Airline passenger traffic in the Twin Cities area has shrunk over the past decade, with fewer people flying through Minneapolis-St. Paul International Airport (MSP) today than 10 years ago. Yet the Metropolitan Airports Commission (MAC) is projecting a passenger explosion over the next two decades.

In the latest revision of its long-term comprehensive plan, the MAC said Thursday that it expects 54 million passengers a year to fly through MSP by 2035. With 32 million using the airport in 2014, that's an increase of 68 percent.

Passenger traffic has yet to match the peak set in 2004, when nearly 37 million passengers used the airport. So where are all those extra fliers going to come from?

Through economic growth locally and nationally, said the MAC's Patrick Hogan. The last decade — which included the Great Recession as well as a spate of airline bankruptcies and mergers — was an "anomaly," Hogan said.

"The Twin Cities will continue to grow," he said. "Will it be 54 million? We don't know. That's our best guess based on the data we're looking at."

The MAC works with a consultant to develop its projections, Hogan said. They look at a variety of state, regional and national sources to come up with what they believe is a realistic projection.

Meanwhile, an airline industry consultant said the MAC projections on passenger traffic were "way out of line."

"There's no way they're going to grow 68 percent," said Mike Boyd, president of Colorado-based Boyd Group International.

Boyd said it's no longer useful to project airline traffic based on general trends in demographics and the economy. His own projections call for Twin Cities passenger traffic to grow 27 percent over the next decade. If everything goes well, he said, the airport could see a 45 percent to 50 percent boost by 2035.

"Minneapolis is not going to be a growth hub for Delta [Air Lines]," Boyd said. "When you compare it to places like Detroit — which has a larger industrial base — or Atlanta, Minneapolis doesn't have a lot of the horsepower that it would need to grow 68 percent."

If those tens of millions of new passengers were to materialize, they would weigh heavily on a facility that is already straining to meet travelers' needs, officials said Thursday in advance of a MAC meeting to discuss the projections.

"These numbers are key to understanding where and at what point the airport may experience deficiencies at its facilities," the MAC said in a statement.

The run-up to the comprehensive plan revision has included 31 briefings with city governments and others throughout this year.

A draft of the plan will be published in September. In October, more input will be taken from the public in two meetings.

Hogan said the comprehensive plan doesn't authorize any construction; it's a planning document only. Any future large-scale construction projects at the airport are likely to focus on the outside of the terminals, rather than the inside, Hogan said.

Improvements could include "a lot more parking," as well as more road capacity and more curbside space to pick up and drop off passengers.