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Why haven’t we encountered aliens yet? The answer could be climate change

Enrico Fermi, when asked about intelligent life on other planets, famously replied, “Where are they?” Any civilisation advanced enough to undertake interstellar travel would, he argued, in a brief period…

Feeling the heat. ESO/L. Calçada, CC BY

Enrico Fermi, when asked about intelligent life on other planets, famously replied, “Where are they?” Any civilisation advanced enough to undertake interstellar travel would, he argued, in a brief period of cosmic time, populate its entire galaxy. Yet, we haven’t made any contact with such life. This has become the famous "Fermi Paradox”.

Various explanations for why we don’t see aliens have been proposed – perhaps interstellar travel is impossible or maybe civilisations are always self-destructive. But with every new discovery of a potentially habitable planet, the Fermi Paradox becomes increasingly mysterious. There could be hundreds of millions of potentially habitable worlds in the Milky Way alone.

This impression is only reinforced by the recent discovery of a “Mega-Earth”, a rocky planet 17 times more massive than the Earth but with only a thin atmosphere. Previously, it was thought that worlds this large would hold onto an atmosphere so thick that their surfaces would experience uninhabitable temperatures and pressures. But if this isn’t true, there is a whole new category of potentially habitable real estate in the cosmos.

Finding ET

So why don’t we see advanced civilisations swarming across the universe? One problem may be climate change. It is not that advanced civilisations always destroy themselves by over-heating their biospheres (although that is a possibility). Instead, because stars become brighter as they age, most planets with an initially life-friendly climate will become uninhabitably hot long before intelligent life emerges.

The Earth has had 4 billion years of good weather despite our sun burning a lot more fuel than when Earth was formed. We can estimate the amount of warming this should have produced thanks to the scientific effort to predict the consequences of man-made greenhouse-gas emissions.

These models predict that our planet should warm by a few degrees centigrade for each percentage increase in heating at Earth’s surface. This is roughly the increased heating produced by carbon dioxide at the levels expected for the end of the 21st century. (Incidentally, that is where the IPCC prediction of global warming of around 3°C centigrade comes from.)

Over the past half-billion years, a time period for which we have reasonable records of Earth’s climate, the sun’s surface temperature increased by 4% and terrestrial temperatures should have risen by roughly 10°C. But the geological record shows that, if anything, on average temperatures fell.

Simple extrapolations show that over the whole history of life, temperatures should have risen by almost 100°C. If that were true, early life must have emerged upon a completely frozen planet. Yet, the young Earth had liquid water on its surface. So what’s going on?

Get lucky

The answer is that it us not just the sun that has changed. The Earth also evolved, with the appearance of land plants around 400m years ago changing atmospheric composition and the amount of heat Earth reflects back into space. There has also been geological change with the continental area steadily growing through time as volcanic activity added to the land-mass and this, too, had an effect on the atmosphere and Earth’s reflectivity.

Remarkably, biological and geological evolution have generally produced cooling and this has compensated for the warming effect of our ageing sun. There have been times when compensation was too slow or too fast, and the Earth warmed or cooled, but not once since life first emerged has liquid water completely disappeared from the surface.

Our planet has therefore miraculously moderated climate change for four billion years. This observation led to the development of the Gaia hypothesis that a complex biosphere automatically regulates the environment in its own interests. However, Gaia lacks a credible mechanism and has probably confused cause and effect: a reasonably stable environment is a precondition for a complex biosphere not the other way around.

Other inhabited planets in the universe must also have found ways to prevent global warming. Watery worlds suitable for life will have climates that, like the Earth, are highly sensitive to changing circumstances. The repeated cancelling of star-induced warming by “geobiological” cooling, required to keep such planets habitable, will have needed many coincidences and the vast majority of such planets will have run out of luck long before sentient beings evolved.

However, the universe is immense and a few rare worlds will have had the necessary good fortune. It may just be that Earth is one of those lucky planets – a precious, fragile jewel in space. So, perhaps inevitably, climate change will remain a bane of the continued existence of life on such planets.


Next, read this: Habitable exoplanets are bad news for humanity.

Join the conversation

28 Comments sorted by

  1. Comment removed by moderator.

  2. Thomas Goodey

    Researcher

    The title of this article is very comical, probably unintentionally, and it has certainly given me a good laugh at breakfast today! Surprisingly, the actual content is quite reasonable; indeed, it is a meaningful contribution to Fermi Paradox speculation.

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    1. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Thomas Goodey

      Hi Thomas
      I think the title was intentionally tongue in cheek. It's not mine, it came from an editor, but I have to say that I like it. I'm glad you found the rest of the article interesting.

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    2. Thomas Goodey

      Researcher

      In reply to David Waltham

      I have never understood this thing about the authors of articles not having any control over the title the articles go by, but I must believe it, because I have been assured several times that it is true.

