Near-real-time estimation of fossil fuel CO2 emissions from China based on atmospheric observations on Hateruma and Yonaguni Islands, Japan

We developed a near-real-time estimation method for temporal changes in fossil fuel CO₂ (FFCO₂) emissions from China for 3 months [January, February, March (JFM)] based on atmospheric CO₂ and CH₄ observations on Hateruma Island (HAT, 24.06° N, 123.81° E) and Yonaguni Island (YON, 24.47° N, 123.01° E), Japan. These two remote islands are in the downwind region of continental East Asia during winter because of the East Asian monsoon. Previous studies have revealed that monthly averages of synoptic-scale variability ratios of atmospheric CO₂ and CH₄ (ΔCO₂/ΔCH₄) observed at HAT and YON in JFM are sensitive to changes in continental emissions. From the analysis based on an atmospheric transport model with all components of CO₂ and CH₄ fluxes, we found that the ΔCO₂/ΔCH₄ ratio was linearly related to the FFCO₂/CH₄ emission ratio in China because calculating the variability ratio canceled out the transport influences. Using the simulated linear relationship, we converted the observed ΔCO₂/ΔCH₄ ratios into FFCO₂/CH₄ emission ratios in China. The change rates of the emission ratios for 2020-2022 were calculated relative to those for the preceding 9-year period (2011-2019), during which relatively stable ΔCO₂/ΔCH₄ ratios were observed. These changes in the emission ratios can be read as FFCO₂ emission changes under the assumption of no interannual variations in CH₄ emissions and biospheric CO₂ fluxes for JFM. The resulting average changes in the FFCO₂ emissions in January, February, and March 2020 were 17 ± 8%, - 36 ± 7%, and - 12 ± 8%, respectively, (- 10 ± 9% for JFM overall) relative to 2011-2019. These results were generally consistent with previous estimates. The emission changes for January, February, and March were 18 ± 8%, - 2 ± 10%, and 29 ± 12%, respectively, in 2021 (15 ± 10% for JFM overall) and 20 ± 9%, - 3 ± 10%, and - 10 ± 9%, respectively, in 2022 (2 ± 9% for JFM overall). These results suggest that the FFCO₂ emissions from China rebounded to the normal level or set a new high record in early 2021 after a reduction during the COVID-19 lockdown. In addition, the estimated reduction in March 2022 might be attributed to the influence of a new wave of COVID-19 infections in Shanghai.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s40645-023-00542-6.

Detection of fossil-fuel CO2 plummet in China due to COVID-19 by observation at Hateruma

The COVID-19 pandemic caused drastic reductions in carbon dioxide (CO₂) emissions, but due to its large atmospheric reservoir and long lifetime, no detectable signal has been observed in the atmospheric CO₂ growth rate. Using the variabilities in CO₂ (ΔCO₂) and methane (ΔCH₄) observed at Hateruma Island, Japan during 1997–2020, we show a traceable CO₂ emission reduction in China during February–March 2020. The monitoring station at Hateruma Island observes the outflow of Chinese emissions during winter and spring. A systematic increase in the ΔCO₂/ΔCH₄ ratio, governed by synoptic wind variability, well corroborated the increase in China’s fossil-fuel CO₂ (FFCO₂) emissions during 1997–2019. However, the ΔCO₂/ΔCH₄ ratios showed significant decreases of 29 ± 11 and 16 ± 11 mol mol⁻¹ in February and March 2020, respectively, relative to the 2011–2019 average of 131 ± 11 mol mol⁻¹. By projecting these observed ΔCO₂/ΔCH₄ ratios on transport model simulations, we estimated reductions of 32 ± 12% and 19 ± 15% in the FFCO₂ emissions in China for February and March 2020, respectively, compared to the expected emissions. Our data are consistent with the abrupt decrease in the economic activity in February, a slight recovery in March, and return to normal in April, which was calculated based on the COVID-19 lockdowns and mobility restriction datasets.

A Global Synthesis Inversion Analysis of Recent Variability in CO2 Fluxes Using GOSAT and In Situ Observations

