Evaluating methane emissions in a warming Arctic

: High-latitude insights from in-situ and satellite observations

Abstract

The Arctic is warming at four times the global average, causing unprecedented environmental
changes and potentially increasing methane (CH4) emissions from tundra. The cold season
(September – May) has emerged as a period of significant methane release, yet it remains
understudied. As the second most important greenhouse gas, understanding CH4 emissions,
trends, and drivers in the Arctic is crucial for climate policy.
Emissions are estimated via top-down atmospheric inversions using an in-situ network of
stations measuring atmospheric CH4, and drivers are analysed through in-situ measurements of
fluxes and environmental conditions. Both types of coverage are currently spatially inadequate,
but expanding the number surface sites is practically challenging. Satellite datasets could
enhance this coverage.
Firstly, this thesis uses in-situ atmospheric CH4 observations to estimate Arctic tundra
emissions through a wind sector approach and a high-resolution atmospheric inversion. Unlike
previous studies, a summertime emission trend of 0.018 ± 0.005 Tg yr−2
from 2000 to 2021
was identified in the North Slope of Alaska (NSA). Additionally, 34%–50% of yearly emissions
occurred in the late season (September–December) across three different high-Arctic regions,
highlighting widespread and underrepresented emissions during this period.
Secondly, high-resolution satellite observations from the TROPOspheric Monitoring Instrument (TROPOMI) were used to estimate CH4 emissions from the NSA in high-resolution
inversions. Despite their potential, these satellite inversions underestimated emissions compared
to benchmark in-situ inversions and were not robust to inversion system changes. This indicates that satellite observations cannot yet replace in-situ observations for Arctic monitoring,
underscoring the need for more data validation, better uncertainty estimates, and improved
atmospheric transport modelling.
Finally, satellite-derived datasets of wetland extent and freeze/thaw state were evaluated
for their ability to investigate the drivers of late season emissions over the NSA. Results suggest
these satellite datasets are currently insufficient for fully capturing late season drivers

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Anita L Ganesan (Supervisor) & David A Richards (Supervisor)