Throughout a solar cycle (roughly 11 years), there are variations in the level of irradiation from the Sun, which has an impact on the chemical composition of the atmosphere and the temperatures of both the atmosphere and Earth’s surface. During the last cycle, spectrometers on the Solar Radiation and Climate Experiment satellite (SORCE) collected surprising data that disagrees with our expectations about how the solar cycle affects climate.

In a recent issue of Nature, Joanna Haigh and colleagues report that the largest deviation from predicted activities of the solar cycle occurred between 2004 and 2007, when the Sun’s activity was in a decline. Data from the Spectral Irradiance Monitor (SIM) instrument showed a four- to six-fold larger decrease in ultraviolet irradiation, and an increase in visible irradiation, compared to predictions from a leading solar model. A second instrument on the SORCE satellite, the Solar Stellar Irradiance Comparison Experiment (SOLSTICE), also observed the drastic variations in the ultraviolet region.

The wavelength of solar irradiation determines what part of the atmosphere is affected the most. Ultraviolet irradiation leads to chemical reactions that produce more ozone in the stratosphere and warm up the stratosphere and mesosphere. Irradiation in the visible wavelengths penetrates further, leading to heating of the Earth’s surface, troposphere, and lower stratosphere.

The authors modeled atmospheric photochemistry and radiative transfer based on the SIM data. Their results indicate that there would have been an increase in ozone concentration below 45km in the stratosphere, while ozone concentrations would be lower above the stratosphere. The Aura-MLS satellite’s ozone measurements corroborate the author’s findings.

Haigh’s paper does raise some doubts about empirical models for the solar cycle, which are based on factors like the number and area of sunspots. It also suggests that the impact of solar activity on surface climate is likely to be out of phase with the peaks and valleys of the 11 year cycle. However, as the authors have pointed out themselves, their data is based on one solar cycle, which could be an outlier. If this trend continues in subsequent cycles, the existing views of the solar cycle will have to be revised.

Nature, 2010. DOI: 10.1038/nature09426