Abstract
The powerful El Niño event of 2015–2016 – the third most intense since the 1950s – has exerted a large impact on the Earth’s natural climate system. The column-averaged CO2 dry-air mole fraction (XCO2) observations from satellites and ground-based networks are analyzed together with in situ observations for the period of September 2014 to October 2016. From the differences between satellite (OCO-2) observations and simulations using an atmospheric chemistry-transport model, we estimate that, relative to the mean annual fluxes for 2014, the most recent El Niño has contributed to an excess CO2 emission from the Earth’s surface (land + ocean) to the atmosphere in the range of 2.4 ± 0.2 PgC (1 Pg = 1015 g) over the period of July 2015 to June 2016. The excess CO2 flux is resulted primarily from reduction in vegetation uptake due to drought, and to a lesser degree from increased biomass burning. It is about the half of the CO2 flux anomaly (range: 4.4–6.7 PgC) estimated for the 1997/1998 El Niño. The annual total sink is estimated to be 3.9 ± 0.2 PgC for the assumed fossil fuel emission of 10.1 PgC. The major uncertainty in attribution arise from error in anthropogenic emission trends, satellite data and atmospheric transport.
Highlights
It is about the half of the CO2 flux anomaly estimated for the 1997/1998 El Niño
Details on observational data selection, atmospheric chemistry-transport model (ACTM) simulations and their processing are given in the Methods section
The OCO-2 minus ACTM results are shown for three combinations of terrestrial and oceanic CO2 fluxes, namely, CYC64 (Fig. 1b), IAV84 (Fig. 1c) and IAV84 + Global Fire Assimilation System (GFAS) (Fig. 1d)
Summary
It is about the half of the CO2 flux anomaly (range: 4.4–6.7 PgC) estimated for the 1997/1998 El Niño. Uncertainties in estimates of regional sources (+ve flux) and sinks (−ve flux) of CO2 and other greenhouse gases, derived from direct inventory methods or inferred from atmospheric observations, have hindered the development of effective policy for reduction of emissions from anthropogenic activity[1]. The other factor that affects estimates of CO2 fluxes from XCO2 measurements is the biases in the inverse methods using chemistry-transport models (CTMs). The role of such bias has been illustrated using the XCO2 observations from the first dedicated Greenhouse Gases Observing Satellite “IBUKI” (GOSAT), which was launched on 23 January 2009 by the Japan Aerospace Exploration Agency (JAXA)[8]. The modeling components include a priori flux and uncertainty assumptions, screening and treatment of observational data, and uncertainties in transport models[4]
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