Heat is land effect - Occurs when warmer temperatures are experienced in urban landscapes compared to adjacent rural areas as a result of solar energy retention on constructed surfaces. Principal surfaces that contribute to the heat island effect include streets, sidewalks, parking lots and buildings. Trees, green roofs (see above), reflective coloring, vegetated landscaping, and bodies of water mitigate the heat island effect.

Hybrid – Also known as Hybrid-Electric Vehicles (HEVs), combine the benefits of gasoline engines and electric motors and can be configured to obtain different objectives, such as improved fuel economy, increased power, or additional auxiliary power for electronic devices and power tools.

Hybrid Vehicles - Use a gasoline engine to power an electric generator.  The generator, together with storage batteries, powers electric motors that drive the vehicle's wheels. Hybrid vehicles use less gasoline and produce less pollution compared to conventional vehicles of similar size.

Hydroelectric energy – Electric energy produced by moving water.

Hydrofluorocarbons – Used as solvents and cleaners in the semiconductor industry, among others; experts say that they possess global warming potentials that are thousands of times greater than CO2.