What’s the hottest Earth’s ever been?

This article was first published in August 2014, and it has been updated to include new research published since then. This article is one of a three-part series on past temperatures. One is about how warm the Earth has been “lately.” The other is about the coldest Earth's ever been.

Our 4.54-billion-year-old planet probably experienced its hottest temperatures in its earliest days, when it was still colliding with other rocky debris (planetesimals) careening around the solar system. The heat of these collisions would have kept Earth molten, with top-of-the-atmosphere temperatures upward of 3,600° Fahrenheit.

Even after those first scorching millennia, however, the planet has often been much warmer than it is now. (No, that doesn’t mean current warming is natural or that it won’t, overall, be harmful to humans and other life. We discuss that further later in the article.) One of the warmest times was during the geologic period known as the Neoproterozoic, between 600 and 800 million years ago. Conditions were also sometimes sweltering between 500 million and 250 million years ago. And within the last 100 million years, two major heat spikes occurred: the Cretaceous Hot Greenhouse (about 92 million years ago), and the Paleocene-Eocene Thermal Maximum (about 56 million years ago).

Cartoon by Emily Greenhalgh, NOAA Climate.gov.

History of hot

Temperature records from thermometers and weather stations exist only for a tiny portion of our planet's 4.54-billion-year-long life. By studying indirect clues—the chemical and structural signatures 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. During the time known as the Hadean (yes, because it was like Hades), Earth’s collisions with other large planetesimals in our young solar system—including a Mars-sized one whose impact with Earth likely created the Moon—would have melted and vaporized most rock at the surface. 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. Following the collision that spawned the Moon, the planet was estimated to have been around 2,300 Kelvin (3,680°F).

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. 

Even after collisions stopped, and the planet had tens of millions of years to cool, surface temperatures were likely more than 400° Fahrenheit. Zircon crystals from Australia, only about 150 million years younger than the Earth itself, hint that our planet may have cooled faster than scientists previously 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. When else has Earth’s surface sweltered?

Thawing the freezer

Between 600 and 800 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. Obviously, the Earth didn’t remain stuck in the freezer, so how did the planet thaw? 

Even while ice sheets covered more and more of Earth’s surface, tectonic plates continued to drift and collide, so volcanic activity also continued. Volcanoes emit the greenhouse gas carbon dioxide. At various times in Earth’s history, continental-scale expanses of magma—known as large igneous provinces—may have released much greater quantities of greenhouse gases than today’s volcanoes. In our current, mostly ice-free world, the natural weathering of silicate rock by rainfall consumes carbon dioxide over geologic time scales of tens to hundreds of thousands of years. During the frigid conditions of the Neoproterozoic, rainfall became rare, and most rocks were covered with ice. With seismic activity churning out carbon dioxide and little or no rainfall to weather rocks and consume the greenhouse gas, temperatures climbed, unabated. Remarkably, our planet may have experienced some of its hottest temperatures sandwiched between some of its coldest temperatures.

What evidence do scientists have that all this actually happened some 700 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 635 million years ago, that is commonly found at the margin of glaciers (diamictite). Image from teaching slides available at SnowballEarth.org.

The fact that these thick, calcium-rich rock layers sit directly on top of rock piles left behind by retreating glaciers indicates that temperatures rose significantly near the end of the Neoproterozoic, perhaps reaching a global average higher than 90° Fahrenheit. (Today's global average is lower than 60°F.) To be clear, the “rapid” change from cold conditions to hot conditions would have been fast in geologic terms but not necessarily in human terms. Changes in carbon dioxide levels and global temperature may have taken thousands of years, and the accumulation of cap carbonate rocks might have taken as long as hundreds of thousands of years.

The tropical Arctic

A Smithsonian Institution project has tried to reconstruct temperatures for the Phanerozoic Eon, or roughly the last half a billion years. Preliminary results released in 2019 showed warm temperatures dominating most of that time, with global temperatures repeatedly rising above 80°F and even 90°F—much too warm for ice sheets or perennial sea ice. About 250 million years ago, around the equator of the supercontinent Pangea, deserts predominated, thanks to a combination of heat and aridity.

Geologists and paleontologists have found that in the last 100 million years, global temperatures have peaked twice. One spike was the Cretaceous Hot Greenhouse roughly 92 million years ago, about 25 million years before Earth’s last dinosaurs went extinct. A 2024 paleoclimate reconstruction named PhanDA (Phanerozoic Data Assimilation) found a strong correlation between temperature and carbon dioxide during the Paleozoic (from the start of the reconstruction at 485 million years ago through 250 million years ago) and the Cenozoic (the last 65 million years) but did not find a similarly strong link during the age of the dinosaurs. So, the precise cause of the Cretaceous Hot Greenhouse will need continued research, but the fossils indicate a very warm planet. 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.

Another hothouse period was the Paleocene-Eocene Thermal Maximum (PETM) about 56 million years ago. 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. Though not quite as hot as the Cretaceous hothouse, the PETM saw average global surface temperature rise about 6°C (11°F) in less than 10 millennia. That figured among the fastest periods of warming observed in the geological record, but as the Smithsonian’s Scott Wing explains, continued high greenhouse gas emissions and the projected amount of warming they are likely to cause over the next few centuries could amount to roughly the same amount of warming at a rate 10 times faster.

Around the time of the Paleocene-Eocene Thermal Maximum, much of the continental United States had a sub-tropical environment. This fossil palm is from Fossil Butte National Monument, Wyoming. Image courtesy U.S. National Park Service.

During the PETM, the global mean temperature appears to have risen by as much as 5-8°C (9-14°F) to an average temperature as high as 34°C (93°F). (Again, today’s global average is shy of 60°F.) 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.

Global surface 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, in this case, oxygen isotope ratios from fossil foraminifera, single-celled marine organisms. "Q" stands for Quaternary. Graphic produced using data from Zachos and Hansen, with help from Dr. Carrie Morrill, Director of the World Data Service for Paleoclimatology.

It is still uncertain where all the 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. In recent years, evidence has emerged that huge volcanic eruptions occurred in North America at the time of the PETM, and these volcanoes were a likely source of at least some of the carbon dioxide.

Like nothing we’ve experienced

Earth’s hottest periods—the Hadean, the late Neoproterozoic, the Cretaceous Hot Greenhouse, the PETM—all occurred millions of years before humans existed. That doesn’t mean we aren’t warming the planet now. In fact, studying past hothouse periods confirms the major role carbon dioxide plays in heating the Earth–no matter where it comes from. 

Since 1800, carbon dioxide has risen 100-200 times as fast as when the Earth thawed out of its most recent glacial period between 17,500 and 11,500 years ago. Since 1850, global surface temperatures have risen roughly 0.11°F (0.06°C) per decade. Since 1982, they have risen 0.36°F (0.20°C) per decade. Earth’s fossil record links sudden climate change to extinction, for instance in marine invertebrates and marine mammals. Humans won’t likely go extinct, but losing the climate stability that has persisted throughout the entire history of civilization will mean unprecedented adaptation challenges: sea level rise and loss of coastal infrastructure, shifts in regional climate that affect food production, loss of dependable water supplies, more floods and wildfires, and an increased spread of infectious diseases.

No people lived on Earth for most of its history; we played no part in the planet’s hothouse conditions of the ancient past. But the planet’s history shows us that carbon dioxide has a strong influence on climate, and that rapid climate change causes major dislocation to animals and plants. We can learn from Earth’s history how our climate system works. These lessons from the past make it clear that we are now the ones changing global climate. Our role is undeniable, but so is our ability to change the future in ways that will make life better for people and other living things in generations to come.

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