Several thousand years ago, during the last ice age, sea levels were lower, so much lower so that a land bridge formed between Alaska and Russia. It is commonly thought that this land bridge was the origin of the American Indians.
After the ice age ended, the ice that covered North America (and Eurasia) melted, sea levels increased, and Russia and Alaska were separated by the Bering Strait
Or, tectonic plates could have been pushed up out of the water.
The earth is always warming and cooling and always has been, it has nothing to do with human activities.