"Even under the most optimistic projections of global emissions reductions, Boston faces serious risk from climate change and must adapt."

So states a massive new report from the city, one which details the challenges officials expect to face in the future, and which seeks to provide a framework for how Boston should best adapt.

"The challenge of climate change is here, in Boston, now," Mayor Marty Walsh writes to open the final report of his Climate Ready Boston initiative. "As the century progresses, the effects of climate change will grow."

The report's climate projection consensus forecasts a further increase in extreme temperatures and precipitation, and more regular flooding and damaging storms as sea levels continue to rise.

For instance:

"Compared to the period from 1971 to 2000, when an average of 11 days per year were over 90 degrees, there may be as many as 40 days over 90 degrees by 2030 and 90 days by 2070—nearly the entire summer." "As sea levels continue to rise, severely damaging floods will [by the late century] shift from a rare occurrence to a monthly reality."

The report — stretching 400 pages, and involving dozens of stakeholders — details how climate change could affect individual city neighborhoods, and includes figures on potentially vulnerable populations, buildings and transportation.

"That's the grim part," Austin Blackmon, the city's chief of environment, energy and open space, said a news conference Thursday. "But the good news is we know there are policies in place, we know there are policies we can enact to make sure Boston continues to thrive over the timeframe of this report."

The report offers 11 strategies city officials see to reducing Boston's "vulnerability now and over the next few decades," including retrofitting existing buildings, updating zoning and building regulations to support climate readiness, and creating a coastal protection system to address flood risk.

The coastal protection system is the most far-reaching idea — a system of seawalls and gates in and around Boston Harbor that could temporarily close during storms and sea surges.

"This is a big question, about whether something out there is feasible," said Bud Ris, a senior adviser to the Barr Foundation, which funded the report. "And a project like that is probably going to take 30 or 40 years."

A study to analyze the need and feasibility alone could cost $3 million; building a system of seawalls and harbor gates -- billions of dollars more.

The report comes at an uncertain time for climate advocates, given the incoming Trump administration.

The president-elect's pick to head the Environmental Protection Agency, announced just Wednesday, has been battling President Obama's climate change initiatives in court and has said the debate over global warming "is far from settled," despite the overwhelming scientific consensus.

With reporting by WBUR's Bruce Gellerman