Predicting climate accurately depends on getting a host of those boundary conditions correct, many of which relate to the atmosphere's energy. They include the amount and strength of sunlight reaching the Earth, the reflectivity of the Earth's surface, the movement of heat in the oceans, and the opacity of the atmosphere to terrestrial radiation as a result of greenhouse gases. And for this reason, getting long-term, accurate observations of, for example, the variations in the sun's output of energy over time is critical for understanding past climate change. Uncertainties in how the sun's output will change in the coming decades limits our ability to predict future climate with complete confidence. However, such decadal variations in the sun's output are irrelevant to predicting tomorrow's weather.