In decades of public debate about global warming, one assumption has been accepted by virtually all factions: that tackling it would necessarily be costly. But a new report casts doubt on that idea, declaring that the necessary fixes could wind up being effectively free.

A global commission will announce its finding on Tuesday that an ambitious series of measures to limit emissions would cost $4 trillion or so over the next 15 years, an increase of roughly 5 percent over the amount that would likely be spent anyway on new power plants, transit systems and other infrastructure.

When the secondary benefits of greener policies — like lower fuel costs, fewer premature deaths from air pollution and reduced medical bills — are taken into account, the changes might wind up saving money, according to the findings of the group, the Global Commission on the Economy and Climate.

“We are proposing a way to have the same or even more economic growth, and at the same time have environmental responsibility,” said the chairman of the commission, Felipe Calderón, the former president of Mexico and an economist. “We need to fix this problem of climate change, because it’s affecting all of us.”