Fengzi,

A simplistic word,
Global warming is very slow, So far we've only warmed the earth ~0.63 degrees Celsius.

This isn't an amount that we would notice. But, 1) the world dosn't warm evenly the poles will feel the biggest effect at first. 2) The importance of this warming isn't that it will be a lil hotter. It's that this small change is significant for all kinds of earth systems. Systems that get destabilized by the change.

By destabilizing these systems (systems like the gulf stream, Jet stream and others) Causes climate change. Climate Change is what we are being subject too and is what were worried about.

If you have any doubt, check out how fast records are getting broken around the globe.
All kinds of records: Hottest hots, coldest colds, wettest, driest, stormiest and so on.

It’s quite concerning.