Honolulu Mayor Kirk Caldwell issued a formal directive Monday requiring city departments and agencies to take action to meet the growing impacts of climate change and sea level rise.

Under the mandate, city managers are required to propose revisions to shoreline development rules and construction standards, and take other measures that aim to protect and prepare Oahu for the climate-driven changes expected in the coming decades.

“If we’re going to thrive as a community, whether it be the city and county of Honolulu or any other city on this planet, action needs to be taken now,” Caldwell said at a Honolulu Hale news conference. “The science is irrefutable. And we must adjust.”

The directive, Caldwell said, comes in response to a city Climate Change Commission brief that was developed from the state’s 300-page report on climate change and sea level rise.

The report, adopted in December, forecasts a future of coastal flooding, erosion and property damage affecting hundreds of businesses and public buildings, along with the displacement of 13,300 residents by midcentury and thousands more by the end of the century.

The report projects flooding from climate change-driven global sea level rises of 3.2 feet by midcentury and some 6 feet by the end of the century.

Oahu is the most vulnerable of the Hawaiian Islands, according to the report, with some 9,400 acres within the 3.2-foot sea level rise exposure area, over half of which is designated for urban uses. What’s more, nearly 18 miles of Oahu’s coastal roads would become impassible.

Charles Fletcher, University of Hawaii oceanography professor and vice chairman of the five-member city Climate Change Commission, said the mean sea level won’t exactly rise 3.2 feet by midcentury. Rather, there will be regular king tide events arriving decades ahead of the higher mean sea level.

Those king tides, he said, will cause dramatic erosion, flooding of intersections and roads, and serious groundwater inundation.

“We need to prepare for those, and we can’t do it if we wait until the end of the century,” Fletcher said.

Caldwell’s directive requires all city departments and agencies under his jurisdiction to view climate change and the need to prepare for it as an urgent matter, taking action to reduce greenhouse gas emissions as well as protect and prepare the city for the physical and economic impacts ahead.

It calls for elevating or relocating infrastructure and critical facilities, developing new coastal development regulations and shoreline setbacks, and striving to “conserve a natural, unarmored shoreline wherever possible.”

“Permitting permanent shoreline armoring is generally inconsistent with this directive and should only be considered as a last resort where it supports significant public benefits and will result in insignificant negative impacts to coastal resources and natural shoreline processes,” the directive says.

Caldwell said he recently returned from the World Cities Summit in Singapore, where mayors from around the globe — and especially those from the Asia-Pacific region — are taking action on climate change.

“It is the issue of our time, bar none,” he said Monday. “Things are changing. We can sit on the sidelines and report it, talk about it and commiserate, or we can take action.”

Caldwell suggested there likely will be a time in the coming decades when the island will have to allow its beaches to migrate inland.

“We must liberate our beaches in order to save them,” he said.

In a joint statement, the leaders of the state’s Climate Change and Adaptation Commission — state Land Board Chairwoman Suzanne Case and Leo Asuncion, head of the State Office of Planning — welcomed the mayor’s edict as an important step forward.

Gov. David Ige issued a statement as well:

“Fighting global climate change will take a coordinated effort, and I applaud Mayor Caldwell’s initiatives to plan for and mitigate sea level rise in the City and County of Honolulu.”