Sources for images used in this section, as they appear, from top to bottom:

• Thomas Robert Malthus, Magnus Manske, Wikimedia
• The Great Trees, Albert Bierstatdt, Wikimedia
• Gifford Pinchot 3c03915u, Pirie MacDonald, Library of Congress
• S118 CCC Men on Truck, Civilian Conservation Corps, Wikimedia
• Noranda1942, Joseph Herman Bolduc, Wikimedia
• Cleveland Ohio aerial view Ken Winters, U.S. Army Corps of Engineers, Wikimedia
• Ozone still 2000 09 06, Srbauer, Wikimedia
• Uscapitolindaylight, Kmccoy, Wikimedia
• Copenhagen - COP15, Marc Kjerland, Wikimedia