Been pondering whether it was worth bother blogging this but I haven’t written for a while and in the end I decided the title was too good a pun to pass on (I never claimed to have high standards).
The paper "Trajectories of the Earth System in the Anthropocene" had entirely passed me by when it came out, though it did seem to attract a bit of press coverage eg with the BBC saying
Researchers believe we could soon cross a threshold leading to boiling hot temperatures and towering seas in the centuries to come.
Even if countries succeed in meeting their CO2 targets, we could still lurch on to this “irreversible pathway”.
Their study shows it could happen if global temperatures rise by 2C.
An international team of climate researchers, writing in the journal, Proceedings of the National Academy of Sciences, says the warming expected in the next few decades could turn some of the Earth’s natural forces – that currently protect us – into our enemies.
and continues in a similar vein quoting an author
“What we are saying is that when we reach 2 degrees of warming, we may be at a point where we hand over the control mechanism to Planet Earth herself,” co-author Prof Johan Rockström, from the Stockholm Resilience Centre, told BBC News.
“We are the ones in control right now, but once we go past 2 degrees, we see that the Earth system tips over from being a friend to a foe. We totally hand over our fate to an Earth system that starts rolling out of equilibrium.”
Like I said, I had missed this, and it was only an odd set of circumstances that led me to read it, about which more below. But first, the paper itself. The illustrious set of authors postulate that once the global temperatures reach about 2C above pre-industrial, a set of positive feedbacks will kick in such that the temperature will continue to rise to about 5C above pre-industrial, even without any further emissions and direct human-induced warming. Ie, once we are going past +2C, we won’t be able to stabilise at any intermediate temperature below +5C.
The paper itself is open access at PNAS. The abstract is slightly more circumspect, claiming only that they "explore the risk":
We explore the risk that self-reinforcing feedbacks could push the Earth System toward a planetary threshold that, if crossed, could prevent stabilization of the climate at intermediate temperature rises and cause continued warming on a "Hothouse Earth" pathway even as human emissions are reduced. Crossing the threshold would lead to a much higher global average temperature than any interglacial in the past 1.2 million years and to sea levels significantly higher than at any time in the Holocene.
(the paper fleshes out these words in numerical terms).
The paper lists a number of possible positive carbon cycle feedbacks and quantifies them as summing to a little under half a degree of additional warming (Table 1 in the paper). The authors then wave their hands, say it could all get much worse, and with one bound Jack was free. End of paper. I went through it again to see what I’d missed, and I really hadn’t. It is just make-believe, they don’t "explore the risk" at all, they just assert it is significant. There’s a couple of nice schematic graphics about tipping points too.
The mildly interesting part is what led me to read the paper at all, 6 months after missing its original publication. An editor contacted me a little while ago to ask if I’d write half a of debate (to form a book chapter) over whether exceeding 2C of warming would lock us onto a trajectory for a much warmer hothouse earth. I was charged with arguing the sceptical side of that claim. I was initially a bit baffled by the proposal as I had not (at that point) thought anyone had claimed anything to the contrary, but it soon became clear what it was all about. I said I’d be happy to oblige, but it turns out that my intended opponents, being two of the co-authors on the paper itself, were not prepared to defend it in those terms.
