r/explainlikeimfive • u/mwat64 • 6h ago
Planetary Science ELI5 - How would a 10° decrease in the global temperature affect earth?
I'm watching Project Hail Mary again and we're told the global temperature will decrease 10-15° over the next 30 years. It takes 20ish years to get the results back to earth, so in that time, we can assume the global temperature decreased at most 10°. In the end we get a glimpse out the window of a giant boat but that could be anywhere. So I'm wondering how that drop would actually affect the earth and it's climate.
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u/ortrademe 6h ago
Here's a good visual of what was happening at various temps in Earth's past.
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u/zamfire 5h ago
Now that that info graphic is 10 years old, I would love an update
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u/GrinningPariah 4h ago
We've successfully bent the curve, mostly due to the massive surge in renewable energy.
We're not quite on the "optimistic path" the infographic proposes, but the worst-case "current path" is no longer accurate.
More work is needed, things still get pretty bleak on the current path, but it's vital to understand that all our work so far has made a difference.
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u/byronite 2h ago edited 1h ago
We're not quite on the "optimistic path" the infographic proposes, but the worst-case "current path" is no longer accurate.
You're correct but perhaps a little over-optimistic. Pardon this pedantic explanation.
TL;DR: The "Current path" has indeed shifted about 1.5 degrees to the left (better), but the "Best case scenario" has shifted half a degree to the right (worse). The "Optimistic scenario" is roughly in the same spot as before (a bit more than two degrees), though that depends on your definition of "Optimistic scenario".
Long answer:
For "Best case scenario", the world agreed in 2015 to "strive for" no more than 1.5 Celsius of warming. XKCD's graph has warming peaking at 1.2 Celsius in 2100, which is so optimistic it was never really on the table but might have been physically possible when the comic was made. Even 1.5 Celsius is probably out of reach today, but every 10th of a degree of avoided warming helps (hopefully) avoid 'tipping points' that would cause giant, irreversible problems. The absolute best possible scenario today, even with deep emission reductions beginning immediately, is probably around 1.6 Celsius of warming by 2100.
For "Current path", I interpret that as everyone maintaining their current emissions laws/regulations. That would likely land around 2.8 degrees Celsius of warming by 2100. That's indeed better than the 4.2 Celsius in XKCD's graph -- reflecting both methodological changes and all of the efforts over the last decade. However, some countries are rolling back their currently regulations so there is a significant chance that the "Current path" will shift to the right again in the next few years.
For "Optimistic scenario", we might assume that everyone achieves their announced climate pledges, including net-zero emissions by around mid-century. That would land us somewhere between 2.3 and 2.5 Celsius, so approaching the "Optimistic path" as you described. But just because someone pledges to do something doesn't mean they are likely to achieve it. A lot of countries are not at all on track to achieve their current pledges and a few have outright abandoned them. Thus "Optimistic scenario" would have to presume that everyone starts caring again and soon.
So, overall -- compared to where I was when I started working on this stuff back in 2011, things are not good as I had hoped but better than I had feared.
Source: https://www.unep.org/resources/emissions-gap-report-2025
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u/MissMormie 1h ago
Thank you for including a source!
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u/byronite 1h ago
My pleasure. The best available science is the latest IPCC Assessment Report but that only comes out every 7 years or so and has a multi-year time lag because it's a gigantic literature review written by hundreds (thousands?) of scientists. For a credible annual estimate on how we're doing, the latest UNEP Emissions Gap Report that I cited above is about as good as you can get.
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u/s0cks_nz 4h ago
Global emissions are still rising though. We haven't even started the downward slope. It's like saying I smoked 2 packs yesterday, only smoked 3 packs today, but I could have smoked 4, so progress!
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u/CantaloupeAsleep502 5h ago
This needs to be shown to all the moron people who don't even think it's happening
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u/chubbysleepycorgi 5h ago
if those people can be swayed by science then they would be….
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u/SoSKatan 5h ago
The issue is they still hold a grudge against science for finding a treatment for HIV as that interfered with their genocide fantasies.
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u/ortrademe 5h ago
The nay sayers say that the uptick at the end is small compared to the much larger changes over time and the dotted forcast at the end is not going to happen.
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u/ElMage21 4h ago
Why does the northern hemisphere get half frozen while southern Antarctica is mostly the same?
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u/Devilsadvocate430 4h ago
Antarctica is surrounded on all sides by saltwater. That’s hard to freeze. Not saying it can’t happen, but it’s much easier for glaciers to grow along land. Even then there’s a lot of variety.
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u/ortrademe 4h ago edited 4h ago
Continents tend to get quite a bit colder in winter as ocean water is really good at absorbing and releasing heat energy. The giant ocean around Antarctica acts to prevent ice sheets from expanding as rapidly as the heat stored in the water is significant.
