スウォンジーの学生が英国気象庁との共同研究で地表モデルの進化に貢献(Swansea student’s collaboration with Met Office to help advance land surface model)

2023-03-23 スウォンジー大学

＜関連情報＞

• https://www.swansea.ac.uk/press-office/news-events/news/2023/03/swansea-students-collaboration-with-met-office-to-help-advance-land-surface-model.php
• https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JG007041

JULES地表面モデルで予測される炭素同位体識別の時空間的変動 Spatio-Temporal Variations in Carbon Isotope Discrimination Predicted by the JULES Land Surface Model

Lewis Palmer, Iain Robertson, Aliénor Lavergne, Deborah Hemming, Neil J. Loader, Giles Young, Darren Davies, Katja Rinne-Garmston, Sietse Los, Jamie Williams
Journal of Geophysical Research: Biogeosciences  Published: 04 December 2022
DOI:https://doi.org/10.1029/2022JG007041

Abstract

Stable carbon isotopes in plants can help evaluate and improve the representation of carbon and water cycles in land-surface models, increasing confidence in projections of vegetation response to climate change. Here, we evaluated the predictive skills of the Joint UK Land Environmental Simulator (JULES) to capture spatio-temporal variations in carbon isotope discrimination (Δ¹³C) reconstructed by tree rings at 12 sites in the United Kingdom over the period 1979–2016. Modeled and measured Δ¹³C time series were compared at each site and their relationships with local climate investigated. Modeled Δ¹³C time series were significantly correlated (p < 0.05) with tree-ring Δ¹³C at eight sites, but JULES underestimated mean Δ¹³C values at all sites, by up to 2.6‰. Differences in mean Δ¹³C may result from post-photosynthetic isotopic fractionations that were not considered in JULES. Inter-annual variability in Δ¹³C was also underestimated by JULES at all sites. While modeled Δ¹³C typically increased over time across the UK, tree-ring Δ¹³C values increased only at five sites located in the northern regions but decreased at the southern-most sites. Considering all sites together, JULES captured the overall influence of environmental drivers on Δ¹³C but failed to capture the direction of change in Δ¹³C caused by air temperature, atmospheric CO₂ and vapor pressure deficit at some sites. Results indicate that the representation of carbon-water coupling in JULES could be improved to reproduce both the trend and magnitude of interannual variability in isotopic records, the influence of local climate on Δ¹³C, and to reduce uncertainties in predicting vegetation-environment interactions.

Plain Language Summary

Carbon has two stable isotopes, ¹²C and ¹³C. During the diffusion of carbon dioxide through the stomata of C₃ plants, the lighter carbon isotope (¹²C) diffuses more easily resulting in leaf internal air that is depleted in ¹³C compared to the ambient air. Tree rings record this discrimination against ¹³C (noted Δ¹³C) which is used to decipher information about past climates, including atmospheric CO₂ concentration when the rings formed. The Joint UK Land Environment Simulator (JULES) is a land-surface model that predicts environmental processes (e.g., the carbon and water cycles). Comparing JULES estimates of Δ¹³C with tree-ring measurements helps to improve simulations, leading to more accurate projections of ecosystem functioning under different climates. We found that JULES accurately predicted tree-ring Δ¹³C in 8 out of 12 UK sites over a 38-year period, with offsets in mean values of up to 2.6‰, but underestimated interannual variations in Δ¹³C at all sites. JULES captured the influence of environmental changes on tree-ring Δ¹³C relatively well, although the trees’ responses at individual sites varied. Results suggest that improvements are needed in JULES to reduce uncertainties in predicting the carbon and water cycles and therefore to produce more accurate projections of future climate-vegetation interactions.

Key Points

• Joint UK Land Environmental Simulator (JULES) predicted well the tree-ring Δ¹³C of oaks in eight out of 12 UK sites but underestimated inter-annual variations and mean values
• JULES captured the environmental drivers of Δ¹³C, but the model failed to reproduce the direction of change in Δ¹³C at some individual sites
• More tree-ring chronologies are needed to improve understanding of spatio-temporal changes in Δ¹³C for UK broadleaf deciduous trees