Abstract
The outstanding tropical land climate characteristic over the past decades is rapid warming, with no significant large-scale precipitation trends. This warming is expected to continue but the effects on tropical vegetation are unknown. El Niño-related heat peaks may provide a test bed for a future hotter world. Here we analyse tropical land carbon cycle responses to the 2015/16 El Niño heat and drought anomalies using an atmospheric transport inversion. Based on the global atmospheric CO2 and fossil fuel emission records, we find no obvious signs of anomalously large carbon release compared with earlier El Niño events, suggesting resilience of tropical vegetation. We find roughly equal net carbon release anomalies from Amazonia and tropical Africa, approximately 0.5 PgC each, and smaller carbon release anomalies from tropical East Asia and southern Africa. Atmospheric CO anomalies reveal substantial fire carbon release from tropical East Asia peaking in October 2015 while fires contribute only a minor amount to the Amazonian carbon flux anomaly. Anomalously large Amazonian carbon flux release is consistent with downregulation of primary productivity during peak negative near-surface water anomaly (October 2015 to March 2016) as diagnosed by solar-induced fluorescence. Finally, we find an unexpected anomalous positive flux to the atmosphere from tropical Africa early in 2016, coincident with substantial CO release.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’.
Highlights
Tropical forests play a vital role in the Earth system, hosting greater than 50% of global terrestrial biodiversity (e.g. [1]), storing two-thirds of global plant biomass (e.g. [2]) and regulating climate by virtue of their exchanges of carbon, water and energy with the atmosphere
We aim to address the following questions: How anomalous is the global CO2 flux anomaly? What are the climate deviations/excesses on land? Where and when do flux anomalies occur and how large are they? How much is due to fire and under what conditions? How much is due to reduction in primary production versus changes in respiration? And are there signs of land vegetation responses outside the usual El Nino patterns given the unprecedented temperatures during the 2015/16 event?
For our prior estimates of air – sea fluxes, we treat separately the fluxes associated with the pre-industrial carbon cycle and uptake of carbon induced by the anthropogenic perturbation of atmospheric CO2 [24,25]
Summary
Tropical forests play a vital role in the Earth system, hosting greater than 50% of global terrestrial biodiversity (e.g. [1]), storing two-thirds of global plant biomass (e.g. [2]) and regulating climate by virtue of their exchanges of carbon, water and energy with the atmosphere. For our prior estimates of air – sea fluxes, we treat separately the fluxes associated with the pre-industrial carbon cycle (two hemispherical loops with CO2 outgassing in the tropics and CO2 uptake at high latitudes) and uptake of carbon induced by the anthropogenic perturbation of atmospheric CO2 (with fluxes steadily increasing and located primarily in the northern Atlantic and Southern Ocean) [24,25] For the former, we use the monthly resolved climatology based on air –sea partial pressure differences and an air– sea gas exchange coefficient parameterization, compiled by Takahashi et al [26], to which we add a constant and spatially uniform flux such that the fluxes are globally in balance on an annual basis. For the comparison of anomalies for specific areas and three-monthly periods, we calculated region mean anomalies including only those pixels for which retrievals exist
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