An extremely important and challenging paper, ‘Coupling of water and carbon fluxes via the terrestrial biosphere and its significance to the Earth’s climate system’, has just been published in the Journal of Geophysical Research (Vol. 112, 2007: doi:10.1029/2007JD008431). The paper is by Paul R. Ferguson and the eminent, Professor Ján Veizer, of the Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, University of Ottawa, Canada. The ‘Abstract’ is as follows:Apparently, Hansen's Scenario B 'success' (wrong assumptions, not bad forecast) is enough to convince Climate Babblers that models work. One thing that amazes me with these people's faith in models is their lack of understanding of back-fitting, what it means and the statistical no-no that it is.
“Terrestrial water vapor fluxes represent one of the largest movements of mass and energy in the Earth’s outer spheres, yet the relative contributions of abiotic water vapor fluxes and those that are regulated solely by the physiology of plants remain poorly constrained. By interpreting differences in the oxygen-18 and deuterium content of precipitation and river water, a methodology was developed to partition plant transpiration (T) from the evaporative flux that occurs directly from soils and water bodies (E d) and plant surfaces (I n). The methodology was applied to fifteen large watersheds in North America, South America, Africa, Australia, and New Guinea, and results indicated that approximately two thirds of the annual water flux from the ‘water-limited’ ecosystems that are typical of higher-latitude regions could be attributed to T. In contrast to ‘water-limited’ watersheds, where T comprised 55% of annual precipitation, T in high-rainfall, densely vegetated regions of the tropics represented a smaller proportion of precipitation and was relatively constant, defining a plateau beyond which additional water input by precipitation did not correspond to higher T values. In response to variable water input by precipitation, estimates of T behaved similarly to net primary productivity, suggesting that in conformity with small-scale measurements, the terrestrial water and carbon cycles are inherently coupled via the biosphere. Although the estimates of T are admittedly first-order, they offer a conceptual perspective on the dynamics of energy exchange between terrestrial systems and the atmosphere, where the carbon cycle is essentially driven by solar energy via the water cycle intermediary.”
“Given the close correspondence between T and NPP and the disparity in terms of mass exchange between these fluxes, it seems probable that the larger water cycle controls the much smaller carbon cycle, not vice versa, and other limitations to plant growth, such as atmospheric carbon dioxide, nutrients, and temperature are likely superimposed phenomena. Intuitively, the validity of this assertion should be apparent and cannot be considered novel, as biological studies have established the connection between the water and carbon cycles at a variety of spatial and temporal scales. Nonetheless, few regional estimates of T are available and despite the limitations inherent to this methodology, results are informative and emphasize the inter-dependency between solar radiation, water vapor, and carbon dioxide in determining the interaction between the terrestrial biosphere and the atmosphere, and ultimately, the Earth’s climate.”