Abstract
Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) provides observations of atmospheric methane (CH4) at an unprecedented combination of high spatial resolution and daily global coverage. Hu et al. (2018) reported unexpectedly large methane enhancements over South Sudan in these observations. Here we assess methane emissions from the wetlands of South Sudan using 2 years (December 2017–November 2019) of TROPOMI total column methane observations. We estimate annual wetland emissions of 7.4 ± 3.2 Tg yr−1, which agrees with the multiyear GOSAT inversions of Lunt et al. (2019) but is an order of magnitude larger than estimates from wetland process models. This disagreement may be explained by the underestimation (by up to 4 times) of inundation extent by the hydrological schemes used in those models. We investigate the seasonal cycle of the emissions and find the lowest emissions during the June–August season when the process models show the largest emissions. Using satellite-altimetry-based river water height measurements, we infer that this seasonal mismatch is likely due to a seasonal mismatch in inundation extent. In models, inundation extent is controlled by regional precipitation scaled to static wetland extent maps, whereas the actual inundation extent is driven by water inflow from rivers like the White Nile and the Sobat. We find the lowest emissions in the highest precipitation and lowest temperature season (June–August, JJA) when models estimate large emissions. In general, our emission estimates show better agreement in terms of both seasonal cycle and annual mean with model estimates that use a stronger temperature dependence. This suggests that temperature might be a stronger control for the South Sudan wetlands emissions than currently assumed by models. Our findings demonstrate the use of satellite instruments for quantifying emissions from inaccessible and uncertain tropical wetlands, providing clues for the improvement of process models and thereby improving our understanding of the currently uncertain contribution of wetlands to the global methane budget.
Highlights
Reducing anthropogenic methane emissions has been recognized as an important requirement for achieving the 2015 Paris Agreement target of limiting global temperature rise to below 2 ◦C relative to pre-industrial times (Ganesan et al, 2019)
Saunois et al (2016) provide global methane emission estimates for all source categories combined of 540–568 Tg yr−1 using top-down approaches and 596–884 Tg yr−1 using bottom-up approaches for the period 2003–2012
This study aims to infer the scale of the wetland methane emissions from South Sudan from TROPOspheric Monitoring Instrument (TROPOMI) observations using a simplified emission quantification method and investigate its relationship to the results of wetland process models and seasonally varying climatological conditions
Summary
Reducing anthropogenic methane emissions has been recognized as an important requirement for achieving the 2015 Paris Agreement target of limiting global temperature rise to below 2 ◦C relative to pre-industrial times (Ganesan et al, 2019). Lunt et al (2019) used methane observations from the Japanese Greenhouse gases Observing Satellite (GOSAT) in inverse modeling to infer emissions from tropical Africa during 2010–2016 They found that emissions from South Sudan were more than 3 times larger than the ensemble mean estimates from the Wetcharts process model (Bloom et al, 2017). They found that emissions from the Sudd wetlands in the region increased rapidly from 2.4– 4.2 Tg yr−1 in 2010–2011 to 5.2–6.9 Tg yr−1 in 2016, likely because of an inundation extent expansion due to an increase in water inflow from the White Nile River.
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