Forecasts show that Silicon Valley is at risk even under optimistic scenarios where rapid cuts in greenhouse gas emissions avoid the most severe increases

Technology giants including Facebook and Google face the prospect of their prestigious Silicon Valley headquarters becoming swamped by water as rising sea levels threaten to submerge much of the property development boom gripping San Francisco and the Bay Area.

Sea level forecasts by a coalition of scientists show that the Silicon Valley bases for Facebook, Google and Cisco are at risk of being cut off or even flooded, even under optimistic scenarios where rapid cuts in greenhouse gas emissions avoid the most severe sea level increases.

Without significant adaptation, Facebook’s new campus appears most at risk. The 430,000 sq ft complex – topped with a nine-acre garden rooftop – is an extension of its Menlo Park base and was crafted by architect Frank Gehry. Located near the San Francisco Bay shoreline, the offices are designed to house 2,800 staff.

“Facebook is very vulnerable,” said Lindy Lowe, a senior planner at California’s Bay Conservation and Development Commission. “They built on a very low site – I don’t know why they chose to build there. Facebook thinks they can pay enough to protect themselves.

Facebook thinks they can pay enough to protect themselves Lindy Lowe

“The temporary flooding within the campus can probably be addressed, but the temporary flooding onto the roadway can’t be addressed by them. I think they realize that is the weakest link for them. We’ll see how dedicated they are to that facility.”

Facebook has elevated its office to spare it from flooding, but even with a 1.6ft rise in sea levels by the end of the century – which is towards the lower end of projections – the area around it will be inundated. Much sooner, within the coming decades, the roads leading into the complex will flood so regularly that major adaptions will be required to keep the site viable. Facebook didn’t respond to repeated requests to comment on the issue.

The situation is a little better for Google, located in Mountain View and also unwilling to discuss sea level rise, and Cisco, headquartered in San Jose. But should the Antarctic ice sheet disintegrate, as outlined in a recent scientific paper, seas will be pushed up beyond 6ft and swamp both businesses.

The situation is similarly stark for Salesforce, which would see its San Francisco base submerged under the worst sea level rise scenario. Meanwhile, Airbnb, located near the vulnerable Mission Bay area, will have its headquarters gain a much closer bayside view simply by staying put.

“Even with a small increase, the sea comes into the 101 highway by the Googleplex – whole areas could be screwed up,” said professor Kristina Hill, an environmental planning and urban design expert at UC Berkeley. “Google and Facebook will have to redo their campuses. I don’t think there’s been much success in getting Google to support adaption, it’s not really on their radar.”

Nearly $100bn in commercial and residential property around the Bay Area is at risk from sea level rises and severe storms without adaption, with an estimated $21bn in new developments planned for areas vulnerable to inundation and storm surges. San Francisco’s airport, the new $1bn stadium for the Golden State Warriors, the Giants’ baseball stadium AT&T Park and sections of the Union Pacific railroad – a vital artery for the region – face significant long-term peril.

“There are projects that are problematic that are being approved,” Lowe said. “We have a lot of infrastructure pushed right up against the shoreline. There are parts of the shoreline in trouble at a 2-3ft (increase). When you get to 4-5ft, things start to go, significantly. Like, everything. It’s a tipping point.”

Facebook Twitter Pinterest The situation is a little better for Google, located in Mountain View and also unwilling to discuss sea level rise. Photograph: Sipa USA/REX

While not facing as severe a situation as Miami Beach, the Bay Area has some major challenges. Much of the region’s key infrastructure has been built upon low-lying wetlands that have been filled in. Liquefaction – a process where the earthquakes can weaken soils, turning them to “Jello”, according to BCDC coastal planner Wendy Goodfriend – risks water spouting up from underground onto the streets.

“The geological basis of the area is a lot of loose material, such as gravel and sand,” Hill said. “Underground freshwater could be pushed up into pipes that are typically cracked due to seismic activity. When those pipes fill up, rainwater won’t drain away properly and you’ll get basements filling up with sewage.

“Areas of land have subsided after they were filled in. San Jose has sunk by 15ft since it was a peach orchard. This isn’t a great combination with sea level rises and means that we are in a vulnerable situation.”

Several recent reports have warned that San Francisco is unprepared for sea level rise and some critics, including Hill, have accused the city of prizing investment in its booming property market – where house prices have leapt 62% in the past four years – over all else.

“People’s lives and property are at stake and we’ve been doing some fairly insane things in developing these areas,” Hill said.

She said plans for Mission Bay (soon to be home of the Warriors) and Treasure Island (a chunk of land near the Golden Gate Bridge earmarked for 8,000 homes and three hotels) were “fantasy projects”.

“The developers’ interest is to sell the properties, ideally before they open,” she said. “Everyone is invested in a quick turnaround, including the developers and the government. The people who are on the hook are those who will buy or rent these places for the next 25 years.”

The city of San Francisco insists it is taking sea level rise seriously and is putting serious mitigation measures in place. Its own planning documents show it expects parts of the bay shoreline to be submerged, as well as strips of the western coast to be eaten up by the rising Pacific Ocean. Public assets including the airport, fire department and port are at risk.

Without adaptation measures, the frequency of temporary flooding will increase with rising seas in low-lying areas San Francisco planning documents

“Without adaptation measures, the frequency of temporary flooding will increase with rising seas in low-lying bayshore areas, until permanent inundation is reached,” the document states, warning there could be up to an extra 108 inches of water if dire sea level rises are temporarily exacerbated by an extremely strong storm.

“We know sea level rise isn’t going to stop at 2100 so we are focused on adaptive solutions, such as earth berms, landscape solutions and levees that can be raised in the future,” said Gil Kelley, director of citywide planning for San Francisco.

“Some of the early planning done in the Mission Bay district was done without much attention to sea level rise and they will need some sort of perimeter fence. We are open to protection, adaption or retreat – all three approaches are valid.”

Kelley said new developments have to provide environmental plans that include sea level concerns and insists that areas such as Treasure Island will be safe. But even if buildings are raised, the areas around them remain vulnerable. There is no comprehensive plan to deal with this yet, so later this year San Francisco will hold the Bay Area Resiliency Design Challenge, inspired by New York’s response to Hurricane Sandy in 2013.

“We don’t have the solutions yet, which is why we are trying to learn,” Kelley said. “We can plan for this. We have a big scientific community and can engage them through a global design challenge.”

Ideas already abound, including Hill’s proposal to dump dredged material into the edge of the bay to make it shallower, thereby reducing the impact of waves and storms. More outlandishly, some call for the Golden Gate Bridge to be complemented by an underwater barrier to completely shut off water flowing into the bay. Whatever the solution, it needs to happen soon.

“There are some areas that just won’t be defendable; it will be hard to adapt because the water will come from everywhere,” Lowe said. “The longer we wait to come up with an approach, the more likely we’ll be in retreat mode. Retreat is usually not intentional.”