According to the IPPCC, we are already experiencing the effects of a 1C increase in average global temperature including increased extreme storm activity, loss of Arctic ice, rising sea levels and coral reef die back.
If society undertakes "rapid and far reaching changes by 2030" we may be able to limit Climate Change to 1.5C versus 2.0C. The difference of global impacts of so doing would include:
coral reefs would decline by up to 90% vs 99%
by 2100, global sea rises would be 10cm lower
the likelihood of the Arctic Oceans being empty of sea ice in summer would occur once per century, rather than once per decade