Earth is getting warmer than at any time in the last 50 million years, according to new research.
The planet has experienced four distinct climate states since the Age of the Dinosaurs. Scientists have named them as “Hothouse”, “Warmhouse”, “Coolhouse” and “Icehouse”.
They have been mapped in unprecedented detail for the first time using seabed rocks collected from drilling expeditions over the last five decades.
For the past three million years, Earth has been in an ‘Icehouse’ characterised by alternating glacial and interglacial periods.
Modern humans evolved during this time, but greenhouse gas emissions and other human activities are now driving the planet toward a ‘Warmhouse’ and ‘Hothouse.’
This has not seen since the Eocene epoch, which ended about 34 million years ago. During the early Eocene, there were no polar ice caps, and average global temperatures were 9 to 14 degrees Celsius higher than today.
Co author Professor James Zachos, of the University of California at Santa Cruz, said: “The IPCC (Intergovernmental Panel on Climate Change) projections for 2300 in the ‘business-as-usual’ scenario will potentially bring global temperature to a level the planet has not seen in 50 million years.”
The ‘ climate reference curve’ called CENOGRID can be studied like a colourful barcode – drawing comparisons between the past, present and future.
(Image: Westerhold et al., CENOGRID)
It puts current changes in context. The continuous record reveals natural climate variability due to changes in Earth’s orbit around the sun is much smaller than projected future warming due to greenhouse gas emissions.
Lead author Dr Thomas Westerhold, of the University of Bremen in Germany, said: “We now know more accurately when it was warmer or colder and have a better understanding of the underlying dynamics and the processes that drive them.
“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.”
These major climate states persisted for millions and sometimes tens of millions of years.
It sheds fresh light on the impact of manmade global warming – and how exceptional it is, warn the international team.
Prof Zachos said: “We have known for a long time 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.
“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.
“Now that we have succeeded in capturing the natural climate variability, we can see the projected anthropogenic warming will be much greater than that.”
Unveiled in Science, the tool shows how the planet gradual cooling over the last 50 million years has now been dramatically reversed.
The distinctive climatic ‘beat’ of each climatic state is driven by greenhouse gas concentrations and polar ice volume.
Higher CO2 and little-to-no global ice volume is seen during the Hothouse and Warmhouse compared to the Coolhouse and Icehouse.
Prof Zachos said: “In an extreme greenhouse world with no ice, there won’t be any feedbacks involving the ice sheets, and that changes the dynamics of the climate.”
The researchers analysed oxygen and carbon isotopes from tiny microfossils found in the ocean sediments.
These are chemical ‘signatures’ of past climates recorded in the shells of microscopic plankton, called foraminifera.
The CENOGRID is the clearest and most accurate view of past climate conditions to date, providing information about past deep-sea temperatures, global ice volumes and the carbon cycle.
Co-author Dr Anna Joy Drury, of University College London, said: “We use CENOGRID to understand what Earth’s normal range of natural climate change and variability is and how quickly Earth recovered from past events.
“While we show the Earth previously experienced warm climate states, these were characterised by extreme climate events and were radically different from our modern world.
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“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.”
Most of the major climate transitions in the past 66 million years – after a giant asteroid strike killed the dinosaurs – have been associated with changes in greenhouse gas levels.
Previous research by Prof Zachos found a period of rapid global warming around 50 million years ago which drove the climate into a Hothouse state was caused by a massive release of carbon into the atmosphere.
Similarly, in the late Eocene, as atmospheric carbon dioxide levels were dropping, ice sheets began to form in Antarctica and the climate transitioned to a Coolhouse state.
Prof Zachos added: “The climate can become unstable when it is nearing one of these transitions, and we see less predictable responses to orbital forcing, so that is something we would like to better understand.”
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