Basically, I never understood why (or rather how) a global increase of 1.5° (Celsius) can have as big an impact on the world as it does. How is that seemingly small increase melting the poles so much so that the coastline of many countries in the world might even be pushed back?
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The average temperature of Earth is something like 290 kelvin. So a 1.5 C increase (which is also a 1.5 K increase) is something like half a percent.
I think the thrust of your question is “why does half a percent matter?”
Well, let’s look at sea level. Seawater doesn’t expand linearly with temperature, but let’s imagine for a sec that it did, and that the amount of water on Earth increased by 0.5%. That’s an amount equal to about 20% of the Earth’s total fresh water, an amount a little lower than the total volume of every lake, river, and aquifer on Earth. Or, in absolute terms, it’s about 7 million cubic kilometers of water.
If you added that 0.5% extra water into the ocean, you’d raise sea levels by (7 million cubic km) / (area of oceans) = about 20 meters. [Here’s a map of the state of Florida](https://i.pinimg.com/originals/59/3c/74/593c747813ee646c79b9227a50bd4443.jpg) – everything in blue and purple is underwater, and bright greens are now beachfront. If sea levels *dropped* by that much, a thin land bridge would connect Great Britain to continental Europe, Florida would be double or more the size it is today, and shallow seas all around the world would be new coastline.
In the long-term, worst-case climate scenarios, we end up melting all the ice in Earth’s ice caps and causing (over centuries) a sea level rise several times worse than that.
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