How Simulations of the Land Carbon Sink Are Biased by Ignoring Fluvial Carbon Transfers: A Case Study for the Amazon Basin

Philippe Ciais,Pierre Regnier,Pierre Friedlingstein,Bertrand Guenet,Ronny Lauerwald

doi:10.1016/j.oneear.2020.07.009

Abstract

Summary Land-surface models are important tools for simulation of the past, present, and future capacity of terrestrial ecosystems to absorb anthropogenic CO2 emissions. However, fluvial carbon (C) transfers are presently neglected in these models. Using the Amazon basin as a case study, we show that this negligence leads to significant underestimation of the net uptake of atmospheric C while terrestrial C storage changes are overestimated. These biases arise from the fact that C—in reality, leached from soils and exported through the river network—is instead represented as partly being respired and partly being stored in soils. Moreover, these biases scale mainly to the fluvial C export to the coast, despite aquatic CO2 emission to the atmosphere being the major pathway of riverine C exports. We further show that fluvial C transfers may change significantly in response to changes in either hydrology or in atmospheric C uptake by vegetation.

Highlights

The inland water network plays an important role in the global carbon (C) cycle, as a major transfer route of land-derived C to the ocean as well as an efficient biogeochemical reactor where large amounts of terrestrial organic C inputs are processed, feeding a net CO2 evasion that is larger than the exports of C to the coast.[1]
Present-Day (1981–2010) C Budget of the Amazon Basin The suitability of a land-surface model to predict changes in net ecosystem exchange (NEE) and net biome production (NBP) at the centennial timescale depends on its ability to reproduce present-day terrestrial C stocks
According to our simulation results, the C stored in the terrestrial biomass of the Amazon basin amounts to 91 Pg C, of which 87 Pg C is attributed to tropical rain forest

Summary

Introduction

The inland water network plays an important role in the global carbon (C) cycle, as a major transfer route of land-derived C to the ocean as well as an efficient biogeochemical reactor where large amounts of terrestrial organic C inputs are processed, feeding a net CO2 evasion that is larger than the exports of C to the coast.[1] In quantitative terms, both the total inputs of C from terrestrial ecosystem into inland waters and the CO2 evasion from inland waters remain largely uncertain at global scale, as demonstrated by different estimates published over the last decade (see review by Drake et al.[2]).

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