The Eiffel Tower and the Arc de Triomphe in Paris have temporarily been lit in green as a reminder to the world to enact the Paris Agreement on climate change.

The eco-colored warning on Paris' most recognizable monuments Friday night was the joint initiative of the Paris Mayor Anne Hidalgo and French Environment Minister Segolene Royal.

The accord enters into force Friday — faster than anyone had anticipated — after a year with remarkable success in international efforts to slash man-made emissions of carbon dioxide and other global warming gases.

But agreeing on those goals was the easy part. Achieving them will require a transformational shift in the world's energy system, which despite strong gains by clean energy sources such as wind and solar power still depends on fossil fuels.