The precise contribution of the two major sinks for anthropogenic CO₂ emissions, terrestrial vegetation and the ocean, and their location and year-to-year variability are not well understood. Top-down estimates of the spatiotemporal variations in emissions and uptake of CO₂ are expected to benefit from the increasing measurement density brought by recent in situ and remote CO₂ observations. We uniquely apply a batch Bayesian synthesis inversion at relatively high resolution to in situ surface observations and bias-corrected GOSAT satellite column CO₂ retrievals to deduce the global distributions of natural CO₂ fluxes during 2009-2010. Our objectives include evaluating bottom-up prior flux estimates, assessing the value added by the satellite data, and examining the impacts of inversion technique and assumptions on posterior fluxes and uncertainties. The GOSAT inversion is generally better constrained than the in situ inversion, with smaller posterior regional flux uncertainties and correlations, because of greater spatial coverage, except over North America and high-latitude ocean. Complementarity of the in situ and GOSAT data enhances uncertainty reductions in a joint inversion; however, spatial and temporal gaps in sampling still limit the ability to accurately resolve fluxes down to the sub-continental scale. The GOSAT inversion produces a shift in the global CO₂ sink from the tropics to the north and south relative to the prior, and an increased source in the tropics of ~2 Pg C y^-1 relative to the in situ inversion, similar to what is seen in studies using other inversion approaches. This result may be driven by sampling and residual retrieval biases in the GOSAT data, as suggested by significant discrepancies between posterior CO₂ distributions and surface in situ and HIPPO mission aircraft data. While the shift in the global sink appears to be a robust feature of the inversions, the partitioning of the sink between land and ocean in the inversions using either in situ or GOSAT data is found to be sensitive to prior uncertainties because of negative correlations in the flux errors. The GOSAT inversion indicates significantly less CO₂ uptake in summer of 2010 than in 2009 across northern regions, consistent with the impact of observed severe heat waves and drought. However, observations from an in situ network in Siberia imply that the GOSAT inversion exaggerates the 2010-2009 difference in uptake in that region, while the prior CASA-GFED model of net ecosystem production and fire emissions reasonably estimates that quantity. The prior, in situ posterior, and GOSAT posterior all indicate greater uptake over North America in spring to early summer of 2010 than in 2009, consistent with wetter conditions. The GOSAT inversion does not show the expected impact on fluxes of a 2010 drought in the Amazon; evaluation of posterior mole fractions against local aircraft profiles suggests that time-varying GOSAT coverage can bias estimation of flux interannual variability in this region.

Land use change and El Niño-Southern Oscillation drive decadal carbon balance shifts in Southeast Asia

An integrated understanding of the biogeochemical consequences of climate extremes and land use changes is needed to constrain land-surface feedbacks to atmospheric CO₂ from associated climate change. Past assessments of the global carbon balance have shown particularly high uncertainty in Southeast Asia. Here, we use a combination of model ensembles to show that intensified land use change made Southeast Asia a strong source of CO₂ from the 1980s to 1990s, whereas the region was close to carbon neutral in the 2000s due to an enhanced CO₂ fertilization effect and absence of moderate-to-strong El Niño events. Our findings suggest that despite ongoing deforestation, CO₂ emissions were substantially decreased during the 2000s, largely owing to milder climate that restores photosynthetic capacity and suppresses peat and deforestation fire emissions. The occurrence of strong El Niño events after 2009 suggests that the region has returned to conditions of increased vulnerability of carbon stocks.

The carbon balance in Southeast Asia is highly uncertain. Here, the authors show that land use changes and occurrence of strong El Niño control decadal shifts in the carbon balance of this region.

Long-term carbon sink in Borneo's forests halted by drought and vulnerable to edge effects

Less than half of anthropogenic carbon dioxide emissions remain in the atmosphere. While carbon balance models imply large carbon uptake in tropical forests, direct on-the-ground observations are still lacking in Southeast Asia. Here, using long-term plot monitoring records of up to half a century, we find that intact forests in Borneo gained 0.43 Mg C ha⁻¹ per year (95% CI 0.14–0.72, mean period 1988–2010) in above-ground live biomass carbon. These results closely match those from African and Amazonian plot networks, suggesting that the world’s remaining intact tropical forests are now en masse out-of-equilibrium. Although both pan-tropical and long-term, the sink in remaining intact forests appears vulnerable to climate and land use changes. Across Borneo the 1997–1998 El Niño drought temporarily halted the carbon sink by increasing tree mortality, while fragmentation persistently offset the sink and turned many edge-affected forests into a carbon source to the atmosphere.

The existence of a pan-tropical forest carbon sink remains uncertain due to the lack of data from Asia. Here, using direct on-the-ground observations, the authors confirm remaining intact forests in Borneo have provided a long-term carbon sink, but carbon net gains are vulnerable to drought and edge effects.

