I’ve read deeply on Earth history over the last 30 years of my life — both for personal interest, and as an underpinning of my research and teaching. I first got into science because I wanted to be a palaeontologist (a specialist on past life and evolution in deep time), an interest that traces its origins back to a trip to the British Natural History Museum in London when I was four. The reason I chose to attend Macquarie University, out of the three major options in Sydney (the others were UNSW and U Syd), was because of its flexible palaeobiology curriculum. However, once I’d done a few years of undergraduate studies — majoring in biology, geology, astrophysics and computer science — I became convinced that what I really wanted to be was an analyst and modeller, rather than a field-based ‘rock jock’ who used fossils primarily as geological aids.

Still, my interest in past life, and the proximate and ultimate drivers of extinction, hasn’t diminished. Indeed, it’s been a principal research focus of mine over the last 15 years. Perhaps that’s why I found one particular aspect of Hansen’s book (reviewed on BNC here and here) so fascinating — and so worrisome. I’m talking about chapters 11 and 12, entitled “The Venus Syndrome” and “Storms of my Grandchildren”. For me, it was the stand-out element of the book.

Hansen’s thesis, in brief, is that if we burn all of the available fossil fuels, we run a high risk of setting in train amplifying feedbacks that lead to a runaway greenhouse effect.

How could this happen? Is it plausible? Well, perhaps, perhaps not. The ‘Venus syndrome’ hypothesis has certainly been derided by some, but to me, such blasé attitudes are puzzling, on both philosophical and scientific grounds. But let’s explore the issue a little more, before I offer some opinions.

Hansen first raised this issue in public at the 2008 AGU meeting. To quote from a report on that conference:

“The Venus Syndrome [in which Earth undergoes runaway warming and the oceans boil off] is the greatest threat to humanity’s existence. Earth is Goldilock’s choice of the planets — not too hot, not too cold, it’s just right.” In the past there have been several periods where temperatures have dropped so low that the planet entered a “Snowball Earth” state, with ice covering the entire surface of the globe. But that slows the process of weathering by rocks and enables carbon dioxide levels to build up in the atmosphere, eventually leading to warming. According to Hansen, there is no escape from the Venus Syndrome, which could occur for a forcing of 10-20 watts per square metre. For comparison, the net forcing today is between 0 and 3 watts per square metre. Although in the past carbon dioxide levels have reached 4000 parts per million (ppm) without a runaway warming effect, solar irradiance was lower. And today humans are increasing carbon dioxide levels at 2 ppm per year, 10,000 times faster than natural rates, which does not allow time for feedback effects to kick in. “If we burn all the coal, we might kick in a runaway greenhouse effect, and if we burn all the tar shale and tar sands we definitely will,” said Hansen, who reckons we could decide to leave coal in the ground or use it only with carbon dioxide capture and storage. “We’re going to have to figure out how to power ourselves without it anyhow so why not do it sooner rather than later?”

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Here is another excellent description of the idea, from Milan Ilnyckyj:

At one point, Venus had liquid water on its surface. Then, the sun grew brighter and Venus warmed. Its oceans evaporated and huge amounts of carbon dioxide (CO2) got baked out of the crust. The heat made the water break up into hydrogen and oxygen: the oxygen bonded with carbon to make more CO2, and much of the hydrogen escaped into space. Venus became permanently hostile to life, with surface temperatures of 450°C. Could burning all of Earth’s fossil fuels produce the same outcome? Some people take comfort from the fact that there have been times in the history of the planet when greenhouse gas concentrations were much higher than now. The world was very different, but there was no runaway greenhouse and life endured. James Hansen devotes the entire tenth chapter of Storms of My Grandchildren to considering whether thisassessment is valid. Three things give him pause: 1. The sun is brighter now than it was during past periods with very high greenhouse gas concentrations. The 2% additional brightness corresponds to a forcing of about 4 watts per square metre and is akin to a doubling of CO2 concentrations. 2. For various reasons, the greenhouse gas concentrations in past hot periods may not have been as high as we thought. 3. We are introducing greenhouse gases into the atmosphere far more quickly than natural processes ever did. This might cause fast (positive) feedback effects to manifest themselves forcefully, before slower (negative) feedback effects can get going. He also explains that the sharp warming that took place during the Paleocene–Eocene Thermal Maximum (PETM) were not caused by fossil fuels (which remained underground), but rather by the release of methane from permafrost and clathrates. If human emissions warm the planet enough to release that methane again, it could add a PETM-level warming on top of the warming caused by human beings. Hansen’s conclusions are, frankly, terrifying: The paleoclimate record does not provide a case with a climate forcing of the magnitude and speed that will occur if fossil fuels are all burned. Models are nowhere near the stage at which they can predict reliably when major ice sheet disintegration will begin. Nor can we say how close we are to methane hydrate instability. But these are questions of when, not if. If we burn all the fossil fuels, the ice sheets almost surely will melt entirely, with the final sea level rise about 75 meters (250 feet), with most of that possibly occurring within a time scale of centuries. Methane hydrates are likely to be more extensive and vulnerable now than they were in the early Cenozoic. It is difficult to imagine how the methane clathrates could survive, once the ocean has had time to warm. In that event a PETM-like warming could be added on top of the fossil fuel warming. After the ice is gone, would Earth proceed to the Venus syndrome, a runaway greenhouse effect that would destroy all life on the planet, perhaps permanently? While that is difficult to say based on present information, I’ve come to conclude that if we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty.

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My favourite part of the book is that which follows the dry scientific description of the Venus Syndrome. Hansen presents a short ‘science fiction’ story — a parable, if you will, centred around a sentient alien species called the Claron. Having detected early TV transmissions from the mid-20th century, the Claron arrive (after a supreme technological feat) at Earth, sometime after the year 2500. However, instead of encountering a verdant blue-green planet, flourishing with life and civilisation, they find a lifeless dust bowl with a blistering surface temperature exceeding 100 degrees Celsius – above the boiling point of water. After working out what must have gone wrong (‘carbocide’), the story story concludes with an act reminiscent of Charlton Heston on the beach, as he shook his fist at the half-buried Statue of Liberty (“Damn you all to hell!). The despairing Claron pilot, having left his companions at an abandoned 21st century Martian base, returns to Earth and plunges his craft into the baking hell hole that was once known as Washington DC.

[Do yourself a favour and read the whole short story, which goes for about 10 pages].

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My closing point is this: There is absolutely no certainty that human action will trigger positive Earth system feedbacks to the extent that all life is eliminated, forever… yet we cannot categorically rule it out. The Venus Syndrome is an unquantifiable and unprovable hypothesis, supported only by a logical — but ultimately subjective — interpretation of the messages of climate past. Hansen might very well has misread the palaeoclimate tea leaves, and those folks who confidently declare it to be nothing more than ‘alarmist speculation’ might well be right. But what if they’re not right? Or what if there’s even a 1% chance that they’re not? (My interpretation of the science is that the odds are ‘better’ than 1%, and when we’re talking about the Venus Syndrome, that’s not comforting). It is nothing short of gross cognitive dissonance to unequivocally disregard this possibility and yet still claim that you give a damn about future generations, especially when (i) we know we have to move beyond fossil fuels at some future point in our civilisation, and (ii) we have already developed the technology to fully enable this transition.

It’s time for some rational risk management, folks.