It was a major carbon sink, but as the climate warms up trees die earlier and release more carbon so it's becoming a carbon emitter. It's obviously hard to calculate an exact plus or minus, but it's not looking good, especially with the amount of deforestation occurring.

For now, the biggest contribution of the amazon rainforest in terms of climate is improving rainfall. If you cut down all that trees that regious would pretty quickly turn into a big desert (in fact, the forest is actually recent in geological terms)