Emissions of methane (CH4) from tropical ecosystems, and how they
respond to changes in climate, represent one of the biggest
uncertainties associated with the global CH4 budget. Historically,
this ...has been due to the dearth of pan-tropical in situ measurements,
which is particularly acute in Africa. By virtue of their superior
spatial coverage, satellite observations of atmospheric CH4
columns can help to narrow down some of the uncertainties in the
tropical CH4 emission budget. We use proxy column retrievals of
atmospheric CH4 (XCH4) from the Japanese Greenhouse gases
Observing Satellite (GOSAT) and the nested version of the GEOS-Chem
atmospheric chemistry and transport model
(0.5∘×0.625∘) to infer emissions from tropical
Africa between 2010 and 2016. Proxy retrievals of XCH4 are less
sensitive to scattering due to clouds and aerosol than full physics retrievals, but the method assumes
that the global distribution of carbon dioxide (CO2) is known. We
explore the sensitivity of inferred a posteriori emissions to
this source of systematic error by using two different XCH4 data
products that are determined using different model CO2 fields. We
infer monthly emissions from GOSAT XCH4 data using a hierarchical
Bayesian framework, allowing us to report seasonal cycles and trends
in annual mean values. We find mean tropical African emissions between 2010 and 2016 range
from 76 (74–78) to 80 (78–82) Tg yr−1,
depending on the proxy XCH4 data used, with larger differences in
Northern Hemisphere Africa than Southern Hemisphere Africa. We find a
robust positive linear trend in tropical African CH4 emissions for our 7-year study period, with values of 1.5 (1.1–1.9) Tg yr−1
or 2.1 (1.7–2.5) Tg yr−1, depending on the CO2 data
product used in the proxy retrieval. This linear emissions trend accounts for around a third of the global emissions growth rate during this period. A substantial portion of this increase is due to a short-term increase in emissions of 3 Tg yr−1 between 2011 and 2015 from the Sudd in South Sudan. Using satellite land surface temperature anomalies and altimetry data, we find this increase in CH4 emissions is consistent with an increase in wetland extent due to increased inflow from the White Nile, although the data indicate that the Sudd was anomalously dry at the start of our inversion period. We find a strong seasonality in emissions across Northern Hemisphere Africa, with the timing of the seasonal emissions peak coincident with the seasonal peak in ground water storage. In contrast, we find that a posteriori CH4 emissions from the wetland area of the Congo Basin are approximately constant throughout the year, consistent with less temporal variability in wetland extent, and significantly smaller than a priori estimates.
Carbon tetrachloride (CCl4) is an ozone‐depleting substance, accounting for about 10% of the chlorine in the troposphere. Under the terms of the Montreal Protocol, its production for dispersive uses ...was banned from 2010. In this work we show that, despite the controls on production being introduced, CCl4 emissions from the eastern part of China did not decline between 2009 and 2016. This finding is in contrast to a recent bottom‐up estimate, which predicted a significant decrease in emissions after the introduction of production controls. We find eastern Asian emissions of CCl4 to be 16 (9–24) Gg/year on average between 2009 and 2016, with the primary source regions being in eastern China. The spatial distribution of emissions that we derive suggests that the source distribution of CCl4 in China changed during the 8‐year study period, indicating a new source or sources of emissions from China's Shandong province after 2012.
Plain Language Summary
Carbon tetrachloride is one of several man‐made gases that contribute to the depletion of the ozone layer high in the atmosphere. Because of this, restrictions were introduced on the use of this ozone‐depleting substance, with the expectation that production should by now be close to 0. However, the slower than expected rate of decline of carbon tetrachloride in the atmosphere shows this is not the case, and a large portion of global emissions are unaccounted for. In this study we use atmospheric measurements of carbon tetrachloride from a site in East Asia to identify the magnitude and location of emissions from this region between 2009 and 2016. We find that there are significant ongoing emissions from eastern China and that these account for a large part of the missing emissions from global estimates. The presence of continued sources of this important ozone‐depleting substance indicates that more could be done to speed up the recovery of the ozone layer.
Key Points
Emissions from eastern Asia region account for around 40% of global CCl4 emissions
There has been no sustained decrease in emissions from the region since the introduction of production controls in 2010
Main source regions are in Jiangsu and Shandong provinces of China
Eastern Boundary Upwelling Systems (EBUSs) are coastal hotspots of the potent greenhouse gas nitrous oxide (N2O). However, estimates of their emissions suffer from large uncertainties due to their ...significant spatial and temporal heterogeneity. Here, we derive the first multiyear, monthly resolution N2O emissions from three of the four major EBUSs using high‐frequency coastal atmospheric measurements and an inverse method. We find average combined N2O emissions from the northern California, Benguela, and southern Canary upwelling systems to be 57.7 (51.4–63.9) Gg‐N yr−1. We also find an offshore region near the Benguela EBUS that exhibits large pulses of emissions with emissions that reach 677 Gg‐N yr−1 in 1 month. Our findings highlight that atmospheric measurements coupled with inverse modeling can capture the large variability in EBUS emissions by quantifying emissions over large spatial distances and over long time periods compared to previous methods using traditional oceanographic measurements.
