A green planet versus a desert world: estimating the maximum effect of vegetation on the land surface climate, Kleidon 2000

This climate model simulation illustrates how the biosphere affects the climate system. With “maximum vegetation,” more water is absorbed in the ground, allowing for evaporation to cool the land surface while also recycling more rain. This simulation resulted in an average temperature reduction over land of 1.2C.

The authors describe their approach:

We quantify the maximum possible influence of vegetation on the global climate by conducting two extreme climate model simulations: in a first simulation (‘desert world’), values representative of a desert are used for the land surface parameters for all non-glaciated land regions. At the other extreme, a second simulation is performed (‘green planet’) in which values are used which are most beneficial for the biosphere’s productivity [Kleidon 2000: 471].

They describe the effects of maximum vegetation on the water cycle, stating that over land:

…the hydrological cycle is more active, with precipitation roughly increasing by 100%, evapotranspiration by more than 200% and the mean moisture content of the atmosphere (or precipitable water) increasing by 30%. These increases can be understood by enhanced recycling of soil water as a response of both, (i) more absorbed radiation at the surface so that more energy is available for evapotranspiration and (ii) larger soil water storage capacities (SWCs) which enhance water availability during dry periods. This increased recycling also leads to an overall decrease in continental runoff by about 25% [Kleidon 2000: 476].

Changes in the water cycle result in land surface temperature changes:

The substantial increase in evapotranspiration is associated with differences in the surface energy balance, primarily concerning the partitioning between sensible and latent heat. The latent heat flux increases by the same amount (more than 200%) as evapotranspiration and the sensible heat flux decreases to 30% of its original value. … Subsequently, the increased latent heat flux leads to more efficient cooling of the surface, resulting in temperatures reduced by 1.2 K^[13] [Kleidon 2000: 477-478].