From Watts Up With That:

https://wattsupwiththat.com/2019/05/01/polar-bears-which-narrative-to-believe/

"The claim that less sea ice will cause polar bears to go extinct is just one narrative, not tested science. From a historical perspective, Derocher’s claim that two-thirds of all polar bears could go extinct by 2030 is laughable. Numerous researchers have reported Arctic temperatures averaged about 3 degrees higher than today between 10,000 and 6,000 years ago and sea ice extent was far less for thousands of years. Clearly, polar bears did not go extinct, and history does not support Derocher’s narrative.

Most importantly, Arctic studies show less sea ice promotes more photosynthesis. After sea ice had recently decreased by 9%, Stanford scientists determined productivity increased by 30%. More photosynthesis provides more food for fish. More fish feed more seals and fatter seals feed more polar bears.

Conversely, there is solid evidence that thick ice is detrimental to seals and bears."