The Jurassic period, stretching from about 201 to 145 million years ago, was a warm greenhouse world with no permanent polar ice sheets, CO2 levels roughly three to four times higher than today’s, and tropical or subtropical conditions reaching much farther toward the poles than anything we experience now. Global average temperatures were significantly warmer than the present, and the planet looked radically different: continents were still partially joined, shallow seas flooded vast stretches of land, and forests grew within reach of both poles.
A Greenhouse Planet
The Jurassic was one of the warmest extended intervals in Earth’s history. The U.S. Geological Survey describes the period as having “warm tropical greenhouse conditions” worldwide. There were no large ice sheets at either pole, and the temperature gradient between the equator and high latitudes was much gentler than it is today. Regions that are now temperate or even arctic supported lush vegetation year-round.
Exactly how hot it got is harder to pin down than you might expect, because thermometers obviously weren’t around. Scientists rely on chemical signatures preserved in rocks, fossils, and ocean sediments. Today’s global average surface temperature sits below 60°F (about 15°C). During the Jurassic, estimates generally place global averages several degrees Celsius higher, though exact figures vary by sub-period and method. The key point: it was warm enough for reptiles and tropical-style plants to thrive at latitudes where polar bears live today.
Carbon Dioxide Was Far Higher Than Today
One of the biggest reasons the Jurassic stayed so warm was atmospheric carbon dioxide. Modern CO2 levels hover around 420 parts per million (ppm). During the Late Jurassic, CO2 concentrations were roughly 1,200 to 1,500 ppm, based on chemical analysis of sauropod dinosaur tooth enamel published in the Proceedings of the National Academy of Sciences. Depending on assumptions about ancient plant productivity, estimates range from about 830 ppm on the low end to as high as 1,800 ppm. Some earlier intervals may have been even higher.
All that extra CO2 acted as a powerful greenhouse blanket, trapping heat and keeping the planet warm from equator to pole. It also meant oceans were more acidic and plant growth was turbocharged in many regions, which in turn supported the enormous herbivorous dinosaurs that define the period.
Oxygen Levels Were Rising
The Jurassic also saw important changes in the air animals breathed. After a catastrophic drop at the end of the Permian period (around 250 million years ago), when oxygen may have plunged from about 30% to roughly 10% of the atmosphere, levels gradually recovered. By around 200 million years ago, near the start of the Jurassic, oxygen began a significant climb. Today’s atmosphere is about 21% oxygen, and that rise was helped along by a major boost right around the Jurassic boundary. Researchers have tied this oxygen increase to the ecological rise of small mammals and birds during the period.
Monsoon Rains and Extreme Seasons
Rainfall in the Jurassic didn’t fall evenly across the globe. The supercontinent Pangaea was still partially intact at the start of the period, and its sheer size generated powerful monsoonal circulation patterns, much like a scaled-up version of today’s Asian monsoon. Modeling experiments and rock records both point to extreme seasonality: long dry stretches punctuated by intense rainy seasons.
Some of the most vivid evidence comes from the Navajo Sandstone in the American Southwest, which preserves ancient sand dunes from the Early Jurassic. Within those dunes, researchers identified slump features caused by heavy rainfall. Over one 36-year stretch of dune migration, 24 separate rain-triggered slumps appeared, 20 of them consistent with summer monsoon events and the remaining four linked to winter storms. These Jurassic dunes essentially recorded a prehistoric rain calendar, confirming that annual monsoon rains were a real and recurring feature of the climate.
At the global scale, the pattern was clear: drier conditions near the equator and wetter conditions at mid to high latitudes. Evaporite minerals (the kind that form when water evaporates in arid environments) dominated low-latitude rock records, while coal deposits, which require swampy, wet conditions, were concentrated at higher latitudes.
How Pangaea’s Breakup Reshaped the Climate
At the start of the Jurassic, most of Earth’s land was still clustered together as Pangaea. The deep interior of this supercontinent was brutally dry, far from any ocean moisture. As Pangaea began to fragment over the course of the Jurassic, new seaways opened up and ocean currents shifted. Moist oceanic air could penetrate deeper into the newly separated landmasses, gradually reducing the extreme aridity that had characterized Pangaea’s interior.
This breakup also raised sea levels. As tectonic rifting created new mid-ocean ridges, the ridges displaced ocean water upward and onto continental margins. Shallow seas flooded large portions of what is now Europe, Africa, and South America. These “epicontinental seas” moderated nearby land temperatures and added moisture to the atmosphere, pushing the climate in many regions toward warmer and wetter conditions. By the succeeding Cretaceous period, sea levels would reach 75 to 250 meters above present-day levels, and the trend was already well underway during the Late Jurassic.
Polar Regions Were Warm but Not Ice-Free Year-Round
One of the most striking features of the Jurassic climate is that forests extended all the way to the poles. Trees grew at latitudes where today you’d find nothing but ice and tundra. There were no large glacial ice sheets anywhere on the planet during most of the period.
That said, “no ice caps” doesn’t mean it never froze. Research published in Science Advances found evidence of seasonal freezing at high latitudes, even under the Jurassic’s extreme CO2 levels. In the Junggar Basin of northwestern China, which sat at roughly 71°N during the early Mesozoic, scientists discovered lake ice-rafted debris: rocks and sediment that had been carried and dropped by floating lake ice. This tells us that winters at high latitudes still brought freezing temperatures, even while the same regions supported forests the rest of the year. The freezing was seasonal, though. None of the evidence points to permanent ice, ice sheets, or glaciers.
Climate models back this up, consistently showing below-freezing winter temperatures at high latitudes for the Late Triassic and Jurassic, but only seasonally. It took extraordinarily high CO2 levels, above roughly 4,500 ppm, to eliminate freezing entirely in these simulations.
What Plants and Dinosaurs Tell Us
Fossil plants are one of the best thermometers for ancient climate. During the Late Jurassic, plant diversity followed a distinctive pattern: it was lowest near the equator, peaked in the midlatitudes, and then declined again toward the poles. This mirrors what you’d expect from a world with a dry tropical belt and lusher conditions farther north and south.
Dinosaur fossils, interestingly, show the opposite distribution. Most dinosaur remains come from drier, lower-latitude environments where open savanna-like vegetation dominated. Fewer dinosaur fossils turn up in the wetter midlatitude and high-latitude zones, even though those regions had richer plant life and abundant coal-forming swamps. Researchers believe this is partly a preservation bias (bones fossilize better in drier sediments) but also reflects real habitat preferences, with many large dinosaurs favoring more open landscapes.
The overall picture is a world that was warmer, wetter at the margins, drier at the center, and far more uniform in temperature from equator to pole than our modern Earth. It was a planet built for giant reptiles, towering conifers, and shallow tropical seas, shaped by high CO2, active volcanism, and a continental arrangement utterly unlike today’s.