Net carbon emissions from African biosphere dominate pan-tropical atmospheric CO2 signal

Paul I Palmer,Liang Feng,Peter Somkuti,David Baker,Hartmut Bösch,Frédéric Chevallier

doi:10.1038/s41467-019-11097-w

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 (CO2), interpreted using independent models, are consistent with the land tropics being a net annual carbon emission of ({mathrm{median}}_{{mathrm{minimum}}}^{{mathrm{maximum}}})1.03_{ - 0.20}^{ + 1.73} and 1.60_{ + 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} PgC in 2015 and 1.65_{ + 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 CO2 record from 2009 onwards.

Highlights

Tropical ecosystems are large carbon stores that are vulnerable to climate change
We find that using column observations of XCO2 from gases Observing SATellite11 (GOSAT) and Orbiting Carbon Observatory12 (OCO-2) results in more consistent a posteriori CO2 flux estimates over the tropics (Fig. 1b–f), with a smaller inter-model spread of estimates[26], and a better agreement on the phase of the seasonal cycle than using only in situ observations of CO2 (Fig. 1a–d)
The amplitude of the seasonal cycle of a posteriori CO2 fluxes over the northern and southern tropical lands inferred by the satellite data is generally much larger than that inferred from the in situ data (Fig. 1b–e), with the exception of LSCE that is driven by a priori fluxes from the ORCHIDEE model (“Methods”)

Summary

Introduction

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. Our knowledge of the tropical carbon budget has improved significantly over the past few decades mainly due to networks of sample plot measurements[4], micro-meteorological measurements of carbon fluxes of forest ecosystems[5], remote sensing of vegetation state or of land use change[6], and sparselydistributed ground-based mole fraction measurements[7,8] of atmospheric CO2. We report our results over land as net biosphere fluxes (“Methods”), representing the net carbon flux exchange with the atmosphere from above-ground biomass and soils across subcontinental regions To interpret these CO2 fluxes in terms of the underlying land surface processes, we use correlative satellite data products (Methods): vegetation indices that provide information about leaf phenology[13]; changes in water storage[14]; a measure of photosynthesis[15]; and formaldehyde columns that provide information about the location and timing of fires[16]. We use dry matter (DM) burned estimates[17] inferred from remotely sensed land surface properties, and analysed meteorological fields of surface temperature and precipitation from the GEOS-5

Methods

Results

Conclusion