img src: BBC
The discussion of people influencing the climate of the world is almost always focused on the modern world. There is good reason. We are observing the extent modern civilization is changing the world's climate through carbon emissions. We are getting front row seats and the discussion of those changes is lively, to say the least. At times it's a bit overly dramatic, but most important discussions these days often are. In the era of the internet, that's hardly surprising.
However, people have been affecting the environment for some time. Most agree the industrial revolution had a huge impact on the environment and climate. There has been discussion of how Native Americans impacted their environment as well. It is almost certain that people have been influencing the climate for a long, long time. Long before the even the foundation of the United States.
Evidence suggests the time period we call the Holocene, when the Pleistocene ice age ended and our modern climate began, was originally nothing more than an 'interglacial.' These were periods when the climate suddenly warmed in the Pleistocene and became at times nearly as warm as the current time. However, they were relatively short in time frame and lasted twenty thousand years, plus or minus, before returning to the ice age with its giant glaciers and very cold climate.
The very first agriculturalists may have completely changed the climate. The early farmers cleared away the forests to create fields for their crops. These forests stored more carbon than the seasonal crops did. This was followed, later, by the creation of rice paddies. The cultivation of rice produces a significant amount of methane, a greenhouse gas. In fact, methane is a very potent greenhouse gas. Far more than even carbon dioxide is.
These greenhouse gases and continued agriculture prevented the descent back into the ice age. It also changed the climate so much it may have derailed the ice age cycle of glaciation and deglaciation long before it would have naturally. Previous ice ages have lasted for tens of millions of years. The Pleistocene ice age ended after about two million years.
Ancient farmers spared us from glaciers but profoundly changed Earth's climate
https://www.eurekalert.org/pub_releases/2018-09/uow-afs090618.php