As little as 12,000 years ago Canada was covered by a thick blanket of ice. Whatever caused its retreat was certainly not influenced by man. There are other astronomical cycles that occur over thousands of years. Also due to orbital eccentricity, the sun moves closer to or further from the earth in a 21,000-year cycle. Many natural terrestrial events have important influence on climate.
Volcanoes pour millions of tons of emanations into the atmosphere.
Natural forest fires are a major pollutant. Oceanic currents and the melting of polar ice play a role. The interplay of so many variables, some poorly understood, make the predictive value of climate models tenuous. Man-made atmospheric pollution is a serious problem. What is debatable is the relative amount of greenhouse gases produced by man versus those form natural sources.