Arguments
Alarming new study makes today’s climate change more comparable to Earth’s worst mass extinction
Posted on 2 April 2014 by howardlee
The Permian Mass Extinction 251.9 million years ago, otherwise known as “The Great Dying,” was the closest this planet has come to extinguishing all complex life on Earth. Around 90% of all species died out in this single event, a worse toll even than the Cretaceous extinction that wiped out the dinosaurs.
For years the cause of the Permian Mass Extinction has been linked to massive volcanic eruptions in Siberia. Volcanic CO2 and a cocktail of noxious gasses combined with burning coal and geothermally-baked methane emissions to enact a combination of toxic effects and, most importantly, ocean acidification and global warming. It led to a world where equatorial regions and the tropics were too hot for complex life to survive. That’s a fact so astonishing it bears repeating: global warming led to a large portion of planet Earth being lethally hot on land and in the oceans! The cascading extinctions in ecosystems across the planet unfolded over 61,000 years, and it took 10 million years for the planet to recover! For comparison, our distant ancestors separated from apes only 7 million years ago.
Until recently the scale of the Permian Mass Extinction was seen as just too massive, its duration far too long, and dating too imprecise for a sensible comparison to be made with today’s climate change. No longer.
In “High-precision timeline for Earth’s most severe extinction,” published in PNAS on February 10, authors Seth Burgess, Samuel Bowring, and Shu-zhong Shen employed new dating techniques on Permian-Triassic rocks in China, bringing unprecedented precision to our understanding of the event. They have dramatically shortened the timeframe for the initial carbon emissions that triggered the mass extinction from roughly 150,000 years to between 2,100 and 18,800 years. This new timeframe is crucial because it brings the timescale of the Permian Extinction event’s carbon emissions shorter by two orders of magnitude, into the ballpark of human emission rates for the first time.
How does this relate to today’s global warming?
Climate and CO2 have changed hand-in-hand through most of geological time. Mostly these changes happened slowly enough that the long-term feedbacks of Earth’s climate system had time to process them. This was true during the orbitally-induced glacial-interglacial cycles in the ice ages. In warmer interglacials, more intense insolation in northern hemisphere summers led to warmer oceans which were in equilibrium with slightly more CO2 in the atmosphere by adjusting their carbonate levels. In glacial times with less intense northern hemisphere summer insolation, the cooler oceans dissolved more CO2, and carbonate levels adjusted accordingly. The changes occurred over gentle timescales of tens of thousands to hundreds of thousands of years – plenty slow enough for slow feedbacks like the deep oceans and ice sheets to keep pace.
How oceans processed the slow glacial-interglacial changes in the ice ages. CCD = Carbonate Compensation Depth, CO32- = carbonate. Based on text in Zeebe, Annual Reviews 2012.
Rapid carbon belches, such as in the Permian and today, occur within the timeframe of fast feedbacks (surface ocean, water vapor, clouds, dust, biosphere, lapse rate, etc) but before the vast deep ocean reservoir and rock weathering can cut-in to buffer the changes. The carbon overwhelms the surface ocean and biosphere reservoirs so it has nowhere to go but the atmosphere, where it builds up rapidly, creating strong global warming via the greenhouse effect. The surface oceans turn acidic as they become increasingly saturated in CO2. The oceans warm, so sea levels rise. Those symptoms should sound familiar.
How oceans get overwhelmed by rapid large CO2 emissions from Large Igneous Province (LIP) eruptions and human emissions. CCD = Carbonate Compensation Depth, CO32- = carbonate. Based on text in Zeebe, Annual Reviews 2012.
Burgess et al’s paper brings the Permian into line with many other global-warming extinction events, like the Triassic, the Toarcian, the Cretaceous Ocean Anoxic Events, The PETM, and the Columbia River Basalts, whose time frames have been progressively reduced as more sophisticated dating has been applied to them. They all produced the same symptoms as today’s climate change – rapid global warming, ocean acidification, and sea level rises, together with oxygen-less ocean dead zones and extinctions. They were all (possibly excluding the PETM - see below) triggered by rare volcanic outpourings called “Large Igneous Provinces,” (LIPs) that emitted massive volumes of CO2 and methane at rates comparable to today’s emissions. The PETM may also have been triggered by a LIP, although that is still debated.
Can we seriously expect Earth’s climate to behave differently today than it did at all those times in the past?
