WASHINGTON — Up and down the coastline, rising seas and climate change are transforming a fixture of American homeownership that dates back generations: the classic 30-year mortgage.

Home buyers are increasingly using mortgages that make it easier for them to stop making their monthly payments and walk away from the loan if the home floods or becomes unsellable or unlivable. More banks are getting buyers in coastal areas to make bigger down payments — often as much as 40 percent of the purchase price, up from the traditional 20 percent — a sign that lenders have awakened to climate dangers and want to put less of their own money at risk.

And in one of the clearest signs that banks are worried about global warming, they are increasingly getting these mortgages off their own books by selling them to government-backed buyers like Fannie Mae, where taxpayers would be on the hook financially if any of the loans fail.

“Conventional mortgages have survived many financial crises, but they may not survive […]