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“The practical concern for humanity is the high climate sensitivity and the eventual climate response that may be reached if all fossil fuels are burned” – Hansen et al. September 2013
WHO: James Hansen, Makiko Sato, The Earth Institute, Columbia University, New York, NY
Gary Russell, NASA Goddard Institute for Space Studies, New York, NY
Pushker Kharecha, The Earth Institute, Columbia University, NASA Goddard Institute for Space Studies, New York, NY
WHAT: Using deep ocean oxygen isotope ratios to determine the sensitivity of climate forcing for sea levels and surface temperatures.
WHEN: September 2013
WHERE: Philosophical Transactions of the Royal Society A (Phil Trans R Soc A) Vol. 371, No. 2001
TITLE: Climate sensitivity, sea level and atmospheric carbon dioxide (open access)
Ok, firstly, let us just take a moment to geek out about how awesome science is. This paper has looked at what our planet was like millions of years ago by studying at the amount of different oxygen and carbon types in the shells of foraminifera that have been buried at the bottom of the ocean since they died millions of years ago. Science can tell us not only how old they are by dating the carbon in their fossilised bodies, but also what the temperature was too. That is awesome.
Foraminifera from Japan (Wikimedia commons)
The lead author of this paper – Dr. James Hansen is pretty much the Godfather of climate science. He’s been doing climate models looking at the possible effects of extra carbon in our atmosphere since he basically had to do them by hand in the 1980s before we had the internet. He knows his stuff. And so far, he’s been right with his projections.
The paper (which is a very long read at 25 pages) focuses on the Cenozoic climate, which is the period of time from 65.5 million years ago to present. The Cenozoic is the period after the Cretaceous (so we’re talking mammals here, not dinosaurs) and includes the Palaeocene-Eocene thermal maximum where the deep ocean was 12oC warmer than today as well as the cooling from there that led to the formation of the Greenland and Antarctic ice sheets.
The period of time studied by the paper (bottom axis is million years before present) (from paper)
What does this show us? The warming that eventually led to the Palaeocene-Eocene thermal maximum started around 3,000 years before there was a massive carbon release. The researchers think this carbon release was from methane hydrates in the ocean venting, because there was a lag in the warming in the intermediate ocean after the carbon release.
The thermal maximum had global surface temperatures around 5oC warmer than today, and there was about 4,000 – 7,000 Gigatonnes (Gt) of carbon that was released into the atmosphere to force that kind of warming.
After this warming happened there were ‘hyperthermal’ events (where the temperature spiked again) as the planet slowly cooled, showing how long the recovery time for the planet was from this greenhouse warmed state.
In the warmed world of the Palaeocene-Eocene maximum, sea levels were probably 120m higher than they are now. The researchers found that there’s a snowball effect with changes in ocean temperatures where a -1oC difference in deep ocean temperatures was enough to trigger the last ice age, while sea levels were 5- 10m higher when temperatures were ‘barely warmer than the Holocene’ (which is us – we live in the Holocene).
The researchers found that during the Pliocene, (about 5million years ago) sea levels were 15m higher than today, which they point out means that the East and West Antarctic ice sheets are likely to be unstable at temperatures we will reach this century from burning fossil fuels.
From the data they then tried to work out what the sensitivity of the atmosphere is to extra carbon. This is important to know, because we’re currently changing the chemical composition of the atmosphere much faster than ever before. The previous greenhouse warming that the planet experienced occurred over millennial time scales – the current rate that we’re pumping carbon into the atmosphere is causing change over only hundreds of years.
To work out how sensitive the climate is to being forced by carbon, the researchers used a simplified model where the atmosphere was split into 24 layers to test the rapid equilibrium responses to forcing.
They wanted to find out if we could be in danger of runaway climate change – the most extreme version of which happened on the planet Venus where runaway climate change amplified by water vapour led to a new stable average temperature of 450oC and the carbon was baked onto the surface of the planet and all the water evaporated into the sky. Obviously, humanity will want to avoid that one… Good news is there isn’t enough carbon on this planet for humans to accidentally do that to ourselves until the sun does it to us in a billion years or so.
We’ve avoided this for now (NASA NSSDC Photo Gallery)
The researchers then tested the response to doubling and halving the CO2 in the system, from the 1950 concentration of 310ppm of CO2 in the atmosphere. They found that three halving gives you a ‘snowball Earth’ response of mass glaciations, while in the other direction 1-4x CO2 is when all the snow disappears, which speeds up the feedback (because snow reflects heat) making the fast feedback sensitivity 5oC of global warming. For 8-32x CO2 the sensitivity is approx. 8oC with water vapour feedbacks (what happened on Venus but a smaller scale).
But what do any of these numbers mean?
As the paper says; ‘the practical concern for humanity is the high climate sensitivity and the eventual climate response that may be reached if all fossil fuels are burned’.
So here’s the lesson we need to learn from the Palaeocene-Eocene thermal maximum. For global warming we can assume that 75% of it is from CO2, and the remaining 25% is from other greenhouse gasses like methane and nitrous oxide. If we burn all the fossil fuels we have left in the ground, that’s about 10-15,000Gt of carbon that we could put in the atmosphere.
That gives us 5x the CO2 from 1950, or 1,400ppm. This will give us 16oC of global warming. It will be a world where there’s an average temperature of 20oC on land and 30oC at the poles (the current average is 14oC). Keep in mind also, that 6oC of warming is generally enough for a mass extinction like the dinosaurs.
This will eliminate grain production across most of the globe and seriously increase the amount of water vapour in the air, which means it’s getting more humid (also the water vapour will destroy most of the ozone layer too).
A wet bulb temperature is the temperature with the humidity included. Humans generally live with wet bulb temperatures between 26-27oC up to 31oC in the tropics. A wet bulb temperature of 35oC or above means the body can’t cool down and results in ‘lethal hyperthermia’ which is scientist for it’s so hot and sticky that you die from the heat.
Burning all the fossil fuels will result in a planet with wet bulb temperatures routinely above 35oC, which means we’ll have cooked the atmosphere enough that we’ll end up cooking ourselves.
If the climate has a low sensitivity to this kind of forcing, it will take 4.8x CO2concentrations to cause an unlivable climate. If the climate is more sensitive, it will take less than that to cook ourselves.
Oh, and the other kicker? The Palaeocene-Eocene thermal maximum took millions of years to take place, so the mammals survived by evolving to be smaller. Our climate change is only taking hundreds of years, which is not enough time for any plants or animals to evolve and adapt.
Basically, if we burn all the fossil fuels, we’re all going down and taking the rest of the species on the planet with us, and we really will be the dumbest smart species ever to cause our own extinction.
So far, James Hansen has been correct with his climate projections. So when he says we can’t burn all the fossil fuels because if we do we’ll cook the planet, I say we pay close attention to what he says. Oh, and we should stop burning carbon.
