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What does Neptune's brightening mean for global warming?

What the science says...

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The sun has been cooling while Neptune was warming.

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.

This argument is part of a greater one that other planets are warming. If this is happening throughout the solar system, clearly it must be the sun causing the rise in temperatures – including here on Earth.

It is curious that the theory depends so much on sparse information – what we know about the climates on other planets and their history – yet its proponents resolutely ignore the most compelling evidence against the notion. Over the last fifty years, the sun’s output has decreased slightly: it is radiating less heat. We can measure the various activities of the sun pretty accurately from here on Earth, or from orbit above it, so it is hard to ignore the discrepancy between the facts and the sceptical argument that the sun is causing the rise in temperatures.

TSI from 1880 to 1978 from Solanki. TSI from 1979 to 2009 from PMOD.

But if the sun’s output has levelled off or even diminished, then what is causing other planets to warm up? Are they warming at all?

The planets and moons that are claimed to be warming total roughly eight out of dozens of large bodies in the solar system. Some, like Uranus, may be cooling. All the outer planets have vastly longer orbital periods than Earth, so any climate change on them may be seasonal. Saturn and its moons take 30 Earth years to orbit the Sun, so three decades of observations equates to only 1 Saturnian year. Uranus has an 84-year orbit and 98° axial tilt, so its seasons are extreme. Neptune has not yet completed a single orbit since its discovery in 1846.

This is a round-up of the planets said by sceptics to be experiencing climate change:

  • Neptune: observations of changes in luminosity on the surface of both Neptune and its largest moon, Triton, have been taken to indicate warming caused by increased solar activity. In fact, the brightening is due to the planet’s seasons changing, but very slowly. Summer is coming to Neptune’s southern hemisphere, bringing more sunlight, as it does every 164 years.
  • Jupiter: the notion that Jupiter is warming is actually based on predictions, since no warming has actually been observed. Climate models predict temperature increases along the equator and cooling at the poles. It is believed these changes will be catalysed by storms that merge into one super-storm, inhibiting the planet’s ability to mix heat. Sceptical arguments have ignored the fact this is not a phenomenon we have observed, and that the modelled forcing is storm and dust movements, not changes in solar radiation.
  • Mars: the notion that Mars is warming came from an unfortunate conflation of weather and climate. Based on two pictures taken 22 years apart, assumptions were made that have not proved to be reliable. There is currently no evidence to support claims that Mars is warming at all.
  • Pluto: the warming exhibited by Pluto is not really understood. Pluto’s seasons are the least understood of all: its existence has only been known for a third of its 248 -year orbit, and it has never been visited by a space probe. The ‘evidence’ for climate change consists of just two observations made in 1988 and 2002. That’s equivalent to observing the Earth’s weather for just three weeks out of the year. Various theories suggest its highly elliptical orbit may play a part, as could the large angle of its rotational axis. One recent paper suggests the length of Pluto’s orbit is a key factor, as with Neptune. Sunlight at Pluto is 900 times weaker than it is at the Earth.

Claims that solar system bodies are heating up due to increased solar activity are clearly wrong. The sun’s output has declined in recent decades. Only Pluto and Neptune are exhibiting increased brightness. Heating attributed to other solar bodies remains unproven.

Last updated on 15 September 2010 by gpwayne.

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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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