Prior to the late 1800's, people believed the Earth's climate was static and never changing, and it wasn't until a few scientists in the second half of the 19th century began to wonder how giant boulders were in places where they shouldn't be.
Some of those scientists correctly hypothesized glaciers moved them there during ice ages, and over the course of the next century, climate science began to take off.
Scientists around the world made discoveries about the Earth's atmosphere and how it retained heat. Some mathematicians even (correctly) calculated how much heat carbon dioxide retained on our planet.
Even then, however, there were contrasting theories ranging from volcanic activity, sunspots, ocean currents and the Earth's revolution around the sun. None of those theories, however, held up or made as much of an impact as carbon dioxide and other greenhouse gases in our atmosphere.
Today, 98 percent of climate scientists agree additional greenhouse gases from burning fossil fuels is the primary reason for the current accelerated warming of the planet.
After all, using the pre-industrial 1881 to 1910 average, the world hasn't seen a month with below average temperatures since December 1964.