GOOD NEWS? In the Journal of Preventive Medicine on Monday, health-policy experts estimated that 42 percent of American adults will be obese in 2030. That’s a 9-point drop from an earlier projection.

Forgive us if we don’t cheer at the prospect of adding 32 million to the total number of Americans who are dozens of pounds too heavy, a count that stood in 2010 at 78 million. Forty-two percent — or even 36 percent, the 2010 rate — is a public-health emergency that hurts not only the morbidly overweight — who face higher rates of diabetes, heart disease and other deadly illnesses — but everyone else who has to help pay for treating the consequences of obesity, a preventable condition.

Reuters reports that the country already devotes a fifth of its health-care spending, $190 billion a year, to obesity-related costs. If America could just maintain its current obesity rate, it would save $550 billion over the next two decades, the researchers noted Monday.

It’s not easy to transform deeply ingrained habits and attitudes, particularly given humans’ biological predisposition to gobble calories and remain at rest. But as a National Academy of Sciences report released on Tuesday indicates, there are policies worth trying.

The government should eliminate programs that encourage unhealthy eating, such as agriculture supports, and direct the money it spends on school lunches or Supplemental Nutritional Assistance Program (SNAP) benefits — formerly known as food stamps — toward healthier foods. The feds must approach the issue delicately, not dictating diets to the poor. Yet there are reasonable ways to use federal leverage — SNAP spending last year totaled $72 billion — to encourage purchases of oranges over orange soda.

With healthier lunches, the controlled environment in public schools can be a primary tool in fighting obesity. Start by removing sugary soda and candy from vending machines. Provide more time for exercise, instruction on why it matters and education on healthy eating that isn’t a joke. And when students leave campus, local planners should ensure they have access to sidewalks, ballfields and bike paths, which would make communities more livable for everyone.

The most progress might come when people have sharper incentives to improve their habits. Public-health advocates suggest a tax on sugary drinks or candy. Insurance plans can reward healthful habits, as could Medicaid. At the least, simple things such as screening for unhealthy eating and lack of exercise should become routine.

Like the battle against tobacco, the fight against obesity might be a slow, hard campaign in changing minds and behaviors. But it is no less important.