Earth Hour is a global action against climate change.
It started in Australia in 2007 and it has grown to a global day of action. Last year 135 countries across every continent took part.
Organisers hope this year will be even more successful.
More top news
Mr Kim has made two recent visits to China.
The study looked at the development of 422 children over the course of eight years.
The president said they will come into force if China refuses to change its practices.