Neptune and Global Warming

One of those nasty global warming skeptics is pointing to an article about changing brightness of Neptune:

Incredibly, an article has appeared in a recent issue of Geophysical Research Letters showing a stunning relationship between the solar output, Neptune’s brightness, and heaven forbid, the temperature of the Earth. With its obvious implications to the greenhouse debate, we are certain you have never heard of the work and never will outside World Climate Report.

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In the recent article, Hammel and Lockwood, from the Space Science Institute in Colorado and the Lowell Observatory, note that measurements of visible light from Neptune have been taken at the Lowell Observatory in Flagstaff, Arizona since 1950. Obviously, light from Neptune can be related to seasons on the planet, small variations in Neptune’s orbit, the apparent tilt of the axis as viewed from the Earth, the varying distance from Neptune to Earth, and of course, changes in the atmosphere near the Lowell Observatory. Astronomers are clever, they are fully aware of these complications, and they adjust the measurements accordingly.

As seen in Figure 1, Neptune has been getting brighter since around 1980; furthermore, infrared measurements of the planet since 1980 show that the planet has been warming steadily from 1980 to 2004. As they say on Neptune, global warming has become an inconvenient truth. But with no one to blame, Hammel and Lockwood explored how variations in the output of the Sun might control variations in the brightness of Neptune.

Figure 1 (a) represents the corrected visible light from Neptune from 1950 to 2006; (b) shows the temperature anomalies of the Earth; (c) shows the total solar irradiance as a percent variation by year; (d) shows the ultraviolet emission from the Sun (Source: Hammel and Lockwood (2007)).

What would seem so simple statistically is complicated by the degrees of freedom in the various time series which is related to the serial correlation in the data (e.g., next year’s value is highly dependent on this year’s value). Nonetheless, they find that the correlation coefficient between solar irradiance and Neptune’s brightness is near 0.90 (1.00 is perfect). The same relationship is found between the Earth’s temperature anomalies and the solar output. Hammel and Lockwood note “In other words, the Earth temperature values are as well correlated with solar irradiance (r = 0.89) as they are with Neptune’s blue brightness (|r| > 0.90), assuming a 10-year lag of the Neptune values.” The temporal lag is needed to account for the large mass of Neptune that would require years to adjust to any changes in solar output.

I went to find the article, and while you have to pay to read the whole article, the abstract is free:

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.

Now, Neptune has an interesting history to its brightness variations, and there's a lot of published papers on the subject. This letter published in 1991--with one of the same authors--observes that an existing Neptune and solar cycle relationship had recently changed:

FOR almost two decades, Neptune's brightness varied inversely, at the level of a few per cent, with the solar cycle. The anticorrela-tion was so striking that some causal mechanism seemed necessary, and several suggestions were made^1,2. Two different but plausible ideas involving solar-induced global changes in Neptune's atmosphere were a cyclic darkening ('tanning') of stratospheric aerosols caused by varying ultraviolet radiation³ and a variation in the rate of ion-induced nucleation of atmospheric aerosols due to the modulation of galactic cosmic-ray flux by solar activity⁴. In 1990, with the current solar cycle near its peak, however, Neptune departed unexpectedly from the previous cyclic behaviour, attaining its greatest brightness since 1972. Further observations will be needed to decide if the present deviation signals a unique atmospheric phenomenon, and to see if the cyclic anticorrelation will be restored.

It certainly raises questions about the connections that this recent paper mentions--but notice that even back in 1991, there was a recognition that cosmic-ray flux modulation caused by solar activity might be influencing cloud cover on Neptune. Regular readers will know that I have mentioned previously the research that suggests that global warming here on Earth is related to solar output altering cosmic ray flux. More solar output not only means more sunlight warming up the Earth, but less cosmic ray flux, and less cloud cover--increasing the amount of sunlight that reaches the surface.

I found some of this material over at Classical Values, including a discussion of how NASA has observed some evidence of changes in solar activity that suggest that the solar output increases are about to stop:

May 10, 2006: The Sun's Great Conveyor Belt has slowed to a record-low crawl, according to research by NASA solar physicist David Hathaway. "It's off the bottom of the charts," he says. "This has important repercussions for future solar activity."

The Great Conveyor Belt is a massive circulating current of fire (hot plasma) within the Sun. It has two branches, north and south, each taking about 40 years to perform one complete circuit. Researchers believe the turning of the belt controls the sunspot cycle, and that's why the slowdown is important.

Right: The sun's "Great Conveyor Belt" in profile.

"Normally, the conveyor belt moves about 1 meter per second—walking pace," says Hathaway. "That's how it has been since the late 19th century." In recent years, however, the belt has decelerated to 0.75 m/s in the north and 0.35 m/s in the south. "We've never seen speeds so low."

According to theory and observation, the speed of the belt foretells the intensity of sunspot activity ~20 years in the future. A slow belt means lower solar activity; a fast belt means stronger activity. The reasons for this are explained in the Science@NASA story Solar Storm Warning.

"The slowdown we see now means that Solar Cycle 25, peaking around the year 2022, could be one of the weakest in centuries," says Hathaway.

This is interesting news for astronauts. Solar Cycle 25 is when the Vision for Space Exploration should be in full flower, with men and women back on the Moon preparing to go to Mars. A weak solar cycle means they won't have to worry so much about solar flares and radiation storms.

As Simon at Classical Values observes:

If sunspots are going to decline in the near future the global warming era may be over. Especially if the sun's effect on Clouds turns out to be affected by solar activity as some scientists have experimentally proved.

Perhaps we have reached the point where the AGW True Believers need to get their tax structure in place now, or worry about the polar bears attacking them when they hold conferences in New York City.