Multiple States in the Vegetation-Atmosphere System during the Early Eocene

Model simulations suggest that different initial conditions can lead to multiple stable vegetation-atmosphere states in the present-day Sahara. Here, we explore the stability of the vegetation-atmosphere system in the warm, nearly ice-free early Eocene climate. Using the MPI-ESM, we simulate the early Eocene vegetation starting from two different states: Continents are either completely covered by forest or completely barren, devoid of any vegetation. The soil albedo is similar to vegetation albedo. Hence, the albedo effect of vegetation is negligible. Without the albedo effect, the Charney effect which is suggested to cause multiple stable vegetation states in the present-day Sahara is absent. In our simulations, the hydrological effect of vegetation plays the major role. We perform the same simulations with preindustrial conditions to compare the stability of the vegetation-atmosphere system in both climate states.

A desert evolves in Central Asia in both early Eocene simulations. This Asian desert is larger when the simulation starts from bare soil instead forest. Bare soil causes a dry climate in Central Asia in the beginning of the simulation. In the dry climate, vegetation does not establish. Forest enhances evaporation relative to bare soil leading to a stronger Asian monsoon and higher precipitation rates. The increased precipitation sustains plant growth and a smaller Asian desert evolves than in the simulation started from bare soil. Moreover, the stronger Asian monsoon affects global climate. Therefore, the two vegetation states in Central Asia accompany two globally different vegetation-atmosphere states.

In the preindustrial climate, the Sahara is larger when the initial vegetation is bare soil instead of forest. The same hydrological effect causes the multiple vegetation states the Sahara as in the early Eocene Asian desert. However, the multiple stable vegetation states in the Sahara do not affect the global climate. This result emphasises that the vegetation-atmosphere system is more sensitive to the initial vegetation cover during the early Eocene than today.