A study of forests that existed 56 million years ago has shown how Earth's ecosystems can respond to a sharp increase in the content of carbon dioxide (CO₂) in the atmosphere. Researchers have restored the structure of ancient forests during the Paleocene-Eocene Thermal Maximum, a period of sharp global warming, which in its nature is closest to current climate change. The study was published in the journal Science.

Researchers found that during that period, a significant part of the vegetation became thinner. Against the background of increasing CO₂ concentration, rising temperatures and decreasing precipitation, forest stands lost density, and many plant species moved north. This intensified soil erosion and disrupted the natural water cycle.

To reconstruct the structure of ancient forests, researchers have used for the first time fragments of fossilized leaf cuticles, a protective layer that protects the cells of the epidermis of plants. Based on the shape of these cells, scientists were able to determine how dense the vegetation was.

To do this, they first studied modern forests in Central and South America. The researchers compared the shape of leaf cells with the leaf area index (LAI), an indicator that characterizes the density of forest cover. Then they applied the resulting model to fragments of fossilized leaves that are 56 million years old.

The main source of the material was organic-rich sediments in the Hanna Basin in the US state of Wyoming. In ancient times, giant metasequoias, sycamores, alders, palms, and several other subtropical and tropical plants grew here. Today, in their place is a mostly dry landscape with shrub vegetation.

The reconstruction showed that during the Paleocene-Eocene Thermal Maximum, the forest canopy became more open: large trees became fewer, and vegetation density decreased.

Researchers believe that what happened 56 million years ago may serve as a warning for modern times. According to scientists, today the CO₂ content in the atmosphere is growing about an order of magnitude faster than during that ancient climatic event.

Initially, the increase in CO₂ promoted the growth of vegetation and led to the global “greening” of the Earth. However, as warming intensified, this trend began to reverse. Droughts, forest fires, insect invasions, and the spread of pathogens increase tree mortality, and in some regions the opposite process is already observed: the “browning” of vegetation cover.

Of particular concern, researchers consider the state of the Amazonian forests, where changes are already being observed that resemble some of the processes discovered in the fossil forests of Wyoming.

At the same time, today’s forests are exposed to a much larger number of simultaneous pressures than ecosystems during ancient warming: in addition to increasing temperatures and CO₂ concentrations, they are affected by deforestation, fires, habitat fragmentation, invasive species, and land use change.

Scientists warn that deforestation can trigger a chain reaction: the destruction of trees reduces the ability of ecosystems to absorb carbon, which, in turn, can further accelerate climate change.