Kottke is reading Charles Mann's 1493 and quotes William Ruddiman's theory (pdf) that Columbus' arrival was a contributing factor to the Little Ice Age, a period of cooling between the 16th and 19th centuries:

In the Americas before [Columbus], the primary tool was fire. For weeks on end, smoke from Indian bonfires shrouded Florida, California, and the Great Plains. … Enter now the Columbian Exchange. Eurasian bacteria, viruses, and parasites sweep through the Americas, killing huge numbers of people — and unraveling the millenia-old network of human intervention. Flames subside to embers across the Western Hemisphere as Indian torches are stilled. In the forests, fire-hating trees like oak and hickory muscle aside fire-loving species like loblolly, longleaf, and slash pine, which are so dependent on regular burning that their cones will only open and release seed when exposed to flame.

The result?

The regular fires and forest regrowth resulted in less carbon dioxide in the atmosphere and the atmosphere traps less heat. It's like global warming in reverse.