Secretary of State John Kerry warned Wednesday that climate change is the biggest threat planet Earth has faced yet, and said it threatens to make the current list of human problems far worse if not stopped.

“If you think current conflicts are all-consuming, imagine what happens when we add food shortages, water shortages, stronger storms, longer droughts, steady rises in sea levels, which are already being predicted, and entire countries swallowed by the sea,” he said in a speech at the University of Oxford in the United Kingdom.