A group of climate scientists have compiled a high-fidelity and continuous record of variations in Earth’s climate extending 66 million into the past, and the results aren’t optimistic. The planet could eventually warm to levels it hasn’t seen in 34 million years as a result of climate change, they argued.

The new global “climate reference curve” created by the team was achieved by bringing together research from twelve international laboratories. They used sample material from the ocean floor collected over more than five decades of international scientific drilling expeditions around the world.

While the framework of a global climate reference curve has existed since 2001, climate records from many new sediment cores greatly improved in recent years. Over the last two decades, scientific drilling specifically targeted older geological strata, giving researchers access to better material to reconstruct the climate many years into Earth’s history.

The sediments analyzed in the study, some 66 million years old, are packed with the preserved shells of tiny organisms that can tell scientists about the temperature and chemical composition of the ocean when they were formed. The researchers used these proxies to reconstruct Earth’s climate history, going all the way back to the time when dinosaurs were still alive.

Using advanced mathematical analysis, the researchers identified four climatic states, classified as Hothouse, Warmhouse, Coolhouse and Icehouse. The distinctive climate pattern of each state is defined by greenhouse gas concentration and polar ice volume, with high CO2 and little ice volume during Hothouse and Warmhouse.

Temperatures on Earth during the warmest of the climate states in the past were more than 10ºC hotter than they are today, the study showed. It took the planet thousands or even millions of years to reach those levels, in sharp contrast to the climate change the world is now experiencing as a consequence of human activity.

The world has been in an icehouse state for several million years but that’s now changing. If we don’t act to reduce our greenhouse gas emissions, in a few centuries the Earth could once again break a threshold, moving to a Hothouse state. Global warming has already caused temperatures to go up over 1ºC compared to pre-industrial levels.

“If you look at the worst-case scenario [by 2300], the change in mean global temperature is larger than most of the natural variability going back over the last 66 million years related to changes in the Earth’s orbit,” Jim Zachos, a paleoclimatologist at the University of California, Santa Cruz, and a co-author of the study, told Scientific American.

The researchers warned that without immediate and stringent action Earth will face some of the strongest and fastest climate change the planet has ever seen. Countries have committed to limit the temperature increase to 2ºC as part of the Paris Agreement but almost everyone is lagging behind on that target.

“It’s a significant advance in Earth science,” said Zachos, claiming that the study provides a valuable framework for many areas of research. It will be useful to test climate models but also for geophysicists studying different aspects of Earth dynamics and paleontologists studying how changing environments drive the evolution of species.

The study was published in the journal Science.