Wetland Methane Emissions to Increase by Up to 60% by the End of the Century: Impact on Climate Goals

This article is an automatically translated version of the original Japanese article. Please refer to the Japanese version for the most accurate information.

Source: Nature Geoscience "Emergent constraints on future methane emissions from global wetlands" (Background Information)

Overview

A study using the latest terrestrial ecosystem model ensembles and observation data predicts that global methane (CH4) emissions from wetlands will increase by 50–60% by the 2090s compared to the 2010s due to global warming. This increase acts as a powerful climate feedback and could partially offset national methane reduction efforts. Consequently, it is essential to account for this increase in natural emissions in future mitigation strategies.

Key Points

1. Quantitative Prediction of Emissions Associated with Temperature Rise

It is estimated that for every 1°C increase in terrestrial surface temperature, methane emissions from wetlands (eCH4) increase by 24 ± 10 Tg (teragrams) per year. Under a high-emission scenario (RCP 8.5), emissions are projected to expand significantly by the end of this century, making the "positive feedback" loop—where emissions accelerate warming—even more pronounced.

2. Reducing Uncertainty via Emergent Constraints

Conventional model predictions have had an extremely wide range of uncertainty, spanning from 20% to 250%. This study employed the "Emergent Constraint (*)" method, which uses observation data (FLUXNET-CH4) to constrain the models. This approach reduced the uncertainty of future projections by approximately 41%, enabling more reliable estimations.

(*) "Emergent Constraint" is a methodology that uses relationships between observable quantities and future projections that emerge from model ensembles. While there is no single standard Japanese translation, it can be simply described as an "observation-based constraint method."

3. Impact on the Global Methane Pledge

The natural increase in wetland methane by the 2030s is projected to be equivalent to 8–10% of 2020 levels of anthropogenic methane emissions. This could effectively diminish the impact of the reduction targets committed to by countries under the "Global Methane Pledge," suggesting the need for more ambitious countermeasures.

Background

Methane is a potent Greenhouse Gas (GHG) with over 80 times the warming potential of carbon dioxide (CO2) over a 20-year span (*), and wetlands are its largest natural source (accounting for 20–30% of total emissions). There have been concerns about a vicious cycle where warming intensifies microbial activity in wetlands, leading to further methane release. However, due to complex biophysical processes, the uncertainty of future predictions has been a challenge. Scientific knowledge is required to accurately grasp natural emission fluctuations and reflect them in mitigation measures to achieve the goals of the Paris Agreement.

(*) For more information on Global Warming Potential (GWP), please refer to the following article:

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Details

Predicted Figures and Regional Contributions to Wetland Methane Emissions

Summarizing the prediction results from the model ensemble (FUMEMIP), the contribution from the tropics is particularly significant.

Structured Analysis Results

• Primary Mechanisms for Emission Increase:
  • Temperature Dependency: Microbial methane production is sensitive to temperature; the characteristic where emissions increase several-fold with a 10°C rise ($q_{10}$) is a factor.
  • CO2 Fertilization Effect: Rising atmospheric CO2 concentrations promote plant photosynthesis, increasing the supply of organic matter from roots, which provides more substrate for methane production. This effect is estimated to account for approximately 62% of the total increase.
• Factors Driving Prediction Uncertainty:
  • Inundation Area Fluctuations: Projections of the expansion or contraction of wetland areas (inundated areas) due to future changes in precipitation patterns are the largest source of uncertainty.
  • Observation Data Bias: The current observation network (FLUXNET) is skewed toward mid-to-high latitudes, resulting in a lack of data from the tropics, which is the primary source of emissions.
• New Potential Hotspots:
  • In addition to the traditional Amazon Basin, Congo Basin, and Southeast Asia, the Tibetan Plateau and Southern China are emerging as significant future methane emission sources.

Summary

This study clarifies that the increase in wetland methane due to warming is of a scale that could potentially negate humanity's emission reduction efforts. While uncertainties remain, the improvement in prediction accuracy achieved through the Emergent Constraint method (an observation-based constraint method) in this paper represents an important advancement in climate policy. Moving forward, there is a need to strengthen monitoring in the tropics and build more resilient Net Zero strategies that incorporate these natural feedbacks.

Amidst increasing geopolitical risks, such as in the Strait of Hormuz, we have recently heard voices suggesting that "this is no time to worry about climate change or decarbonization." We believe it is precisely in such circumstances that it is vital to continue updating our understanding with the latest scientific knowledge.