Neptune’s brightness and solar variation

To those who accuse me of having a particular bias on the issue of anthropogenic global warming, I offer the following, sent my way by planetary astronomer Heidi Hammel, the subject of a recent children’s book I wrote in the “Women’s Adventures of Science” series.

Science thrives on questions that challenge the consensus view. In this case, the analysis of changes in Neptune’s brightness suggest that a solar component may be in play. If further research bears out Heidi’s hypothesis, then our understanding of Earth’s recent warming may need to be refined to include a greater role for solar phenomena than are included in the current consensus view.

The paper in question appears in Geophysical Research Letters, vol. 34, L08203 and its full title is “Suggestive correlations between the brightness of Neptune, solar variability, and Earth’s temperature” by H. B. Hammel and G. W. Lockwood. Though written in the usual dry style of research science journals, it is clear and well-organized so that most readers with a background in the physical sciences can understand it.

With apologies to the authors for any misinterpretation, here is my understanding of its main points:

An examination of a long-term (1950-2006) set of data from the Lowell Observatory reveals a brightening trend of Neptune. Since this represents less than a third of a Neptunian year, and since Neptune has a polar inclination of 29 degrees and reached southern summer solstice in 2005, the trend has a seasonal component that must be accounted for. In addition, Neptune reached its maximum distance from the sun in the 1960s, which would also lead to brightening as viewed from Earth in recent decades.

However, even after accounting for those factors and for variations over the 11-year solar sunspot cycle, the data suggest other factors may be driving Neptune’s changes in brightness, which happen to correlate with Earth’s warming trend.

Certainly those changes may be due to factors on Neptune that have yet to be understood, but changes in the sun must also be considered. The solar variations are not simply in intensity (total solar irradiance), which can account for no more than about 1% of the observed increase in Neptune’s brightness.

“However,” write Hammel and Lockwood, “other components of the solar output might have an impact on the atmospheres of both Neptune and Earth (e.g., ultraviolet flux, magnetic field strength, cosmic ray shielding).”

They do not dispute that Earth’s recent warming is primarily caused by human influences and note that “(s)purious coincidences among low-frequency signals in natural phenomena are notoriously common.”

For me, their take-home message is this:

“…(E)ven for the well-studied Earth temperature variability, the steady rise in temperature since the mid 1970s is not fully understood but has an anthropogenic component due in part or entirely to rising greenhouse gases, in combination with changes induced by sulfate and volcanic aerosols, and/or other forcing factors. Total solar irradiance seems to be ruled out as a driving factor in temperature variations, although other components of solar output may still play a role.”

Because global warming is becoming an urgent issue, we need to find ways to mitigate its effects. We must begin by addressing its primary (anthropogenic) cause, which is the burning of fossil fuels. But we must also understand the phenomenon well enough to put our efforts into perspective.

Do we need to do more because solar effects are adding to the warming, or can we expect solar variation to make our task easier in the coming decades?

We can’t change the Sun, but we can understand it. Hammel and Lockwood think that Neptune may be sending us a message, and they believe that their observations and the observations of others “during the next decade may provide a critical test of the hypothesis that Neptune’s brightness changes are related to some component of solar variability.”

We can’t wait for their results to begin acting, but we need to stay abreast of such research so that our future actions are based on the best available scientific understanding.