The Effects of Solar Variability on the Earth's Atmosphere

The Sun's ultraviolet radiation varies on all time scales: a few minutes in the case of solar flares, the 27-day solar rotation period, the 11-year solar activity (sunspot) cycle, and extended periods of centuries and longer. While much of the solar visible light varies by a small amount, about 0.1% over the 11-year solar cycle as does the total solar irradiance (TSI), the ultraviolet radiation varies much more, ranging from 10% for the Sun’s chromosphere emissions in the middle and far ultraviolet to more than a factor of 10 for the hot coronal emissions in the extreme ultraviolet. While the solar visible light primarily deposits its energy at Earth on its surface or in the oceans, the ultraviolet radiation shorter than 300 nm is completely deposited into Earth’s atmosphere. Being mainly caused by ozone and molecular oxygen, this absorption is very wavelength dependent. Discussions have been carried on for decades about how the solar ultraviolet radiation could have a top-down influence on climate change through the ultraviolet radiation first being deposited into Earth’s stratosphere and then slowly propagating down to the troposphere. Recent studies and modeling efforts indicate that this top-down mechanism is indeed one of the ways that the solar variability can influence climate, including affecting the El Nino Southern Oscillation (ENSO), which is a dominant source of interannual climate variability around the world.