Leaders of countries and regions across the Americas vowed to put a price on greenhouse gases as a key instrument in the fight against climate change.

Canada, Colombia, Costa Rica, Chile and Mexico pledged to apply the cost of carbon dioxide discharges to guide public investment decisions and to encourage private companies to follow suit through internal pricing systems. The governors of California and Washington, as well as the premiers of Alberta, British Columbia, Nova Scotia, Ontario and Quebec, also joined the declaration signed at a climate summit in Paris on Dec. 12.

All those jurisdictions already have or are...