As an airliner prepares to land, a bird takes off at the Gravelly Point park near Reagan National Airport in Arlington, Va. on December 23, 2018. (Photo by Michael S. Williamson/The Washington Post)

Vehicles pass through the heavy fog and air pollution that cover Skopje valley in the early morning, in Skopje, The Former Yugoslav Republic of Macedonia, 14 December 2017. According to media reports, Skopje is considered one of the most polluted cities in the world. (Photo by Georgi Licovski/EPA/EFE)

An artist gets ready backstage to perform an Indian art form of dance called “Ottamthullal” at the annual eight-day long Vrischikolsavam festival, which features a colourful procession of decorated elephants along with drum and trumpets concerts, at Sree Poornathrayeesa temple in Kochi, India, December 7, 2018. (Photo by Sivaram V/Reuters)

A man stands near smoke billowing from a brick kiln in Karachi, Pakistan, 14 December 2018. According to the UN report, carbon dioxide (CO2) emissions have increased for the first time in four years. The United Nations Conference COP24, held in Katowice, Poland, until 14 December 2018, searches for a common position in the fight against climate change. (Photo by Shahzaib Akber/EPA/EFE)

Catwalks hang over a section of the newly constructed eastern span of the San Francisco-Oakland Bay Bridge during a media tour of the self-anchored suspension span tower on August 29, 2011 in Oakland, California. Contruction crews have erected twelve foot wide catwalks that connect to the San Francisco-Oakland Bay Bridge self-anchored suspension span's tower and crews will begin to lay the nearly one mile of main cable beginning in early 2012. The bridge has been under constrution since 2002 with an estimated price tag of $6.3 billion and will have the world's tallest Self-Anchored Suspension (SAS) tower once completed. (Photo by Justin Sullivan/Getty Images)

A performer poses with “The Gronkle” and “Deadly Nadder” at the How to Train Your Dragon Media Call at Fox Studios on November 29, 2011 in Sydney, Australia. The “How to Train Your Dragon Arena Spectacular” world premiere will take place in Australia in March 2012. (Photo by Brendon Thorne/Getty Images)

Performers wearing traditional outfits demonstrate the Coopers' Dance on Marienplatz square on January 7, 2012 in Munich, Germany. The Coopers' Dance, known in German as the Schaefflertanz, is performed only every seven years and dates back to 1702. (Photo by Johannes Simon/Getty Images)

Men cover their heads from the sun while riding on a motorcycle during a hot day in Cairo, Egypt, August 17, 2015. A heatwave killed at least 61 people across Egypt from Sunday to Tuesday and caused nearly 600 people to be admitted to hospital, Egypt's health ministry said on state news agency MENA on Wednesday. (Photo by Amr Abdallah Dalsh/Reuters)