As the most prominent example, the EU ETS showed a steady decline in GHG emissions until 2016, though also registered a rise in emissions in 2017 for the first time in seven years. Although challenges around certificate trading markets still exist, the EU ETS is the only emissions trading system globally that has proven its feasibility and works to achieve the ambitious climate objectives set out by the European Union. An indicator for this success is the carbon price in the EU ETS, which has reached a 10-year high. Other prominent examples of emissions trading systems include the Western Climate Initiative (WCI) and the Regional Greenhouse Gas Initiative (RGGI), both in the United States.