If we're talking changing the atmosphere, probably not. But humans have been altering local ecologies for thousands of years. Gary shows some examples. A couple of others...

The spread of the Sahara desert has been aided by lousy grazing practices. Some people theorize that the large mammals of the Americas began to disappear as soon as humans crossed the Bering Strait.

Before the Europeans brought the horse to America, some Native Americans used to hunt buffalo by driving herds over cliffs. The precision hunting techniques made possible by the horse might have given the buffalo a short reprieve...before white men started shooting them in droves.