They're not trying to stop all greenhouse gas emissions. Just excess greenhouse gas emissions. The hope is that the temperature will gradually decrease back to near natural levels, thus averting further sea level rises and severe weather.

I know a few degrees C doesn't seem like much, but that's just because earth is so ridiculously huge. The epa has a good example of this here.

> For reference, an increase of 1 unit on this graph (1 × 10^22 joules) is equal to approximately 17 times the total amount of energy used by all the people on Earth in a year.

We'd have to build 17 duplicates of every power plant on earth to generate as much energy as the ocean absorbed. How can this be? Well, as it turns out there's a much bigger source of heat called The Sun.

Our little itty bit of extra carbon dioxide traps a little itty bit of extra energy from the sun. How much of a little itty bit? Well, from the data? 17x every power plant humanity has running right now. Relative to the giant ball of fusion we orbit? Tiny. Relative to us as a species? Pretty big.

edit: an exponent