As negotiators at United Nations climate talks in Poland this week hammer out a rulebook to curb greenhouse-gas emissions, some of the biggest boosters of the 2015 Paris accord are undermining efforts back home to curb global warming.

China is ramping up coal-fired electricity generation despite pledges to cut emissions, according to clean-energy advocates. Canadian provinces are challenging federal carbon-price rules and adopting local policies that go against national emissions goals. And the European Union is bickering...