Carbon hasn't entered our atmosphere this quickly in at least 66 million years — since an asteroid slammed into our planet and wiped out the dinosaurs, or perhaps even earlier. Our addiction to fossil fuels has pushed the planet into a "no-analogue" state that's "likely to result in widespread future extinctions", an exceedingly humourless study published today in Nature Geoscience concludes. It's no secret that industrial society is racking up a serious carbon bill, nor that our CO2 emissions — roughly 2000 billion tonnes since the start of the industrial revolution — are warming and changing the planet. But to understand just how exceptional this moment in Earth's history is, we need to look at the geologic record.
That's what Richard Zeebe of the University of Hawaii at Manoa and his colleagues did, and what they found is truly terrifying. Modern carbon emissions outpace anything our planet has seen over the entire Cenozoic — the period that began after the K-T extinction — by at least a factor of ten.
For a long time, geologists have considered the Palaeocene — Eocene Thermal Maximum (PETM) a close analogue to modern global warming. Fifty-six million years ago as the supercontinent Pangea was breaking apart, global temperatures rose at least 5C, possibly due to a massive release of methane from the seafloor. But the timescale of high carbon emissions during the PETM isn't well known: did all that methane gush skyward over the course of decades, or did it leak slowly for thousands of years? Answering that question can help us determine whether the PETM really was comparable to the present.
In their study, Zeebe and his colleagues re-evaluated the carbon-13 and oxygen-18 isotope records — which track atmospheric carbon concentrations and global temperature, respectively — from sediment cores collected on the New Jersey coast. They learned that carbon emissions and global warming occurred nearly simultaneously during the PETM, suggesting a slow release of greenhouse gases. (If billions of tonnes of methane had poured out of the ground in a sudden burst, Earth's climate would have taken some time to catch up.)
Using climate and carbon cycle models, the researchers calculated that somewhere between 2000 and 4500 billion tonnes of carbon were released over a period of at least 4000 years. The annual emissions rate was somewhere between 0.6 and 1.1 billion tonnes per year.
Today, humanity is offloading 10 billion tonnes of carbon into the atmosphere each year — and that rate is still going up, despite our recent resolution to end fossil fuel consumption this century. We're emitting carbon way, way faster than the planet was during the PETM.
And if the most abrupt global warming episode of the Cenozoic doesn't measure up to the present?
Well, we have to look even deeper into the geologic past. At the K-T boundary 66 million years ago, a six-mile wide asteroid slammed into the planet with the force of a billion Hiroshima bombs, kicking off a period of intense volcanic activity that lasted approximately a half million years. It's possible that this dramatic chapter in Earth's history saw a comparable carbon release. But we can't be sure. "It's not well known if or how much carbon was released [at the K-T boundary]," Zeebe told Gizmodo in an email, adding that "geologic records are getting progressively worse for older events".
Still, it's unsettling to think that we can't find any comparison to the present since the dinosaurs kicked the bucket. The authors note that life during the PETM would have had some time to adapt to global climate change and ocean acidification, given the slow rise in atmospheric carbon concentrations. Life during the present? Maybe not so much.
"If anthropogenic emissions rates have no analogue in Earth's recent history, then unforeseeable future responses of the climate system are possible," the researchers write.
If I had to make a prediction? We're going the way of the dinosaurs.
Top: Sarychev eruption in 2009, as seen from the International Space Station. Image: NASA