Plain Language Summary
Eastern Boundary Upwelling Systems (EBUSs) are important emissions hotspots of marine nitrous oxide to the atmosphere, where it acts as a greenhouse gas and ozone depleting substance. Emissions from the EBUSs are highly episodic, and most previous estimates are snapshots derived from ship‐based measurements. The variability in emissions combined with the sparsity of measurements makes EBUS emission estimates highly uncertain. Here, we use multiyear, near‐continuous atmospheric measurements from coastal stations and an inverse modeling framework to derive emissions from three of the four major EBUSs. Our results quantify the significant spatial and temporal variability in emissions, which is not well‐represented in global studies of marine nitrous oxide emissions.
Key Points
Eastern Boundary Upwelling System (EBUS) N2O emissions are episodic, and methods are needed to capture their variability in space and time
Previous upscaled estimates of EBUS emissions based on sparse measurements may be inaccurate
N2O emissions from the northern California upwelling system vary with PDO phase
High frequency, in situ observations from 11 globally distributed sites for the period 1994–2014 and archived air measurements dating from 1978 onward have been used to determine the global growth ...rate of 1,1-difluoroethane (HFC-152a, CH3CHF2). These observations have been combined with a range of atmospheric transport models to derive global emission estimates in a top-down approach. HFC-152a is a greenhouse gas with a short atmospheric lifetime of about 1.5 years. Since it does not contain chlorine or bromine, HFC-152a makes no direct contribution to the destruction of stratospheric ozone and is therefore used as a substitute for the ozone depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). The concentration of HFC-152a has grown substantially since the first direct measurements in 1994, reaching a maximum annual global growth rate of 0.84 ± 0.05 ppt yr−1 in 2006, implying a substantial increase in emissions up to 2006. However, since 2007, the annual rate of growth has slowed to 0.38 ± 0.04 ppt yr−1 in 2010 with a further decline to an annual average rate of growth in 2013–2014 of −0.06 ± 0.05 ppt yr−1. The annual average Northern Hemisphere (NH) mole fraction in 1994 was 1.2 ppt rising to an annual average mole fraction of 10.1 ppt in 2014. Average annual mole fractions in the Southern Hemisphere (SH) in 1998 and 2014 were 0.84 and 4.5 ppt, respectively. We estimate global emissions of HFC-152a have risen from 7.3 ± 5.6 Gg yr−1 in 1994 to a maximum of 54.4 ± 17.1 Gg yr−1 in 2011, declining to 52.5 ± 20.1 Gg yr−1 in 2014 or 7.2 ± 2.8 Tg-CO2 eq yr−1. Analysis of mole fraction enhancements above regional background atmospheric levels suggests substantial emissions from North America, Asia, and Europe. Global HFC emissions (so called “bottom up” emissions) reported by the United Nations Framework Convention on Climate Change (UNFCCC) are based on cumulative national emission data reported to the UNFCCC, which in turn are based on national consumption data. There appears to be a significant underestimate ( > 20 Gg) of “bottom-up” reported emissions of HFC-152a, possibly arising from largely underestimated USA emissions and undeclared Asian emissions.
The global atmospheric methane growth rates reported by NOAA for
2020 and 2021 are the largest since systematic measurements began in 1983.
