For the sun to do this to Neptune (and Pluto), the effect on earth would be 900 times as much. That is, the oceans would boil. Boil off.
Not necessarily because you are assuming increased solar output would effect the planets the same way.
Neptune and Earth are two totally different planets in size, atmospheric makeup, distance from the sun, orbital eccentricity, etc. So you wouldn't expect them to react in the same exact matter over the same period of time to changes in solar variability.
See some basic Neptune atmospheric chemistry
Clouds (On Neptune) form this way: solar ultraviolet radiation destroys methane high in Neptune's atmosphere by converting it to hydrocarbons such as ethane, acetylene, and haze particles of more complex polymers. The haze particles freeze and become ice particles. These ice particles eventually fall into the warmer layers of the atmosphere, where they evaporate back into gases, mix with hydrogen gas and are reformed as methane. Methane clouds then rise high into the atmosphere.
This fits perfectly on how a more active sun would cause global warming on Neptune
What is happening is an increase in solar output means increased UV radiation which will destroy more methane in the atmosphere which then freeze out of the atmosphere as more complex hydrocarbons, which will release heat (from solidification) thus warming the atmosphere. Then as these hydrocarbons sink they will react with hydrogen and reform into methane which will again release heat and warm up the atmosphere further. The newly created methane as it rises will form into clouds increasing Neptune's brightness, as the clouds dissipate the cycle repeats with greater intensities, which = Global warming on Neptune kicked off by a more active sun.
“...the effect on earth would be 900 times as much. That is, the oceans would boil. Boil off.”
Don’t laugh, this is a serious comment to yours...our greenhouse gasses protect us.