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Washington County could add 100,000 residents over the next 25 years and represent about 5 percent of the state's population, according to projections by the State Demographic Center.

The estimates, which use different methodology than the Metropolitan Council, project that Washington County will be Minnesota's seventh-fastest growing between 2015 and 2040, with its share of the state's population jumping 17 percent in that span. Scott is projected to be the fastest-growing county, the estimates show.

Demographers cautioned against drawing broad conclusions from the findings, which are based on data collected up to 2012 and are unofficial, saying they were just projections. Official estimates will be released later this year.

Just 13 years ago, 4.1 percent of the state's population, or 201,130 people, lived in Washington County, once a sparsely populated rural outpost. Its share of the population is projected to increase from 4.7 percent in 2015 to 5.5 percent by 2040 if current trends continue.

"At the state level, right now the proportion of growth that's due to birth is still above that due to migration, but that is declining over time," said State Demographer Susan Brower. Washington County's population could jump by 100,000 over the next 30 years, Brower said.

Overall, by 2040 the state's population is projected to increase by 14 percent from 5.4 million in 2013 to 6.2 million.

Much of the growth in the county will be fueled by immigrants and births, analysts say. But Brower said these gains will be dampened somewhat by domestic migration, with more people expected to move out than move in over the next 30 years.

Analysts say the county could face a major demographic challenge in the coming decades, with a shrinking workforce unable to support a rapidly graying population.

A separate study, released last year by the Washington County Housing and Redevelopment Authority said that, by 2035, about a third of the county's population will be 55 or older. One in three residents will be older than 55 in 2035, compared with one in five today, the study said.

"We are already seeing a kind of retirement boom going on and that is going to cause some stress on the current workforce," said Todd Graham, a demographer with the Met Council. "The workforce challenge is can you prepare workforce workers to backfill the vacancies left open by retirees."

In 2010, there were about seven working-age adults for each person over 65. By 2035, that ratio will drop to about 3 to 1, the lowest in the metropolitan area, the housing authority study found.

Officials say that to keep pace with the projected growth, communities will need more senior housing and a modern transit system. Officials are also seeking to address the county's changing transportation and water needs.

The Demographic Center's findings are the latest in a series of reports trying to sort out the long-term effects of the dizzying growth of the past decade, in which residential and commercial developments went up on what used to be farmland.

The Met Council's population projections, released in September, were more conservative, predicting a population of 310,220 by 2040. Graham explained the discrepancy in projections by noting that the council uses a different methodology.

"There are parts of Washington County where we expect much less development, because there are some townships in Washington County that plan to remain rural. These townships don't have the land availability for city services to enable suburbanization," Graham said.

Some town officials said the council's estimates have proved overly optimistic in the past.

"When the projections were made, when growth was allocated to Hugo, it was prior to the recession … we had widespread development and the widespread thought at the time was that it would continue," said Bryan Bear, Hugo's city administrator.

But Bear — whose city has for years had the fastest rate of population growth in the county — said the Met Council has since "ratcheted back projections for all of the developing suburbs, so while they're still projecting growth, they're not projecting that much growth for those suburbs."

Libor Jany • 651-925-5033 Twitter:@StribJany