The human population grew from 1.6 billion to 6.1 billion people in the 20th century. As humans globally - particularly in developed countries - are the consumers & creators of emissions that cause climate change, more people means higher emissions rates. Additionally, trends in employment have drawn increasing numbers to cities. For the first time in the history of mankind, more people globally live in cities than in rural area and are living increasingly mechanized lives. This series visually explores population density in cities of Spain, Hawaii (US), Netherlands, Morocco & California (US).