Some have pointed out that since we began our modern climate change in an “icehouse” era with ice sheets to melt and low starting CO2 levels, we might not generate a Permian-like hothouse. In addition, since the Permian, calcareous algae have changed the way deep oceans process carbonate, providing more of a buffer. But that buffer only comes into play if the deep oceans come into play, which most estimates consider won’t happen for a few more centuries.
All in all, the parallels between the many mass extinction events in the geological record and today’s climate change offer no comfort about the legacy we’re leaving for our children and our grandchildren. Rather they stand as signposts for an increasingly scary future.
Surely this terrible study should be stopped immediately!
I thought you should know that when you rest your cursor on PNAS it provides a definition of PNA: "Pacific-North American pattern" instead of "The Proceedings of the National Academy of Sciences of the United States of America."
Another great article about the threat humanity faces that some among the poulation do not care to better understand.
I appreciate that the focus of this site is the improvement of understanding related to global warming/climate change, however, I believe it is important to frequently include mention that burning fossil fuels produces more negative consequences than the excess CO2. Also, many other unsustainable and damaging activities have 'developed' in the 'developed' nations and are being developed in the developing nations. The socioeconomics of popularity and profitability have led to the rapid development of unsustainable and damaging activities any way they can be gotten away with. That is what needs to be challenged. And it is why efforts to lead to better understanding of CO2 impacts have prompted such persistent and aggressive attacks.
These 'developed and developing' activities clearly cannot be continued by even just a few humans through the hundreds of millions of years that this amazing planet should be able to support a robust diversity of life. In addition, the fighting to try to get the most benefit from the limited unsustainable and damaging activities creates massive social justice harm, including the collateral death of bystanders in the vicious conflicts that erupt over control of these opportunities.
Keep on raising awareness of the concern regarding CO2, but I recommend adding in these other reasons the current 'developed ways' are so fatally flawed. Reduced CO2 is only part of the solution (and why I am opposed to suggestions that Carbon Capture should be considered a part of the solution, it should only be an added required temporary action on top of rapidly curtailing the burning of fossil fuels).
So many other popular and profitable activities need to be curtailed, and that is why issues like CO2 emissions, social justice (including things like the patently obvious need to eliminate the production and use of land-mines), environmental protection, and reduction of consumption of artificial mass-production face such hostile attack. All of these clear indications of the unacceptability of what has been developed threaten those who want to continue to benefit as much as possible any way they can get away with.
Can you give some numbers pertaining to CO2 levels during the Permian extinction event?
Terranova @4, based on figure 3A of Royer (2006), late Permian CO2 levels were around 2300 ppmv, while end Permian and very early Triassic levels were around 3300 ppmv. These figures have very low temporal resolution, and my not be accurate for the million years on either side of the Permian/Triassic extinction - but I will use them as a working hypothesis.
Based on these figures, there was a CO2 forcing relative to the preindustrial era of 11.3 W/m^2 prior to the end Permian extinction, rising to 13.2 W/m^2 after. Of course, the sun was also less active at the time of the end Permian extinction - 2.14% less active to be precise. That equates to a solar forcing of -5.1 W/m^2. The net change in forcing was, therefore, from 6.2 to 8.1 W/m^2; equivalent to the change in forcing from 464 to 610 ppmv, ie, the change in forcing from about twenty years from now to early to mid twenty-second century with BAU.
Of course, these figures are far from definitive. For a start, the temporal resolution of the CO2 concentration figures are far too inadequate to draw any strong conclusion. Further, I have not accounted for changes in albedo due to changes in position of the continents, and the lack of large ice sheets at or near sea level in the late Permian. I certainly would not conclude from these figures that we are facing an end Permian extinction with BAU, although I cannot exclude it either. What I can conclude is that any argument that start with the high CO2 levels in the Permian and concludes that nothing similar could happen now is simplistic in the extreme. It almost certainly does not factor in the cooler Permian Sun, let alone a host of other relevant factors.
Terranova: to add to what Tom said,
Emissions volumes Svensen 2013 and 2009 has estimated 10E3 to 10E4 Gt Carbon but he can't separate out the individual components: "magmatic volatiles depend on both total content and oxygen fugacity (CO2/CO, SO2/H2S), whereas the sediment degassing is mostly depending on the sediment type and the organic carbon content. Since both carbonates and organic matter (+petroleum) is present in the Tunguska Basin, CO2+CH4 are generated but CO2 should dominate in the carbonate dominated lithologies" (Svensen pers comm).
