Reading notes-8

Understanding and quantifying the global methane (CH4) buget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2).

The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at the imporving and regularly updating the global methane budget.

Following Saunois et al.(2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations).

For the 2008-2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 TgCH4yr-1 (range 550-594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 TgCH4yr-1 or ~ 60% is attributed to anthropogenic sources, that is emissions caused by direct human activity(i.e. anthropogenic emissions; range 336-376 TgCH4yr-1 or 50%-65%).

The mean annual total emission for the new decade (2008-2017) is 29 TgCh4yr-1 larger than our estimate for the previous decade (2000-2009), and 24TgCH4yr-1 larger than the one reported in the previous budget for 2003-2012 (Saunois et al.,2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30% larger global emissions (737 TgCH4yr-1, range 594-881) than top-down inversion methods.

Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overstimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (~65% of the global budget, <30°N) compared to mid-latitudes(~30%, 30-60°N) and high northern latitudes (~4%,60-90°N).

The most important sources of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some of our global source estimates are smaller than those in previously published bugets （Saunois et al., 2016; Kirschke et al.,2013）.

In particular wetland emissions are about 35TgCH4yr-1 lower due to imporved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 TgCH4yr-1 by 8TgCH4yr-1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5% compared to Saunois et al.,(2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors.

Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii)further development of process-based models for inland-water emissions;(iii) intensification of methane observations at local scales and urban-scale monitoring to constrain botton-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning.

引自“The global methane budget 2000-2017”

标签：atmospheric,notes,global,budget,down,emissions,Reading,methane
来源： https://blog.csdn.net/xiaoxinxin2017/article/details/123581857