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They don't posses any advanced knowledge, their knowledge is in fact irrelevant, just their possible actions (it doesn't matter how irrational those actions might be) that would cause significant environmental changes.

  • Any kind of climate change. Making Earth less hospitable, making a very earth-like world more like Earth - it doesn't matter.
  • A long period of time is allowed (let's say a maximum timeframe is 100 000 years). Natural climate change doesn't count. Reaching industrialization doesn't count, so let's assume we halt their civilization's development at an earlier point if it takes them too long.
  • Whole mankind globally is involved.
  • If it would still not be possible, what changes to the world would allow it?

How could mankind of ancient times cause significant climate change?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Jan 4 '18 at 21:32

16 Answers 16

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Large scale albedo changes

Albedo is the reflectivity of solar radiation off the surface of a planet or moon or something. It represents how much of the solar radiation is sent back into space as opposed to how much is retained.

The planetary albedo of the Earth is about 0.367. However, this varies greatly from place to place. Cumulus clouds and fresh snow can have an albedo of 0.7 or more; that is, 70% of incident radiation is reflected right back into space. On the other hand the ocean's albedo is about 0.08, which means that 92% of the solar radiation striking the ocean stays with it.

There are certain things that humans can do the change the albedo. Most plausibly for ancient man, a forest might have an albedo of 0.1 while dry bare soil might be 0.3. Thus, if Man could change the environment from one to the other over a large enough area, he could change the Earth's albedo.

How much area is reasonable for Man to change?

There were some large scale deforestation incidents in the ancient world. Possibly the biggest was the reduction of the Gangetic Plains. 10,000 years ago the area from Delhi to Bengal was covered in moist deciduous tropical forest; an area of around 500,000 km$^2$. This was ultimately replaced by cropland over the period from roughly 1000 BC to 1000 AD. Crops and irrigated soil have an albedo not too much higher than a forest, so the climate change impact wasn't much. But that Ganges plain is actually pretty dry; had humans not spent so much time irrigating it could have ended up a barren semi-desert like the African Sahel.

This 500,000 km$^2$ represents 0.1% of the Earth's surface. If .1% of the Earth's surface increased in albedo by 20 percentage points, the overall Earth's albedo would have changed from 0.367 to about 0.3677; not much.

But what is super self destructive humans did this to similar decidous tropical forests around the world? There are 2.7 million km$^2$ of miombo woodland in southern Africa; about 0.7 million km$^2$ more in India's northern Deccan Plateau; 0.4 million in Indochina; 0.3 million on the Pacific coast of Central America; and 0.9 million in the Dry Chaco of Argentina and Bolivia. This is about 5.5 million km$^2$ of forest total, or up to 1.1% of the Earth's surface. The albedo effect of turning all this forest into barrens would be from 0.367 to 0.375.

What does that albedo change do?

Albedo can be used along with stellar luminosity and orbital distance to calculate planetary effective temperature using

$$T_{eff} = \sqrt[4]{\frac{1}{4}\frac{L(1-a)}{4\pi\epsilon\sigma R^2}}$$ where $\sigma$ is the Stefan-Boltzman constant ($3.67\times10^{-8} \text{W m}^{-2}\text{K}^{-4}$); $\epsilon$ is planetary emissivity (0.96 for Earth); $R$ is distance from the sun ($1.50\times10^{11} \text{ m}$); $L$ is the luminosity of the sun ($3.83\times10^{26} \text{ W}$); and $a$ is albedo.

For Earth's albedo of 0.367, $T_{eff} = 276.5 \text{ K}$. This is a bit low; planetary average temp is more like 285 K, and the difference is mostly due to the greenhouse effect. But it is close enough to demonstrate the changes. When albedo raises to 0.375, $T_{eff} = 275.6 \text{ K}$. This is about 1 degree of cooling from albedo changes, or about the same magnitude as the heating that we have currently caused due to climate change.

Conclusion

The ancients had it in their power to change the climate the same amount that we have changed it today. However, this will require them to cut down some 5 million km$^2$ of forest; an area about twice the size of Argentina. And not just cut it down, leave it fallow so its turns into a scrubby desert with lots of exposed dirt.

