It is often said that humans have caused the Earth to warm at an unprecedented rate. However researchers have discovered another period, some 55m years ago, when massive volcanic eruptions pumped so much carbon into the atmosphere that the planet warmed at what geologists would think of as breakneck speed.

The good news is that most plants and animals survived the warm spell. The planet has experienced several mass extinctions – and this wasn’t one of them. But there’s a catch: even after carbon levels returned to their previous levels, the climate took 200,000 years to return to normal.

Geologists have a name for this earlier period of sudden warming: the Palaeocene-Eocene Thermal Maximum. The PETM, as we’ll call it, occurred 55.5–55.3 million years ago. According to new research published in the journal Nature Geoscience it involved global warming of between 5 and 8°C over a period of 200,000 years.

The massive carbon injections responsible for the PETM probably originated in volcanic eruptions in the North Atlantic and the burning of organic-rich rocks through which lava passed, which further triggered the melting of frozen methane stored at the bottom of the deepest oceans.

Lessons from the PETM

There are obvious analogies between the PETM and the present-day situation, even despite the lack of fossil-fuel burning humans 55m years ago. The study shows the PETM was caused by annual carbon emissions of at least 900 million tons (900 MT) over the 200,000 years. That is ten times less than the 9500 MT carbon humans are releasing into the atmosphere every year. Surely cause for concern?

The world hasn’t been as hot ever since.Glen Fergus, CC BY-SA

However, it is a little misleading to suggest emissions of just 10% of current levels resulted in warming of 5°C or more. It is possible to zoom out too far, even when assessing climate change. Considering that CO₂ only sticks around in the atmosphere for 1,000 years at most, to achieve as much as 8°C warming the bulk of PETM carbon must have been delivered to the atmosphere in a very short time, during which the long-term average was greatly exceeded.

The researchers identify two such “pulses”, each lasting no more than 1,500 years. Volcanic eruptions are not predictable – and certainly not consistent – but they would fit the profile of these “pulses”. It is these massive, rapid injections of carbon into the atmosphere that overwhelmed Earth’s otherwise extremely efficient oceanic carbon sink: it takes time to deal with such blows.

The study suggests that it took 200,000 years before Earth returned to normal (probably hindered by volcanic eruptions) – a duration that suggests Earth will not recover from its current stresses any time soon.

“Nice” warming?

An intriguing aspect of the PETM warming is that there was no mass extinction. Perhaps ecosystems were resilient having evolved in the aftermath of the great extinction of the dinosaurs some 10m years earlier. Perhaps the warming didn’t last long enough. Perhaps warming just made things “nice”.

But global warming hasn’t usually been particularly good news for the planet’s inhabitants. When massive volcanic eruptions in present-day Siberia generated a comparable increase in global temperature 250m years ago, it caused the greatest crisis in Earth’s history. Around 95% of the planet’s species were wiped out in what is known as the Permian-Triassic extinction, or the Great Dying. Earth was definitely not “nice” and remained inhospitably hot for millions of years. In comparison, the PETM looks like a tea party.

Is humankind off the hook? After all, we are only emitting a bit of carbon each year, nothing like the massive doses administered by enormous eruptions in geological history. No. The excellent high-resolution palaeoclimate records now emerging indicate that global warming has precedent in the rock record, and it always takes Earth a long time to recover.

The impact of 5-8°C global warming today is hard to define. It is beyond the worst-case scenarios of most climate models – and yet it is not beyond the bounds of possibility.

Let’s do some rough calculations – and simplify things by talking about CO₂ alone: today, Earth’s atmosphere contains at least 3000 gigatons of CO₂. Humans inject a further 30000 megatons every year (1% of the mass in the atmosphere). Volcanoes add only a fraction of the human contribution.

By contrast, the total CO₂ release from the Siberian Traps and the rocks burned by its lava is estimated at 30,000 to 100,000 gigatons. That’s ten to thirty times the total amount of carbon currently in the atmosphere. At current rates it will take humans just 1000 - 3000 years to produce this amount (it took the Siberian Traps much longer) and by the year 3014 Earth could be facing another catastrophe. Does that sound a long time away? For a geologist, these are incredibly short timescales.

Our saving grace appears to be that we are coming out of an ice-age with an atmosphere relatively low in CO₂, and that we are going to run out of fossil fuels before the above scenario can happen. However, geologists have shown that massive injections of carbon to the atmosphere can change the climate quite rapidly, and we are already well along the road. We are now seeing major changes to our weather and time will tell if this is the manifestation of longer term climate change.

Undoubtedly Earth’s climate will change as we continue to emit. Our generation might not see the impact of that change, but we need to decide what we want for the future of Earth. It is time to learn from Earth’s past – episodes such as the PETM, and the Permian-Triassic – as we look to its future.

David Bond receives funding from the Natural Environment Research Council (NERC).