It appears as if the Earth System, a most complex system controlled by a concert of physical, chemical, economic, social and even sentimental processes, is drifting away from conditions favourable for humankind to survive. Because being expelled from Earth is not in the interest of mankind, measures are to be taken, to secure our planet's habitability, now and in future. But what are the right decisions and actions to be made - when and where? Decision makers of today carry a tremendous and unprecedented responsibility for mankind on Earth. Science is obliged to make available and understandable the knowledge of the complex nature of the Earth system. Knowledge constitutes the very basis of decision making processes.