Enhancing land carbon sink by carbon dioxide fertilization


Elevated atmospheric carbon dioxide concentration increases land carbon sink, partially offsetting carbon dioxide emissions from human activities such as fossil fuel, cement production, and land-use change. This process is known as carbon dioxide fertilization, playing a key role in climate change mitigation and sustainable development of human society.
Quantifying the carbon dioxide fertilization on the terrestrial carbon sink is critical for scientific understanding and predicting of climate-carbon cycle interactions. To answer this question, model simulation and field experiment observation are two important ways, but facing great challenges. 
Model results show considerable divergence in carbon dioxide fertilization effect on land carbon sink. Site-scale field carbon dioxide enrichment experiment is not able to directly clarify regional and global effect. New approaches are needed to link the carbon dioxide fertilization effect in site-scale field experiment and large-scale model simulations.
Recently, the international team led by Prof. PIAO Shilong from the Institute of Tibetan Plateau Research of the Chinese Academy of Sciences quantified the sensitivity of the terrestrial carbon dioxide sink to elevated atmospheric carbon dioxide concentration for the past five decades, using the simulations from 12 terrestrial ecosystem models and the observations from seven field carbon dioxide enrichment experiments.  The team's findings were published in Nature Geoscience on 2nd September 2019.
The team applied novel mathematical strategy to improve the comparability between the field carbon dioxide enrichment experiment observation (a short-term response to a stepwise increase of carbon dioxide) and the terrestrial ecosystem model simulation (a progressive response to a gradual rise of carbon dioxide).
They found that the sensitivity of northern temperate carbon sink to rising carbon dioxide is linearly related to the site-scale sensitivity across the models. Based on this emergent relationship and field experiment observations as constraint, the study estimated that the northern temperate carbon dioxide sink increases by 0.64 billion tons of carbon per year for per hundred parts per million increase in carbon dioxide in air.
Extrapolating worldwide, this northern temperate sensitivity projects the global terrestrial dioxide carbon sink to increase by 3.5 billion tons of carbon per year for an increase in carbon dioxide of 100 parts per million. This value suggests that carbon dioxide fertilization alone explains most of the increase in global land carbon sink since the 1960s.
“This study reduces uncertainty in the understanding of the carbon dioxide fertilization effect on the terrestrial carbon sink,” said Prof. PIAO, “The new approach and techniques in this study are also very useful to the scientific community in future research and studies.”
“To explain further mechanisms underlying carbon dioxide fertilization effect, more longer-term field experiments are required, particularly in boreal and tropical ecosystems. Joint effort between experimentalists and modelers also remains necessary,” said first author Dr. LIU Yongwen who works at the Institute of Tibetan Plateau Research, Chinese Academy of Sciences.
This study was supported by the Strategic Priority Research Program (A) of the Chinese Academy of Sciences, the National Key R&D Program of China, the National Natural Science Foundation of China, and the other programs.