As for the CO2 levels - more recent proxy data than Tom quoted put late Permian CO2 levels broadly similar to today. Mid Permian levels were higher at around 1000 to 2000 ppm, but early Permian CO2 levels were at today's levels or below (there was a significant ice age then).
The Permian sun was about 1.6% less bright (taking a 30% increase over 4.6 billion years and scaling it to 252million years). That's more than the TSI variation in our modern solar cycle but it was close enough to modern levels by the Permian.
The rates that those grreenhouse gasses were emitted depends on taking those estimated volumes and dividing them by the timeframes of emission - which is why dating is axiomatic. The biggest individual flow and pipe degassing events are considered by workers in the field to have taken place over a 1-100 year timeframe. By showing that the Permian emissions occurred faster than the slow compensation mechanisms (weathering etc) and at rates in the ballpark of modern emissions, from CO2 levels not far off modern values (geologically speaking), Burgess et al have shown that - as Tom says - we can't rule out a Permian-like (or Triassic-like, etc) event at the extreme end, with business-as-usual emissions continuing.
And no, it wasn't triggered by microbes, as a recent (April-1st) paper has suggested. I plan a follow up post on why that isn't plausible, although microbes may well have had a role.
The takeaway from this article was that big, geologically-rapid greenhouse gas emissions have a long track record of being very destructive to the planet. We repeat that exercise at our peril.
One Planet Forever - to your point that it is about more than CO2, it's a valid point. We have many fingers in the machinery of our climate, and the environment generally.
And to that point, Burgess et al suggest that the effects of the Permian extinction unravelled in a cascade over 60,000 plus years, in a kind of domino effect across different niches and ecosystems.
Even if we cease all CO2 emissions today, we would still have multiple other effects ongoing with climatic and environmental effects (soot, methane, land use, etc etc). But CO2 is the major, most existential threat right now, so I believe it is correct to focus effort on it's reduction commensurate with the risk it poses to our kids and our grandkids and their descendents.
howardlee@6
Are you talking about this (Rothman et al 2014) paper (also press release)? Both links seem to be living happily at the time herein and it seems strange that such apparent April's Fool joke is still there, not debunked/taken out. To be precise, the press release is dated 31 March 2014 while the article approval date is February 4, 2014 so I would not say it is April's Fool based on those dates.
In any case, (Rothman et al 2014) does not invalidate the conclusions from the study at hand here. In the end it does not matter what was the direct source of carbon, with respect ot the efects such release. I note however, that if we assume 100% of that release came from volcanoes, and that the rate of release was close to current antropogenic release, then we conclude that such volcanic activity be an extreme outlier - 100 times faster than the natural rate of CO2 outgassing, and lasting continuously for few centuries or 10 times faster for few millenia. Is it geologically possible?
howardlee @6:
You have scaled the rise in solar luminosity with time linearly, whereas the best simple approximation is:
L=L0/(1+0.4*((T0-T)/T0)
where L0 is the current luminosity, T0 is the current time, and L and T are the luminosity and time at the time of interest. I believe this formular breaks down prior to 4 billion years ago, but otherwise is accurate. My estimate of 2.14% less luminosity is based on that formula, for a time of 250 million years ago. For 252 million years ago the reduction is 2.16%.
Based on Breecker et al, I have estimated approximate CO2 levels of 390 ppmv for the late Permian, and 1,390 ppmv for the early Triassic immediately following the Permian/Triassic extinction. With the solar luminosity estimate, that becomes equivalent to a change from 150 to 530 ppmv today. The 150 is a very low value, suggesting ice age conditions prevailed in the late Permian, something known not to be true. Consequently, if Breecker et al are correct, either the Earth's albedo was less at the time, or the continental configuration discouraged ice age conditions, or both.
Regardless, the change in forcing is equivalent to a change in forcing from 280 to 1000 ppmv. Because greenhouse forcing is a log function of CO2 concentration, the lower estimated CO2 levels reduce the apparent threat of a Permian extinction event hothouse, while still leaving it well within the range of an aggressive (or sustained) BAU. At the same time, they increase the apparent risk of Permian extinction event like ocean acidification levels.
Again, all calculations are for indicative purposes only. I certainly lack sufficient information on Permian albedo etc to make exact comparisons.
