Natural forcings. Solar, ocean currents, location of landmasses, even greenhouse gas levels.
The key is that none of the "natural" forcings (which are EXTENSIVELY discussed in all the literature) line up with the current warming. The sun is not in a proper part of its cycle to cause warming, the ocean currents are not changing independent of the warming (so not driving the warming) etc.
Human activity, however CAN account for the warming we've seen.
THis set of graphs shows that:
The data doesn't make any sense until you invoke "anthropogenic" factors (human activity).