Stolerov Arctic Meth

Critical Review pubs.acs.org/est Review of Methane Mitigation Technologies with Application to Rapid Release of Methane from the Arctic Joshuah K. Stolaroff,* Subarna Bhattacharyya, Clara A. Smith, William L. Bourcier, Philip J. Cameron-Smith, and Roger D. Aines Lawrence Livermore National Laboratory, Livermore, California, United States *S Supporting Information ABSTRACT: Methane is the most important greenhouse gas after carbon dioxide, with particular influence on near-term climate change. It poses increasing risk in the future from both direct anthropogenic sources and potential rapid release from the Arctic. A range of mitigation (emissions control) technologies have been developed for anthropogenic sources that can be developed for further application, including to Arctic sources. Significant gaps in understanding remain of the mechanisms, magnitude, and likelihood of rapid methane release from the Arctic. Methane may be released by several pathways, including lakes, wetlands, and oceans, and may be either uniform over large areas or concentrated in patches. Across Arctic sources, bubbles originating in the sediment are the most important mechanism for methane to reach the atmosphere. Most known technologies operate on confined gas streams of 0.1% methane or more, and may be applicable to limited Arctic sources where methane is concentrated in pockets. However, some mitigation strategies developed for rice paddies and agricultural soils are promising for Arctic wetlands and thawing permafrost. Other mitigation strategies specific to the Arctic have been proposed but have yet to be studied. Overall, we identify four avenues of research and development that can serve the dual purposes of addressing current methane sources and potential Arctic sources: (1) methane release detection and quantification, (2) mitigation units for small and remote methane streams, (3) mitigation methods for dilute (<1000 ppm) methane streams, and (4) understanding methanotroph and methanogen ecology. ■ INTRODUCTION indicates it has played a role in past warming events in the 7,9−11 Methane is the most important greenhouse gas (GHG) after paleoclimate record. Although the major cause remains carbon dioxide (CO ). It contributes 14% of current GHG disputed, Arctic methane is proposed as a driver of the 2 − emissions by the most common measure, and 30% of current Paleocene Eocene Thermal Maximum, when global temper- ° net climate forcing.1,2 Although emissions of methane are much atures rose by about 6 C, triggering mass extinctions. Contributing to the risk, the climatic response to methane smaller than those of CO2 by mass, methane is far more potent a GHG, even as it is shorter-lived in the atmosphere. Averaged release is superlinear: additional emissions deplete the · over the (most common) 100-year time scale, methane is 25 abundance of the OH radical in the atmosphere, the primary methane sink, and thereby increase the lifetime of atmospheric times more potent than CO2 per unit mass. Over a 20-year time scale, which is relevant to the near-term threat of crossing a methane. Additionally, atmospheric feedbacks with ozone, 2 water vapor, and clouds add to the forcing from methane climate tipping point, methane is 72 times more potent. 12,13 Recent research suggests that methane is more potent still emissions. when accounting for its indirect effect on aerosol formation.3 If the beginnings of a rapid methane release are detected, it In addition to the warming effect of current forcing and would very likely be desirable to stage a large-scale emissions, methane plays a role in climatic feedback technological intervention to contain Arctic methane and mechanisms that can exacerbate warming and even lead to avoid triggering the feedback loop. Such an intervention would ff abrupt, catastrophic climate change in the future. This risk is not replace general e orts to reduce GHG emissions and primarily associated with the rapid release of carbon stores in mitigate climate change, but could be considered an emergency the Arctic due to warming, leading to higher atmospheric counter-measure to avoid the worst outcomes. methane levels, especially in the Arctic. Warming due to higher Arctic concentrations, in turn, leads to additional methane Received: December 27, 2011 − releases in a positive feedback cycle.4 7 Revised: May 10, 2012 A major release of Arctic methane would have a devastating Accepted: May 17, 2012 impact on the global climate and air quality,8 and evidence Published: May 17, 2012 © 2012 American Chemical Society 6455 dx.doi.org/10.1021/es204686w | Environ. Sci. Technol. 