The U.N. secretary-general is challenging investors around the world to at least double their investments in clean energy by 2020.

Ban Ki-moon told an investor summit on climate risk Wednesday that increasing investment in clean energy is critical in following up on the landmark agreement to tackle climate change reached in Paris last year.

Ban says that about $330 billion was invested in clean energy last year, but that is far from what he calls the "clean trillion" needed per year in the decades to come.

The U.N. chief also says he has invited all of the world's heads of states and government to attend an April 22 signing ceremony for the climate agreement, and that 55 countries' signatures are needed to put the agreement into effect.