The carbon budget of South Asia

Abstract

The source and sinks of carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) due to anthropogenic and natural biospheric activities were estimated for the South Asia region (Bangladesh, Bhutan, India, Nepal, Pakistan and Sri Lanka). Flux estimates were based on top-down methods that use inversions of atmospheric data, and bottom-up methods that use field observations, satellite data, and terrestrial ecosystem models. Based on atmospheric CO<sub>2</sub> inversions, the net biospheric CO<sub>2</sub> flux in South Asia (equivalent to the Net Biome Productivity, NBP) was a sink, estimated at −104 ± 150 Tg C yr<sup>−1</sup> during 2007–2008. Based on the bottom-up approach, the net biospheric CO<sub>2</sub> flux is estimated to be −191 ± 193 Tg C yr<sup>−1</sup> during the period of 2000–2009. This last net flux results from the following flux components: (1) the Net Ecosystem Productivity, NEP (net primary production minus heterotrophic respiration) of −220 ± 186 Tg C yr<sup>−1</sup> (2) the annual net carbon flux from land-use change of −14 ± 50 Tg C yr<sup>−1</sup>, which resulted from a sink of −16 Tg C yr<sup>−1</sup> due to the establishment of tree plantations and wood harvest, and a source of 2 Tg C yr<sup>−1</sup> due to the expansion of croplands; (3) the riverine export flux from terrestrial ecosystems to the coastal oceans of +42.9 Tg C yr<sup>−1</sup>; and (4) the net CO<sub>2</sub> emission due to biomass burning of +44.1 ± 13.7 Tg C yr<sup>−1</sup>. Including the emissions from the combustion of fossil fuels of 444 Tg C yr<sup>−1</sup> for the decades of 2000s, we estimate a net CO<sub>2</sub> land-to-atmosphere flux of 297 Tg C yr<sup>−1</sup>. In addition to CO<sub>2</sub>, a fraction of the sequestered carbon in terrestrial ecosystems is released to the atmosphere as CH<sub>4</sub>. Based on bottom-up and top-down estimates, and chemistry-transport modeling, we estimate that 37 ± 3.7 Tg C-CH<sub>4</sub> yr<sup>−1</sup> were released to atmosphere from South Asia during the 2000s. Taking all CO<sub>2</sub> and CH<sub>4</sub> fluxes together, our best estimate of the net land-to-atmosphere CO<sub>2</sub>-equivalent flux is a net source of 334 Tg C yr<sup>−1</sup> for the South Asia region during the 2000s. If CH<sub>4</sub> emissions are weighted by radiative forcing of molecular CH<sub>4</sub>, the total CO<sub>2</sub>-equivalent flux increases to 1148 Tg C yr<sup>−1</sup> suggesting there is great potential of reducing CH<sub>4</sub> emissions for stabilizing greenhouse g...

New data‐driven estimation of terrestrial CO2 fluxes in Asia using a standardized database of eddy covariance measurements, remote sensing data, and support vector regression

“…SVR‐NEE is expressed by the differences between GPP and RE. On the other hand, net CO 2 fluxes estimated by GOSAT L4A (top‐down estimation) include various processes between atmosphere and land, such as land use change, biomass burning, and crop harvesting [e.g., Patra et al ., ; Jung et al ., ]. Therefore, using independent available estimations of each individual land‐atmosphere CO 2 flux component, the differences between GOSAT L4A and SVR‐NEE were examined by using following equations based on Jung et al .…”

“…Terrestrial ecosystems in Asia are heavily influenced by monsoon climate and anthropogenic activities such as land use change [e.g., Piao et al ., ; Patra et al ., ; Ito et al ., ]. In recent decades, significant changes in climate have been observed in Asia, particularly in Siberia (e.g., 1.0–2.0°C increases in temperature from 1961 to 2004 [ Chapin et al ., ]), and potential changes in terrestrial CO 2 fluxes in these regions are expected to occur in the near and distant future [ Ito et al ., ].…”

“…In recent years, multiple terrestrial ecosystem models have been applied to provide robust estimates of CO 2 fluxes and budgets [ Ichii et al ., ; Piao et al ., ]. For example, the Regional Carbon Cycle Assessment and Processes project estimated the terrestrial carbon budget of East Asia [ Piao et al ., ], South Asia [ Patra et al ., ], and Russia [ Dolman et al ., ] by using two different approaches for carbon budget estimation, namely, bottom‐up estimation and top‐down estimation methods. These studies yielded important information about different CO 2 budget estimation methods.…”

Comparing simulated atmospheric carbon dioxide concentration with GOSAT retrievals

“…This uncertainty in our estimation can be partly explained by the lack of the available observation sites in the tropical and subtropical areas of the NH. A greater amount of human activities and local carbon emissions over this region also contribute to this uncertainty, further increasing the bias between the simulated and observed values [5,[52][53][54]. In addition, a large standard deviation (*2.0 ppm) to the simulated-minus-observed difference was found in the Northern middle-to high latitude zones (50°-60°N), mainly due to solar zenith angle restrictions and cloud contamination, which cause large errors in GOSAT XCO 2 [20,34].…”

“…The atmospheric inversion method provides an effective way to quantify the biosphere CO 2 concentration/ flux at the full-coverage spatial resolutions [3][4][5][6][7]. This approach derives the CO 2 concentrations and fluxes jointly from an atmospheric transport model and data assimilation technique.…”