Diseases introduced by Europeans after 1492 are now known to have caused massive population declines in the Americas, and the failure of ancient agricultural systems across huge regions, many of which depended on the regular burning of forests.
Now, researchers, led by Richard Nevle and Dennis Bird have investigated the climate consequences of this massive decline in agriculture and the burning of forests in the Americas, and demonstrated that the carbon stored in soils and vegetation as a result drove global cooling by taking carbon dioxide out of the atmosphere, helping to explain the "Little Ice Age" of the 1500s to 1900s.
This is really a groundbreaking paper, further demonstrating the major role of preindustrial human populations in altering global climate, and further strengthens Bill Ruddiman's Early Anthropocene Hypothesis.