Judith L. Lean
Naval Research Laboratory
Election Year: 2003
Primary Section: 16, Geophysics
Secondary Section: 12, Astronomy
Membership Type: Member
The Earth resides in the astronomical neighborhood of the Sun, whose electromagnetic radiation enables our terrestrial society. Solar energy at visible and infrared wavelengths heats the Earth's surface and powers the climate system. Solar ultraviolet radiation creates ozone in the atmosphere that protects the biosphere from harmful radiation at shorter wavelengths. Radiation in the EUV and X-ray spectrum controls the temperature and composition of the Earth's upper atmosphere and creates the ionosphere. Solar radiation varies continuously. Even the total radiative output- the so-called solar "constant" - exhibits fluctuations associated with the Sun's 11-year activity cycle, and may vary on longer time scales. Measurements and models of total, ultraviolet and extreme ultraviolet irradiance variability enable simulations of a wide variety of variable terrestrial phenomena on time scales from minutes to millennia. The Earth's responses to variations in solar energy output are well established in outer atmospheric layers, where temperature changes of hundreds of degrees occur during the 11-year solar cycle. Ozone levels and the state of the middle atmosphere also respond to variable solar inputs, but much less dramatically. The extent of climate change associated with solar variability remains controversial because substantial empirical evidence is, in general, inconsistent with present understanding of the mechanisms of climate change encapsulated in current general circulation models. The behavior of the extended Sun-Earth system is of increasing importance for commerce and defense, as well as for policy making on issues of global change.