[Image of a world map with no colored countries]

1900

[Image of a world map with a few countries colored in light green]

1950

[Image of a world map with more countries colored in light green, and a few countries colored in yellow and orange]

2000

[Image of a world map with a majority of countries colored in yellow, orange, and red]

2019

[Image of a world map with many countries colored in green, some countries colored in yellow and orange, and fewer countries colored in red]