The hashtag used for the 2015 Paris climate agreement was #COP21.
The tag refers to the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change. The conference took place at Le Bourget near Paris from November 30 to December 12, 2015, and its negotiations produced the Paris Agreement, the landmark international climate treaty.
“COP21” means “Conference of the Parties 21,” not “Paris Agreement 2015.” That is why #COP21 was the event’s principal shorthand and social-media hashtag. The conference also had related campaign tags, but #COP21 was the canonical identifier used for the summit itself.
The agreement sought to keep global warming well below 2°C above pre-industrial levels while pursuing efforts to limit warming to 1.5°C. The treaty was signed in 2016 and entered into force on November 4 of that year. COP22 followed in Marrakech, Morocco, in 2016, which explains the tempting but incorrect alternative #COP22.