Forests Didn't Fare Well During an Ancient Global Warming Event, Offering a 'Cautionary' Tale About Today's Climate Change

About 56 million years ago, Earth experienced one of its most intense periods of global warming, known as the Paleocene-Eocene Thermal Maximum, or PETM. As atmospheric carbon dioxide levels surged, forests suffered a massive decline in canopy cover, and they took more than 100,000 years to recover, researchers report in a study published on August 13 in the journal Science. The findings offer a sobering look into what our planet’s future might look like because of ongoing human-caused climate change.

Predicting the effects of the planet-warming greenhouse gas on vegetation is complicated. Since plants need the gas for photosynthesis, elevated carbon dioxide levels should theoretically lead to enhanced plant growth. But too much heat and its consequences, like extreme weather events and an increase in pathogens, can also stress and kill plants.

That’s why Dunn and her colleagues turned to the past to see what might happen to today’s forests. They collected fossilized leaf fragments hidden in the rocks of Hanna Basin in south-central Wyoming. The specimens preserve the shapes of the leaves’ outermost cells, which can reveal how much sunlight the leaf received as it grew.

Leaves that grow in the shade have longer cells that stretch out looking for light, while those exposed to the sun are shorter and rounder. The information helped the team calculate the density of the forest canopy, which they tracked across the entire PETM. At first, the forest flourished thanks to the elevated carbon dioxide.

But that growth spurt didn’t last. The canopy then quickly lost around 60 percent of its leaf coverage, and it stayed around 35 percent thinner than it started through the rest of the PETM. The Amazon rainforest might be more resilient to climate change than scientists previously thought.

Average tree size increased by 3.3 percent each decade between 1971 and 2015, according to a study published last year. This change affected the surrounding environment too. Ferns replaced the broad-leafed relatives of elms, walnuts and avocado trees.