MONTREAL -- An amendment to the Montreal Protocol has been signed that will lower the levels of a type of greenhouse gas that warms the planet even more quickly than carbon dioxide, Environment Minister Catherine McKenna said Monday.

McKenna says implementing the Kigali Amendment could avoid up to 0.5 degrees Celsius of warming by the end of the century.

The objective of the amendment is to reduce the levels of hydrofluorocarbons, which are used in products such as refrigerators, air conditioners and aerosols.

McKenna told reporters the amendment is important because the chemicals are "very polluting greenhouse gases which were causing huge warming."

"The Kigali Amendment is the most concrete action the world can take right now together to reduce global warming," she said.

"It represents half of a degree of warming that we are going to avoid. It's like closing 300 coal-fired plants."

McKenna added she was thrilled to announce the magic number of 20 countries had been reached in order to ratify the amendment so it can come into effect in January 2019.

The federal minister was in Montreal alongside former prime minister Brian Mulroney to mark the 30th anniversary of the Montreal Protocol, which was aimed at protecting the Earth's ozone layer.

Mulroney was prime minister when the agreement was signed.

During a news conference, Mulroney said he expects U.S. President Donald Trump to eventually change the way he sees a number of issues like the environment.

"You know you come into office with certain set views and then a strange thing happens -- reality intrudes, in trade, the environment, taxes and so on," Mulroney said.

"I think that he'll be moved by the realities of governing and the need to maintain excellent relations with his neighbours and this will have an impact on the decisions he makes."