STOCKHOLM — If governments are serious about the global warming targets they adopted in Paris, scientists say they have two options: eliminating fossil fuels immediately or finding ways to undo their damage to the climate system in the future.

The first is politically impossible — the world is still hooked on using oil, coal and natural gas — which leaves the option of a major cleanup of the atmosphere later this century.

Yet the landmark Paris Agreement, adopted by 195 countries on Dec. 12, makes no reference to that, which has left some observers wondering whether politicians understand the implications of the goals they signed up for.

“I would say it’s the single biggest issue that has to be resolved,” said Glen Peters of the Cicero climate research institute in Oslo, Norway.

Scientists refer to this envisioned cleanup job as negative emissions — removing more greenhouse gases from the atmosphere than humans put in it.

Right now we’re putting in a lot — about 50 billion tons a year, mostly carbon dioxide from the burning of fossil fuels for energy.

There are methods to achieve negative emissions today but they would need to be scaled up to a level that experts say could put climate efforts in conflict with other priorities, such as eradicating hunger. Still, if the Paris climate goals are to be achieved, there’s no way to avoid the issue, said Jan Minx of the Mercator Research Institute on Global Commons and Climate change in Berlin.

“My view is, let’s have this discussion,” he said. “Let’s involve ourselves in developing these technologies. We need to keep learning.”

The Paris Agreement was historic. For the first time all countries agreed to jointly fight climate change, primarily by reducing the emissions of carbon dioxide and other greenhouse gases.

Governments vowed to keep global warming “well below” 2 degrees Celsius (3.6 degrees Fahrenheit) compared with preindustrial times. But even 2 degrees of warming could threaten the existence of low-lying island nations faced with rising seas. So governments agreed to try to limit warming to 1.5 degrees C (2.7 degrees F), which is just half-a-degree above the global average temperature this year.

That goal is so ambitious — some would say far-fetched — that there’s been very little research devoted to it. In Paris, politicians asked scientists to start studying how it can be done.