (fossil fuels), and pyrogenic (i.e., biomass burning) sources. After sustained growth during most of the 20th century, the CH4 growth rate declined during the 1980s to the early 2000s. With some surprise, however, the growth rate rebounded in 2007 to 2020. During this same period, the 13CH4/12CH4 ratio of atmospheric CH4 also declined to suggest the post-2006 CH4 growth was caused by an increase in 13CH4-depleted biogenic emissions. This work provides additional insight into the recent behavior of atmospheric methane by performing a global three-dimensional Bayesian inversion of atmospheric CH4 and 13CH4/12CH4 ratios over the period 1983-2015 using NOAA Global Monitoring Laboratory (GML) CH4 measurements obtained from surface observation sites located worldwide and the GEOS-Chem chemical transport model (CTM).
We compare modeled and observed atmospheric methane (CH4) between 1996 and 2001, focusing on the role of interannually varying (IAV) transport. The comparison uses observations taken at 13 high-frequency (~hourly) in situ and 6 low-frequency (~weekly) flask measurement sites. To simulate atmospheric methane, we use the global 3-D chemical transport model (MATCH) driven by NCEP reanalyzed winds at T62 resolution (~1.8° x 1.8°). For the simulation, both methane surface emissions and atmospheric sink (OH destruction) are prescribed as annually repeating fields; thus, atmospheric transport is the only IAV component in the simulation. MATCH generally reproduces the amplitude and phase of the observed methane seasonal cycles. At the high-frequency sites, the model also captures much of the observed CH4 variability due to transient synoptic events, which are sometimes related to global transport events. For example, the North Atlantic Oscillation (NAO) and El Niño are shown to influence year-to-year methane observations at Mace Head (Ireland) and Cape Matatula (Samoa), respectively. Simulations of individual flask measurements are generally more difficult to interpret at certain sites, partially due to observational undersampling in areas of high methane variability. A model-observational comparison of methane monthly means at seven coincident in situ and flask locations shows a better comparison at the in situ sites. Additional simulations conducted at coarser MATCH resolution (T42, ~2.8° x 2.8°) showed differences from the T62 simulation at sites near strong emissions. This study highlights the importance of using consistent observed meteorology to simulate atmospheric methane, especially when comparing to high-frequency observations.
This open access book discusses the impact of human-induced global climate change on the regional climate and monsoons of the Indian subcontinent, adjoining Indian Ocean and the Himalayas. It documents the regional climate change projections based on the climate models used in the IPCC Fifth Assessment Report (AR5) and climate change modeling studies using the IITM Earth System Model (ESM) and CORDEX South Asia datasets. The IPCC assessment reports, published every 6–7 years, constitute important reference materials for major policy decisions on climate change, adaptation, and mitigation. While the IPCC assessment reports largely provide a global perspective on climate change, the focus on regional climate change aspects is considerably limited. The effects of climate change over the Indian subcontinent involve complex physical processes on different space and time scales, especially given that the mean climate of this region is generally shaped by the Indian monsoon and the unique high-elevation geographical features such as the Himalayas, the Western Ghats, the Tibetan Plateau and the adjoining Indian Ocean, Arabian Sea, and Bay of Bengal. This book also presents policy relevant information based on robust scientific analysis and assessments of the observed and projected future climate change over the Indian region.