National Fossil Day   Explore Nature
National Park Service
US Department of the Interior

Climate has Changed Throughout Earth's History

Using fossils and many geologic clues, geologists have reconstructed Earth's climate going back hundreds of millions of years. Over long time scales (tens- to hundreds- of millions of years), Earth's climate can be broadly characterized as "greenhouse" or "icehouse" climates.

During greenhouse times, there is little, if any, permanent ice on either pole. Warm temperate climates are found at high latitudes. During icehouse conditions, global climate is cool enough to support large ice sheets at one or both poles. Earth's climate has transitioned between these two categories only a few times in the past 540 million years. The most recent transition occurred during the Cenozoic Era.
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We live on an "icehouse" Earth, one that is quite cold compared to other periods in Earth history. Because Earth was warmer in the past doesn't mean that modern climate change is "no big deal."

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Over the past 542 million years, Earth's climate can be broadly categorized as "greenhouse" or "icehouse." This graphic shows the timing and distribution of glacial debris evidence. Glacial evidence at lower latitudes (closer to the equator) suggests colder climates. The shift to icehouse climates during the Cenozoic Era is one of only a few such shifts since the beginning of the Paleozoic Era. Continents were also moving during this time as show by the maps on the right. North America (yellow star) was much closer to the equator during the Paleozoic Era! Graphic modified from Kenworthy (2010), information compiled from Frakes and others (1992). Paleogeography maps modified from Ronald Blakey, Northern Arizona University Department of Geology.

Precambrian Climates (prior to 542 million years ago) View time scale...

Billion-year-old, mound-like, layered stromatolites in Glacier National Park. The stromatolites lived in a warm, shallow, sea (think Bahamas) when North America was south of the equator. NPS Photo.
The Precambrian accounts for more than 80% of Earth's history and is the "Age of Early Life." Climate varied widely during different periods of the Precambrian. Shallow seas—like today's Caribbean—covered some of the early continents. During warm climates, mats, or mounds of algae called stromatolites grew in those shallow seas. Shark Bay, Australia is famous for its modern stromatolites. Billion-year-old stromatolites are found in Glacier National Park (Montana). During the later Precambrian, Earth's climate flip-flopped between very warm periods and very cold periods. The cold periods were so cold that extensive glaciers were found near the equator—a true "snowball" Earth!. A handful of parks preserve fossils from these most ancient of rocks: Precambrian Parks (prior to 542 million years ago)

Paleozoic Era Climates(542 to 251 million years ago) View time scale...

Marine invertebrates (such as this trilobite from Grand Canyon National Park) were fantastically diverse and abundant during the Paleozoic Era. Even though global climate was cool for much of the Paleozoic, North America was near the equator and experienced equatorial climates. U.S. Geological Survey photo by Edwin Dinwiddie McKee.
Global climate was relatively cool during many periods of the "Age of Fishes." However, North America was located near the equator and experienced generally warm climates for much of the Paleozoic Era. Shallow seas advanced and retreated over vast areas of North America, depositing immense amounts of limestone and other marine sediments. These sediments contain a fantastic fossil record of evolving sea life from ubiquitous trilobites to vertebrates. Vertebrates first evolved in the oceans. Land plants and animals first appeared about 350 million years ago. Visitors can walk through the limestone layers and among Paleozoic fossils in many NPS cave parks including Mammoth Cave National Park (Kentucky), Wind Cave National Park (South Dakota), and Jewel Cave National Monument (South Dakota). Many other fossil parks preserve fossils from the Paleozoic Era: Paleozoic Era Parks (542 to 251 million years ago)

Mesozoic Era Climates(251 to 65.5 million years ago) View time scale...

Global climates much warmer than today existed during most of the Mesozoic Era—the "Age of Reptiles." Carbon dioxide was likely many times higher than today contributing to a "greenhouse" planet. During the early part of the Mesozoic, all of Earth's continents were assembled into a supercontinent called Pangaea. Pangaea began to break apart during the Mesozoic and the continents began moving towards their present locations. The last of North America's shallow inland seas covered the continent's interior during the Cretaceous Period of the Mesozoic Era. Lush forests, including newly-evolved flowering plants (angiosperms) blanketed much of North America. Dinosaur National Monument (Utah, Colorado) and Petrified Forest National Park (Arizona) were established to tell fossil stories from the Mesozoic, but there is a rich fossil record of that warm world in many parks: Mesozoic Era Parks (251 to 65.5 million years ago)

Dinosaurs ruled a warm planet during the Mesozoic Era. This Camarasaurus skull is from Dinosaur National Monument. NPS Photo.
The beautiful petrified logs of Petrified Forest National Park are evidence of a thick forest of tall trees. Note the badlands land forms and lack of vegetation in the arid environment—a stark contrast to the forest 220 million years ago. NPS Photo by T. Scott Williams.

Cenozoic Era Climates(the past 65.5 million years) View time scale...

The Cenozoic Era—the "Age of Mammals"—was a time of climatic transition. The early Cenozoic Era was a time of "greenhouse" climates like those the dinosaurs experienced. By 34 million years ago, permanent ice sheets were present at the south pole ushering in "icehouse" conditions. Climate warmed during the Miocene (about 20 million years ago) as mammal populations reached their greatest diversity. Climate then cooled again. In North America, the lush "greenhouse" fossil forests (there were palm trees in Wyoming and banana trees in Oregon) were replaced by open grasslands. Grassland ecosystems are better suited for a cooler, drier "icehouse" climate. By 2 million years ago, Earth's climate was cold enough to support large ice sheets on both poles that were poised to advance and retreat during the "ice ages." Ice advanced to Chicago and would subsequently retreat to Greenland. For the past 800,000 years, this cycle occurred about every 100,000 years. The last great ice advance was about 20,000 years ago. Six national parks were established to tell the fossil stories of the Cenozoic Era: Agate Fossil Beds National Monument (Nebraska), Badlands National Park (South Dakota), Florissant Fossil Beds National Monument (Colorado), Fossil Butte National Monument (Wyoming), Hagerman Fossil Beds National Monument (Idaho), and John Day Fossil Beds National Monument (Oregon). Numerous other parks preserve significant Cenozoic fossils: Cenozoic Era Parks (65.5 million years ago through today)

mural mural
The Cenozoic Era was a time of climatic change. The lush nearly tropical forests of North America (left) gave way to grasslands (right) as climate cooled and dried. The reconstructions are murals from John Day Fossil Beds National Monument. Left mural represents the 44-million-year-old Clarno Nut Beds ecosystem. Mural by Larry Felder for NPS. Right mural represents the 7-million-year-old Rattlesnake ecosystem. Mural by Roger Witter for NPS. Both murals photographed by Will Landon for NPS.

Landscapes and life changed too

Climate was not the only thing changing over the past 4.5 billion years of Earth history. The location and configuration of the continents, mountain ranges, and oceans was changing. Life was changing and evolving. Climate, landforms and life, are the three major components of ecosystems. All living things are adapted to a particular ecosystem "comfort zone" regarding the land they live on, the climate they experience, and the other living things (communities) they interact with. As landforms change, climates change, and the communities change, animals and plants are forced to migrate to more favorable conditions, adapt to the changing conditions, or go extinct. Fossils record these changes over time and provide clues to understanding how modern ecosystems may respond to changing climate.



Last updated: September 22, 2010