Earth’s climate has never stayed constant. Over the last 485 million years, our planet has moved repeatedly between much colder “icehouse” conditions and much warmer “greenhouse” states. A landmark 2024 study in Science now gives us the clearest picture yet of just how wide those swings have been, and what has driven them. The message is double-edged: yes, Earth has endured far hotter worlds than ours, but the speed at which we are pushing the climate today is unlike anything in that vast record.

The reconstruction, called PhanDA (for Phanerozoic Data Assimilation), combines geological temperature evidence with hundreds of climate-model simulations to produce a continuous estimate of Earth’s global mean surface temperature across almost the entire Phanerozoic Eon. Instead of leaning on a single type of proxy or a single model run, the researchers statistically integrated different paleoclimate records—on the order of 150,000 sea-surface temperature estimates—with model outputs spanning a range of carbon dioxide levels and ancient continental configurations. The result is a spatially complete, time-continuous temperature history that captures both the broad arcs and the sharp spikes of Earth’s thermal past.

What PhanDA reveals is striking. Over the last 485 million years, global average temperature has ranged from roughly 11°C to 36°C (about 52°F to 97°F), a wider span than many earlier reconstructions suggested. The coldest values correspond to recent glacial periods within our current icehouse, while the hottest peaks align with notorious greenhouse intervals such as the mid-Cretaceous and the Paleocene–Eocene Thermal Maximum. For most of this time, Earth was warmer than today, often lacking large polar ice sheets altogether. In that sense, our modern climate—cool enough to sustain extensive ice on both poles—is actually on the colder side of Earth’s long-term norm.

One of the study’s clearest findings is the tight connection between atmospheric CO₂ and these temperature swings. The researchers identify carbon dioxide as the dominant control on variations in global climate throughout much of the Phanerozoic. When CO₂ was high, the planet trended toward sweltering greenhouse states; when it was low, ice sheets expanded and global temperatures fell. From this relationship they infer an “apparent” Earth-system sensitivity of about 8°C per doubling of CO₂. That figure is much larger than the roughly 3°C per doubling often cited for near-term, fast-feedback responses, because it includes slow feedbacks—changes in ice sheets, vegetation, and surface reflectivity—that unfold over centuries to millions of years. It helps explain why relatively modest shifts in CO₂ over geological time can correspond to dramatic reorganizations of Earth’s climate system.

But Earth’s ancient climate should not be used to suggest today’s warming is harmless. The crucial difference is how quickly the change occurs. Many natural climate shifts in Earth’s deep history unfolded over thousands to millions of years, giving ecosystems and species time to migrate, adapt, or evolve. Today’s human-driven warming is occurring on a much shorter timescale, increasing the difficulty for ecosystems and species to keep pace. During the PETM, one of the most intense past hyperthermal events, global temperatures rose by roughly 5–8°C over several thousand years. In contrast, human activities have already warmed the planet by about 1.2–1.3°C in just over a century, with annual CO₂ emissions now roughly nine to ten times higher than during such natural carbon-release episodes. Co-author Jessica Tierney has put it plainly: we have never seen carbon dioxide and temperature rise as fast as they are rising now.

That speed matters. Organisms, ecosystems, and human societies evolved under the relatively stable conditions of the Holocene, the last 11,700 years. The Phanerozoic record shows that Earth as a system can tolerate far higher temperatures than we experience today, but it also shows that the transitions between climate states usually played out on timescales that allowed life to track changing conditions. Rapid warming compresses those windows, increasing the risk of widespread stress, collapse, and extinction. The deep-time record is not a comfort; it is a warning that the same CO₂ that has governed Earth’s climate for hundreds of millions of years is now being pushed upward at an unprecedented rate.

Seen in this light, the PhanDA reconstruction does not undermine concern about modern climate change; it sharpens it. It confirms that CO₂ is the master lever of Earth’s long-term climate, that the planet can and has been much hotter than now, and that the current trajectory is steering us toward greenhouse conditions not seen in millions of years—but doing so with a speed that has no analogue in the last 485 million years, The lesson is not that Earth will be fine no matter what we do. It is that the window for orderly adaptation is narrow, and we are closing it fast. TY