土壌が予想以上に炭素を放出し、気候変動モデルに影響を与えている可能性(Soil May Release More Carbon than Expected, Affecting Climate Change Models)

2024-03-07

2024-03-06 タフツ大学

気候モデルの正確性は多くの要因に依存するが、土壌の変化がその中でも重要だとする研究がある。土壌は地球上の80％の炭素を保持し、乾燥と土壌の崩壊によりさらに二酸化炭素や他の温室効果ガスが大気中に放出される可能性がある。これが気候変動を加速させる可能性があり、これを考慮に入れないとモデルの予測が不正確になるおそれがある。

＜関連情報＞

干ばつ、土壌乾燥割れ、温室効果ガス排出の間の増幅するフィードバックループ
Amplifying feedback loop between drought, soil desiccation cracking, and greenhouse gas emissions

Farshid Vahedifard, C Clay Goodman, Varun Paul and Amir AghaKouchak
Environmental Research Letters Published:5 March 2024
DOI:10.1088/1748-9326/ad2c23

The continuous escalation of carbon dioxide (CO₂) emissions into the atmosphere is recognized as the primary catalyst for anthropogenic climate change. In 2021, CO₂ emerged as the predominant contributor to the warming effect of all human-made greenhouse gases (GHGs), accounting for two-thirds of their global heating impact [1]. While the primary anthropogenic source of increased atmospheric CO₂ concentration is the combustion of fossil fuels, the largest terrestrial source of CO₂ emissions is soil [2] where 80% of the total terrestrial carbon is stored. Approximately 62% of soil carbon is in organic form and readily released as CO₂, while the remaining is made up of inorganic carbon (soil inorganic carbon (SIC)) [3]. Here, we postulate that there is an amplifying feedback loop between drought, soil desiccation cracking, and CO₂ emission in a warming climate (figure 1)—a critical aspect that has been overlooked in the existing literature. Further, we argue that the postulated feedback loop affects the emissions of other GHGs, such as methane (CH₄) and nitrous oxide (N₂O), from soils. The urgent need to recognize and characterize this exacerbating feedback loop is twofold. Firstly, it is widely acknowledged that drought accelerates the oxidation of soil organic carbon (SOC) and, thus, increases CO₂ emissions into the atmosphere. Drought-induced soil moisture deficits differentially affect plant processes; while photosynthesis rates may be reduced in plants, leading to decreased carbon uptake, respiration rates can vary. Initially, drought may cause a slight increase in respiration, despite a decline in photosynthesis, leading to increased carbon emissions from the soil. These effects can differ based on ecosystem types, highlighting the complex interplay between drought, photosynthesis, and respiration. Secondly, drought triggers soil desiccation cracking, substantially increasing the permeability of the soil and the interfacial exchange area between the atmosphere and the soil, which, in turn, can considerably increase CO₂ efflux in soil by exposing deeper and older stores of soil carbon. Desiccation cracking threatens earthen infrastructure systems and the natural environment. The problems associated with desiccation cracks are becoming more prevalent as anthropogenic climate change exacerbates the severity and frequency of droughts, heatwaves, and drought-heavy precipitation cycles [4]. As the warming trends continue, more (and possibly older) CO₂ is released from the soil, which can further contribute to global warming. Thus, a chain of events happens in a cascading manner. Failure to consider the hypothesized feedback loop can result in significant inaccuracies when modeling and predicting GHG emissions from soil. It may also lead to underestimating the overall impact of climate change on critical aspects such as soil health, crop production, and the structural integrity of earthen infrastructure.