Abstract
Tropical ecosystems are large carbon stores that are vulnerable to climate change. The sparseness of ground-based measurements has precluded verification of these ecosystems being a net ...annual source (+ve) or sink (−ve) of atmospheric carbon. We show that two independent satellite data sets of atmospheric carbon dioxide (CO
2
), interpreted using independent models, are consistent with the land tropics being a net annual carbon emission of
$$({\mathrm{median}}_{{\mathrm{minimum}}}^{{\mathrm{maximum}}})$$
(
median
minimum
maximum
)
$$1.03_{ - 0.20}^{ + 1.73}$$
1.0
3
-
0.20
+
1.73
and
$$1.60_{ + 1.39}^{ + 2.11}$$
1.6
0
+
1.39
+
2.11
petagrams (PgC) in 2015 and 2016, respectively. These pan-tropical estimates reflect unexpectedly large net emissions from tropical Africa of
$$1.48_{ + 0.80}^{ + 1.95}$$
1.4
8
+
0.80
+
1.95
PgC in 2015 and
$$1.65_{ + 1.14}^{ + 2.42}$$
1.6
5
+
1.14
+
2.42
PgC in 2016. The largest carbon uptake is over the Congo basin, and the two loci of carbon emissions are over western Ethiopia and western tropical Africa, where there are large soil organic carbon stores and where there has been substantial land use change. These signals are present in the space-borne CO
2
record from 2009 onwards.
Large variations in the growth of atmospheric methane, a prominent greenhouse gas, are driven by a diverse range of anthropogenic and natural emissions and by loss from oxidation by the hydroxyl ...radical. We used a decade-long dataset (2010-2019) of satellite observations of methane to show that tropical terrestrial emissions explain more than 80% of the observed changes in the global atmospheric methane growth rate over this period. Using correlative meteorological analyses, we show strong seasonal correlations (r = 0.6-0.8) between large-scale changes in sea surface temperature over the tropical oceans and regional variations in methane emissions (via changes in rainfall and temperature) over tropical South America and tropical Africa. Existing predictive skill for sea surface temperature variations could therefore be used to help forecast variations in global atmospheric methane.
Particulate matter (PM) in the atmosphere and deposited on solar photovoltaic (PV) panels reduce PV energy generation. Reducing anthropogenic PM sources will therefore increase carbon-free energy ...generation and as a cobenefit will improve surface air quality. However, we lack a global understanding of the sectors that would be the most effective at achieving the necessary reductions in PM sources. Here we combine well-evaluated models of solar PV performance and atmospheric composition to show that deep cuts in air pollutant emissions from the residential, on-road, and energy sectors are the most effective approaches to mitigate PM-induced PV energy losses over East and South Asia, and the Tibetan Plateau, Central Asia, and the Arabian Peninsula, and Western Siberia, respectively. Using 2019 PV capacities as a baseline, we find that a 50% reduction in residential emissions would lead to an additional 10.3 TWh yr–1 (US$878 million yr–1) and 2.5 TWh yr–1 (US$196 million yr–1) produced in China and India, respectively.
Limiting the rise in global mean temperatures relies on reducing carbon dioxide (CO
) emissions and on the removal of CO
by land carbon sinks. China is currently the single largest emitter of CO
, ...responsible for approximately 27 per cent (2.67 petagrams of carbon per year) of global fossil fuel emissions in 2017
. Understanding of Chinese land biosphere fluxes has been hampered by sparse data coverage
, which has resulted in a wide range of a posteriori estimates of flux. Here we present recently available data on the atmospheric mole fraction of CO
, measured from six sites across China during 2009 to 2016. Using these data, we estimate a mean Chinese land biosphere sink of -1.11 ± 0.38 petagrams of carbon per year during 2010 to 2016, equivalent to about 45 per cent of our estimate of annual Chinese anthropogenic emissions over that period. Our estimate reflects a previously underestimated land carbon sink over southwest China (Yunnan, Guizhou and Guangxi provinces) throughout the year, and over northeast China (especially Heilongjiang and Jilin provinces) during summer months. These provinces have established a pattern of rapid afforestation of progressively larger regions
, with provincial forest areas increasing by between 0.04 million and 0.44 million hectares per year over the past 10 to 15 years. These large-scale changes reflect the expansion of fast-growing plantation forests that contribute to timber exports and the domestic production of paper
. Space-borne observations of vegetation greenness show a large increase with time over this study period, supporting the timing and increase in the land carbon sink over these afforestation regions.
