Colin Monteath / Smog from wood and coal fires blankets suburbs and city center on autumn dawn, Christchurch, Canterbury, New Zealand

Konrad Wothe / Cruise ship in Juneau harbor, Alaska

Richard Du Toit / Walled townhouse complex showing dense suburban development, Pretoria, South Africa

Patricio Robles Gil / Lake Anosy surrounded by buildings, Antananarivo, capital of Madagascar

Gavin Hellier / Elevated view of Charles Bridge & St Vittus Cathedral, Prague Czech Republic

Mark Moffett / Statue of Christ the Redeemer towers over the 8,150 acre Tijuca National Park, one of the world's largest urban forests, Rio de Janeiro, Atlantic Forest ecosystem, Brazil

Mark Moffett / Statue of Christ the Redeemer towers over the 8,150 acre Tijuca National Park, one of the world's largest urban forests, Rio de Janeiro, Atlantic Forest ecosystem, Brazil

Mark Moffett / Statue of Christ the Redeemer towers over the 8,150 acre Tijuca National Park, one of the world's largest urban forests, Rio de Janeiro, Atlantic Forest ecosystem, Brazil

Colin Monteath / Winter smog over the city of Christchurch caused by an inversion layer and pollution from cars and fires, seen through trees from Port Hills, South Island, New Zealand

Michael & Patricia Fogden / Windmill and buildings in village of Cley-next-the-sea, Norfolk, England

Konrad Wothe / Rock Dove (Columba livia) on Empire State Building, Manhattan, New York

Sebastian Kennerknecht / City of Oakland, Berkeley, the Bay Bridge, and San Francisco at twilight, San Francisco Bay, California