In the 21st century the focus is now shifting again. Atmospheric chemistry is increasingly studied as one part of the Earth system. Before, scientists focused on atmospheric chemistry in isolation. Now, scientists study atmospheric chemistry as one part of a single system with the rest of the atmosphere, biosphere and geosphere. A reason for this is the links between chemistry and climate. For example, changing climate and the recovery of the ozone hole affect each other. Also, the composition of the atmosphere interact with the oceans and terrestrial ecosystems.