Start saving your money, kids.

Unless we start cutting carbon dioxide emissions soon, it’s going to cost today’s young people as much as $535 trillion to clean up the atmosphere by 2100, according to a study published on Tuesday evening. By way of context, that’s around seven times the size of the entire global economy.

In contrast, if the world starts reducing emissions 6 percent a year by 2021, it will only cost $8 to $18.5 trillion to extract enough carbon dioxide to avoid the worst dangers of climate change, or $100 billion per year on the low end.

It isn’t news that lowering greenhouse-gas emissions by switching to clean energy sources will ultimately be far less expensive and risky than any unproven technological means of sucking up carbon dioxide. But the study, led by the well-known climate researcher James Hansen, a professor at the Columbia University Earth Institute, sought hard figures to add scientific weight to a closely watched case in which 21 young plaintiffs have sued the federal government for failing to adequately combat climate change. Earlier this year, a federal judge in Oregon ruled that the case could move forward.

Hansen, who is a plaintiff in that suit, is considered the father of climate research by some, thanks to his early modeling studies as a scientist at NASA and his landmark congressional testimony, which is credited with drawing the first widespread public attention to the issue of global warming.

Still, in some ways the study is more of a thought experiment than a prediction of actual outcomes. Indeed, there are several reasons why it might overestimate eventual costs.

Notably, the first scenario in the study assumes that carbon dioxide emissions will continue to rise by 2 percent per year throughout the century. But there are encouraging signs that they won’t. Global emissions have already been flat for the last three years, according to the International Energy Agency. What’s more, nearly 200 nations signed on to the Paris climate accords, committing to significantly roll back emissions in the coming years—specifically, to 40 percent below 1990 levels by 2030, in the case of the European Union.

In addition, the research found that the first scenario would require extracting more than 1,000 gigatons of carbon dioxide, versus 150 gigatons if we begin cutting emissions in 2021. Of the latter number, about two-thirds could be absorbed from the atmosphere through better agricultural and forestry practices, the study estimates.