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During the past 5 million years significant changes have taken place in the Lake Baikal ecosystem. The climate changed from subtropical, with average annual air temperatures (T air ) of 15-10°C, to a moderate one with T air of $0°C. The modern hydrophysical regime of the lake has been formed with increase in depth, and gradual cooling of the water body from deepwater temperatures (T dw ) of 15-10°C to below 4°C. The processes of cooling of Lake Baikal water in its transition from subtropical climate with deep temperatures greater than those of maximal density (T md ) are similar to modern processes of deepwater cooling of the Mediterranean type of MEDOC convection. Such intensive mechanisms of vertical exchange (MVE) are similar to autumn homothermy in Lake Baikal. However, they are higher than T md . Inverse thermal stratification began to occur in surface layers in cold periods of the year after the deep water had cooled to T md . The character of water exchange also changed; instead of a monomictic lake with warm water from surface to bottom, the lake has become dimictic with lower water below 4°C, i.e. vertical mixing now occurs not once but twice a year. Winter ice cover began about a million years ago.

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