How do we add up?

GOV. ARNOLD Schwarzenegger’s $222-billion proposal to rebuild California’s infrastructure is based on nothing more than an educated guess.

He’s betting that the state’s population is growing so rapidly that it will soon create an insupportable burden for California’s existing roads and schools, and that if steps are not taken to address the problem today, a crisis will ensue tomorrow.

Schwarzenegger didn’t make this up. His concern is based on demographic projections by the state Department of Finance that California will add about 1 million people every two years for the next 20 to 30 years, bringing the total population to 46 million by 2025. If true, that’s about 10 million more people than we have now -- an awful lot of people to fit onto the 101 Freeway during the morning rush.

But is he right? How were those numbers calculated? Is it wise to base a nearly quarter-trillion-dollar gamble on their accuracy?

A lot more than just the size of the population is at stake; population growth (or shrinkage) directly affects the state in many ways, including how many classrooms are needed and how crowded state roads are. The kind of population growth -- the age of residents, their location -- has consequences for California’s future as well.

Over the last 50 years, population growth in the state has varied, ranging from less than 4 million in the 1970s to more than 6 million in the 1980s. Of course, demographers don’t just take averages from previous decades to guess future population growth. They calculate detailed birth and death rates for each age group (most demographers agree on fertility and mortality rates) and apply them to current California residents. They factor in anticipated migration from other states, based on trends found in census data, and immigration from other nations as well, based on historical patterns. Putting all these components together and letting a computer add them up one year at a time, demographers arrive at a profile of the future.

So can these experts be wrong? The further one looks into the future, of course, the greater the probability that actual trends will diverge from projected ones. For example, a major earthquake or recession similar in severity to the one in the early 1990s would knock current projections off track.

As insurance against unhappy guesses, the Public Policy Institute of California, working with the Department of Finance, has developed two alternative projections for 2025 California. One scenario is based on fertility and migration rates elevated 10% above the normal, the other on these rates 10% below normal. Viewed that way, the state’s population could add as few as 8 million people or as many as 12 million over 20 years. Still, even if the slower-growth scenario occurred, a California with 46 million people would still arrive -- in 2030, instead of 2025.

The real challenge (and benefit) of population projections is knitting them into infrastructure planning. Location, for instance, is terribly important because, say, a highway or bridge needs to be put where the population will grow most. Accounting for age and ethnicity is also essential. Baby boomers begin retiring in 2011. By 2030, about one Californian in five is projected to be older than 65. Any infrastructure plan must accommodate the needs of this group.

At the other age extreme, and of even greater importance because of education spending, the number of children is expected to change little over the next 10 years. The number of births in California is already falling, and family sizes are declining among young Latinas, who had 3.4 children in 1990 but one fewer today. As a result, public school enrollment is projected to increase only 4% over the next 10 years, a dramatic slowdown after the 21% rise of the last 10 years.

Polls tell us that the majority of Californians dislike population growth, and some may choose to disbelieve the projections for that reason. But it is foolish to deny continuing growth or to avoid the hard work of planning. That would only invite an unlivable future.