The energy flux from Earth's surface to the atmosphere via evapotranspiration is a significant correction to our simplest energy balance model that had only considered radiative fluxes. Derive the value of the flow of energy via evapotranspiration. Express your answer in units of watts/m². Hint: you will need to know, or derive, how much precipitation falls each year on a typical square meter of Earth's surface and you will need to look up in the Appendix to COW a value for the heat of vaporization of water.