Past, present and future Exchanges of CArbon and Water between the vegetation and the atmosphere: new insights from analysis and modelling of stable carbon ISOtope data

ended

Water-use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. Rising CO2 concentrations and climate change both affect the stomatal regulation of leaf gas exchange and must influence WUE. However, the magnitude of change in plant WUE estimated by different methods differs strongly amongst studies, indicating unresolved issues. Moreover, current land surface models produce inconsistent and often unrealistic magnitudes and patterns of variability in WUE. This project thus aims at developing the use of carbon isotopes (13C) in terrestrial biosphere modelling to better understand and predict terrestrial carbon and water cycling and to evaluate their representation in models. We will perform a detailed investigation of the influences on discrimination against 13C during photosynthesis and their representation on various scales in the Joint UK Land Environment Simulator (JULES) model, which forms the land component of the UK Earth System Model, and implement the resulting discrimination model in JULES. We will then evaluate historical simulations of past changes in WUE and quantify the relative contributions of each forcing (climate and CO2) on WUE trends. Finally, we will perform projections of future changes in WUE following the 1.5°C and 2°C targets in global average temperature rise by 2100 proposed by the United Nations Framework Convention on Climate Change (UNFCCC). This project will contribute to the establishment of an improved theoretical and empirical basis for next-generation land surface modelling, and will address key uncertainties including the magnitude of changes in carbon and water cycling, which are requested by policy makers.