Combining Ground-based and Satellite Observations to Reconstruct Changes in the Functioning of the Terrestrial Biosphere

Vegetation links the planet’s water and carbon cycles. Stomata on plant leaves are the pores through which CO₂ is fixed during photosynthesis, and also the pores through which water is returned to the atmosphere as the transpiration flux. Stomata therefore exert considerable control over both the water and carbon cycles. Unfortunately, the long-term responses of stomata to rising CO₂ and changes in climate are still rather uncertain, despite their significance for future climate. In contrast, Plant Water Use Efficiency (WUE) which is the ratio of the carbon assimilated through photosynthesis to the water lost through transpiration, is a robust diagnostic of the functioning of the land biosphere that is not directly dependent on the uncertain long-term responses of stomata.

We have recently shown that it is possible to get constrained estimates of fractional changes in WUE based purely on changes in atmospheric CO₂ and near surface temperature and humidity. This is achieved by calibrating against eddy covariance flux measurements (that constrain the response of WUE to humidity deficit), and also d¹³C records from tree-rings (that constrain the CO₂-sensitivity of WUE). This talk will show how these ground-based measurements imply very significant changes in WUE, both globally and regionally, from 1900 onwards. Furthermore, we will show how the combination of our reconstructed changes in WUE with satellite-based estimates of Gross Primary Productivity, enable recent changes in plant transpiration to be estimated.