The climate on Earth - do we understand it well enough?

Although it seems impossible to make reliable predictions about climate changes in the near future, an attempt will be made to answer the question posed in the title of this contribution. To this end, a few basic concepts which influence the climate on Earth will be discussed. The principal difficulty is the fact that in the long history of the Earth it is the first time that natural variations of the climate are influenced by human activities as well. And here, the problem arises that besides scientific issues of how these activities (e.g. fossil fuel burning) influence the climate, economic and political considerations are also drawn into the discussion. And this clearly has consequences on how to react to some of the observed changes (e.g. global warming). But emerging technologies to trace radionuclides in the environment at large may still help to get a better understanding of the climate on Earth.