To explore the underlying reasons for these anomalous ...growth rates, we use
newly available methane data from the Japanese Greenhouse gases Observing
SATellite (GOSAT) to estimate methane surface emissions. Relative to
baseline values in 2019, we find that a significant global increase in methane emissions of 27.0 ± 11.3 and 20.8 ± 11.4 Tg is needed to reproduce observed atmospheric methane in 2020 and 2021, respectively,
assuming fixed climatological values for OH. We see the largest annual
increases in methane emissions during 2020 over Eastern Africa (14 ± 3 Tg), tropical Asia (3 ± 4 Tg), tropical South America (5 ± 4 Tg),
and temperate Eurasia (3 ± 3 Tg), and the largest reductions are observed over China
(−6 ± 3 Tg) and India (−2 ± 3 Tg). We find comparable emission
changes in 2021, relative to 2019, except for tropical and temperate South
America where emissions increased by 9 ± 4 and 4 ± 3 Tg,
respectively, and for temperate North America where emissions increased by
5 ± 2 Tg. The elevated contributions we saw in 2020 over the western
half of Africa (−5 ± 3 Tg) are substantially reduced in 2021, compared
to our 2019 baseline. We find statistically significant positive
correlations between anomalies of tropical methane emissions and
groundwater, consistent with recent studies that have highlighted a growing
role for microbial sources over the tropics. Emission reductions over India
and China are expected in 2020 due to the Covid-19 lockdown but continued in
2021, which we do not currently understand. To investigate the role of
reduced OH concentrations during the Covid-19 lockdown in 2020 on the elevated
atmospheric methane growth in 2020–2021, we extended our inversion state
vector to include monthly scaling factors for OH concentrations over six
latitude bands. During 2020, we find that tropospheric OH is reduced by
1.4 ± 1.7 % relative to the corresponding 2019 baseline value. The
corresponding revised global growth of a posteriori methane emissions in 2020 decreased
by 34 % to 17.9 ± 13.2 Tg, relative to the a posteriori value that we inferred
using fixed climatological OH values, consistent with sensitivity tests
using the OH climatology inversion using reduced values for OH. The counter
statement is that 66 % of the global increase in atmospheric methane
during 2020 was due to increased emissions, particularly from tropical
regions. Regional flux differences between the joint methane–OH inversion
and the OH climatology inversion in 2020 are typically much smaller than
10 %. We find that OH is reduced by a much smaller amount during 2021 than in
2020, representing about 10 % of the growth of atmospheric methane in that
year. Therefore, we conclude that most of the observed increase in
atmospheric methane during 2020 and 2021 is due to increased emissions, with
a significant contribution from reduced levels of OH.
The recovery of the stratospheric ozone layer relies on the continued decline in the atmospheric concentrations of ozone-depleting gases such as chlorofluorocarbons
. The atmospheric concentration of ...trichlorofluoromethane (CFC-11), the second-most abundant chlorofluorocarbon, has declined substantially since the mid-1990s
. A recently reported slowdown in the decline of the atmospheric concentration of CFC-11 after 2012, however, suggests that global emissions have increased
. A concurrent increase in CFC-11 emissions from eastern Asia contributes to the global emission increase, but the location and magnitude of this regional source are unknown
. Here, using high-frequency atmospheric observations from Gosan, South Korea, and Hateruma, Japan, together with global monitoring data and atmospheric chemical transport model simulations, we investigate regional CFC-11 emissions from eastern Asia. We show that emissions from eastern mainland China are 7.0 ± 3.0 (±1 standard deviation) gigagrams per year higher in 2014-2017 than in 2008-2012, and that the increase in emissions arises primarily around the northeastern provinces of Shandong and Hebei. This increase accounts for a substantial fraction (at least 40 to 60 per cent) of the global rise in CFC-11 emissions. We find no evidence for a significant increase in CFC-11 emissions from any other eastern Asian countries or other regions of the world where there are available data for the detection of regional emissions. The attribution of any remaining fraction of the global CFC-11 emission rise to other regions is limited by the sparsity of long-term measurements of sufficient frequency near potentially emissive regions. Several considerations suggest that the increase in CFC-11 emissions from eastern mainland China is likely to be the result of new production and use, which is inconsistent with the Montreal Protocol agreement to phase out global chlorofluorocarbon production by 2010.
As the second most populous country and third fastest growing economy, India has emerged as a global economic power. As such, its emissions of greenhouse and ozone-depleting gases are of global ...significance. However, unlike neighbouring China, the Indian sub-continent is very poorly monitored by atmospheric measurement networks. India's halocarbon emissions, here defined as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and chlorocarbons, are not well-known. Previous measurements from the region have been obtained at observatories many hundreds of kilometres from source regions, or at high altitudes, limiting their value for the estimation of regional emission rates. Given the projected rapid growth in demand for refrigerants and solvents in India, emission estimates of these halocarbons are urgently needed to provide a benchmark against which future changes can be evaluated. In this study, we report atmospheric-measurement-derived halocarbon emissions from India. With the exception of dichloromethane, these top-down estimates are the first for India's halocarbons. Air samples were collected at low altitude during an aircraft campaign in June and July 2016, and emissions were derived from measurements of these samples using an inverse modelling framework. These results were evaluated to assess India's progress in phasing out ozone-depleting substances under the Montreal Protocol. India's combined CFC emissions are estimated to be 54 (27–86) Tg CO2 eq. yr−1 (5th and 95th confidence intervals are shown in parentheses). HCFC-22 emissions of 7.8 (6.0–9.9) Gg yr−1 are of similar magnitude to emissions of HFC-134a (8.2 (6.1–10.7) Gg yr−1). We estimate India's HFC-23 emissions to be 1.2 (0.9–1.5) Gg yr−1, and our results are consistent with resumed venting of HFC-23 by HCFC-22 manufacturers following the discontinuation of funding for abatement under the Clean Development Mechanism. We report small emissions of HFC-32 and HFC-143a and provide evidence to suggest that HFC-32 emissions were primarily due to fugitive emissions during manufacturing processes. A lack of significant correlation among HFC species and the small emissions derived for HFC-32 and HFC-143a indicate that in 2016, India's use of refrigerant blends R-410A, R-404A and R-507A was limited, despite extensive consumption elsewhere in the world. We also estimate emissions of the regulated chlorocarbons carbon tetrachloride and methyl chloroform from northern and central India to be 2.3 (1.5–3.4) and 0.07 (0.04–0.10) Gg yr−1 respectively. While the Montreal Protocol has been successful in reducing emissions of many ozone-depleting substances, growth in the global emission rates of the unregulated very short-lived substances poses an ongoing threat to the recovery of the ozone layer. Emissions of dichloromethane are found to be 96.5 (77.8–115.6) Gg yr−1, and our estimate suggests a 5-fold increase in emissions since the last estimate derived from atmospheric data in 2008. We estimate perchloroethene emissions from India and chloroform emissions from northern–central India to be 2.9 (2.5–3.3) and 32.2 (28.3–37.1) Gg yr−1 respectively. Given the rapid growth of India's economy and the likely increase in demand for halocarbons such as HFCs, the implementation of long-term atmospheric monitoring in the region is urgently required. Our results provide a benchmark against which future changes to India's halocarbon emissions may be evaluated.
