Complex sentence: He disputes the atmospheric temperature increases observed in the 1930s and since the 1970s were man-made, and instead are naturally caused, as part of the Earth's climate's cyclical nature.
Split into simpler sentences: He disputes the theory that atmospheric temperature increases observed in the 1930s and since the 1970s were man-made. Instead, he asserts that they are naturally caused, as part of the Earth's climate's cyclical nature.