Michael Bloomberg, the UN secretary-general's special envoy for cities and climate change, hailed the ratification of the Paris Agreement by China and the United States, the world's two biggest emitters of greenhouse gases.

In an email statement, he said that by ratifying the Paris Climate Agreement on Saturday, the US and China are demonstrating that the world's two largest economies will help lead the fight against climate change - but it's imperative that more nations join them.

"Cities are united in this effort, but to succeed, nations must be too - and I urge local leaders to continue pushing their national governments to ratify the Paris Agreement in the months ahead," Bloomberg said in the statement.

Last December, China and the US agreed to cut emissions in a bid to keep the global average rise in temperatures below two degrees Celsius. Both China and US are together responsible for 40 per cent of the world's carbon emissions.