Finally, one curious effect of the change of luminosity is to change the balance of heat between poles and equator. Increased solar luminosity has a far greater warming effect on the poles, while that from GHG warms more in mid-high latitudes. For the same level of forcing, with more GHG forcing and less solar forcing (as in the Permian extinction), we would expect less warming at the equator. Despite this, you point to a paper indicating that the equatorial regions became inimical to life due to heat in the aftermath of the Permian extinction. That strongly suggests that pushing BAU to Permian extinction levels will result in equatorial regions even more inimical to life.
Tom@9,
Can you point the source of your claim? How do you reconcile your claim with the known facts about solar incoming short length solar vs. outgoing long length IR, as measured by satelites? For example anual average here:
where we can clearly see that solar absorbtion dominates on the equator. Therefore, with increased TSI and all other things equal, one would expect the increasing warming over the equator, contrary to your claim.
On the other hand, the satellite date on OLR looks like this (top - absolute values, bottom - S deviation):
The biggest IR is at mid-lattitudes. So that data supports your assertion that "[positive forcing from] GHG warms more in mid-high latitudes"
Sorry Chriskoz, complete brain fart there. I had intended to type "Increased solar luminosity has a far greater warming effect on the equator". If you switch to the correct word, you will find the rest of the paragraph makes far more sense. I apologize for the confusion.
Tom@11,
Thanks. Having read your last paragraph @9 (originally I did not pay attention to it because of your false premise/typo), I need to add that the equatorial extinction as described in (Burgess et al 2014) does not result from the strength of positive climate forcings in that region.
As you note, forcings change the energy budget and the changes are not homegeneous. However the actual local warmings are usually not the same as the local forcings, because the heat transfer within AO. For example, we know that highest rate of warming the Arctic ice is currrently experiencing results from heat transfer via ocean currents, whereas Antarctic ice sits on land so does not enjoy heat exchange as fast as Arctic.
Secondly, the actual local warming (expressed as dT) does not necessarilly correlate with the ensuing local extinction. The T stress on organisms depends mainly on the number and the duration of extreme events expressed as the n-sigma departure from the original T variability to which the organisms are adapted. Over equator hovewer, the variability is much lower than over the poles so even small dT causes large stress.
Finally, heat is not the only factor in the extinctions. As you can see from the pictures in the article, the ocean acidification was the big factor in Permain extinction of the ocean creatures. While even relatively smaller dT changes could still extint land creatures per my second point above.
Yes, because we now have politicians who are committed to legislating physics into submission. Those previous events were politician-free, so there is no comparison to be made with today. Some claim politicians change polarity over multi-year cycles in much of Earth's landmass, but these mysterious 'cycles' have not been adequately explained by political scientists, so may be regarded as nothing more than arm-waving by activists.
Personally, I see little change in polarity between the little red ones and the little blue ones (both spinning more or less to the right), although some of the little green ones exhibit stronger polarity differences (spinning to the left in general) and these have developed thicker shells.
Hi,
I've some question for a thing which I cannot grasp regarding the first picture. The problem I have is that at 280 ppm CO2 in atmosphere and less of it dissolved in oceans, I'd expect lower pH in ocean than at 200 ppm, but the images shows the opposite (pH is higher when atmospheric concentration is at 200 ppm). I guess there must be something more, but it is somehow unclear what mechanism is behind this. I will appreciate more detailed explanation (P.S. if I've missed a link and you consider this off topic, I apologize, but I'd be grateful to point me into right direction).
MP3CE - I recommend you download Prof Zeebe's paper - it explains it all.
But essentially, colder oceans can dissolve more CO2 than warmer oceans.
Tom - I defer to your greater knowledge on luminosity and forcing calculations.
regarding Permian ice-age conditions there was southern glaciation in early to mid Permian, at least at higher elevations. According to this paper, even in the late permian there were frigid conditions with permafrost in the southern part of of the world. We would have to drill down to those specific late Permian data points to figure out what's going on there. According to the Isbell et al paper linked to in this comment, CO2 levels rose markedly at the end Guadalupian - concident with the Emeishan LIP eruptions. The environmental effects of that were ongoing (this paper and this) when the P-T extinction hit.
The Permian supercontinent configuration, with land stretching from pole to pole, would undoubtably have different ocean currents than today, with much more north-south heat distribution than is possible in today's world with the Antarctic Circumpolar Current.
