Plant fossils are evidence of ancient climates. This 6-foot long, 52-million-year-old palm frond was found near Fossil Butte National Monument (Wyoming) and suggests a subtropical climate. Where are palm trees found today? NPS photo.
Fossils provide evidence of Earth's climate and how it has changed over millions of years. Just like detectives work with evidence found at a crime scene to reconstruct what happened, paleontologists look for fossil clues to interpret climate (including temperature and precipitation) of fossil ecosystems.
Paleontologists compare fossil ecosystems to modern ecosystems to get a general idea of the past climate. One major assumption—that may or may not be the case—is that the climate "comfort zone" of the plants or animals being studied is the same today as it was in the past. As an example, descendants of plants that lived together in the fossil ecosystem of Florissant Fossil Beds National Monument are now found in forests from Florida, Mexico, Argentina, southern Texas, southeast Asia, the Pacific coast of North America, the southern Rocky Mountains, and the southern Appalachians!
To get more specific information about climates of the past, paleontologists have a variety of tools to use. This page will highlight just three: tiny fossils from the bottom of the ocean called Foraminifera, fossil plant types and shapes, and ancient soil layers called paleosols. Paleontologists use many different approaches to study past climates and feel more confident about their estimates when different techniques suggest similar past climates.
Tiny "foram" fossils contribute to a giant climate record
Foraminifera (or "forams") are single-celled amoeba-like organisms that make a "shell" (Learn more). Although they are pretty small (some are microscopic, some are the size of large coins) they are extremely abundant and found throughout the world's oceans. The chemical composition of foram "shells" depends on the temperature of the surrounding water and the presence or absence of large polar ice sheets. Because of this relationship, scientists analyze foram shells to estimate ocean temperatures and the existence of large ice sheets, both good indicators of past climate. Although the record goes back much farther, the foram record is commonly used to study climate change over the past 65 million years, following the extinction of dinosaurs. The foram climate record comes from deep ocean sediment not within parks. However, the foram record provides a great global climate framework to compare fossil parks here on land!
Chemical analysis of the oxygen isotopes in "shells" of Foraminifera allow scientists to determine temperature and the presence of large ice sheets over long time periods, creating a climate record like the one above. Climate cooled and dried throughout the Cenozoic Era. Yellow dots and bars represent the time spanned by the Cenozoic fossil parks. The presence or potential presence of ice sheets at the North and South poles is also indicated (whiter shades = increased ice cover). Oxygen isotope data from Zachos and others (2008). Graphic modified from Kenworthy (2010).
Past climate "leaves" behind plant fossil evidence
Leaf shape is one tool paleontologists use to learn about past climates. The percentage of smooth-margined fossil leaf species can be used to estimate an average annual temperature for fossil forests. Forests in warmer climates have a higher percentage of smooth margined leaf species. The photos show fossil leaves from Fossil Butte National Monument (Wyoming). NPS photos courtesy Arvid Aase.
Estimating past climate on land is challenging. Temperature can vary widely not just seasonally, but daily! Precipitation also varies widely as seasons change. Through all these changes, plants are rooted in place and must tolerate such variable conditions. Paleontologists look to plant fossils for clues about ancient climates. Some plants indicate very general climates. For example palm trees suggest frost-free climates. For more-specific climate information, paleontologists study plant shapes (see photo). Modern forests with higher percentages of plant species with smooth-margined leaves experience higher average annual temperature. Additionally, leaves in warmer and wetter climates tend to be larger and thicker than their colder and drier counterparts. Warm-climate leaves also often have a "drip tip" at the end of the leaf. Paleontologists use these clues and other fossil leaf shape and size characteristics to estimate the average yearly temperature, range of temperature, and annual amounts of precipitation. The number of openings on a leaf (called stomata) can also provide information about carbon dioxide—an important greenhouse gas—concentrations.
Fossil soils provide the "dirt" on ancient climates
Ancient soil layers—paleosols—form the dramatic stripes in Badlands National Park (South Dakota). Paleontologists analyze paleosols for clues to ancient climates. NPS Photo.
Soils preserved in the geologic record are called paleosols. Paleontologists describe and analyze paleosols and compare them to modern soils. In this manner, they estimate ancient precipitation amounts and seasonal patterns, average yearly temperature, range of temperature, as well as type and extent of vegetation rooted in the soil. Colors of paleosols are helpful, and are easy to spot. At John Day Fossil Beds National Monument, deep red paleosols were much more common during the warm climates and suggest wetter, more deeply weathered soil layers. Paleosols from colder time periods tend to be brown or green in color. Paleosol layers can be found in many fossil parks, but they are particularly well preserved at John Day Fossil Beds National Monument and Badlands National Park (see photo).