      The article is very interesting, but you omit to say (perhaps because you think it to be obvious) that the underlying reason that "our planet has miraculously moderated climate change for four billion years" is probably simply anthropic.

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    3. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Thomas Goodey

      You are correct that this is an anthropic selection argument. However, in such a short article there's no space to introduce such jargon. I can assure you that I talk a lot about it in my book, "Lucky Planet" and you can learn more about that from my website davidwaltham.com/lucky-planet. Please take a look, I think you'll enjoy it and find much you agree with (and maybe a few things you don't!).

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  3. Nick Holmes

    retired environmental scientist

    Nice article, Dr Waltham. It's an interesting facet of earth-systems science, where we are beginning to understand the nature and action of the various feedback linkages in the global dynamic system. The phase space of the Earth system through time would look extremely interesting.

    The major question that this article triggers is whether planetary systems are necessary self-stabilizing, or whether we just happen to be lucky. Is Earth special in having a suite of features that allow life to have…

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    1. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Nick Holmes

      Hi Nick
      Interesting comments, many of which I do cover in Lucky Planet so I hope you enjoy it. I don't discuss the Fermi Paradox or the Drake equation explicitly although, as I hope the article above makes clear, these are very relevant. So, I'm not really able to comment on how serious Fermi was being. Maybe it was, as you say, a brush-off to dispose of an irritating questioner. All I can say, if that's true, is that Fermi off-the-cuff is more profound than me at my very best.

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    2. Leon Carter

      logged in via email @gmail.com

      In reply to David Waltham

      A seriously intelligent species would work out that it has no long term future on its home planet, and would thus devise technical methods to colonize the galaxy. Fermi was being serious when he pointed out that despite plenty of time in which to do so, none evidently bothered or succeeded.
      We don't yet have a plausible explanation for how life got started here, or what the sufficient starting conditions were.

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    3. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Leon Carter

      Hi Leon
      I can only agree that we don't know how easy it is for life to start even when conditions are suitable. The origin of life may well be a difficult trick to pull off and I discuss this possibility in Lucky Planet. We're certainly not short of potential explanations for the Fermi Paradox! The difficulty is in deciding which one is correct.

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    4. Leon Carter

      logged in via email @gmail.com

      In reply to David Waltham

      Hey David,
      It is an inferred feature of the "paradox" that an advanced civilization figures out the threat posed by stellar evolution and tries to do something about it in a timely fashion. Stellar evolution cannot therefore cannot be the explanation resolving the apparent problem, for it is *the prime motive for colonizing space.*
      The "paradox," in any case, is only in the discrepancy between (a) crude statistical calculations on the probability of an advanced extra-terrestrial civilization evolving and colonizing the galaxy, and, (b) the alien-free reality we observe.
      The Fermi Paradox is similar to the bee "paradox" from the old day back when the fat little hymenopterans weren't supposed to be capable of flight ... because the modellers of flight said so! But there was no paradox in the flight of the bee: it was simply that the models were wrong.
      The model that resolves Fermi's "paradox" is in your book title.

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    5. Nick Holmes

      retired environmental scientist

      In reply to Leon Carter

      With respect, Leon, a seriously intelligent species would ensure that it lasts long enough to worry about a long-term future. Our current technological culture looks very fragile to me and I'm not sure that it's going to last all that long. Ecological (in the wide sense) force majeur will decide the outcomes.

      Sorry to be gloomy about this, but I understand where James Lovelock is coming from. There are significant instabilities in our current economic system which may affect our culture before planetary/ecological factors become important.

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    6. Thomas Goodey

      Researcher

      In reply to Nick Holmes

      You are making cultural assumptions about motivation. What is "seriously intelligent"? A highly advanced species might not be in the least concerned about continuing indefinitely into the future. The advanced attitude might be "apres nous, le deluge!"

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    7. Leon Carter

      logged in via email @gmail.com

      In reply to Thomas Goodey

      Motivation of animated organisms is not a cultural construct, but one determined by states of deficiency in the brain that were fashioned by evolution. These deficient states are know in English as pain & suffering, but the same states are of course universal to all humans and to all animals with a neural network.
      Evolution wires us to suffer if we aren't doing things that are good for reproduction -- things like eating, drinking, improving our status within our tribe, killing or maiming perceived…

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    8. Nick Holmes

      retired environmental scientist

      In reply to Thomas Goodey

      Actually, Thomas, I was making evolutionary assumptions rather than cultural ones. The cultural assumptions that I did make relate to the likelihood of survival of our technological culture in its present form. Leon's comment (below) puts the evolutionary idea over well.

