People claimed to know what caused sickness, and how to cure it, for a very long time. They claimed to know how the Earth was created. They claimed to know the fundamental elements which combined to form all matter.

All of these pre-scientific theories were wrong. But we don't reject modern medicine, cosmology and atomic theory because of it. Humanity's history of doomsday predictions has no more bearing on the science of climate change than the former practice of leeching has on modern treatment of septicaemia.