The energythe Sun emits—technically called “solar irradiance”—currently is much more than it did when the solar system was young. Scientists estimate that, billions of years ago, our Sun only emitted about 75 percent of the energy it now does. Now, however, the amount of solar irradiance is fairly stable, though not completely constant. It can vary by as much as 0.1 percent, which may not sound like much, but such fluctuations are big enough to make the difference between moderate and frigid winters, merely warm or searing-hot summers. Solar irradiance can be affected by sunspots, but the internal workings of the Sun are still so complex that scientists do not fully understand what causes these fluctuations. Despite there being no complete consistency in the Sun’s energy output, astronomers and meteorologists still refer to a “solar constant,” which is the amount of energy of all emissions (measured in watts) per square meter. Readings are taken at the upper extremes of the Earth’s atmosphere, which receives, on average, about 1,366 W/m2.