Georgia’s annual economic output will increase by more than $5.2 billion by 2040 — thanks to reduced commute times, jobs brought downtown and additional development — if a multimodal passenger terminal and green space stretching from the Georgia Dome to the Capitol are built in downtown Atlanta, according to a new study.

A major caveat: The master plan for the site is only in the early stages. And as with any long-term economic forecast, the study, commissioned by the downtown development group Central Atlanta Progress, contains educated guesswork.

“There are so many external shocks that can affect things,” said Bruce Seaman, associate professor of economics at Georgia State University, a forecasting expert who was not involved in the study. “Of course it’s going to be a challenge to go out 30 years.”

The city and state have struggled for years to gain traction on a transit hub envisioned for the area residents know as the “Gulch.” The sunken tract of downtown, spread for acres around CNN Center, is crisscrossed with railroad tracks and parking lots. Late last year, the Georgia Department of Transportation signed a $12.2 million contract for a new master plan with a team of contractors experienced in large-scale developments.

“While it is still very early in the process, we’re proud of the work we’re starting and how it will benefit our hometown,” said Larry Gellerstedt, president and chief executive officer of Cousins Properties.

The study released Friday is the latest salvo in more than 17 years of brainstorming about how to get a multipurpose transit center — a central hub linking MARTA, intercity buses and rail — built downtown.

The study by Bleakly Advisory Group, Economic Development Research Group and Kimley-Horn and Associates made some eye-popping financial predictions about the effects of the passenger terminal. Among them:

● Over the next 30 years, nearly $3.1 billion in net additional investment and building in the Gulch area.

● $2.2 billion saved in travel costs for the region’s residents and businesses.

● Nearly 15,700 more jobs brought to downtown Atlanta and up to 8.6 million square feet of additional development attracted to the Gulch and surrounding area.

“This economic impact study highlights the importance of moving forward with the multimodal passenger terminal as well as its direct impact on economic development in downtown Atlanta and the larger Atlanta region,” said A.J. Robinson, president of Central Atlanta Progress. “The redevelopment effects indicated in this study underscore the importance of the [terminal], not only for downtown but the Atlanta region at large.”