Now, humanity certainly has the capacity to do this, even with stone age technology. All these forests I listed have dry seasons of more than 6 months. It wouldn't be too much work to set everything on fire at the end of the dry season. As a double bonus, all these forests have heavy monsoonal rains during the wet season, so if you timed it just right, you could get the monsoon to wash all the ashes away, leaving parched, unfertile soil that would take centuries to recover.

This seems a bit extreme, but given the sorts of things we have and are doing to the Earth, maybe we should consider ourselves lucky this didn't happen already.

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  • $\begingroup$ Albedo is a great choice! (Of all of the terran-based climate change factors, I find it the easiest to convey in terms of the current world status i.e. less glaciation means less energy reflected back into space which means less glaciation, etc.) $\endgroup$ – DukeZhou Jan 3 '18 at 20:10
  • $\begingroup$ I've read somewhere that it's also speculated that human activity may have been responsible for the deserts in mesopotamia and for the Sahara. $\endgroup$ – Shufflepants Jan 3 '18 at 21:42
  • $\begingroup$ Forest fires seems the least possible effort, but are there enough currently arid places that might become tropical if covered in trees through human effort to make the opposite change? $\endgroup$ – user25818 Jan 4 '18 at 17:03
  • $\begingroup$ @notstoreboughtdirt I tried to look into that when I was researching. It is hard to say what could or could not be turned into trees, is the problem. Some grasslands are maintained by grazing, elephants, or naturally occuring fires. Its tough to say which ones are climatically suitable for forest cover and which aren't. $\endgroup$ – kingledion Jan 4 '18 at 18:19
  • $\begingroup$ I think this answer misses the forest for the trees. Albedo would not be the main driver of climate change following massive-scale deforestation. The major changes instead would be drying and heating related to the loss of evapotranspiration. Note that there are potentially rapid tipping points in this system, because as forests dry out, eventually forest fires will reach a frequency and intensity that converts the area to savanna (in the tropics) or grassland/shrubland (in the temperate zone). $\endgroup$ – Jacob Socolar Jan 5 '18 at 13:27
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Both Iceland and Easter Island were deforested by people who settled there. The Vikings let their farm animals loose and they probably used a fair bit of wood for keeping warm, boats, huts and metallurgy. The combination left the Island deforested. What happened on Easter Island is less clear but similar result.

Whether such an outcome is likely over an entire continent is unclear, because a society generally speaking, develops within a certain amount of sustainability and the deforestation happens when they go somewhere new that can't sustain their way of life. (different predators for example, so farm animals can grow unchecked). Traveling with invasive species can do a lot of harm, even if it's unintended.

Mentioned in the other thread, Large scale agriculture, burning to clear-cut forests could also be a factor. Enough farming and deforestation can drive climate change, though probably not as fast as we're doing it now, but it's possible. It's theorized, that human agriculture had a warming effect on the planet that continues to this day. Both the deforestation and Methane (aka cow-farts) likely have warming effects.

That said, all those effects are likely smaller than our current warming and not huge drivers, so significant change is somewhat unlikely, but if combined with naturally occurring variation due to orbital variation in the same direction, it's reasonably possible.

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  • $\begingroup$ iirc there is evidence of unsustainable deforestation around prehistoric settlements in Britain too $\endgroup$ – jk. Jan 5 '18 at 13:05
  • $\begingroup$ @jk. Pre Roman or when the Romans were there? Just curious. I wasn't aware of that. $\endgroup$ – userLTK Jan 5 '18 at 17:37
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They could do it the same way we are doing it today, by burning large quantities of stored fossil fuel, releasing large amounts of Carbon Dioxide into the atmosphere.

Based on the low population numbers burning them for semi useful purposes are out. Also due to the low technology they are going to be limited to near surface deposits.

The most likely candidates that I can see are large out of control burning events. A few come to mind, there are a disturbing number of underground coal seam fires, where the deposits of coal are lit on fire and keep on burning through the underground seam over years burning huge amounts of coal. Many of these coal seams start near the surface so early low tech mining or surface harvesting, could lead to burning huge amounts of deep underground coal.

This type of underground burning can happen for natural gas and oil sources as well. A semi famous example is the Darvaza gas crater in Turkmenistan, which is known locally as the "Door to Hell", where a Soviet drill site collapsed in 1971 igniting the near surface natural gas, it has been burning ever since.

