The report says that if we do not stop using fossil fuels over the next 5 years (note that the 5 years the time frame to stop using the fossil fuels), it would spell the end of humanity as we know it (this timeline is open, no date is set). It's a slow process mind you. First heatwaves (which we are seeing today) that make living along the equator impossible. You will notice this as a massive wave of immigration (much larger than any war has caused). Then rising food prices of course when agriculture is no longer viable in large parts of the world. Rising sea levels may not be noticabe in your part of the world (like the Nordics for example where the ground is also rising since after the last ice age) but people in the parts where it is a problem, will have to emigrate, further raising food prices and a more dense population where they immigrate.

I know thinking about long time spans is hard. Most can't imagine a time before or after their own lives. But I hope you can understand now why this is not a cliff but a slippery slope. Once we start gliding it might be almost impossible to stop it.