[Paper Review] Low latency carbon budget analysis reveals a large decline of the land carbon sink in 2023

Piyu Ke, Philippe Ciais+14 more|arXiv (Cornell University)|Jul 17, 2024

Atmospheric and Environmental Gas Dynamics4 citations

TL;DR

This study reveals a sharp decline in the global land carbon sink in 2023, with a net uptake of only 0.44 ± 0.21 GtC yr⁻¹—the weakest since 2003—driven primarily by extreme drought and fires in the Amazon, Canada, and Southeast Asia, despite only a 0.6% increase in fossil fuel emissions. Using a low-latency carbon budget framework integrating satellite data, atmospheric inversions (OCO-2), and dynamic vegetation models, the analysis identifies record heat stress and El Niño conditions as key drivers of ecosystem carbon loss.

ABSTRACT

In 2023, the CO2 growth rate was 3.37 +/- 0.11 ppm at Mauna Loa, 86% above the previous year, and hitting a record high since observations began in 1958, while global fossil fuel CO2 emissions only increased by 0.6 +/- 0.5%. This implies an unprecedented weakening of land and ocean sinks, and raises the question of where and why this reduction happened. Here we show a global net land CO2 sink of 0.44 +/- 0.21 GtC yr-1, the weakest since 2003. We used dynamic global vegetation models, satellites fire emissions, an atmospheric inversion based on OCO-2 measurements, and emulators of ocean biogeochemical and data driven models to deliver a fast-track carbon budget in 2023. Those models ensured consistency with previous carbon budgets. Regional flux anomalies from 2015-2022 are consistent between top-down and bottom-up approaches, with the largest abnormal carbon loss in the Amazon during the drought in the second half of 2023 (0.31 +/- 0.19 GtC yr-1), extreme fire emissions of 0.58 +/- 0.10 GtC yr-1 in Canada and a loss in South-East Asia (0.13 +/- 0.12 GtC yr-1). Since 2015, land CO2 uptake north of 20 degree N declined by half to 1.13 +/- 0.24 GtC yr-1 in 2023. Meanwhile, the tropics recovered from the 2015-16 El Nino carbon loss, gained carbon during the La Nina years (2020-2023), then switched to a carbon loss during the 2023 El Nino (0.56 +/- 0.23 GtC yr-1). The ocean sink was stronger than normal in the equatorial eastern Pacific due to reduced upwelling from La Nina's retreat in early 2023 and the development of El Nino later. Land regions exposed to extreme heat in 2023 contributed a gross carbon loss of 1.73 GtC yr-1, indicating that record warming in 2023 had a strong negative impact on the capacity of terrestrial ecosystems to mitigate climate change.

Motivation & Objective

• To rapidly assess the 2023 global carbon budget using low-latency methods to identify the cause of the record CO2 growth rate (3.37 ppm yr⁻¹) despite minimal fossil fuel emission increases.
• To determine the drivers behind the unprecedented weakening of the land carbon sink, particularly in tropical and high-latitude regions.
• To reconcile top-down (atmospheric inversion) and bottom-up (vegetation models, fire emissions) estimates of carbon flux anomalies across regions.
• To quantify the impact of extreme climate events—especially drought, heatwaves, and wildfires—on terrestrial carbon uptake in 2023.
• To assess the role of climate modes such as El Niño and La Niña in modulating land and ocean carbon sinks during the 2023 transition.

Proposed method

• Employed a low-latency carbon budget framework integrating multiple data streams and models to deliver rapid, consistent carbon flux estimates.
• Combined OCO-2 satellite atmospheric CO2 measurements with an atmospheric inversion system to derive top-down regional carbon fluxes.
• Used dynamic global vegetation models (DGVMs) and satellite-based fire emissions data to estimate bottom-up land carbon fluxes.
• Applied emulators of ocean biogeochemical models and data-driven ocean flux estimates to close the global carbon budget.
• Validated regional flux anomalies (2015–2022) against historical consistency between top-down and bottom-up approaches.
• Quantified contributions of extreme heat events to gross carbon loss using temperature anomaly data linked to ecosystem productivity.

Experimental results

Research questions

• RQ1 What caused the record 2023 CO2 growth rate of 3.37 ppm yr⁻¹ despite only a 0.6% increase in fossil fuel emissions?
• RQ2 How did the land carbon sink in 2023 compare to historical trends, and what were the regional drivers of its decline?
• RQ3 To what extent did extreme drought, wildfires, and heat stress contribute to the weakening of terrestrial carbon uptake?
• RQ4 How did the transition from La Niña to El Niño in 2023 affect carbon fluxes in tropical and high-latitude regions?
• RQ5 How consistent are top-down (atmospheric inversion) and bottom-up (vegetation models, fire emissions) estimates of carbon flux anomalies in 2023?

Key findings

• The global land carbon sink in 2023 was 0.44 ± 0.21 GtC yr⁻¹—the weakest since 2003—indicating a major decline in terrestrial carbon sequestration capacity.
• The Amazon region experienced a carbon loss of 0.31 ± 0.19 GtC yr⁻¹ due to severe drought in the second half of 2023, contributing most to the global sink decline.
• Extreme fire emissions in Canada reached 0.58 ± 0.10 GtC yr⁻¹, significantly weakening the land sink in boreal regions.
• Southeast Asia contributed a carbon loss of 0.13 ± 0.12 GtC yr⁻¹, linked to drought and peatland combustion.
• The land sink north of 20°N declined by half to 1.13 ± 0.24 GtC yr⁻¹ in 2023, reflecting reduced sequestration in high-latitude ecosystems.
• Extreme heat in 2023 caused a gross carbon loss of 1.73 GtC yr⁻¹ from heat-stressed land regions, highlighting the vulnerability of ecosystems to climate extremes.