To predict what type of Earth lies ahead of us, we scientists usually turn to complex computer simulations. But how can we test whether these models are remotely accurate? Perhaps the best solution is to turn to instances in the geological past when Earth’s climate experienced similarly rapid warming. One such event is the Palaeocene-Eocene Thermal Maximum (PETM) that occurred 56m years ago.
In our latest research, we have identified the cause of this well-known warm period. Its links to present day climate change are clear.
Just prior to the PETM, Earth looked very different than it does today. The polar regions were devoid of ice sheets, with temperate or even subtropical forests along the coastlines of Antarctica, and Arctic Canada resembling the swamplands of modern Florida. The deep oceans were about 10°C warmer than today, and warm climate zones were all shifted polewards.
Against the background of this “greenhouse world”, the planet warmed by at least a further 5°C over a few thousand years at the onset of the PETM. Life in the deep sea suffered disproportionately; many species went extinct and parts of the deep ocean became anoxic (oxygen depleted). On land, the water cycle strengthened, leading to both floods and droughts. It took about 150,000 years for Earth’s climate to naturally recover from this “fever” and regain some sort of equilibrium.
Here’s the really worrying part: 5°C over a few thousand years is breakneck speed in geological terms, but is still nothing compared to our current rate of warming. In fact, if we keep burning fossil fuels at our current rate, the worst-case scenarios suggest we could hit 5°C by the end of the century.
Blame the volcanoes
So what can the PETM tells us about the future? It has long been suspected that the warm period was triggered by increasing greenhouse gas concentrations in the atmosphere. These gases absorb and trap solar heat, which is why any significant increase unavoidably leads to global warming.
We know there was a huge release of “new” carbon into the atmosphere and oceans at the time, thanks to analysis of 56m-year-old sediments. Yet where this carbon came from has always been disputed. Carbon can be emitted as carbon dioxide or methane (aka CH₄) and both are greenhouse gases. Some say the PETM carbon was methane from marine sediments, while others have advocated methane from thawing Antarctic permafrost or the impact of a large comet releasing carbon from rocks.
In our study recently published in Nature, we identified the distinctive chemical fingerprint of this carbon – it pointed not to methane, but to emissions from intense and prolonged volcanic activity. We also show that atmospheric CO₂ levels more than doubled in less than 25,000 years.
This makes sense: at the same time, Greenland and North America were drifting away from Europe, creating the North Atlantic Ocean and a string of volcanic activity along what is now the Mid-Atlantic Ridge.
We found more than 10,000 Gigatonnes of carbon must have been released into the atmosphere by volcanic activity during the PETM, which is an order of magnitude higher than all currently-accessible fossil fuel reserves taken together.
But the rate of emissions would have been at least 20 times slower than today. Given how much CO₂ was released, the resulting global warming was about what we would predict based on calculations of current climate sensitivity.
So what would volcanoes large enough to affect the climate like this actually look like, in practice? We could imagine a series of sky-blackening eruptions along the lines of Laki in Iceland which caused temperatures to drop across the Northern Hemisphere when it erupted in the 18th century. But, given we know the PETM volcanism largely took place under water and at a slower pace, perhaps the best modern equivalent would be the “black smokers” still found today in the deep North Atlantic – but lots of them.
The carbon released by these vents would bubble up to the surface and kick off a cycle that would eventually affect the oceans themselves. First, extreme PETM warmth led to faster weathering of rocks and soil, which meant more nutrients like phosphorus were being washed into the sea. This in turn stimulated plankton growth. When the plankton died they drifted down to the seafloor and gradually stored that same carbon in deep marine sediments.
While this chain of events aided the removal of carbon from the ancient atmosphere it also led to oxygen starvation in some parts of the deep sea – analogous to the “dead zones” that form today in areas like the Gulf of Mexico where an excess of nutrients is washed into warm water.
We found the PETM was caused by massive carbon emissions from Earth’s interior. It thus has many parallels to today, where we are ratcheting up CO₂ levels in our atmosphere and oceans by burning fossil fuels that have been buried for millions of years. This extra carbon is, in effect, permanent on human timescales. The PETM is giving us an increasingly clearer picture of what Earth will be like if we carry on, and take our planet to places it has not been in at least 56m years.