No one, but no one, claims that the only way warming can be caused is by human activities and waste and combustible engines. Everyone on all sides agrees there is long term climatic variation. The disagreement is whether recent human activities *add* onto that.
The disagreement is not only about whether human activities ADD to climatic change. There is also a possibility that there is no long-term natural climate change and all of the recent change that is worth remarking about is caused by humans.