2012, 46, 6455−6469 Environmental Science & Technology Critical Review Climate-feedback risks aside, reducing methane emissions natural emissions is fuzzy, with human perturbations such as has been identified as a low cost, near-term approach to climate land-use change and climate change influencing emissions from − change mitigation.14 17 Moreover, in light of continuing failure natural systems, but here we use the term to refer only to direct 18 to address global CO2 emissions, recent analyses recommend emissions from human activities. Although many such activities a strategy of aggressive reductions in “short-lived climate contribute to methane emissions, they all essentially operate by forcers,” i.e., methane and black carbon, as an alternative one of three mechanisms: approach to limit warming in the next few decades. Reductions 1. Creation of conditions for methanogenesis. This leads to in short-lived forcers open an emissions path for CO2 that can emissions from enteric fermentation, rice cultivation, still avoid dangerous levels of warming, and have substantial landfills, manure, wastewater treatment, and agricultural cobenefits for human health and crop yields.19,20 soils (listed under “Other agriculture” in Figure 1). Mitigation options have been explored or deployed for the major current sources of methane: livestock, oil and natural gas 2. Extraction and release of fossil methane. This includes systems, landfills, coal mines, animal and human waste, and rice leaks and venting during extraction and distribution of oil paddies. Development of these mitigation technologies has and natural gas, and from coal mining. generally been specific to the type of source, with limited 3. Release of products of incomplete combustion. This application across types. In some cases, technologies that have includes emissions from stationary and mobile combus- been developed for other purposes may be applicable to tion of fossil fuel, biomass burning, and several sources methane mitigation. included under “Other agriculture”: prescribed burning This paper first describes the current sources of methane of savannas, burning of forests for clearing, and field emissions. We then review the mitigation technologies and burning of agricultural wastes. strategies available, with particular attention to common themes Methanogenesis is the conversion of organic material to across sources. Where possible, we group the mitigation methane under anaerobic conditions by methanogenic archaea. technologies according to the following categories of sources: The process accounts for about 60% of anthropogenic fi (1) con ned sources, where the gas is physically contained or in emissions. “Enteric fermentation”, the largest single source of fl a managed ow, (2) area sources, where the gas is emitted emissions, refers to methanogenesis in the guts of ruminant relatively uniformly over an area of land, (3) aqueous sources, livestock (cattle, goats, sheep). A number of strategies have where the gas is created within or must pass through a water been explored to reduce the emissions, not least because the fi column, and (4) uncon ned sources, where the gas is at a methanogens are parasitic on the livestock by taking useful locally elevated concentration but open to the atmosphere. We energy from their feed, though no definitive solution has yet fl then brie y review potential sources of rapid methane release in been found.26,27 Collections of organic wastes (landfills, the Arctic. In light of known technologies and sources of manure ponds, wastewater treatment systems) also emit methane emissions, we assess the options for future develop- methane through methanogenesis. Some mitigation methods ment of mitigation technologies for Arctic sources and for are mature and widely applied in the developed world methane generally. We conclude with recommended directions (especially for landfills, discussed further in the next section) for further work. but the sources remain significant globally. In rice cultivation, soils are usually inundated with water, producing methane in ■ CURRENT SOURCES OF METHANE EMISSIONS the same way as natural wetlands. Other agricultural soils are About two-thirds of current methane emissions are anthro- sometimes a source and sometimes a sink for methane, and on pogenic, with the other third coming primarily from natural balance are a minor contributor. 22 Fossil methane is the primary constituent of natural gas and is wetlands. Figure 1 shows the sources of anthropogenic 21 methane estimated for the year 2010. The emissions data in coextracted, to varying degrees, from oil and coal formations. Figure 1 are based on the most recent source for global Leakage and venting of this gas contributes roughly 30% of methane emissions estimates,21 however several alternative data anthropogenic emissions. Natural gas systems, in turn, − 21,28 sets are available.23 25 The line between anthropogenic and contribute

Load more