Why have issues related to global environmental protection become the priority concern of states since the 1990s?

Issues related to global environmental protection became the priority concern of states since the 1990s because, at the global level, the environmental issues drew the attention of various states at the United Nations Conference on Environment and Development held in Rio de- Janerio, Brazil in June 1992 through Agenda 21: 

1. Rio-Summit 1992 dealt with climatic change, biodiversity and forestry. 

2. Agenda 21 combined economic growth with ecological responsibilities. 

3. Kyoto Protocol set targets for greenhouse emissions. The above-mentioned conferences and summits raised the environmental issues at the global level to take steps by various states to check environmental degradation in a co-operative manner.