A Greenhouse World in the Pyrenees
About 37 million years ago, the landscape of Berguedà was unrecognizable. Instead of the pine and oak forests seen today, the area was a lush, subtropical swamp bordering a warm sea. This period, known as the Priabonian age of the late Eocene epoch, was part of a "greenhouse" world. Global temperatures were significantly higher, and the Pyrenees mountains were still in their final stages of formation. In the low-lying basin, dense vegetation flourished in a warm, humid climate. As plants died, they fell into the stagnant, low-oxygen swamp waters. Instead of decaying completely, the organic matter was slowly compressed and transformed over millions of years into the soft, brown coal known as lignite, which was mined in the region until 1991.
This process created a fossil record. The coal seams and surrounding shales trapped leaves, pollen, and wood, preserving a detailed snapshot of this ancient ecosystem. Scientists use a technique called leaf margin analysis to reconstruct the climate of this lost world. The method is based on a strong correlation observed in modern forests: the proportion of plant species with smooth-edged (entire) leaves is higher in warmer climates, while species with toothed-edged leaves are more common in cooler climates. By analyzing the percentage of entire-margined fossil leaves found in the Berguedà coalfield, researchers determined the mean annual temperature was approximately 23°C. This is a full 10°C warmer than the region's average temperature today.