Final answer:
The most severe and long-lasting impact of climate change will be rising sea levels, which will make coastal cities unlivable and displace human populations. Extreme weather events will also cause loss of life and property damage.
Step-by-step explanation:
The most severe and long-lasting impact of climate change, critically affecting the largest global population numbers, will be rising sea levels. This will make some large coastal cities unlivable and lead to the displacement of human populations. Additionally, extreme weather events caused by climate change, such as hurricanes and forest fires, can cause loss of life and property damage.