How green was my valley

World leaders pondering the possible consequences of man-made climate change would be wise to consider the fate of the Roman Mediterranean.

Computer modelling has revealed that the now-arid region was a far wetter and more hospitable place 2,000 years ago.

The culprit for the subsequent change, the research suggests, was the wholesale clearance of forests that once ringed the sea to make way for agriculture, grazing animals and cities.

Oreste Reale, a scientist at the Institute of Global Environment and Society in Calverton, Maryland, and Jagadish Shukla, president of the institute and professor at George Mason University, Virginia, constructed significantly different atmospheric circulation patterns in the first quantitative simulation of the classical climate.

This revealed evidence for a Roman mini-monsoon that would have soaked northwestern Africa and caused large rises in precipitation over the Nile valley and the Sahel - the strip of land bordering the Sahara, which often suffers drought today.

Levels of rainfall in the Nile valley would have been high enough to sustain groundcover, savanna across the Sahel and oak forests on the northern flanks of the Atlas mountains, from Morocco to Tunisia.

Shukla said: "The current dry conditions are at least in part due to the fact that all the vegetation around the Mediterranean has been deforested - we feel this climate has been very strongly influenced by human activity."

The scientists created a high-resolution general circulation model, published in the journal Global and Planetary Change , using techniques developed by Shukla and colleagues in a series of pioneering climate-simulation exercises carried out over recent years.

The study ignored other influences on climate to test the impact of large shifts in vegetation.

A detailed picture of the lost "green" Mediterranean was built up from pollen analysis, extrapolation of surviving forest relics and study of historical records.

Roman chroniclers referred to North Africa as the "granary of Rome" with vineyards, olive trees and extensive forests populated by elephants. Ruined cities also indicate a wetter climate, with aqueducts, bridges and public baths.

The experiment revealed significantly different patterns of rainfall, winds, moisture fluxes and temperature across the region.

The vegetation reduced the amount of solar heat reflected back into space and increased evaporation. This prompted a northward shift of the intertropical convergence zone, the turbulent equatorial atmospheric region where the trade winds run alongside each other, often leading to heavy precipitation.