      As to what is "seriously intelligent", I haven't really any bons mots to offer, except to note that "apres nous le deluge" might well be the motto of the current neoclassical economics paradigm. Any species that is smart enough for long enough to develop workable interstellar flight is surely not going to be much like humanity in the way in which it sets its priorities. That's one of my quibbles regarding science fiction: the aliens tend to be funny-looking humans.

      All this does actually relate to the article and the Fermi Paradox. And to that element in the Drake equation about how long a techy culture might last. And to some of the findings from behavioural economics about discounted future benefits.

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    9. Thomas Goodey

      Researcher

      In reply to Leon Carter

      This is your point of view as a human being, a product of natural evolution. Your mandate, your raison d'etre, is "survive to reproduction". This is not a function of intelligence per se, except insofar as intelligence is a product of evolution. You are assuming that all "intelligent" organisms, everywhere and everywhen, will follow the same imperative. My point is that this may be a parochial view. Those hypothetical other highly intelligent races may, for one reason or another, dance to a different drummer. They may be artificial rather than natural; they may have changed their own nature; they may know something we don't know; or they may have crystallized, like the entities in Theodore Sturgeon's 'The Dreaming Jewels'. Sturgeon simply asserted that the imperative of those strange creatures was not "Survive!"; it was something else. He didn't tell us what that something else was.

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    10. Thomas Goodey

      Researcher

      In reply to Nick Holmes

      Obviously you haven't read enough of the right science fiction!

      That is my point: a smart species, totally different from mankind, may be quite different from mankind in the way it sets its priorities. In fact, it may no longer be driven by evolutionary culture, if it ever was. Note that we ourselves are nowadays very far from being totally driven by evolutionary imperatives: each human family does not spend its time trying to kill all the other families.

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    11. Nick Holmes

      retired environmental scientist

      In reply to Thomas Goodey

      "In fact, it may no longer be driven by evolutionary culture, if it ever was."

      Depends on how one defines an "evolutionary culture". There will still be selection, natural or otherwise. To be sure, a technological culture like ours manages to stave off many of the dangers of the "natural world", such as disease. But any technological culture will have limits and will meet obstacles that it cannot overcome.

      My contention is still that a species with enough interstellar travel to surprise Fermi…

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    12. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Nick Holmes

      Indeed it has gone a little off topic but an interesting thread none the less. Food for thought and what more could you ask for? Thanks Nick, Thomas and Leon.

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  4. Alexander Rosser

    Philosopher

    Why the anachronistic use of "centigrade" when "Celsius" has been the standard for over 50 years. (Except of course in those backward nations that still use Fahrenheit).

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    1. Thomas Goodey

      Researcher

      In reply to Alexander Rosser

      Although this is a minor nit, I think it is worth picking. Actually I am of the opinion that, in non-mission-critical applications, it is high time to drop even the specification of degrees C, and to allow those who are prone to confusion to become confused. Celsius is the default standard now.

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    2. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Thomas Goodey

      It's interesting what we're individually irritated by. My own personal pet-hate is Km and Kg instead of km and kg. I do not live 100 Kelvin-metres from London! Talking of Kelvins, in an ideal world we'd use these instead of Celsius and then wouldn't have to hear nonsense like "temperatures have doubled" when they've gone from 10 Celsius to 20 Celsius (i.e 283 K to 293 K). In the context of that last sentence, the distinction between Centigrade and Celsius is absolutely critical so I do see where Alexander and Thomas are coming from but, personally, I'm quite comfortable with Centigrade whilst finding Fahrenheit rather quaint.

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  5. Graham Strouts

    logged in via Twitter

    I recently listend to your book Lucky Planet on Audible, found it incredibly stimulating, with a very broad range of science explained from astronomy and cosmology to evolution and geology. Reading this I didn't think Fermi's Paradox was in the book, so it is good to see that part of the jigsaw explained here. It makes perfect sense- if advanced life forms were able to travel across the Galaxy that would be an incredible technological achievement, they would surely have been able to find us by now as well. Thankyou!

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    1. David Waltham

      Reader in Mathematical Geology at Royal Holloway

      In reply to Graham Strouts

      Thank you for your kind words about my book. You are correct that I don't mention Fermi's paradox (or the Drake Equation) in Lucky Planet. There are so many different ways to tell the story and these important ideas just didn't quite fit in once I'd decided to go for "God, Gaia or Goldilocks" as my tack. I was delighted to be able to write a piece for The Conversation which put this right. I'll have to do another piece somewhere that brings in the Drake equation!

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  6. john byatt

    retired and cranky at RAN Veteran

    excellent read

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  7. Leon Hostetler

    logged in via Facebook

    David,

    You have a very interesting hypothesis here. With more than a passing interest in all things exo, I'm surprised I've never heard it before.

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