What is solar variability and its role in the Earth's climate?
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January 8, 2013: In the scheme of galactic objects, the Sun is a remarkably fixed star. While some stars exhibit tremendous pulsations, yo-yoing in size and brightness, and sometimes even exploding, our own Sun's luminosity varies by 0.1 percent during its 11-year solar cycle.
However, it has been recognized among researchers that even these seemingly small changes can have a significant impact on Earth's climate. A new report released by the National Research Council (NRC), titled "Effects of Solar Variability on Earth's Climate," reveals some of the surprisingly complex ways that solar activity can make itself felt on our planet.
Taken by NASA's Solar Dynamics Observatory, these six intense UV images of the Sun track the increasing level of solar activity as the Sun climbs toward the peak of the last 11-year sunspot cycle.
Understanding the connection between the sun and climate requires extensive expertise in fields such as plasma physics, solar activity, atmospheric chemistry and fluid dynamics, energetic particle physics, and even Earth history. No single researcher has the full range of knowledge needed to solve a problem. To make progress, the NRC had to gather dozens of experts from different fields in a workshop. This report summarizes their combined efforts to frame the problem in a truly multidisciplinary context.
One of the participants, Greg Kapp of the Laboratory for Atmospheric and Space Physics at the University of Colorado, noted that while the luminosity changes over the 11-year solar cycle only amounts to about one-tenth of a percent of the Sun's total output. Such a small fraction is still significant. "Even typical short-term changes of 0.1 percent in irradiance exceed all other energy sources (such as natural radioactivity in the Earth's core) combined," he says.
Of particular importance is the sun's intense ultraviolet (EUV) radiation, which peaks over the years around solar maximum. In the relatively narrow band of EUV wavelengths, the solar output varies not by a paltry 0.1%, but by enormous factors of 10 or more. This can strongly affect the chemical and thermal structure of the upper atmosphere.
sun-climate (tsi, strip)
Space measurements of total solar radiation (TSI) show about 0.1% variation with solar activity on time scales of 11 years and shorter. These data have been corrected to compensate for calibration between different instruments used to measure TSI. Source: Courtesy of Greg Cope, University of Colorado.
Several researchers have discussed how changes in the upper atmosphere can trickle down to Earth's surface. There are many "top-down" paths for solar penetration. For example, Charles Jackman of the Goddard Space Flight Center explained how nitrogen oxides (NOx) created by energetic particles from the sun and cosmic rays in the stratosphere can reduce ozone levels by several percent. Because ozone absorbs UV radiation, less ozone means more UV radiation from the sun reaches the Earth's surface.
NOAA's Isaac Held took this a step further. He explained how the loss of stratospheric ozone can change the dynamics of the atmosphere below. "A polar stratospheric cooling associated with ozone loss increases the horizontal temperature gradient near the tropopause," he explains. The troposphere controls the surface westerlies.” In other words, solar activity felt in the upper atmosphere can derail surface storm tracks through a complex series of effects.
Sun-climate (September, strip)
How incoming galactic cosmic rays and solar protons penetrate the atmosphere. Source: C. Jackman, NASA Goddard Space Flight Center, "Effects of Energetic Particle Precipitation on the Atmosphere," presentation to Workshop on Effects of Solar Variability on Earth's Climate, September 9, 2011.
Many of the mechanisms proposed at the workshop had a Rube Goldberg-like quality. They relied on multi-step interactions between multiple layers of the atmosphere and ocean, some relying on chemistry to do their work, others on thermodynamics or the physics of fluids. But just because something is complicated doesn't mean it isn't real.
In fact, Gerald Mill of the National Center for Atmospheric Research (NCAR) provided compelling evidence that solar variability has an effect on climate, particularly in the Pacific Ocean. When the researchers looked at sea surface temperature data during peak sunspot years, the tropical Pacific showed a prominent La Nina-like pattern, cooling by about 1 degree Celsius in the eastern tropical Pacific, according to the report. Additionally, "there are signs of increased precipitation in the Pacific ITCZ (Intertropical Convergence).
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