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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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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.

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 The reason Hammel used Foukal's outdated TSI reconstruction was she was unable to obtain a reliable TSI dataset dating back to the 1940's. The Max Planck Institute have constructed such a TSI dataset.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?

Updated on 2016-10-26 by John Cook.

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