For those who have wondered what life would be like on the alien planets of the Star Wars films, a study conducted at the University of Hamburg, Germany, should now assuage your curiosity. But Klaus Fraedrich and colleagues at Hamburg's Meteorological Institute are not conducting idle speculation about would-be worlds – their work casts light on the role that vegetation plays in the Earth's climate system.

Attempts to simulate our planet's climate using computer models are now routine. But mostly these start out with a planet that closely resembles the present-day Earth, for example to investigate how it might change in the face of such disturbances as human-induced global warming. Fraedrich and colleagues have done something more extreme. They consider an Earth on which the land masses support the maximum amount of vegetation conceivable – like the Forest Moon of Endor in The Return of the Jedi – and one in which all the land is a barren desert, as Luke Skywalker's homeworld of Tatooine.

Whether or not the world has ever been as green as the Hamburg group's "Green Planet", there have certainly been times in the past when the global climate was warmer and wetter, and the vegetation more abundant. And before life colonized the land, this was surely a planet something like the researchers' "Desert World". Desert conditions do not imply a hot climate, however – the Siberian and Mongolian steppes are partly frigid deserts.

Fraedrich and colleagues are not, however, anticipating a return to these extremes – at least, not in the near future. Rather, they chose to model them so that the effects of land plants on climate would be seen as clearly as possible. The insights thus gathered might then be applicable to regions where, for instance, vegetation is disappearing due to deforestation (in Brazil) or encroachment of deserts (in northern Africa).

Plants are not just the beneficiaries but the engineers of a moist climate. Trees pump vast amounts of water up from their roots to their leaves, where much of it is lost by evaporation. This process, called transpiration, allows a typical birch tree to pump around 80 gallons of water each day from the ground into the atmosphere. In addition, plants help to maintain the porous structure of soil and so to retain rainwater. And they prevent soils from being rapidly eroded away by running water after heavy rain.

In this way, vegetation plays a crucial role in the cycling of water from sky to ground and back again – the so-called hydrological cycle. And indeed Fraedrich and colleagues found that there was much more intense cycling of water on their Green Planet relative to Desert World; the climate was wetter. But a wetter world should mean more cloud cover, and that in turn could increase the amount of sunlight reflected back to space, cooling the globe.

The researchers found that this cooling effect was almost balanced out by a warming influence resulting from the covering of bare ground by plants. Because plants are darker than dry desert, vegetation-covered land surface absorbs more of the Sun's heat than does the desert surface.

Because its atmosphere was more moist, the Green Planet also had more intense storms and monsoons than the Desert World – cyclone systems, for instance, are triggered initially by the condensation of water vapour. But temperatures at the surface of the Green Planet are overall slightly lower than those on Desert World, partly because evaporation of water pumped up by plants sucks in heat.

Although the German study, described in the October issue of the Journal of Climate1, uses a highly simplified model of the interactions between climate and vegetation, it gives a broad-brush picture of the changes that we might expect if we continue to let the "desert" replace the forest.