Methane doesn't stay in the atmosphere nearly as long which means that, in theory, it would cause some short term warming, but then the methane would break down

Methane doesn't stay in the atmosphere nearly as long which means that, in theory, it would cause some short term warming, but then the methane would break down and we would slowly return to more normal levels. There are alot of unknowns, and while a very worst case scenario involves runaway methane causing warming, we don't know how likely this is to happen. We do know that CO2 takes hundreds of years to dissipate and will cause the ice caps to melt which will lead to higher sea levels overall.