A major drawback with the Gaia hypothesis is its unsuitability for testing (e.g. Kirchner 1989). In an attempt to overcome this problem, Axel Kleidon (2002) tried to express the
Gaia hypothesis in terms that enable the formulation of testable null hypotheses. He used gross primary production (GPP), which is the global gross uptake of carbon by organisms, to describe biotic activity. GPP seems a fair measure of how beneficial environmental conditions are for life - the more favourable the conditions, the higher the GPP. With this definition, he formulated a set of hypotheses focusing on how GPP for an environment including biotic effects compared to the hypothetical value of GPP for an environment without biotic effects - in effect, a planet with life compared to a lifeless planet. His approach did not focus on whether particular biotic feedbacks are positive or negative, but on the sum of all biotic effects. It would be difficult to construct the environmental conditions without biotic effects in the real world, but numerical simulation models provide a means to do so. Using climate model simulations of extreme vegetation conditions - a Desert World and a Green World (called a Green Planet by its inventors: see Fraedrich et al. 1999; Kleidon et al. 2000) - Kleidon (2002) showed that, overall, terrestrial vegetation generally leads to a climate that is more favourable to carbon uptake (Table 9.1). He concluded that 'life has a strong tendency to affect Earth in a way which enhances the overall benefit (that is, carbon uptake)' (Kleidon 2002).
James W. Kirchner (2002) criticized Kleidon's work on three counts. First, he argued that environmental homeostasis is not always the outcome of biotic feedbacks. Second, he contended that biotic effects do not necessarily lead to a more suitable environment. Third, he complained that beneficial and destructive feedbacks could both evolve by natural selection. Further work involving the setting up of null hypotheses by Timothy Lenton (2002) focussed on the role of biotic effects regarding the response of the Earth system to external perturbations. While these hypotheses are difficult to test, Lenton (2002) argued that the Earth system with life is more resistant and resilient to many perturbations than the Earth system without life, and that life has not survived for 3.8 billion years purely by chance. Tyler Volk (2002) summarized the proposed definitions and null hypotheses by
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