Criskoz - yes I am referrring to the Rothman et al 2014 paper. You are correct, it is a real paper not an April fool, and although it hit the headlines on April 1, it was realeased before. From my reading of the Rothman paper they were attributing the cause of the carbon emissions primarilty on microbes (with fertilization by nickel from the Siberian Volcanics). On the one hand you are correct - the way the climate responded to a huge slug of carbon - irrespecive or source - is a stark warning for us today.
But on the other hand LIPs have a long track record of these kinds of changes, whereas the microbe mutation idea is a 1-off explanation. Aside from establishing the true cause intellectually, establishing the true cause helps us understand how comparable the Permian (or Triassic or Toarcian etc etc) events are to today. A chance microbe mutation a couple of hundred million years ago has no applicability to today.
Yes LIPs are not your average volcanoes - they are a very differnt animal altogether. Every year there are something like 50 to 65 volcanic eruptions, but we haven’t had a LIP eruption in 16 million years. They are apparently related to mantle plumes delivering copious quantities of superhot, superliquid lava from the lower mantle to the surface and injecting in sheets and fissures through the crust - like internal bleeding. Their lava flows are so copious they can flow for over 1,500 km. A single LIP can cover 1% of the planet's surface in lava.
MP3CE - a simplified answer:
Higher atmospheric CO2 concentrations result in more CO2 dissolving into the oceans, despite warmer ocean temperatures (See Dalton's Law of Partial Pressures and Henry's Law). Basically, if you increase the pressure of CO2 in the atmosphere it will dissolve more CO2 into the ocean. Less CO2 in the atmosphere during the last glacial maximum, for instance, meant lower partial pressure and consequently lower dissolved CO2 in the ocean.
Because pH is a negative logarithmic scale, higher pH indicates a lower concentration of hydrogen (hydronium) ions in seawater. With lower atmospheric CO2 at the last glacial maximum, there would have been fewer hydronium ions in seawater. Therefore pH would have been higher than today and, all things being equal, it would have been more conducive to shell-building in marine organisms than today.
The general response indicated in the image above is correct.
chriskoz @12:
1) While local warming rates may not correlate with local forcings due to heat transfer, for increased insolation to not result in a greater warming at the tropics than at the poles, it must drive a mechanism to reduce the escape of heat to space, thereby forcing greater heat transfer towards the poles. In fact it does drive such a mechanism in the water vapour feedback. However, given that the WV feedback essentially doubles the Planck response to forcing, it is a reasonable approximation that for the same forcing, increased CO2 (which also restricts heat escape to space) will drive a greater heat transfer to the poles than will an equivalent increase in insolation. Indeed, given the same global temperature response from both forcings, and given that CO2 forcing does amplify polar temperatures more than does solar forcing, it follows that solar forcing must generate greater warming elsewhere for the average to come out the same.
2) While relative change from conditions to which organisms are adapted to is the major driver of extinction, there are some hard physiological limits. Clearly no organism can survive the permanent lowering of its body temperature much below freezing. The possibility of evolving some sort of antifreeze for the blood (found in some fish) or or merely seasonal activity makes this "hard limit" a bit fuzzy, but the dominance of warm blooded life forms in Arctic ecology shows that very low temperatures present more than a relative impediment to life.
The same occurs for warm temperatures. Very high temperatures restrict the capability of getting rid of excess heat. This is particularly a problem for large warm blooded creatures, but at higher temperatures becomes a problem for large "cold blooded" creatures, which generate internal heat from the function of muscles and organs, as well. For large warm blooded creatures that use evaporative cooling for heat dissipation, the "hard limit" is sustained wet bulb tempertures of 35 C (SkS summary).
Even for small cold blooded creatures, as sustained temperatures exceed 40-50 C, the disorganizing activity of the heat tends to overwhelm their ability to sustain life - but that limit is nowhere near as hard as it is for large cold blooded, or for warm blooded creatures.
3) While Burgess et al may not attribute the extinctions to high temperatures, Sun et al (2012), linked in the OP under "lethally hot" certainly do. They write:
The lower temperature for marine animals is because of induced anoxia rather than heat stress specifically, but for land animals and plants, it is heat stress that is the killer.
