“Atmospheric methane (CH4) is the second-most important greenhouse gas after carbon dioxide (CO2); the Intergovernmental Panel on Climate Change (IPCC) estimates that it was responsible for ∼about 20% of global anthropogenic direct radiative forcing from 2000 to 2010” begins a paper by Maryann R. Sargent et al-Majority of US urban natural gas emissions unaccounted for in inventories. The authors continue “Oil and natural gas systems are estimated to account for 31% of anthropogenic methane emissions in the United States .”
Globally wetlands contributed 30% of global methane
emissions, with oil, gas, and coal activities accounting for 20%. Agriculture,
including enteric
fermentation (cow belching), manure management, and rice cultivation,
made up 24% of emissions, and landfill gas contributed 11%. Sixty-four percent
of methane emissions come from the tropical regions of South America, Asia, and
Africa, with temperate regions accounting for 32% and the Arctic contributing
4%. According to previous studies the amount of methane in
Earth’s atmosphere continues to rise. Concentrations of methane now exceed 1875
parts per billion, about 2.5 times as much as was in the atmosphere in the
1850s.
In the past most efforts to reduce methane emissions in the
United States has targeted oil- and natural gas-production, because the U.S.
Environmental Protection Agency (EPA) had estimated that to be the source of
most of the leakage from natural gas. However, a 2019 NOAA and University of Michigan study using an
instrumented airplane found unexpectedly large emissions over five major cities
along the East Coast. These cities have natural gas distribution systems and
deliver natural gas to households.
This confirmed the earlier work of two scientists, Robert B.
Jackson, who was then Professor of Global Environmental Change at Duke
University and Nathan Phillips, associate professor at Boston University
Department of Earth and Environment collaborated with Robert Ackley of Gas
Safety Inc., and Eric Crosson of Picarro Inc., who studied gas leaks in Boston.
They mapped the gas leaks under the city using a the Picarro high-precision
methane analyzer installed in a GPS-equipped car. Driving all 785 road miles
within city limits, the researchers discovered 3,356 leaks. The leaks were
found to be associated with old cast-iron underground pipes, rather than
neighborhood socioeconomic indicators. Levels of methane in the surface air on
Boston’s streets exceeded 15 times the normal atmospheric background value.
"Bottom-up" estimates measure emissions from
a sample of devices and multiplies the result by the total number of devices.
In contrast, "top-down" measurements can be performed at a regional
scale, such as flying an aircraft upwind and downwind of a study area. In most
studies, the bottom up method is underestimating the emissions. The newly published study mentioned above from
scientists at Harvard, Boston University and NOAA used 8 years of actual
measured methane levels at two locations in the heart of Boston (Boston
University and Copley Square) and 3 sites far outside the city to study methane
emissions in Boston has made some interesting suggestions of the potential
missing sources of methane emissions.
The city of Boston has set a goal of becoming carbon neutral
by 2050 (34), and Massachusetts implemented new laws and regulations between
2014 and 2019 requiring utilities to report and repair large leaks based on
their size (35) (36). A 2014 law required repair of leaks classified as grade 1
and grade 2, as well as making leak data public. A 2018 law required repair of
grade 3 leaks classified as “significant environmental impact” as well. In the
eight year period of this study there has been no significant change in the
number of grade 1 or 2 leaks on the pipeline system and only a slight reduction
in the number of grade 3 leaks. Thus the authors conclude that new leaks are
appearing in the aging Boston pipeline system as fast as old ones are being
fixed. The methane distribution infrastructure is crumbling.
According to the authors of a 2019 study by the Home Energy
Efficiency Team (HEET) predicted that the 2018 Massachusetts law requiring the
repair of leaks deemed “significant environmental impact” could reduce pipeline
emissions by half, based on a finding that 7% of leaks emit half of all gas by
volume. The current study analysis finds that these efforts have not yet
resulted in a measurable change in methane emissions which can be attributed to
the constant number of leaks reported despite ongoing repairs.
Using their measurements and model the scientists found an
average methane loss rate of 2.5% ± 0.5% from 2012 to 2020. They found no
statistically significant trend in loss rate over the 8 year period of the
study, despite the new leak repair regulations going into effect. In
addition, the methane loss rate calculated by the scientists in this study is
three times higher than the 0.8% loss rate indicated by their prior bottom up
inventory for Boston and six times higher than the Massachusetts Department of
Environmental Protection (MassDEP) estimate.
The scientists found a strong, correlation between methane emissions
and consumption in Boston that the scientists said was unexpected. During the
winter when natural gas is in higher use, the methane emission are higher. This
relationship has also been found in Washington, DC and Los Angeles. This
lead the scientists to speculate that either the releases are coming from end
use equipment (heaters, stoves, etc.) in homes or there is a fluctuation in the
pipeline pressure based on demand that causes and increase in leakage from the distribution
system. Only further study of distribution pipeline pressure can answer this.
However, the authors interpreted their data to imply that sources
other than pipelines, such as transmission and appliances, are important
sources of methane emissions and may require future policy action. They estimated
total supply chain losses of 3.3% to 4.7% for methane consumed in urban areas,
which significantly increases the climate impacts of natural gas compared to
Environmental Protection Agency estimates.
The scientists point out that densely populated urban areas could
potentially make big changes in their methane emissions, because they have
concentrated populations, infrastructure, and emissions and in in many cases Democratic/
progressive control and the will to implement emission reductions policies.
Pipelines, transmission infrastructure, household and commercial appliances,
meters, stationary combustion, and service leaks are believed to be the most
significant contributors for urban methane emissions. Bottom-up inventories
estimate that distribution and end use are responsible for 6% of US emissions
from methane this work suggests a higher contribution.
Full Citation:
Majority of US urban natural gas emissions unaccounted for in inventoriesMaryann R. Sargent, Cody Floerchinger, Kathryn McKain, John Budney, Elaine W. Gottlieb, Lucy R. Hutyra, Joseph Rudek, Steven C. Wofsy
Proceedings of the National Academy of Sciences Nov 2021, 118 (44) e2105804118; DOI: 10.1073/pnas.2105804118
Other reading:
R B Jackson, M Saunois, P Bousquet , J G Canadell, B Poulter, A R Stavert , P Bergamaschi, Y Niwa , A Segers and A Tsuruta: Increasing anthropogenic methane emissions arise equally from agricultural and fossil fuel sources, Environ. Res. Lett. 15 (2020) 071002 https://iopscience.iop.org/article/10.1088/1748-9326/ab9ed2/pdf
No comments:
Post a Comment