56 million years ago, tropical forests were green at the poles.

Earth's past lacks a perfect analogy for today's climate change - human-induced warming is happening too fast, writes Science writer Peter Woosen.

The closest analogue to the current climate situation took place 56 million years ago, when greenhouse gases caused warming of at least 5 ° C and pushed rainforests to the poles. But then the warming stretched for 3000, or even 5000 years. Now we're talking about decades..

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The cause of the Paleocene-Eocene Thermal Maximum (PETM) has long been debated. Some researchers cite exotic mechanisms, such as catastrophic releases of methane from the seafloor or even asteroid impacts.. But over the past few years, evidence has emerged of a more prosaic PETM culprit: the carbon-spewing volcanoes that erupted beneath Greenland as it broke away from Europe and drifted toward what is now Canada..

It is believed that the bowels of Greenland are composed of carbon-rich rocks.. At the same time, carbon and most of the methane responsible for warming were accumulated by living organisms.. During rifting (in other words, the formation of land and oceans), carbon could be released in the form of carbon dioxide (CO2). In addition, the muddy rocks on the seafloor also contain carbon that has accumulated in living things, and magma from underwater eruptions helped release it..

That is, in accordance with the hypothesis of the animal-vegetative origin of greenhouse gases in the era of the Paleocene-Eocene thermal maximum, most likely living organisms are responsible, and not just volcanoes spewing greenhouse gases from the depths of the Earth.

Or, after all, volcanoes?

However, there are objections to this hypothesis.. In 2017, researchers analyzed plankton fossils and found that the carbon released during PETM was heavier than previously thought.. For some researchers, this indicated that the source of carbon was not living organisms.. “Given the current state of knowledge, this is likely volcanism,” says Markus Gutjar, a geochemist at the GEOMAR Helmholtz Ocean Research Center in Kiel, who led the study..

Greenland separated from Europe during PETM when a mantle plume passed under the island. As with any volcanism, this process should have led to the release of CO2. However, according to researchers, eruptions during rifting produced only one-fifth of the more than 10,000 gigatonnes of carbon needed to explain PETM warming..

Today's Horn of Africa is Greenland 60 million years ago.

Currently, there is another powerful source of carbon dioxide, in addition to human activity.. When the earth's crust is ruptured by rifting, carbon can be erupted as carbonatite lava, which contains much more CO2 than standard volcanic lava..

Such a process is happening right now in East Africa, where a fault is tearing the Horn of Africa off the rest of the continent, says James Muirhead, a structural geologist at the University of Auckland.. “At the very edge of the craton, we get these carbonatite lavas,” he says.. “And next to the craton we get high CO2 fluxes”.

The exact same carbon dioxide-spewing hotspot was Greenland 60 million years ago..

Warming against the backdrop of cooling.

The modern interglacial, which replaced the period of cooling, the so-called " The Holocene Climatic Optimum (also called the Atlantic Optimum), with temperatures 1 to 3 degrees Celsius warmer than today, lasted from about nine to five thousand years ago..

Estimates of the likely duration of the Holocene range from 10,000 years to 20,000 years. So the end of the Holocene and the new ice age are expected in a few thousand years..