The largely voluntary pledge and review system where states make their own plans for emissions reduction was introduced in 1991, but abandoned before the 1997 Kyoto treaty, where the focus was on securing agreement for "top down" emissions targets. The approach was revived at Copenhagen, and gained further prominence with the 2015 Paris Agreement, though pledges came to be called nationally determined contributions (NDCs). These are meant to be re-submitted in enhanced form every 5 years. How effective this approach is remains to be seen. The first submission of nations elevated NDCs is due to take place at the 2021 Glasgow conference. There are plans to agree on an international system for carbon price related trading, also at the 2021 Glasgow COP meeting.