Last year, developing countries — including China, India, and Mexico — invested more in renewable energy than developed ones, including the US and those in Europe.

This is according to a UN-backed report released on March 24.

Part of the reason for this switch was because of enormous investment in China. More than a third of all the renewables investments around the world in 2015 occurred there.

But it's also because clean-energy spending slowed in Europe — down 21% from $62 billion in 2014 to $48.8 billion in 2015, according to the report.

Ulf Moslener, professor for sustainable-energy finance at the Frankfurt School and coauthor of the report, explained why Europe's investments are declining.

"These economies already have a significant part of renewables, and are now struggling with adapting their energy systems to these new technologies," he said at a press conference. "Storage will play a large role in fixing that."