Atmospheric methane was abundant billions of years ago. Then somehow life appears and transforms the composition of our atmosphere.
Now humans are terraforming us back towards the initial stage.
The brightening is caused by accumulation of helium ash in the Sun's core. This causes the core's equilibrium state to be denser and hotter, with more energy being produced.
In another billion years or so, this will drive the Earth into a runaway greenhouse effect. The oceans will evaporate and the hydrogen will be lost to space. Not long after, the Earth will be a dead planet devoid of any life, even microorganisms. You sometimes see statements about how when the Sun expands into a red giant it will destroy life on Earth, but the Earth will have long been lifeless by that time.
What evidence do you have for the sloppiness and how does the current model fail to explain the deviation in observations?
If we make the assumption that our model applies everywhere and at every time (colloquially, that the laws of physics don't change), we can ask our models what happened in the past. Under our assumption, that's probably quite close to what actually happened. This assumption is called the "principle of induction", the "uniformitarian principle", the "cosmological principle", the "Copernican principle", and many other things besides.
Ash is the name given to the energetic alpha-particles or helium nuclei produced by fusion reactions in a deuterium-tritium plasma, even though helium bears no physical resemblance to ash from a fire.
So who is right?
Sun has been getting about 8% brighter (8% more energy output) every 1 billion years.
The article linked by one of your commenters is super interesting (assuming it is correct).
Life itself is very much this: one giant, ongoing, turbulent chemical reaction, brewing in a pot the size of a planet.