A recent paper that there is a Suggestive correlations between the brightness of Neptune, solar variability, and Earth’s temperature (Hammel and Lockwood, 2007).

Abstract: Long-term photometric measurements of Neptune show variations of brightness over half a century. Seasonal change in Neptune’s atmosphere may partially explain a general rise in the long-term light curve, but cannot explain its detailed variations. This leads us to consider the possibility of solar-driven changes, i.e., changes incurred by innate solar variability perhaps coupled with changing seasonal insolation. Although correlations between Neptune’s brightness and Earth’s temperature anomaly—and between Neptune and two models of solar variability—are visually compelling, at this time they are not statistically significant due to the limited degrees of freedom of the various time series. Nevertheless, the striking similarity of the temporal patterns of variation should not be ignored simply because of low formal statistical significance. If changing brightnesses and temperatures of two different planets are correlated, then some planetary climate changes may be due to variations in the solar system environment.

Update May 10, 2007
See also Stoat.
Update May 17, 2007
See also More Neptune Brightness and Solar Variability.

The problem is, they get their total solar irradiance (TSI) from a model. In and of itself, a model is a good tool. But in this case, its use is misleading. The model for TSI is taken from Foukal (2002). Foukal says “that variations in total solar irradiance are found to be closely proportional to the difference in projected areas of dark sunspots and of bright magnetic plage elements in active regions and in enhanced network area. This difference varies from cycle to cycle and is not simply related to cycle amplitude itself.” I’m not exactly sure what that means. Let’s look at the original paper, and its reconstructed TSI.

A comparison of variable solar total and ultraviolet irradiance outputs in the 20th century

Abstract: Differences in time- variation between total and ultraviolet solar irradiance could help in separating their influence on climate. We present the first models based on area measurements of magnetic plages from CaK spectroheliograms obtained between 1915–1999. Correlation of our time series of UV irradiance with global temperature, T, accounts for only 20% of the global temperature variance during the 20th century. Correlation of our total irradiance time series with T accounts statistically for 80% of the variance in global temperature over that period, although the irradiance variation amplitude is insufficient to influence global warming in present-day climate models. This interesting difference has been obscured in past modelling by additional components introduced to represent secular variations, which are no longer supported by current observational evidence. Future irradiance models emphasizing the more securely- based contributions of photospheric magnetic structures seem to provide better prospects for improved physical understanding of sun-climate links.

From World Climate Report: Neptune News. Originally from Hammel and Lockwood (2007) as adapted from Foukal (2002).

The panel of interest is (C). We see a reconstructed TSI dating from 1920. Remember the model attempts to reconstruct TSI based upon dark sunspots and bright magnetic plage elements. So as long as these measurements were made, no problem. The problem comes when we look at how the model compares to direct observations, which started in the early 1980s. Below is an image of a TSI composite from Greg Kopp.

From Greg’s TSI Page

We see that the actual TSI from 1980 onward has oscillated dramatically, but in general has stayed about the same; some may even say it has decreased. If we go back to the original Foukal paper, the model TSI from 1980 onwards looks a lot like the TSI composite above [not shown]. So why does the model TSI have that crazy upward curve on it? My guess is that when they 11-year smoothed it, they did not properly account for the end of the time series, which was increasing at about the year 2000. Since this (newest) paper was published in 2007, they had access to the latest TSI data, which shows a decrease from 2000-2007. I should point out that the non-smoothed model data does show an increase from 1920-2000, but since there has been no observed increase from 1980-2000 as shown in the direct measurements, there is probably something wrong with the model.

Reference:

Foukal, P. (2002), A comparison of variable solar total and ultraviolet irradiance outputs in the 20th century, Geophys. Res. Lett., 29(23), 2089, doi:10.1029/2002GL015474.

Hammel, H. B., and G. W. Lockwood, 2007. Suggestive correlations between the brightness of Neptune, solar variability, and Earth’s temperature, Geophysical Research Letters, 34, L08203, doi:10.1029/2006GL028764.

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