From:
http://en.wikipedia.org/wiki/Little_Ice_Age
Decreased human populations
Some researchers have proposed that human influences on climate began earlier than is normally supposed and that major population declines in Eurasia and the Americas reduced this impact, leading to a cooling trend. William Ruddiman has proposed that somewhat reduced populations of Europe, East Asia, and the Middle East during and after the Black Death caused a decrease in agricultural activity. He suggests reforestation took place, allowing more carbon dioxide uptake from the atmosphere, which may have been a factor in the cooling noted during the Little Ice Age. Ruddiman further hypothesizes that a reduced population in the Americas after European contact in the early sixteenth century could have had a similar effect.[78][79] Faust, Gnecco, Mannstein and Stamm (2005)[80] and Nevle (2011)[81] supported depopulation in the Americas as a factor, asserting that humans had cleared considerable amounts of forests to support agriculture in the Americas before the arrival of Europeans brought on a population collapse. A 2008 study of sediment cores and soil samples further suggests that carbon dioxide uptake via reforestation in the Americas could have contributed to the Little Ice Age.[82] The depopulation is linked to a drop in carbon dioxide levels observed at Law Dome, Antarctica.[80]