While Sun et al deal strictly with the Triassic, including the aftermath of the Permian/Triassic extinction, Burgess et al show that CO2 levels at the extinction event where higher even, than those at the end Smithian with its 40 C tropical waters. (As a side note, those 40 C waters may have been restricted to the proto-tethys, a very large shallow sea stradling the equator, and may not have been typical of oceanic tropical water.) As it happens, CO2 concentrations were higher during the Permian extinction than the end Smithian, with presumably higher temperatures as a result:
(Corrected version of Burgess et al, 2014 Fig 3)
howardlee @16, I agree with you about the ocean currents. With respect to the Permian taiga forest like conditions, that would be consistent with a forcing equivalent to modern forcings rather than with a forcing less than during the last glacial as suggested by the revised CO2 estimates. Having said that, that estimate assumes modern alebedo. Had the albedo decreased to 0.25, the net solar forcing would represent an increase of 11.5 W/m^2 rather than a decrease of 5.2 W/m^2. The unknowns are too large to say anything definitive.
I will say this, though. The forcing calculations show that a change in conditions equivalent to that which drove the Permian extinction event cannot be excluded based on "high" Permian CO2 levels; but nor is it certain that we face one. Based on Sherwood and Huber, we would require four doublings of CO2 at "likely" estimates of climate sensitivity to reach such conditions; but could reach them with two doublings at climate sensitivities within the IPCC "90% confidence interval". That is, a BAU approach with declining conventional fossil fuels being replaced by unconventional fossil fuels (shale oil, tar sands) and diesel manufactured from coal could bring about such conditions from, at a rough estimate, 150-300 years from now.
We could get Permian extinction levels of species loss, however, within 100 years with without such an aggressive BAU approach when coupled with other factors (notably overfishing).
Tom - Ii think the climate sensitivity question goes to the heart of the issue. We have on the one hand sensitivities based mainly on modeling and the instrumental record that are 1.5-4.5°C (2.7-8.1°F) (IPCC AR5).
On the other hand, looking at the geological record, it suggests that actual Earth System Sensitivity is double that. For transient sensitivity - relevant to the year 2100 - we have to bridge the gap between what we are doing now to that long term sensitivity. Zeebe does this in this paper: "even if the fast-feedback sensitivity is no more than 3 K per
CO2 doubling, there will likely be additional long-term warming from slow climate feedbacks"
Even though The AR5 study included the paleoclimate sensitivity in this paper i worry that by heavily weighting the study with benign glacial-interglacial changes which happen within the slow feeback timeframe, we may have generated an overly benign estimate of climate sensitivity. The carbon-belch scenario - with atmospheric emissions overwhelming the surface ocean and fast feebacks before deep oceans come into play would - intuitively - suggest a much higher sensitivity. That's why study of LIP-generated climate change could be crucial for understanding what's in store for us, more so than glacial-interglacial changes.
I often wonder whether we're really the first 'intelligent' life on Earth.
After all, we haven't been around long enough to leave any fossils of ourselves. Even if we had, all but Siberia will have been subducted by continental drift before 'intelligent' life evolves again another 250m yrs.
They'll never even know we were here, butt for one or two rare 'London Hammer' type relics in the Mongolian Steppes.
This would explain the (unexplained) drop in the 13C/12C isotope ratio before the Great Dying. Another lifeform - like us - might have madly burned fossil fuels for 200 years before triggering their own extinction.
It would also explain the Fermi paradox.
The reason we've never found evidence of intelligent life in the universe is that every 250m yrs or so, sentience springs up for the briefest moment - just a hundred thousand years. Then it discovers fire, and carbon-based fuels, and the acquisitive behaviours and superstitions that helped it thrive, wipe it out in a cosmic blink.
Over an over. Across the universe. Little flashes of thought that never manage to coincide.
In 1000 years or more the surface of this planet will be too hot to live on. A remnant of a mutated human species (already happening due to mutagenic chemicals in the environment) will then resort to living in caves, of which there are some very large systems. They will subsist on GM fungi the size of Saguaro (spelling??) cacti.
Psychokinesis removes the need for large muscles. Hence a slim build. A diet of fungi removes the need for a large mouth and teeth, resulting in an almond shaped face.
Dim light underground requires very large eyes. Life in relative darkness results in a typical pallor.
Development of travel backwards in time via flying saucers results in these descendants appearing in our time in order to collect pre-mutation DNA from cows and people.
Wishing to warn us of this future they send symbolic messages in the form of crop circles. Telepathy has by their time removed the need for words and a written language, hence they resort to such symbolic communication.
We call these people 'E.T.'s. They are of course 'endoterrestrials'.
[DB] This venue is based on credible evidence for claims. Thus, it is about science, not science fiction. Please keep comments on-topic.