A new model of how the Sun's 11-year cycle affects climate leads to slight changes in model results on atmospheric chemistry, but temperature and wind results are consistent with the previous model.

The Sun’s impact on the climate is a hot and tangled topic. Mounting evidence suggests that the 11-year solar cycle can affect climate and temperatures—the most famous example being Europe’s Little Ice Age, when the Sun went through several nearly sunspotless cycles from 1645 to 1715. Of course, there is a much larger factor acting on Earth’s climate: the warming trend from the turn of the 20th century, mostly due to human carbon emissions. On shorter timescales, however, the Sun’s solar cycle can have a significant impact on the physics and chemistry of Earth’s atmosphere.

Now scientists have an upgraded model to help them figure out exactly how these processes work.

Over the course of the 11-year cycle, the rotation of the Sun slowly twists its magnetic field into knots, creating dark sunspots. Although the overall brightness of the Sun varies by only 0.1%, the twisted bundles of magnetic energy can boost its ultraviolet (UV) radiation by 4%–8% at the solar cycle’s peak. These powerful UV rays trigger chemical reactions in the stratosphere that bind oxygen atoms and molecules to form ozone. Since ozone itself is a good absorber of UV radiation, it can heat the stratosphere near the equator, which affects the winds that circle the globe.

Increased solar activity also excites Earth’s magnetic field, sending high-energy particles hurtling into the upper atmosphere. During the long polar night, this can generate large amounts of the nitrogen compounds nitric oxide (NO) and nitrogen dioxide (NO 2 ), which eventually descend into the stratosphere and destroy ozone.

To study the effects of these often-competing processes, scientists construct simulations using models such as the Whole Atmosphere Community Climate Model (WACCM) produced by the National Center for Atmospheric Research. Peck et al., at the University of Colorado Boulder, use the latest version, WACCM4, and benchmark its treatment of the solar cycle against its predecessor, WACCM3.

Reassuringly, they find that the results are largely similar to the previous model, with a few new twists. As before, when solar activity peaks, the increased UV light boosts ozone levels in the stratosphere over the equator and midlatitudes by 2%–3%. However, the atmosphere’s vertical circulation is stronger in WACCM4, which brings twice as much NO and NO 2 down into the stratosphere over Antarctica, more than doubling the destruction of ozone there.

One complication is a long-standing bias in the model, called the cold pole problem, which seems to be exacerbated in WACCM4. This results in stratospheric winter temperatures over the South Pole that are too low and an Antarctic polar vortex that is too strong. However, the Arctic polar vortex is significantly more accurate in WACCM4 than in the previous version.

Overall, the wind and temperature results from the model are mostly consistent with the previous version. The authors say that the results validate WACCM4 and lay the groundwork for a new round of studies that may refine our understanding of the solar cycle’s impact on climate. (Journal of Advances in Modeling Earth Systems (JAMES), doi:10.1002/2014MS000387, 2015)

—Mark Zastrow, Freelance Writer

Citation: Zastrow, M. (2015), Model of solar cycle’s impact on climate gets upgrade, Eos, 96, doi:10.1029/2015EO040587. Published on 7 December 2015.