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KATOWICE, Poland — Leaders from around the world arrived Monday for ceremonial opening of the United Nations' annual climate change conference in a Polish city built around mining coal, widely seen as a main culprit behind global warming.

The conference in Katowice is expected to work out how governments can report on their efforts to reduce green gas emission and keep global warming well below 3.6 degrees Fahrenheit, as agreed in 2015 in Paris.

Arriving for two weeks of talks on tackling climate change, conference participants cast off hats, scarves and heavy coats as they entered cavernous halls in Katowice heated by coal-fired power plants nearby.

Coal is center-stage at the U.N. summit.

While the Polish government claims Katowice is in the process of transforming into a green city, power plant chimneys pumped plumes of smoke into a dull December sky and monitoring sites showed elevated levels of air pollution.

A coal miner works in a shaft around 1100 yards below the surface at the KWK Pniowek coal mine in Pawlowice, Poland, on Friday. Around 3,900 miners descend below ground there to churn out 13,500 tons of high-grade coal each day that will be destined for coking at steel mills across Europe. The COP24 climate conference is being held in nearby Katowice, Poland. Sean Gallup / Getty Images

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Poland, which is presiding over the meeting, plans to use Monday's official opening event to promote a declaration calling for a "just transition" for fossil fuel industries that face cuts and closures amid efforts to reduce greenhouse gas emissions.

Environmental activists have expressed concerns about the non-binding declaration, arguing that it could be cited as justification for propping up dying industries instead of investing in renewable energy sources. Some also have questioned why coal companies are among the meeting's sponsors.