Hothouse Earth: Here’s What the Science Actually Does – and Doesn’t – Say
By Richard Betts
A new scientific paper proposing a scenario of unstoppable climate change has gone viral, thanks to its evocative description of a "Hothouse Earth."
Much of the media coverage suggests that we face an imminent and unavoidable extreme climate catastrophe. But as a climate scientist who has carried out similar research myself, I am aware that this latest work is a lot more nuanced than the headlines imply. So what does the hothouse paper actually say, and how did the authors draw their conclusions?
First, it's important to note that the paper is a "perspective" piece—an essay based on knowledge of the scientific literature, rather than new modeling or data analysis. Leading Earth System scientist Will Steffen and his 15 co-authors draw on a diverse set of literature to paint a picture of how a chain of self-reinforcing changes might potentially be initiated, eventually leading to very large climate warming and sea level rise.
One example would be the thawing of Arctic permafrost, which releases methane into the atmosphere. As methane is a greenhouse gas, this means the Earth retains more heat, causing more permafrost to thaw, and so on. Other possible self-reinforcing processes include the large-scale die-back of forests, the melting of sea ice, or the loss of ice sheets on land.
Global map of potential tipping cascades, with arrows showing potential interactions. Steffen et al / PNAS
Hothouse or Stabilized?
Steffen and colleagues introduce the term "Hothouse Earth" to emphasize that these extreme conditions would be outside those that have occurred over the past few hundred thousand years, which have been cycles of ice ages with milder periods in between. They also present an alternative scenario of a "Stabilized Earth" where these changes are not triggered, and the climate remains similar to now.
The authors make the case that there is a level of global warming which is a critical threshold between these two scenarios. Beyond this point, the Earth System might conceivably become set on a pathway that makes the extreme "hothouse" conditions inevitable in the long term. They argue—or perhaps speculate—that the process of irreversible self-reinforcing changes could in theory start at levels of global warming as low as 2°C above pre-industrial levels, which could be reached around the middle of this century (we are already at around 1°C). They also acknowledge large uncertainty in this estimate, and say that it represents a "risk averse approach".
A key point is that, even if the self-perpetuating changes do begin within a few decades, the process would take a long time to fully kick in—centuries or millennia.
Steffen and colleagues support their suggestion of a threshold at 2°C through reference to previously-published scientific work. These include other review papers which themselves drew on wider literature, and an "expert elicitation" study in which scientists were asked to estimate the levels of global warming at which "tipping points" for these key climate processes might be passed (I was one of those consulted).
The authors argue that 2°C can still be avoided if humanity takes concerted action to reduce its warming effect on the climate. In a similar way that the "Hothouse Earth" scenario involves huge changes in the climate system with multiple effects of one process leading to another, the concerted global action to avoid 2°C would, they suggest, also involve huge changes in the human system, again with several fundamental steps leading from one change to another.
Don't Ignore the Caveats
Personally, I found this an interesting and important think piece that was well worth reading. But since this is not actually new research, why is it getting so much coverage? I suspect that one reason is the use of the vivid "Hothouse Earth" term at a time when everyone's talking about heatwaves. Another is that it's clearly a dramatic narrative, and not surprisingly this has led to some sensationalist articles.
With some exceptions, much of the highest-profile coverage of the essay presents the scenario as definite and imminent. The impression is given that 2°C is a definite "point of no return", and that beyond that the "hothouse" scenario will rapidly arrive. Many articles ignore the caveats that the 2°C threshold is extremely uncertain, and that even if it were correct, the extreme conditions would not occur for centuries or millennia.
Some articles do however emphasise the more tentative nature of the work, and some push back against this overselling of the doomsday scenario, arguing that provoking fear or despair is counterproductive.
One thing that strikes me about the scientific literature on "tipping points" is that there are a lot of review papers like this that end up citing the same studies and each other – indeed, my colleagues and I wrote one a while ago. There is a great deal of interesting, insightful research going on using theoretical methods and calculations with large approximations. However, we have yet to see an equivalent level of research in the highly-complex Earth System Models which generate the kind of detailed climate projections used for addressing policy-relevant questions by the Intergovernmental Panel on Climate Change (IPCC).
Steffen and colleagues have made a good start at addressing such questions, going as far as they can on the basis of the existing literature, but their essay should motivate new research to help narrow down the huge uncertainties. This will help us see better whether "Hothouse Earth" is our destiny, or mere speculation. In the meantime, awareness of the risks—however tentative—can still help us decide how to manage our impact on the global climate.
Reposted with permission from our media associate The Conversation.
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By Aaron W Hunter
A chance discovery of a beautifully preserved fossil in the desert landscape of Morocco has solved one of the great mysteries of biology and paleontology: how starfish evolved their arms.
The Pompeii of palaeontology. Aaron Hunter, Author provided<h2></h2><p>Although starfish might appear very robust animals, they are typically made up of lots of hard parts attached by ligaments and soft tissue which, upon death, quickly degrade. This means we rely on places like the Fezouata formations to provide snapshots of their evolution.</p><p>The starfish fossil record is patchy, especially at the critical time when many of these animal groups first appeared. Sorting out how each of the various types of ancient starfish relate to each other is like putting a puzzle together when many of the parts are missing.</p><h2>The Oldest Starfish</h2><p><em><a href="https://www.biorxiv.org/content/10.1101/216101v1.full.pdf" target="_blank" rel="noopener noreferrer">Cantabrigiaster</a></em> is the most primitive starfish-like animal to be discovered in the fossil record. It was discovered in 2003, but it has taken over 17 years to work out its true significance.</p><p>What makes <em>Cantabrigiaster</em> unique is that it lacks almost all the characteristics we find in brittle stars and starfish.</p><p>Starfish and brittle stars belong to the family Asterozoa. Their ancestors, the Somasteroids were especially fragile - before <em>Cantabrigiaster</em> we only had a handful of specimens. The celebrated Moroccan paleontologist Mohamed <a href="https://doi.org/10.1016/j.palaeo.2016.06.041" target="_blank" rel="noopener noreferrer">Ben Moula</a> and his local team was instrumental in discovering <a href="https://www.sciencedirect.com/science/article/abs/pii/S0031018216302334?via%3Dihub" target="_blank" rel="noopener noreferrer">these amazing fossils</a> near the town of Zagora, in Morocco.</p><h2>The Breakthrough</h2><p>Our breakthrough moment came when I compared the arms of <em>Cantabrigiaster</em> with those of modern sea lilles, filter feeders with long feathery arms that tend to be attached to the sea floor by a stem or stalk.</p><p>The striking similarity between these modern filter feeders and the ancient starfish led our team from the University of Cambridge and Harvard University to create a new analysis. We applied a biological model to the features of all the current early Asterozoa fossils in existence, along with a sample of their closest relatives.</p>
Cantabrigiaster is the most primitive starfish-like animal to be discovered in the fossil record. Aaron Hunter, Author provided<p>Our results demonstrate <em>Cantabrigiaster</em> is the most primitive of all the Asterozoa, and most likely evolved from ancient animals called crinoids that lived 250 million years before dinosaurs. The five arms of starfish are a relic left over from these ancestors. In the case of <em>Cantabrigiaster</em>, and its starfish descendants, it evolved by flipping upside-down so its arms are face down on the sediment to feed.</p><p>Although we sampled a relatively small numbers of those ancestors, one of the unexpected outcomes was it provided an idea of how they could be related to each other. Paleontologists studying echinoderms are often lost in detail as all the different groups are so radically different from each other, so it is hard to tell which evolved first.</p>
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