Pre-Columbian reforestation in Amazonia An early 17th-century temporary reduction in global atmospheric carbon dioxide (CO ₂ ) levels was previously attributed to reforestation in Amazonia after the catastrophic loss of life of the indigenous population caused by diseases brought by European invaders. Using fossil pollen data from Amazonian lake sediments with temporal resolution over the past millennium, Bush et al. found that forest recovery began 300 to 600 years before the population crash. The more recent nadir in atmospheric CO ₂ was not associated with rapid reforestation at that time. The vegetation changes appear to be the result of changing patterns of land use in the centuries preceding the European arrival and the resulting devastation, whereas the cause of the CO ₂ decline remains enigmatic. Science , this issue p. 484