Inoltre, il tetto di emissioni totali per il terzo periodo di scambio prevede nel [] una riduzione dei gas serra nei settori di riferimento pari al 21% dei valori del []. Il sistema è stato recentemente esteso alle compagnie aeree, ma tale novità entrerà in vigore solo a partire dal 2012. <ref>[http://www.nytimes.com/2008/10/25/business/worldbusiness/25emissions.html?scp=3&sq=airlines%20emissions&st=cse Europe Forcing Airlines to Buy Emissions Permits], The New York Times, October 24, 2008</ref><ref name="EC 2007">[http://europa.eu/press_room/presspacks/energy/comm2007_02_en.pdf Limiting global climate change to 2 degrees Celsius - The way ahead for 2020 and beyond], Communication from the Commission to the Council, the European Parliament, the European Economic and Social Committee and the Committee of the Regions, Brussels, 10 October 2007.</ref>
The first phase of the EU scheme was created to operate apart from international climate change treaties such as the pre-existing [[United Nations Framework Convention on Climate Change]] (UNFCCC, 1992) or the [[Kyoto Protocol]] that was subsequently (1997) established under it. When the Kyoto Protocol came into force on 16 February 2005, Phase I of the EU ETS had already become operational. The EU later agreed to incorporate Kyoto flexible mechanism certificates as compliance tools within the EU ETS. The "Linking Directive" allows operators to use a certain amount of Kyoto certificates from flexible mechanism projects in order to cover their emissions.
The Kyoto flexible mechanisms are: