1. Ideal black-body properties for the Earth 2. Grey-body properties with the absorption of 0.7 3. Spectrally sensitive absorption such that the absorption averaged over the sun's energy spectrum is 0.7. The reduced emissivity at longer wavelengths, due to greenhouse gases, is modeled as a decreased absorption of 0.6 averaged over wavelengths corresponding to the Earth's radiative emission 4. How much change in the latter absorption is required to heat the Earth's temperature by 2C