You sure about that? Most likely they are having an extremely measurable effect, it's just the effect is maintaining the status quo. Do you really think that the climate in Africa wouldn't change if forests were permitted to grow? Remember that climate is a complex dynamical process full of feedback loops. If anything, given the size of the landmass, allowing the savannahs to become dense forest would be a huge source of carbon capture.

Also I deny the premise that all climate change is bad prima facie, although again because complex dynamical system it can be impossible to predict the eventual outcomes. Nevertheless, a climate change like greening the Sahara could at least potentially be a net positive for humanity.