For many reasons.

First of all, 1.5°C spread out over the entire atmosphere means that there is significantly more energy inside the atmosphere. This manifests as extreme weather such as storms, floods and hurricanes.

Second, this increase in temperature increases the amount of polar ice that melts, which increases the sea levels, which can lead to more flooding or the disappearance of entire regions or island nations.

Third, some ecosystems are very very sensitive to temperature changes, because they have evolved under a certain temperature for a long time.

OK cool but now I want to know what kind of event would stretch out all the DNA in my body between the earth and the sun