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What does Neptune's brightening mean for global warming?

What the science says...

Select a level... Basic Intermediate

Neptune's orbit is 164 years so observations (1950 to present day) span less than a third of a Neptunian year. Climate modelling of Neptune suggests its brightening is a seasonal response. Eg - Neptune's southern hemisphere is heading into summer.

Climate Myth...

Neptune is warming

In April 2007, Heidi Hammel published a study comparing Neptune's brightening to Earth's warming and solar variations. She concluded that while they don't correlate statistically, the patterns are visually compelling and planetary climate changes may be due to solar variations.

The crux of this study is the correlation between solar irradiance and temperatures on Earth and Neptune. However, the solar data comes from an erroneous TSI reconstruction by Peter Foukal that contradicts direct satellite measurements, sunspot numbers and solar radio flux or flare activity. Foukal used an uncalibrated sunspot dataset compiled from different observatories, which were unfortunately not consistent with each other. After 1976 when Greenwich Observatory stopped observing, Foukal found a rapid increase in irradiance, due to the fact that the sunspot areas recorded by the USAF's SOON network are too small.

Since then, Foukal has concluded that solar variations are too small to have contributed appreciably to accelerated global warming over the past 30 years. The reason Hammel used Foukal's outdated TSI reconstruction was she was unable to obtain a reliable TSI dataset dating back to the 1940's. The Max Planck Institute have constructed such a TSI dataset. Comparing it to Foukal's TSI as well as Earth's and Neptune's temperature plots (the red line is the updated TSI data) shows there is no correlation in solar activity over the past 30 years.

So if the sun isn't getting hotter, what's causing Neptune's warming? Neptune's orbit is 164 years so observations (1950 to present day) span less than a third of a Neptunian year. Climate modelling of Neptune suggests its brightening is a seasonal response (Sromovsky 2003). Eg - Neptune's southern hemisphere is heading into summer.

If you wish to determine whether solar variations are causing global warming on Earth, you're better off studying Earth's climate for which we have a wealth of data (as opposed to a single dataset of brightness levels over 50 years). You can check out the many studies of solar influence on Earth's climate on our page Are solar variations causing global warming?

Last updated on 7 January 2017 by John Cook. View Archives

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Many thanks to Heidi Hammel for kindly answering my questions re her Neptune research. Thanks also to Sami Solanki for his advice and generously sharing the Max Planck TSI reconstruction data.


Comments 1 to 7:

  1. rrrrriiiiiiiigggggggggghhhhhhhhhhhhhttttttt So its one feeble excuse after another. Here we have a general solar system trend and any excuse is enough not to blame it on the sun. Its this static equilibrium light and air model that is to blame here. If you thought instead in terms of accumulating and decumulating joules imbedded in the planet than this levelling of in the suns activity would be immaterial. So long as the sun was strong enough to keep the planets accumulating joules.
  2. The reported connection between brightenings of Neptune and of the Sun is suspect even from cursory inspection of the vastly different ordinate scales in the Hammel and Lockwood figure you show. The fractional size of the Neptune brightening ( ~ 0.15 mags, or 15%) is roughly ONE THOUSAND times greater than the Sun's brightening ( by ~ 0.02%).I pointed out this huge disparity to Heidi when she first sent me a draft, and it is mentioned as a problem in her paper.This disparity by almost three orders of magnitude overshadows whatever correlation may exist between irradiance or global temperature. Second, I have to comment on your quote from Sami Solanki that our TSI time series, used by Heidi, is erroneous. His basis is that our model overlooks the scale change in spot area measurements after the RGO program closed. The plot you present comparing our model to the one provided to you from MPI begins to show divergence not around 1976 (when RGO spot areas stopped), but only around 1985. Also, the MPI model plotted by Solanki and Fligge ( GRL, 25,341,1998)shows a similar rise of irradiance into the 1990's as our model, even though it uses the "corrected" spot areas preferred by MPI. For both these reasons it is hard to believe that this claimed spot area scale change (which is not widely accepted anyway) is the main reason our model shows solar brightening that disagrees with radiometry. Frankly, it is unclear at this point which to believe - the model or the radiometry. We are, after all, dealing with changes in TSI of a few hundredths of one percent over multi-decadal time scales. This is at the edge of what even optimists believe about the stability of the radiometry.Both radiometry and models have been subject to so much tweaking since 1978 that, as a 35 -yr veteran of this field, I have both a healthy regard and skepticism for both approaches. The lack of recent TSI increase seen in the radiometry happens to be more acceptable at present, but the error bars are still large enough to raise questions. The main argument against TSI driving of recent global warming still comes not from the direction of TSI change, but by its insufficient MAGNITUDE to drive the recently accelerated global temperature increase ( e.g. Foukal et al., Nature, 443, 161, 2006). Peter Foukal
  3. Late thanks but well deserved for Dr. Foukal. That puts everything in perspective.
  4. TSI is your argument? Do you really think that its only TSI that affects Plantes Climate? WOW!
  5. Thebadmentor@4 Can you suggest another forcing that affects both Neptune and the Earth. This article is a counter-argument to the suggestion that solar focing is responsible for climate change on Earth as it is also causing climate change on Neptune (except it isn't). It is the skeptics that seem to think only TSI affects planetary climate ("its the sun"). P.S. Yes, I do know about Svensmark's cosmic ray theory as well, but you would have a hard time applying that argument to Mars, which is also used as an example in such arguments.
  6. Dikran Marsupial @5, I suspect Svenmark would have a hard time applying his theory to Neptune as well. Is he some how going to increase Neptune's 100% cloud cover?
    Response: [Dikran Marsupial] Good point! ;o)
  7. Max Plank institute link is broken (source moved).

    (posted in the corrct thread this time :D )

    ... maybe Sromovsky 2003 link too?

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