The basic reason is that before 2007 their were not enough energy in the liquid methane molecules to break the bond between them after 2007 the energy of the molecules increases which breaks the bond and the liquid methane changes phase.
Why didn’t the liquid methane change phase before 2007?
It had been summer since 2002, but the lake didn’t evaporate until 2007.
The molecular attraction between methane molecules is stronger the amount of energy absorbed by liquid methane was not enough to break the inter molecular forces until 2007.
After 2007, the sun had transferred enough energy so that the kinetic energy of the methane molecules increased enough to overcome the attraction between them.
The liquid methane evaporated and the molecules started moving away from each other.
Thus the liquid methane starts to change phase after 2007 by getting enough energy from the sun.
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