Unsaturation
A term used to describe the presence of double bonds between two adjacent carbon atoms of hydrocarbon groups within fatty acid chains.  Limited increases in unsaturation are thought to increase the stability of membranes.

Urban CO2 dome
A region of higher carbon dioxide concentration (relative to the surrounding countryside) in a metropolitan area.

Urban heat island
A region of warmer air temperature (relative to the surrounding countryside) in a metropolitan area.  Urban heat islands have been documented to exist in cities with as few as a thousand inhabitants.