Footprints from the past: Blueprint for the future?

Humans have influenced their natural environment as long as they have existed. For thousands of years this influence was negligible or temporary. From time to time humans left their marks on the surrounding landscape, but these "marks" generally disappeared as ecosystems recovered to a more or less "pristine" state. The changes are not well known because the mysterious rises and declines of ancient cultures are poorly documented. During the last 300 years, however, the influence of humans on their surrounding landscape has increased at a staggering rate. Rapid increases in population numbers boosted the need for food and fodder. Land needed for crops and ranching expanded mostly at the cost of forests and natural grass-lands. Estimates of the decrease in global forested area during the last 300 years range from 8 to 13 million km(2), corresponding with 15% to 25% of the forest extent in 1700. Approximately 15 to 19 million km(2) of natural grassland and savanna were also domesticated to some extent. Much of the uncertainty in these estimates can be explained by the use of different classification schemes of historical inventories, different methodological approaches, and the absence of present-day technologies, such as satellites, for measurement of change. Nevertheless, these land-use changes are large enough to have contributed significantly to an enhancement of the greenhouse effect. The International Panel on Climate Change (IPCC) estimates that roughly one fifth of the total anthropogenic emissions of greenhouse gases during the 1990s were from changes in land use.