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The interesting thing is that these large scale burning events, could actually lower temperatures in the short term by releasing huge amounts of soot into the atmosphere, similar to a large volcanic eruption, as well as causing acid rain and other environmental problems.

More near surface deposits or earlier, widespread, not very safe, use of fossil fuels would make these events more likely and have a bigger effect on global climate.

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  • $\begingroup$ Not arguing, honestly asking - how ancient are we talking here, and when did people know how to uncover fossil fuels and have the ability to dig it up in any relevant quantity? $\endgroup$ – JPhi1618 Jan 5 '18 at 15:30
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    $\begingroup$ @JPhi1618 4000 BC is the earliest known use of coal, but really as soon as people are using fire they could find that these rocks burn. The issue with coal seam fires is you don't have to mine it, the seam can extend to the surface allowing Neolithic people to potentially start a fire burning more coal than they could mine. $\endgroup$ – Josh King Jan 5 '18 at 17:08
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Okay, so our ancient people really have it out for future generations. I'd start with Viking technology - they're going to want to be able to sail long distances, although this isn't strictly necessary. Other answers have mentioned deforestation, but that alone won't actually have that huge of an impact. They can certainly put some effort into that, but what's really going give you the most bang for your medieval buck is:

Burning Peat Bogs and Coal Seams

It turns out that burning coal seams represent 3% of the world's global carbon emissions. If we actively search for and ignite these coal seams, we can increase this number. Peat bogs have similar effects. I recommend that we concentrate our search in the Arctic. Greenland especially. Recent fires in Greenland have been a cause for concern among many scientists. These fires do of course cause major release of carbon from the burning peat and coal, but they also release black soot, which lowers the albedo of the surrounding ice, which can cause widespread melting. Once the ice is gone, it's possible that the permafrost would start melting as well, releasing large amounts of methane, a very potent greenhouse gas. If you really really want to get the most out of this, I would recommend that your ancient climate scientists find a way to distribute this soot over as large an area as possible.

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The most obvious is large scale deforestation.

Changes rain patterns, erosion, heat absorption, soil properties, changes the makeup of the atmosphere, etc. And it actually happened, too.

Almost the opposite is large scale irrigation, which also happened in ancient times

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    $\begingroup$ And overgrazing, too. $\endgroup$ – AlexP Jan 3 '18 at 12:12
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Realistically, man in ancient times was too few in numbers to make a meaningful impact even with today's technology unless they knew exactly what they were doing and it was deliberate.

That said, the easiest way to have an impact on climate change is to add carbon to the atmosphere in great numbers. About the only way that ancient man could do that is to burn trees, forests, whatever coal they could get their hands on.

Let's assume that they set fire to every forest they could find. If man was sufficiently spread out and had access to every forest on Earth, it's possible this could be done. It would have devastating effects on the ecology and it would be bad for them in terms of their own survival, not to mention that the wood going up in smoke would have been extremely useful. But, if they did that in a concerted and sustained effort, burning back even the regrowth, it's possible that you could put enough carbon in the atmosphere through fires to have an effect.

My concern about the above is that I don't know if this would represent significant climate change. I'd need to know more about the forest assets on the Earth at the given time, what access to coal they had (ancient man certainly had some coal at his disposal) in order to put numbers down, but the fact that the forests would have a role to play in any recovery (stripping out the carbon via photosynthesis) would tend to indicate that you'd be adding carbon AND reducing the ability to remove it from the atmosphere at the same time.

Also, with their numbers and limited communication strategies, this kind of concerted global effort seems highly unlikely. That said, if that issue was addressed, it's possible for ancient man to have an impact on climate; it's just not clear if that impact would be significant.

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  • $\begingroup$ Nice answer. It reminds me of Tolkien's Mordor in a way--I could see a mass movement based on burning every forest and reducing the world to ash. If the world had a single massive continent, I could see the effects being plausibly global. $\endgroup$ – DukeZhou Jan 3 '18 at 20:12
  • $\begingroup$ Yes, good analogy DukeZhou. A single continent was WAY before humans, but it would be a plausible solution to the communication issue. $\endgroup$ – Tim B II Jan 3 '18 at 21:20
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The easiest way to change the environment in a large way is to introduce (or eradicate) a species that drastically changes the ecosystem.

