Each year, humans spew billions of tons of carbon dioxide into the air. About half of it remains aloft. The rest is absorbed by ocean water and plants, slowing the buildup of CO2 and other gasses that are heating the planet.

But this natural check on global warming might not continue.

In a Puerto Rico rainforest, an experiment is underway to figure out what might happen to carbon stored in tropical jungles as temperatures rise. Reporter Daniel Grossman (@grossmanmedia) has more.

This story was supported by the Pulitzer Center on Crisis Reporting.