Does human activity affect the climate? The article The Ice Age That Wasn't by Richard A. Lovett appeared in the April 2007 edition of Analog Science Fiction and Fact (yes, I'm that far behind in reading that magazine). It first lays out reasons why the earth's temperature naturally and regularly fluctuates due to variations in the earth's orbit. The cycle is about 22,000 years. The fluctuation is enough to produce ice ages. We hit a peak in that cycle about 11,000 years ago, which means we should be in an ice age right about … now. But it didn't happen. The drop was steady and according to schedule until about 5,000 years ago when temperatures began to rise again.
The significance of that date? The beginning of rice cultivation in Asia and with that the release of more swamp methane into the air (rice grows in flooded paddies that in some ways act as swamps). That was followed by the deforestation of Europe (complete about 2,000 years ago) and the release of all that carbon.
There are three dips in the steady rise in temperature. All three correspond to devastating plagues -- bubonic plague at the time of the collapse of the Roman Empire, the Black Death of the 14th Century, and the European diseases that swept through the Americas in the 16th Century. Reduce the number of humans and our effect on climate is reduced.
The tantalizing question is: Did the Industrial Revolution accelerate the process that lead to warming? Sorry, that's outside the scope of this article.