Invasive species can end up growing out of control because the ecosystem they were introduced to has not evolved defense mechanisms. They may then decimate existing species through predation or competition.

In the real world there are many examples of humans transporting a species from one place to another where the grow out of control. I don't think this has led to huge climate change, but it is easy to imagine a situation where this could happen. e.g. introducing wood-eating beetles might destroy forests, or eradicating apex predators may cause an explosion of herbivores that consume all the plants.

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    $\begingroup$ They certainly affect the environment. Can you provide any evidence that an invasive species affected the climate of a region? $\endgroup$ – sphennings Jan 3 '18 at 22:35
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No need to think about what-would, it's enough to focus on things we've already done:

There is theory that we've started changing the climate ~10000 years ago, when we've invented agriculture. What was the result of it? That we've stopped the ice age. You can start researching it from Wikipedia article on "early anthropocene"

Another, more recent, more drastic, however more mechanized event was the dust bowl of 1930. On the one hand it's commonly thought as made possible by mechanization of farming. On the other, it was done with nothing else but plowing, so it's imaginable that given enough effort it could be replicated with nothing else but a horse-drawn plough (I can't find atm any references if deep plowing is possible with a horse or if the results are good enough to outweigh working the horse so hard)

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Kill lots of people

According to Julia Pongratz, Genghis Khan and his Mongol hordes killed so many people that it led to 700 million tons of carbon absorbed from the atmosphere.

... But in the case of the Mongol invasions, which had the biggest impact of the four events studied, re-growth on depopulated lands stockpiled nearly 700 million tons of carbon absorbed from the atmosphere. This is equivalent to the world’s total annual demand for gasoline today. ...

https://carnegiescience.edu/news/war-plague-no-match-deforestation-driving-co2-buildup

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    $\begingroup$ @Caleb The article says Genghis Khan killed so many people that the forests grew back. If deforestation leads to climate change, surely reforestation would also change the climate. I could have answered ancient forest cult spreads worldwide but humans are pretty good at killing. $\endgroup$ – Biff MaGriff Jan 5 '18 at 20:47
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One simple way has happened many times in the past and even into modern times.

Slash and burn agriculture is one of the oldest methods, basically they chop a new bit of forest down and set fire to it. The ash fertilizes the land and they plant their gardens. As recently as the 80's or 90's one of these fires got out of control in Indonesia due to poor management and peaty soil and the smoke changed the weather locally for quite a while as well as wiping out huge tracts of forest. Scale this up and you can have permanent climate change.

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There's some real-life examples. As one example for all, there is a hypothesis (Lewis, Maslin) that a significant cooling occurred all over the world (and particularly in Europe, where it led to crop failures, epidemics, all the usual stuff) when Native Americans started to die out during/following the Columbian exchange.

Despite the still common picture of the natives as being "in harmony with nature" and such, they engaged in massive deforestation practices, just like the Europeans. And when they died out, the forests returned with a vengeance. This, according to the hypothesis, caused a substantial temporary drop in the levels of carbon dioxide. A natural forest is carbon neutral on average - they capture carbon dioxide from air, but also release it when the trees die and decay. But this is not true of an "invasion" like this - you have lots of new, healthy and young trees, and very little decay. As the forests matured and began to again be destroyed by the new colonists, levels started raising again. We can't tell how long the recovery would last naturally, since it was interrupted about two centuries later with the arrival of the industrial revolution.

In short:

  • It happened as a result of a civilisation collapse
  • The effects appeared throughout a period of just a couple of years
  • The effects lasted for centuries at least
  • It had a significant impact on agricultural output, but not nearly enough to destroy Europe, much less humans all over the world - though it should be noted that there were also significant benefits coming from the colonisation that may have reduced the severity somewhat

If true, it also shows one way you can significantly alter climate through one big change where a series of small changes over large enough timescales would have little or no effect (there was no "symmetrical" heating when the natives originally cut those trees - probably because the expansion of agriculture progressed over hundreds of years). This could perhaps be exacerbated by some weird rituals that would make such grand changes to the environment periodically - like burning down massive areas of forest, and then ten years later fostering a replanting of the forest, rinse and repeat.

