With the world's fight against global climate change gathering steam, pressure is growing on Japan, the world's fifth-largest emitter, which has not ratified the Paris climate change treaty that will formally come into force on Nov 4.

As of Wednesday, 73 countries and the European Union, which account for more than 55 percent of global greenhouse gas emissions, had ratified the accord, meeting the threshold for the pact to take effect.

And on Wednesday, US President Barack Obama urged Japan and other nations that have not yet ratified the treaty to do so "as soon as possible".