Thanks for the question! Our knowledge of Graham Hancock’s work is currently limited to a few internet searches (Dan) and a few episodes of his Netflix show (Constantin), but we wanted to make sure we got around to answering this.

A first important point is that our own work focuses on catastrophes that occurred much further back in time: many millions of years rather than tens of thousands, and way before humans even evolved as a species. So our own technical knowledge is still rather different than that needed to evaluate archaeological claims about past civilizations, and to productively wade into the debate surrounding his work.

The events themselves are also rather different in scale: any extinction that may have occurred during the Younger Dryas is still relatively minor in the grand history of life (if otherwise, this would have been observed in the fossil record), while some of the events we’ve been considering genuinely wiped out a large fraction of species present at the time.

On the whole (and speaking now more generally), we do think that questions of past climate changes and societal collapse are interesting ones that deserve to be looked at. Beyond pure intellectual interest, better understanding whether/how climate changes caused collapse in the past seems quite important for humanity’s future.

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This is the only question…shame to have it ignored like this. Why waste our time with an AMA?

The present rate of CO2 increase is much larger than in past disruption events. However, the critical rate of change seen in past events is only part of the story. First. the timescale of the current situation (about a century) is much shorter than past events. Second, natural processes in the oceans tend to damp perturbations of CO2, at a roughly 10,000 year timescale. The upshot is that the critical rate of the modern event must be rescaled by a factor of about 100/10000 = 0.01 to be compared to past catastrophes. When that rescaling is done, our modern disruption event, if it continues throughout this century, looks fairly similar to the runup to extreme warming events of the past, including those associated with mass extinction. A rough estimate is that the tipping point would occur late in this century. For more detail, see our papers here and here.

But that too is only part of the story. If the tipping point is real, our own calculations suggest a roughly 10000-yr trajectory during which things become progressively worse—but only if there were no negative feedbacks beyond those we currently understand that would act to arrest the trajectory. And we would imagine that new technologies—or simply improved scientific understanding—might contribute towards goading the Earth system in the right direction.

Thanks for your question! Clearly the answer is to do both. But how much to each? That depends on relative costs and benefits, time horizons, how easy or hard it is to actually implement certain policy changes, etc. --- and unfortunately these are all things we have no particular expertise in. Nevertheless we think that this discussion is important; democracy probably has an important role to play here.

Thank you for raising these important questions and for expressing them through your own personal experience. Our go-to-points about CO2 emissions and global environmental catastrophe are largely made in other comments in this thread. Depending on context, other scientists will emphasize other points and other issues. We know that reducing and eventually stopping CO2 emissions is necessary for stabilizing temperatures, and also for reducing the risk of the kinds of events we’ve studied, but how to best do this and balance it with other important trade-offs is complicated and involves a number of things outside of our particular expertise. We hope that this helps at least a little.

Thanks for the great question! One interesting phenomenon that really matches the idea of the canary is that of “early warning signals”: basically, mathematical theory predicts that a system that’s about to cross a tipping point will start fluctuating more slowly, and this can be measured. There is a lot of work going into assessing elements of the climate system that might have tipping points, and searching for such early warning signals: one recent example is the Atlantic Meridional Overturning Circulation (AMOC; think Gulf stream). Recent work (e.g. Boers 2021) suggests that the AMOC is currently exhibiting early warning signals for collapse.

With respect to the global-scale disruption events, we could say that the canary is the historical record itself. Appropriately analyzed, the record suggests that there is a critical rate of CO2 perturbation at which mass extinction occurs, and that we are on track to cross the threshold by 2100 (see also this comment). Nevertheless, since it may take thousands of years for the ultimate consequences to play out, actions we take today to mitigate this could still have an effect.

Thanks for the question! Unfortunately the question of how well equipped we are to survive the coming changes is pretty complex and somewhat outside of our specific expertise (we’ve been focusing more on the changes themselves). Understanding this involves not just scientific knowledge from a range of disciplines but also knowledge from social sciences about how societies will respond, what’s possible in terms of economics and policy, and so on.

We agree with you that the food question is a really important one. We’re also not experts on agriculture, but there are some other researchers and research institutes doing great work on this. Off the top of our heads, one prominent recent study is that of Gerten et al. (2020): they argue that it should in principle be possible to transform the global food system to feed 10 billion people in away that’s relatively globally sustainable. Whether this is going to be possible in practice, of course, is another matter.

Maybe the most important thing individuals can do (in democracies) is to vote and support initiatives that advocate for solutions to climate change. We have not worked with policymakers but we have
much respect and admiration for their difficult work.

