By the end of the 15th century, between 40 and 80 million people are thought to have been living in the Americas. Many of them burned trees to make room for crops, leaving behind charcoal deposits that have been found in the soils of Mexico, Nicaragua, and other countries. About 500 years ago, this charcoal accumulation plummeted as the people themselves disappeared. Smallpox, diphtheria, and other diseases from Europe ultimately wiped out as much as 90% of the indigenous population. [It has been speculated that many of the early European settlers thought of North America as a vast unpopulated or under-populated region because of the lack of awareness of the devastation wrought by diseases from early European explorers.]
As the population was decimated, trees returned, reforesting an area at least the size of California, researchers have estimated. This new growth could have soaked up between 2 and 17 billion tons of carbon dioxide from the air. This supposition is supported by Ice cores from Antarctica which contain air bubbles that show a drop in carbon dioxide around this time. These bubbles suggest that levels of the greenhouse gas decreased by 6 to 10 parts per million between 1525 and the early 1600's.
It is this decrease in atmospheric carbon dioxide that may have helped to trigger the Little Ice Age, researchers argue.
Critics point to natural processes that may have also played a role in cooling off Europe: a decrease in solar activity, an increase in volcanic activity, and/or colder oceans capable of absorbing more carbon dioxide. Some or all of these processes may have triggered or supported the Little Ice Age.
The study on the reforestation of North America and the Little Ice Age can be found at http://hol.sagepub.com/content/21/5/853.abstract.
Other related studies on this topic can be found at http://gsa.confex.com/gsa/2011AM/finalprogram/abstract_196092.htm and http://hol.sagepub.com/content/21/5/775.