“The California coast is pretty unprotected,” said Helen Fricker, a glaciologist at the Scripps Institution of Oceanography who co-authored the study. “There’s not really anywhere for the water to go except onto the land.”

In oceanfront cities like Newport Beach, big waves that occur during high tides regularly cause coastal flooding. Now that city is raising the height of its seawalls by 6 inches, with the option to add on later if necessary.

“I liken it to Legos,” said Newport Beach City Manager David Kiff. “You snap one on, and then you snap something on, on top of that, when and if you need to do so.”

How much Newport Beach will have to extend its walls is uncertain, and completely depends on global efforts to curb greenhouse gas emissions.

If nothing is done and we continue emitting greenhouse gases at the rate we are now, sea levels are likely to rise in Southern California by up to 3.6 feet by 2100, according to the report.

Even if we dramatically cut our greenhouse gas emissions and keep global temperatures from rising more than 2 degrees Celsius above pre-industrial levels (the goal of the 2015 Paris Agreement), sea levels will still rise, although not as much: up to 2.5 feet by 2100.

'The California coast is pretty unprotected.' Helen Fricker, glaciologist

It's uncertain whether people and governments around the world can cut enough greenhouse gases to avoid the worst-case scenario. In March, President Donald Trump announced efforts to undo President Obama's Clean Power Plan, which would have required nearly all states to devise plans to cut carbon emissions from power plants. The plan was intended to play a major role in U.S. efforts to meet its goal under the Paris climate accord of cutting emissions by 2025 by roughly a quarter of what they were in 2005.

Because people have so much influence over the outcome, and because scientists aren’t sure how ice sheets in Antarctica and Greenland will respond to warming temperatures, it’s still unclear exactly how much the sea will rise. But one thing is clear: We can’t stop the sea from rising anymore.

“Antarctica and Greenland are not going to stop losing mass,” Fricker said. “It’s not like we’re suddenly going to see a downturn in sea level.”

That’s why the report’s authors recommend coastal communities take action now.

“Waiting for scientific certainty is neither a safe nor prudent option," they write.