Abstract
Recent work has highlighted the large role of methane emissions from the Sudd wetland and surrounding ecosystems on the global atmospheric growth rate of methane since 2010. These emissions ...are driven by high rainfall over basin catchments linked with the positive phase of the Indian Ocean Dipole. We reconstruct flood inundation for the Sudd wetland over a 38-year period at a spatial resolution of 30 m using a new satellite Earth Observation (EO) wetland mapping tool. We reveal considerable changes in the wet season extent of the wetland, including an increase >300% since 2019 compared to the median 1984–2022 extent. We report major increases in flood extent within grassland-dominated floodplains outside of the area currently defined Sudd wetland region. These year-to-year changes in wetland extent are corroborated with total water storage anomalies inferred from satellite data (Pearson correlation
R
= 0.92), Lake Victoria levels (
R
= 0.73), and anomalies in reported annual mean global methane growth rates since 2009 (
R
= 0.88). Our analysis shows that flood water inundation is dominated by inundated vegetation and aquatic vegetation, accounting for an average of 40% and 50% of total extent, respectively, compared to open water that accounted for just 9% of inundation in a typical year. This is consistent with recent studies that report wetland methane emissions are focused on areas with inundated vegetation. Our findings also support recent studies that highlight the significant role of the Sudd wetland in driving anomalously large global atmospheric annual growth rates, 2020–2022. By capturing high resolution information on inundated vegetation, our EO wetland mapping tool has significant potential for improved wetland emission estimates of methane. Vascular plants common in the Sudd wetland, e.g. macrophytes including
Phragmites Australis
and
Cyperus Papyrus
, seem to play a key role in methane emissions and we recommend they should be the focus of future research.
The 2015/2016 El Niño was the first major climate variation when there were a range of satellite observations that simultaneously observed land, ocean and atmospheric properties associated with the ...carbon cycle. These data are beginning to provide new insights into the varied responses of land ecosystems to El Niño, but we are far from fully exploiting the information embodied by these data. Here, we briefly review the atmospheric and terrestrial satellite data that are available to study the carbon cycle. We also outline recommendations for future research, particularly the closer integration of satellite data with forest biometric datasets that provide detailed information about carbon dynamics on a range of timescales.
This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.
We show that transport differences between two commonly used global chemical transport models, GEOS‐Chem and TM5, lead to systematic space‐time differences in modeled distributions of carbon dioxide ...and sulfur hexafluoride. The distribution of differences suggests inconsistencies between the transport simulated by the models, most likely due to the representation of vertical motion. We further demonstrate that these transport differences result in systematic differences in surface CO2 flux estimated by a collection of global atmospheric inverse models using TM5 and GEOS‐Chem and constrained by in situ and satellite observations. While the impact on inferred surface fluxes is most easily illustrated in the magnitude of the seasonal cycle of surface CO2 exchange, it is the annual carbon budgets that are particularly relevant for carbon cycle science and policy. We show that inverse model flux estimates for large zonal bands can have systematic biases of up to 1.7 PgC/year due to large‐scale transport uncertainty. These uncertainties will propagate directly into analysis of the annual meridional CO2 flux gradient between the tropics and northern midlatitudes, a key metric for understanding the location, and more importantly the processes, responsible for the annual global carbon sink. The research suggests that variability among transport models remains the largest source of uncertainty across global flux inversion systems and highlights the importance both of using model ensembles and of using independent constraints to evaluate simulated transport.
Key Points
There are systematic differences in transport between two commonly used chemical transport models, TM5 and GEOS‐Chem
These systematic differences lead to significant and meaningful posterior flux uncertainties in atmospheric CO2 flux inversions
The Orbiting Carbon Observatory-2 has been on orbit since 2014, and its global coverage holds the potential to reveal new information about the carbon cycle through the use of top-down atmospheric ...inversion methods combined with column average CO2 retrievals. We employ a large ensemble of atmospheric inversions utilizing different transport models, data assimilation techniques, and prior flux distributions in order to quantify the satellite-informed fluxes from OCO-2 Version 7r land observations and their uncertainties at continental scales. Additionally, we use in situ measurements to provide a baseline against which to compare the satellite-constrained results. We find that within the ensemble spread, in situ observations, and satellite retrievals constrain a similar global total carbon sink of 3.7±0.5 PgC yr−1, and 1.5±0.6 PgC yr−1 for global land, for the 2015–2016 annual mean. This agreement breaks down in smaller regions, and we discuss the differences between the experiments. Of particular interest is the difference between the different assimilation constraints in the tropics, with the largest differences occurring in tropical Africa, which could be an indication of the global perturbation from the 2015–2016 El Niño. Evaluation of posterior concentrations using TCCON and aircraft observations gives some limited insight into the quality of the different assimilation constraints, but the lack of such data in the tropics inhibits our ability to make strong conclusions there.
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.
Wetlands are the largest individual source of methane (CH₄), but the magnitude and distribution of this source are poorly understood on continental scales. We isolated the wetland and rice paddy ...contributions to spaceborne CH₄ measurements over 2003-2005 using satellite observations of gravity anomalies, a proxy for water-table depth Γ, and surface temperature analyses TS. We find that tropical and higher-latitude CH₄ variations are largely described by Γ and TS variations, respectively. Our work suggests that tropical wetlands contribute 52 to 58% of global emissions, with the remainder coming from the extra-tropics, 2% of which is from Arctic latitudes. We estimate a 7% rise in wetland CH₄ emissions over 2003-2007, due to warming of mid-latitude and Arctic wetland regions, which we find is consistent with recent changes in atmospheric CH₄.
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