Abstract:The global mean temperature variation obeys the rule of square root, i. e.,thetemperature variation within 10 years is in inverse proportion to and that within 10-105 years is in direct proportion to the square root of time scale. Three guesses are thusmade: (1)the climatic variation within 10-105 years is controlled by the earth heat fluxvariation; (2)the oscillation of the climatic system is an inherent property of its stabili-ty,which is determined by the stability of its outer parameters; (3)the real climatic stateis always vacillating around its critical state of stability,tending toward a kind of ex-treme value principle, rather than the real "highest efficiency".