The most valuable thing in a coal mine is not the coal.

Exploring coal mines can be an urgent undertaking, for the fossils, once revealed, do not linger. Once Peabody was done mining a section of coal seam at Vermilion Grove, the company put wire fencing against the ceiling to keep loose pieces of the shale roof from falling to the tunnel floor. Researchers needed to bring flashlights, take pictures, collect samples, and leave before the formation crumbled away.

The good news is that coal science is a renewable resource. Once the old formation is gone, an entirely new fossil exposure is uncovered. At Cerrejón, tropical rains every spring and summer cut huge erosion channels in the shale and mudstone, sending fossils and bones to a watery grave in the crater far below. But in November, researchers can find a whole new set of skeletal remains drying in the sun.

The fossil plants and animals tell scientists a lot about what the prehistoric world was like during each glacial cycle. But researchers are taking the analysis a step further. Global warming and cooling occurs in part as a result of periodic changes in the Earth’s orientation during orbit. But the warming trends are accompanied by dramatic rises in atmospheric carbon dioxide. The key question—for our time—is whether CO₂ is the driver.

Coal is “organic climate,” said paleoclimatologist Isabel Montanez of the University of California-Davis. “It tells us when it was wet and when it was dry.” And coal can be a tool in reconstructing atmospheric CO₂ at particular geological moments. “It’s an environmental dipstick,” Montanez said, “that allows me to tie sea levels to the CO₂ in the atmosphere to continental climate.”

Montanez stays in touch with Elrick and other network members to find where to explore next. For the past two years she has been analyzing fossil plant stomata—the pores through which plants absorb the CO₂ they need for photosynthesis. Her samples come from coal mines in many parts of the world; much of the recent work has been done in the Donets Basin, in the Ukraine. The “stuff on the bottom” of a coal seam is the tail end of a glacial period, she said. Carbon dioxide is low and the plants have “lots more stomata” to get what they need. “Above the coal, there are fewer and fewer stomata,” she added.

That’s when CO₂ is at its highest levels—and when when the floods come. Then everything in the swamps dies or gets washed out to sea where plankton suck up all the loose carbon, and atmospheric CO₂ plunges once again.

So what does this mean for us today? Quite a lot, it turns out. The Earth is in an “interglacial” period right now. Carbon dioxide is rising and the planet is warming. Ice melts, the polar permafrost and tundra thaw out, microbes on land have their own feast of dead vegetation and begin to emit methane—a powerful greenhouse gas—in large enough quantities to increase warming.

The coal tells us that, by itself, there is nothing unusual about climate change, “and when we tell people about it, they think that because it’s a natural process, they don’t have to worry about CO₂ emissions,” Montanez said. “But what we’re doing is not natural.” The natural cycle takes several hundred thousand years. Plants and animals have time to evolve. With the atmospheric changes occurring these days, however, this cycle could happen in centuries. Many plants and critters will not have enough time to adapt.

The coal also tells us that as atmospheric CO₂ rises, the warming effect accelerates. Add the auto emissions and the exhaust from coal-fired power plants for an extra jolt of CO₂, and it’s like spiking the punch. “It will make a huge difference, there’s no doubt about that,” Montanez said. “The question is when.”