COP21 in Paris was a reasonable success. World leaders are finally understanding the threat posed by global warming. However, not all of them are convinced that it is the human consumption of fossil greenhouse gases that has a direct impact on our climate. One of the arguments for the climate not being influenced by increased CO2 concentrations is that in the past – in at least one case – the Earth has experienced a powerful ice age at a time when the globe was otherwise seemingly in a super-greenhouse environment with an estimated CO2 pressure up to 16x the pre-industrial level. This occurred 445 million years ago towards the end of the geological period Ordovicium. However, the high CO2 estimate for the early part of the Paleoikum is primarily based on models. New data, based on geological evidence, however, indicate that the climate was not at all as hot as previously believed. On the contrary, it has probably reminded a lot of the globe has endured over the last several million years. The lecture will therefore deal in particular with palontological methods and evidence of this paradigm shift in the climate of the clock.