A decadal inversion of CO2 using the Global Eulerian-Lagrangian Coupled Atmospheric model (GELCA): sensitivity to the ground-based observation network

We present an assimilation system for atmospheric carbon dioxide (CO₂) using a Global Eulerian-Lagrangian Coupled Atmospheric model (GELCA), and demonstrate its capability to capture the observed atmospheric CO₂ mixing ratios and to estimate CO₂ fluxes. With the efficient data handling scheme in GELCA, our system assimilates non-smoothed CO₂ data from observational data products such as the Observation Package (ObsPack) data products as constraints on surface fluxes. We conducted sensitivity tests to examine the impact of the site selections and the prior uncertainty settings of observation on the inversion results. For these sensitivity tests, we made five different site/data selections from the ObsPack product. In all cases, the time series of the global net CO₂ flux to the atmosphere stayed close to values calculated from the growth rate of the observed global mean atmospheric CO₂ mixing ratio. At regional scales, estimated seasonal CO₂ fluxes were altered, depending on the CO₂ data selected for assimilation. Uncertainty reductions (URs) were determined at the regional scale and compared among cases. As measures of the model-data mismatch, we used the model-data bias, root-mean-square error, and the linear correlation. For most observation sites, the model-data mismatch was reasonably small. Regarding regional flux estimates, tropical Asia was one of the regions that showed a significant impact from the observation network settings. We found that the surface fluxes in tropical Asia were the most sensitive to the use of aircraft measurements over the Pacific, and the seasonal cycle agreed better with the results of bottom-up studies when the aircraft measurements were assimilated. These results confirm the importance of these aircraft observations, especially for constraining surface fluxes in the tropics.

Top-down assessment of the Asian carbon budget since the mid 1990s

Increasing atmospheric carbon dioxide (CO₂) is the principal driver of anthropogenic climate change. Asia is an important region for the global carbon budget, with 4 of the world's 10 largest national emitters of CO₂. Using an ensemble of seven atmospheric inverse systems, we estimated land biosphere fluxes (natural, land-use change and fires) based on atmospheric observations of CO₂ concentration. The Asian land biosphere was a net sink of −0.46 (−0.70–0.24) PgC per year (median and range) for 1996–2012 and was mostly located in East Asia, while in South and Southeast Asia the land biosphere was close to carbon neutral. In East Asia, the annual CO₂ sink increased between 1996–2001 and 2008–2012 by 0.56 (0.30–0.81) PgC, accounting for ∼35% of the increase in the global land biosphere sink. Uncertainty in the fossil fuel emissions contributes significantly (32%) to the uncertainty in land biosphere sink change.

Land biosphere uptake of carbon is important in mitigating the anthropogenic increase in atmospheric CO₂ and its climate forcing. Here, the authors show that land biosphere uptake of carbon in Asia has increased substantially since the mid 1990s, likely owing to reforestation and regional climate change.

Canopy-level stomatal narrowing in adult Fagus sylvatica under O3 stress - means of preventing enhanced O3 uptake under high O3 exposure?

Spatio-temporally consistent O(3) doses are demonstrated in adult Fagus sylvatica from the Kranzberg Forest free-air fumigation experiment, covering cross-canopy and whole-seasonal scopes through sap flow measurement. Given O(3)-driven closure of stomata, we hypothesized enhanced whole-tree level O(3) influx to be prevented under enhanced O(3) exposure. Although foliage transpiration rate was lowered under twice-ambient O(3) around noon by 30% along with canopy conductance, the hypothesis was falsified, as O(3) influx was raised by 25%. Nevertheless, the twice-ambient/ambient ratio of O(3) uptake was smaller by about 20% than that of O(3) exposure, suggesting stomatal limitation of uptake. The O(3) response was traceable from leaves across branches to the canopy, where peak transpiration rates resembled those of shade rather than sun branches. Rainy/overcast-day and nightly O(3) uptake is quantified and discussed. Whole-seasonal canopy-level validation of modelled with sap flow-derived O(3) flux becomes available in assessing O(3) risk for forest trees.

On understanding the land-ocean CO2 contrast over the Bay of Bengal: a case study during 2009 summer monsoon

Ship-based observations of atmospheric carbon dioxide (CO2) concentration over the Bay of Bengal (BoB) between 17 July 2009 and 17 Aug 2009 offered an excellent opportunity to evaluate the land-ocean contrast of surface CO2 and facilitated its comparison with model simulated CO2 concentrations. Elevated values of CO2 with large variability near the coastal region and relatively low values with correspondingly lower variability over the open ocean suggest that this observed CO2 variability over the ocean essentially captures the differences in terrestrial and oceanic CO2 fluxes. Although the region under investigation is well known for its atmospheric intraseasonal oscillations of Indian summer monsoon during July and August, the limited duration of observations performed from a moving ship in a research cruise, is not able to capture any high-frequency variability of atmospheric CO2 concentrations. But band-passed sea surface temperature and wind anomalies do indicate strong intraseasonal variability over the study region during the observational period. The synoptic data, albeit quite short in duration, thus offer a clear benchmark for abrupt variability of CO2 concentration between land and ocean.