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This article was first published in Augustand it has been updated to include new research published since then. Our 4. Even after those first scorching millennia, however, the planet has often been much warmer than it is now.
One of the warmest times was during the geologic period known as the Neoproterozoic, between and million years ago. Conditions were also frequently sweltering between million and million years ago. And within the last million years, two major heat spikes occurred: the Cretaceous Hot Greenhouse about 92 million years agoand the Paleocene-Eocene Thermal Maximum about 56 million years ago. Temperature records from thermometers and weather stations exist only for a tiny portion of our planet's 4. By studying indirect clues—the chemical and structural atures of rocks, fossils, and crystals, ocean sediments, fossilized reefs, tree rings, and ice cores—however, scientists can infer past temperatures.
None of these techniques help with the very early Earth. Because no rocks on Earth have survived from so long ago, scientists have estimated early Earth conditions based on observations of the Moon and on astronomical models. What the collision that spawned Earth's Moon may have looked like.
Collisions between Earth and rocky debris in the early solar system would have kept the surface molten and surface temperatures blistering. Image courtesy NASA. Zircon crystals from Australia, only about million years younger than the Earth itself, hint that our planet may have cooled faster than scientists ly thought. Still, in its infancy, Earth would have experienced temperatures far higher than we humans could possibly survive.
But suppose we exclude the violent and scorching years when Earth first formed. Between and million years ago—a period of time geologists call the Neoproterozoic—evidence suggests the Earth underwent an ice age so cold that ice sheets not only capped the polar latitudes, but may have extended all the way to sea level near the equator. Reflecting ever more sunlight back into space as they expanded, the ice sheets cooled the climate and reinforced their own growth.
A geologic history of Earth since its formation 4. Fossils reveal not only ancient plants and animals, but also ancient climates. Used with permission. Volcanoes emit the greenhouse gas carbon dioxide. In our current, mostly ice-free world, the natural weathering of silicate rock by rainfall consumes carbon dioxide over geologic time scales. During the frigid conditions of the Neoproterozoic, rainfall became rare.
With volcanoes churning out carbon dioxide and little or no rainfall to weather rocks and consume the greenhouse gas, temperatures climbed. What evidence do scientists have that all this actually happened some million years ago? Some of the best evidence is " cap carbonates " lying directly over Neoproterozoic-age glacial deposits.
Cap carbonates—layers of calcium-rich rock such as limestone—only form in warm water. Rock formation in Namibia that shows a type of rock that only forms in warm water cap dolostone lying directly over a type of jumbled sedimentary rock, dated to million years ago, that is commonly found at the margin of glaciers diamictite. Image from teaching slides available at SnowballEarth. A Smithsonian Institution project has tried to reconstruct temperatures for the Phanerozoic Eon, or roughly the last half a billion years.
About million years ago, around the equator of the supercontinent Pangea, it was even too hot for peat swamps!
Preliminary from a Smithsonian Institution project led by Scott Wing and Brian Huber, showing Earth's average surface temperature over the past million years. For most of the time, global temperatures appear to have been too warm red portions of line for persistent polar ice caps. The most recent 50 million years are an exception. Geologists and paleontologists have found that, in the last million years, global temperatures have peaked twice.
Widespread volcanic activity may have boosted atmospheric carbon dioxide. Temperatures were so high that champsosaurs crocodile-like reptiles lived as far north as the Canadian Arctic, and warm-temperature forests thrived near the South Pole. During much of the Paleocene and early Eocene, the poles were free of ice caps, and palm trees and crocodiles lived above the Arctic Circle. Around the time of the Paleocene-Eocene Thermal Maximum, much of the continental United States had a sub-tropical environment.
Image courtesy U. National Park Service. At roughly the same time, paleoclimate data like fossilized phytoplankton and ocean sediments record a massive release of carbon dioxide into the atmosphere, at least doubling or possibly even quadrupling the background concentrations.
Deep ocean temperatures were generally high throughout the Paleocene and Eocene, with a particularly warm spike at the boundary between the two geological epochs around 56 million years ago. Temperatures in the distant past are inferred from proxies oxygen isotope ratios from fossil foraminifera. The calculation of temperatures before 35 million years ago red assumes an ice-free ocean and does not apply to more recent conditions gray.
It is still uncertain where all Could have been so hot carbon dioxide came from and what the exact sequence of events was. Scientists have considered the drying up of large inland seas, volcanic activity, thawing permafrost, release of methane from warming ocean sediments, huge wildfires, and even—briefly—a comet. Those ancient climates would have been like nothing our species has ever seen. Modern human civilization, with its permanent agriculture and settlements, has developed over just the past 10, years or so. The period has generally been one of low temperatures and relative global if not regional climate stability.
British Geological Survey. Greenhouse Earth — the story of ancient climate change. Accessed June 13, Engber, D. What's the hottest the Earth has ever gotten? Hearling, T. R, Gabbott, S. An early Cambrian greenhouse climate. Science Advances4 5easar Hoffman, P. Snowball Earth. Accessed February 4, The snowball Earth hypothesis: testing the limits of global change.
Huber, B. The rise and fall of the Cretaceous Hot Greenhouse climate. Global and Planetary Change, Summary for Policymakers. Klages, J. Temperate rainforests near the South Pole during peak Cretaceous warmth. Nature, 81— Lindsey, R. Ancient crystals suggest earlier ocean. McInerney, F. Annual Review of Earth and Planetary Sciences39 1— Paleoclimatology: How can we infer past climates? Accessed June 9, Retallack, G. Permian and Triassic greenhouse crises. Gondwana Research24 1 Royer, D. CO 2 as a primary driver of Phanerozoic climate. GSA Today, 14 3 Scientific American Frontiers. Deep freeze.
Sleep, N. The Hadean-Archaean Environment. Cold Spring Harbor Perspectives in Biology2 6. Sun, Y. Lethally hot temperatures during the early Triassic Greenhouse. Science, Terrestrial Paleoclimate. Eocene latitudinal gradients. Stanford University School of Earth Sciences.
University of California Museum of Paleontology. Voosen, P. Project traces million years of roller-coaster climate. What's the hottest Earth's ever been? Michon Scott. Rebecca Lindsey. June 18, Share This:. Long-term Cycles. Greenhouse Gases.Could have been so hot
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