We present a method to derive atmospheric-observation-based estimates of carbon dioxide (CO2) fluxes
at the national scale, demonstrated using data from a network of surface
tall-tower sites across ...the UK and Ireland over the period 2013–2014. The
inversion is carried out using simulations from a Lagrangian chemical
transport model and an innovative hierarchical Bayesian Markov chain Monte
Carlo (MCMC) framework, which addresses some of the traditional problems
faced by inverse modelling studies, such as subjectivity in the
specification of model and prior uncertainties. Biospheric fluxes related to
gross primary productivity and terrestrial ecosystem respiration are solved
separately in the inversion and then combined a posteriori to determine net
ecosystem exchange of CO2. Two different models, Data
Assimilation Linked Ecosystem Carbon (DALEC) and Joint UK Land Environment Simulator (JULES),
provide prior estimates for these fluxes. We carry out separate inversions
to assess the impact of these different priors on the posterior flux
estimates and evaluate the differences between the prior and posterior
estimates in terms of missing model components. The Numerical Atmospheric
dispersion Modelling Environment (NAME) is used to relate fluxes to the
measurements taken across the regional network. Posterior CO2 estimates
from the two inversions agree within estimated uncertainties, despite large
differences in the prior fluxes from the different models. With our method,
averaging results from 2013 and 2014, we find a total annual net biospheric
flux for the UK of 8±79 Tg CO2 yr−1 (DALEC prior) and
64±85 Tg CO2 yr−1 (JULES prior), where negative values represent an
uptake of CO2. These biospheric CO2 estimates show that annual UK
biospheric sources and sinks are roughly in balance. These annual mean
estimates consistently indicate a greater net release of CO2 than the
prior estimates, which show much more pronounced uptake in summer months.
Atmospheric measurements can be used as a tool to evaluate national greenhouse gas inventories through inverse modelling. Using 8 years of continuous methane (CH4) concentration data, this work ...assesses the United Kingdom's (UK) CH4 emissions over the period 2013–2020. Using two different inversion methods, we find mean emissions of 2.10 ± 0.09 and 2.12 ± 0.26 Tg yr−1 between 2013 and 2020, an overall trend of −0.05 ± 0.01 and −0.06 ± 0.04 Tg yr−2 and a 2 %–3 % decrease each year. This compares with the mean emissions of 2.23 Tg yr−1 and the trend of −0.03 Tg yr−2 (1 % annual decrease) reported in the UK's 2021 inventory between 2013 and 2019. We examine how sensitive these estimates are to various components of the inversion set-up, such as the measurement network configuration, the prior emissions estimate, the inversion method and the atmospheric transport model used. We find the decreasing trend to be due, primarily, to a reduction in emissions from England, which accounts for 70 % of the UK CH4 emissions. Comparisons during 2015 demonstrate consistency when different atmospheric transport models are used to map the relationship between sources and atmospheric observations at the aggregation level of the UK. The posterior annual national means and negative trend are found to be consistent across changes in network configuration. We show, using only two monitoring sites, that the same conclusions on mean UK emissions and negative trend would be reached as using the full six-site network, albeit with larger posterior uncertainties. However, emissions estimates from Scotland fail to converge on the same posterior under different inversion set-ups, highlighting a shortcoming of the current observation network in monitoring all of the UK. Although CH4 emissions in 2020 are estimated to have declined relative to previous years, this decrease is in line with the longer-term emissions trend and is not necessarily a response to national lockdowns.
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