Photo taken on Dec. 5, 2019 shows the Sydney airport in smog, which was caused by bushfires in northern New South Wales, Australia. (Photos: Xinhua)

Photo taken on Dec. 5, 2019 shows the Sydney airport in smog, which was caused by bushfires in northern New South Wales, Australia.

Photo taken from aboard a plane on Dec. 5, 2019 shows the city of Sydney in smog, which was caused by bushfires in northern New South Wales, Australia.

Photo taken on Dec. 5, 2019 shows the Sydney airport in smog, which was caused by bushfires in northern New South Wales, Australia.