What are the most serious environmental problems nowadays?

Every day we read terrifying articles about global warming that caused ice melting in Greenland or rise of sea level somewhere in the Pacific Ocean. You can hardly find a person in the word that has not heard about it. Scientists are constantly organizing various conferences, trying to attract public attention.
Another great environmental problem is plastic pollution. Since the beginning of last century, humanity has thrown out so much plastic that we need at least several hundred years to decompose it.