Beijing choked under a thick layer of toxic smog Saturday, after the Chinese capital issued its second-ever red alert and put its emergency response plan into action.

The notice from the capital's environmental bureau ordered factories to close and pulled half of all private cars off the streets, among other measures, as extreme levels of pollution in the city's air were observed for the third time this month.

Beijing issued its second-ever red alert for smog Friday ahead of severe pollution forecast to hit China's capital, weeks after putting its emergency response plan into action for the first time.

The notice from the capital's environmental bureau orders factories to close and pulls half of all private cars off the streets, among other measures, as bad air floods into the city for the third time this month.

"Let’s eat ugly!”

Of the various calls to action at the United Nations climate conference this week, that one by Nicolas Chabanne might be among the catchiest catchphrases.

China's push for a global climate pact was due in part to its own increasingly pressing need to solve serious environmental problems, observers said Sunday.

China, the world's biggest source of climate-changing gases, was blamed for obstructing the last high-level climate talks in Copenhagen in 2009. This time around, it sent strong political signals it wanted a deal ahead of and during the Paris negotiations that ended Saturday with the agreement to keep global temperatures from rising another degree Celsius (1.8 Fahrenheit) between now and 2100.

The U.N. secretary general Ban Ki-moon has said the international climate talks that are edging towards a conclusion in Paris have been the most complicated and difficult negotiations he has ever been involved in.

Ban said that differences still remain among the nearly 200 governments searching for a climate deal in Paris but he urged negotiators to set aside their national interests to reach a compromise.

In today’s fractious world, getting 195 nations to agree on anything, even whether the sun rises in the east, isn’t easy. So, in many ways, the climate change deal that emerged from Paris over the weekend, a month after terrorist attacks traumatized the city, represents a remarkable achievement on behalf of humanity.

But the things that made the Paris agreement so broadly acceptable to so many countries — its voluntary nature, its lack of enforcement tools, and the many “requests” and “urges” throughout the 31-page text — are the same things that threaten its effectiveness.

PARIS, 9 December, 2015 – Some of the world’s coldest places, on land and sea, may be plunged into an unstoppable transition to a climate system most scientists believe has not existed for 35-50 million years.

The almost immediate consequences would include the loss of reliable water resources for millions of people, and the start of a process leading to ultimate sea level rise of 4-10 metres or more.

Air pollution in Iran's capital hit its worst level -- for a day without a sandstorm -- in at least nine months on Monday, prompting warnings that all citizens should stay indoors.

"Tehran's air is unhealthy for all," said the city's transport and traffic department on a municipality-owned website.

Pope Francis on Sunday hailed the U.N. climate accord reached in Paris but warned the key now lay in its implementation, especially in help for the poor.

"The climate conference has just ended in Paris with an agreement that many describe as historic," the pontiff said at Angelus prayers in St. Peter's Square.

In the future, China will shut down a factory before it even pollutes -- or so it hopes, as it deploys big data in the fight against bad air.

In Beijing's environmental bureau, a team of engineers tend to giant mainframe computers that keep a watchful eye on the city's pollution.