A 15-Million-Year-Old Climate Analogue
In the dry riverbeds, or "ramblas," of Murcia, paleontologists have found a record of Earth's past. The fossil leaves discovered at sites like Rambla de los Molinos are from the Miocene Epoch, which lasted from about 23 to 5.3 million years ago. Preserved in fine-grained lacustrine (lake) sediments, these fossils are so perfectly mineralized that they retain their cellular structure, including veins and the microscopic pores used for gas exchange, known as stomata.
These plant remains, from species related to modern oaks and poplars, provide a direct snapshot of an ancient ecosystem. The period they belong to, specifically the Middle Miocene Climatic Optimum (around 17 to 14 million years ago), matters to climate scientists. It represents one of the most recent times in Earth's history when atmospheric carbon dioxide levels were similar to what they are today, yet the global climate was significantly warmer. The geological setting in the Mula basin of Murcia consists of sedimentary rocks that chronicle the environmental shifts from the late Mesozoic through the Miocene, providing context for these fossil finds.
Reading a Leaf's Climate Story
The way to understand the Miocene climate lies in the stomata. Plants regulate the density of these pores based on the amount of CO2 in the atmosphere; when CO2 is abundant, leaves develop fewer stomata. By counting the stomatal density on these fossil leaves and comparing them to their nearest living relatives, scientists can produce a reliable estimate of atmospheric CO2 concentration. Analysis of Miocene fossils from Spain and other locations indicates CO2 levels were between 300 and 600 parts per million (ppm). The specific findings from some sites show a level of 400 ppm—similar to modern concentrations.
Despite the similar CO2 levels, global mean temperatures during the Miocene Climatic Optimum were approximately 3°C to 7°C higher than today. This apparent contradiction is explained by differences in Earth's orbital parameters, known as Milankovitch cycles. These cycles describe long-term variations in Earth's eccentricity (the shape of its orbit), axial tilt, and precession (the wobble of its axis). During the Middle Miocene, these orbital conditions were configured to allow Earth to absorb more solar radiation, leading to a warmer planet even with CO2 levels that we would consider moderate today. This ancient world is a natural experiment, showing how orbital forcing and greenhouse gases interact and affect global climate.