No problem, we just burn more coal then, oh wait ..

There's a slight variation of what the GP is proposing, called methane pyrolysis [1]. In that, you separate the hydrogen and the carbon from methane, and then you can burn the hydrogen to get energy, and store the carbon in a landfill. Burning hydrogen creates more energy than it takes to split the methane, so overall the process is exothermic. Right now that process is not economical yet (you need some catalysts, and the carbon doesn't just come out, it sticks to something, you need to wash it with some acids, then retrieve the acids, etc). But active research is ongoing, so there's a chance this will become viable in the next few years.

[1] https://en.wikipedia.org/wiki/Pyrolysis#Methane_pyrolysis_fo...

Easier, but not safer. Pure carbon (mixed with something inert, to prevent burning) doesn't need armed guard for the next 500 years at least.

IMHO, the easiest and cheapest way to store carbon is to grow algae to make hydrocarbons (oils), separate them mechanically, then spread them over a soil as fertilizer, or pump them underground, or burn part of the algae to make a kind of charcoal (to prevent biodegradation).