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As Luaan mentioned, "natives living in harmony with nature" is a myth. Native South American tribes used to set up camp, hunt until nothing much larger than a hand was left, then move.

So, the general idea is that Australia was a large forest until the aborigines' ancestors arrived, say 40K years ago, and turned it all into grassland (which subsequently turned to desert; expect the same in the Amazon now). E.g., this article argues it has timed monsoons and similar large-scale events.

Basically, you don't need tech you just need anything large-scale. For example the monsoons: Mis-time them sufficiently, and things become unlivable --- 11months drought followed by a month of floods destroying whatever is left; with water running off an impenetrable ground so you next month your reserve runs dry.

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Goats.

Having goats enough, and time (it already happened more than once in ancient times, and is still happening today), a grassland near enough to the equilibrium can be turned into a desert. The process is self-sustaining (goats migrate to graze on still fertile land, extending the desert).

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Perhaps excessive population, leading to overconsumption of natural resources, and the eradication of natural habitats, would be the simplest (though most boring) reasoning as to how ancient humans could disturb the climate. Every human consumes a piece of nature, and the more technology there is, the more aggressive the consumption is, in terms of both natural resources and energy demand.

There are some similarities to the behavioral sink study:

https://en.wikipedia.org/wiki/Behavioral_sink

Further more, as resources are consumed, there's a direct influence in climate via CO2 emissions, and so forth, particularly when you observe deforestation:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1871823/

So, even if it isn't an immediate collapse, the climate of the whole world could possibly be severely altered just through resource consumption, begotten by a massive population bloom.

As for the reverse...

A drastic depopulation event could reduce the energy toll. It wouldn't necessarily change the climate, so much as help halt the change of it.

Triggering volcanic activity can reduce the temperature of the planet by sheltering it from sunlight. Granted, that approach could have severely damaging consequences.

http://www.newsweek.com/artificially-cool-earth-volcano-eruptions-extremely-dangerous-711203

Additionally, reforestation could conceivably effect climate by reducing CO2 in the atmosphere, managing watershed, and creating evaporative patterns that can influence rainfall.

https://www.theguardian.com/environment/2012/nov/29/planting-trees-climate-change

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Could humanity be set up by a natural phenomenon? Let's say there is a period of good weather that leads to a worldwide population increase. Then an eruption or comet causes a global but short term cold period. People all over the world fell trees to build defenses. They build ships to find new place to live or to hunt more distant sea animals. Some might burn brush to make hunting easier. Burn trees to make charcoal for forging weapons. If your earth-like planet starts out with less forests and more people the effect could be devastating for the environment.

This happened on small scale in the 1250s. Irish chronicles say 1253 was a great year for food production. An eruption in Indonesia in 1257 caused a two year famine (and left deposits in both the Arctic and Antarctic). The Mississippian culture in the midwest US began its decline. This leads into the little ice age here on Earth.

Other answers describe what humans could do to change climate. My answer is about getting them all to do it at the same time.

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Redirecting rivers may be some help by causing desertification and changes in local rainfall. Directing them into blind valleys may cause salt pans that reflect sunlight locally which may not be desired.

The best way to melt ice is to cover it with something that will absorb more sunlight. A black cloth over the edges of the north and south icecaps that is moved in as it melts would eventually raise sea levels and change salinity enough to interrupt the normal temperate currents from the traditionally habitable zones. They zones may move or almost disappear. Alternately slag and ash may be used but this will be covered with snow so something that will be shaken clean and lifted above new snow by thousands of conscript workers. As soon as the ice layer is melted and the ground surface is reached it could be covered with black pitch, slag or stones to increase the local air temperature and the black cloth repositioned over the retreating ice sheet. This could be tested first on all inland glaciers as the system is deployed.

Deforestation may also work due to albedo changes. It may also affect the greenhouse CO2 levels, however it may eventually be cancelled by some virulent ocean plankton that starts to grow in the increased CO2 reducing atmospheric CO2 levels. Alternately if all plants are killed then animals will have no oxygen to breath. A plague that eats all plant life should be enough to test the theory :-/

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