1. burning releases stored carbon dioxide
2. fewer trees mean less rainfall in that area and creates atmospheric dust that affects the earth’s albedo (its reflectivity, and thus its absorption of sunlight)

NASA has published satellite photos of Amazon fires in the summers of 2005 and 2006. From the intro to this article: Smoke Over the Amazon

“In 2005, at the tail end of a multi-year dry spell, the annual dry season in the Amazon south of the equator ballooned into an unrelenting drought. Agricultural fires, now a fixture on the Amazon frontier from August through mid-October, invaded adjacent forests on a large scale.”