Thanks for your question! Whatever we do, over hundreds of millions of years there will probably be a mass extinction at some point, most simply because there have been 5 mass extinctions in the last 500 million years. That said, there appears to be an unprecedented rate of species loss today, and many would argue that we are already in the sixth mass extinction (see for example Elizabeth Kolbert’s book, or this paper).

Changes in land use probably account for much modern species loss. Climate change and ocean acidification (which we’ve focused on in our work) may eventually contribute to much more. The best way to prepare for this as a society is clearly to act in ways that limit disruptions to the global environment. At the individual level, the question becomes about how to best influence the actions of societies, which is unfortunately outside of our domain of expertise.

The greatest threat to humanity stemming from climate change may be the possibility of social strife (due, e.g., to displaced populations) and its interaction with a host of other problems that results in widespread warfare. This is an important but still poorly understood subject.

Thanks for your question. We think that 100% renewable energy might be an elusive goal, but anything we can do to put less CO2 in the atmosphere is of interest. As scientists we recognize that many industries contribute disproportionately to CO2 emissions. The question then becomes how to develop appropriate policies to deal with problems in which different groups may have different views as to what is important and should be prioritized. As we say elsewhere in this thread, the democratic process probably has an important role to play here.

Thanks for your question! Severe volcanic eruptions would be minor perturbations (in terms of CO2 injection) compared to the fossil-fuel burning. Chemical fires even more so, but clearly they are problems by themselves. Nuclear detonation would bring us towards an entirely new set of serious risks independent of climate change.

First, thank you for doing such important work! It’s hard for us to give advice specifically for your situation (we’d probably just ask you for your thoughts!)

Nevertheless, in case it’s helpful: we’ve found that even talking about it is useful by itself. Extrapolating from our own lives, for the little it may be worth, it seems like often a more thorough awareness of what the negative issues are can help us better deal with them.

Thanks for your questions!

1. A large part of the answer is curiosity. We were aware of the great disruptions of the geologic past and wondered whether a quantitative theory could be devised that helped us relate past disruptions to modern environmental change. Of course were were also motivated by the practical importance of the question. But curiosity came first. Perhaps the most important consequence of our conclusions is that they’ve provided a new impetus to study the past, not just because it helps us understand how the Earth came to its pre-industrial state, but also because it helps us understand the risk of disturbing it.

2. Most fundamentally, the experience has led to a deeper respect for and interest in the people doing this important work!

Thanks for the question! See our other response here.

10 - 12 inches (0.25 - 0.30 meters) in the next 30 years, which is about as much as in the last 100 years, according to https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html.

The ozone depletion was largely caused by CFCs. The reason the ozone is healing is a massive worldwide effort to cut the use of CFCs called “the Montreal protocol.“

I’m not sure why you’re bringing deforestation into this. Deforestation is bad, we should stop that too.

There were two major volcanic eruptions in 536 and 542 AD which led to a period known as the Late Antiquity Little Ice Age. It is suspected poorer diets from poorer harvests led to the Plague of Justinian wrecking havoc with a weakened population.

While there was a decline the Avars, Sassanid and Gupta civilisations, the vacuum led to the rise of Turks, Mongols and Islam. The strategy they have in common is that, at the time, the peoples lived nomadic lifestyles on the fringes of deserts. With the Sun being ‘feeble’, as Roman senator Cassiodrus puts it, these people could be more productive during the day.

The estimate of 3/4 largely derives from the fossil record of marine animals. And that means that if a particular fossil species has not yet been discovered, we cannot know if it went extinct.

Our own work does not predict extinction in 2100. Instead it identifies the end of the present century as a time when the total CO2 added to the oceans will likely exceed a value that in the past has been associated with mass extinctions. See also our other comment here.

This is extremely difficult to answer. Our own work has focused on a small piece of the puzzle: what happened in the geologic past, and how does it inform us about our problems today? But past events played out in the absence of human societies, and what could happen to human societies over long term (1000s to millions of years) is fascinating but very poorly understood problem.

Thanks for your question! Promoting ecosystem flourishing on a local scale is a lovely goal, but outside of our particular expertise. And no, we’ve not read the book yet.

Among the many ways to answer this question, we’ll choose a path consistent with our topic: rates of change matter. So while plants may ultimately flourish in a world with higher CO2 levels, the fast
rates of change of our current situation risk triggering instabilities in the Earth system and/or very bad outcomes for human society.