HUMANS AS GEOLOGIC AGENTS: A DEEP-TIME PERSPECTIVE

Mean rates of cropland soil loss are thought to exceed rates of soil generation by up to an order of magnitude, but appreciating the importance of humans as global geologic agents necessitates knowledge of denudation rates imposed on land surfaces solely by natural processes. Amounts of weathering debris that comprise continental and oceanic sedimentary rocks indicate average subaerial denudation over the last half billion years of Earth history has lowered continental surfaces by a few tens of meters per million years. In comparison, construction and agricultural activities currently result in the transport of enough soil and rock to lower all ice-free continental surfaces by several hundred meters per million years. Humans are now an order of magnitude more influential at moving sediment than the sum of all other natural processes operating on the surface of our planet. Relations between temporal trends in land use and global population indicate that humans became the prime agents of erosion sometime during the latter part of the first millennium AD.