A barcode of Earth’s climatic past

Climate scientists have compiled the first continuous, high-fidelity record of variations in Earth’s climate extending back 66 million years to the time of the last great extinction.

The global “reference curve” highlights four distinctive climatic states and the natural million- and thousand-year variability that Earth’s climate has experienced.

It brings together research from 12 international laboratories using sample material from the ocean floor collected over more than five decades of scientific drilling expeditions by the International Ocean Discovery Program (IODP) and its predecessors.

The resulting colourful barcode – officially called CENOGRID (CENOzoic Global Reference benthic foraminifera carbon and oxygen Isotope Dataset) – provides a snapshot of past climate change, the researchers say, and a possible guide to the future.

“We now know more accurately when it was warmer or colder on the planet and we also have a better understanding of the underlying dynamics,” says Thomas Westerhold from Germany’s University of Bremen, first author of a paper in the journal Science.

“The time from 66 to 34 million years ago, when the planet was significantly warmer than it is today, is of particular interest, as it represents a parallel in the past to what future anthropogenic change could lead to.”

The challenge was to determine past climate variations on a time scale fine enough to see the variability attributable to orbital variations.

“We’ve known for a long time that the glacial-interglacial cycles are paced by changes in Earth’s orbit, which alter the amount of solar energy reaching Earth’s surface, and astronomers have been computing these orbital variations back in time,” says co-author James Zachos, from the University of California, Santa Cruz, US.

“As we reconstructed past climates, we could see long-term coarse changes quite well. We also knew there should be finer-scale rhythmic variability due to orbital variations, but for a long time it was considered impossible to recover that signal.”

The rough framework of the curve has existed since 2001, the researchers say, but climate records from new sediment cores have improved greatly in recent years. In particular, some recent drilling has specifically targeted geological strata older than 34 million years.

The team examined evidence preserved in oxygen and carbon isotopes, which provides information about the past deep-sea temperatures, global ice volumes and the carbon cycle. These clues are stored in the shells of microorganisms that once lived on the sea floor.

They then used advanced mathematical analysis to identify the four climatic states they call Hothouse, Warmhouse, Coolhouse and Icehouse, which are recognised by the characteristic pattern of their climate variability.

The distinctive climatic “beat’ of each state is driven by greenhouse gas concentrations and polar ice volume, with higher CO₂ and little-to-no global ice volume during the Hothouse and Warmhouse compared to the Coolhouse and Icehouse.

“While we show that the Earth previously experienced warm climate states, these were characterised by extreme climate events and were radically different from our modern world,” says co-author Anna Joy Drury, from University College London, UK.

“Since the peak warmth of the Hothouse, Earth’s climate has gradually cooled over the last 50 million years, but the present and predicted rapid anthropogenic changes reverse this trend and, if unabated, far exceed the natural variability of the last 66 million years.

“CENOGRID’s window into the past provides context for the ongoing anthropogenic change and how exceptional it is.”