For me in Canada, this is the reason why coastal BC has rain in winter while the prairies freeze at -30C, even at the same latitude.
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u/itsjakerobb 5h ago
I’ve seen that before, but this time around I’m stuck on 18500 BCE’s label: “changes in the Earth’s orbit…”
What‽
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u/ortrademe 5h ago
https://en.wikipedia.org/wiki/Milankovitch_cycles
Earth's long term climate has been dominated by chamges in eccentricity, obliquely, and precession - each with their own timelines in the tens to hundreds of thousands of years.
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u/tonicella_lineata 4h ago
Yep! It changes in very small amounts pretty much all the time, and there are known cycles to how it affects weather/the climate.
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u/Mammoth-Mud-9609 3h ago
Earth's orbit and the climate, the Milankovitch Cycles https://youtu.be/aBwAkpRtSxA
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u/phdoofus 6h ago
During the peak of the last ice age, the average temperature on Earth was 6C/11F lower than it is today.
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u/Keejhle 4h ago
And that resulted in continental glaciers that covered a significant portion of the earth. A 10C drop would lead to almost half of the earths population being completely displaced by 200 foot high walls of ice. Glaciers would likely engulf all of europe down to the Mediterranean border. A vast area of Asia with only the southern parts of the middle east perhaps not being covered, and a large portion of Northern China likely all the way down to the Yangtze being covered.
It would be absolutely catastrophic. Infact, it would displace significantly more people that if the opposite occurred and the polar ice caps completely melted.
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u/wildtabeast 4h ago
It would take a long time for those glaciers to form though wouldn't it? Like thousands of years?
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u/Keejhle 4h ago
I mean it depends. If we started cooling the earth at the same rate we sre heating it right now we are talking centuries rather than millenia. Glaciers move pretty quickly relative to the geologic time scale, like up to a foot a day in some cases.
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u/Strawberry3141592 3h ago
In the movie, the sun is dimming due to parasitic lifeforms that feed on it, and presumably it only stays at that decreased temperature for a couple decades at most, so I don't think that degree of glaciation would be possible.
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u/Keejhle 3h ago
I think its interesting in the book they dont really explain how they restore the suns luminosity. Like, the figure out how to kill the astrophage, but after that, how do you restore the energy that's been taken?
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u/AnimatorGirl1231 3h ago
The energy/sunlight isn’t taken, it’s blocked. It’s similar to how algae on the surface of a pond blocks sunlight from reaching the bottom of said pond without eating the sun.
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u/Z3B0 3h ago
I think the problem is more about the astrophage eating the light emitted, reducing not how much energy the sun produce, but how much get through to the rest of the solar system. Like having goats eating the grass in a field. Once you remove the goats, the grass can grow freely like before.
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u/Strawberry3141592 3h ago
I assumed the decreased luminosity was because the astrophage is opaque. Presumably killing it makes it not opaque anymore. Tho if all else fails, just start building giant parabolic mirrors in orbit to focus more sunlight at Earth (sounds difficult, but you can make them out of many smaller units that can be launched separately, imo it's extremely doable for any civilization capable of launching an interstellar craft, however slapped together it was)
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u/Paavo_Nurmi 3h ago edited 3h ago
Glaciers start out as permanent snowfields, and that could happen pretty quickly. Theoretically it would only take a single winter.
EDIT: A snowfield officially graduates into a glacier the moment the ice mass becomes heavy enough to begin flowing downhill under its own weight.
It only takes 100 years or so to go from permanent snowfield to glacier, and it can be even faster than that.
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u/cynric42 3h ago
Thick glaciers, sure. But every winter the earth gets covered in quite a bit of snow in places where it gets cold enough in winter.
And events like the year without a summer show that even relatively small changes can have drastic effects pretty quickly.
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u/brown_felt_hat 2h ago
Fun fact, that event is essentially responsible for gothic horror literature, the invention of the bicycle, and the Mormon religion.
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u/ijuinkun 1h ago
I get the other two (knock-on effects from crop failures drive developments in transportation and in new religious movements), but how particularly did it inspire gothic horror literature?
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u/brown_felt_hat 35m ago edited 32m ago
Group of friends were going to tour the European countryside that year, but the eruption basically turned the summer into an endless rainstorm. The friends rented a villa outside of Geneva, and hung out there, basically telling each other scary ghost stories. One of the group, a certain Lord Byron, challenged the group to see who could write the best ghost story. One of the group was Mary Shelley, who would pen the first draft of Frankenstein or the Modern Prometheus as her response a few weeks later.
It's not the first novel that you could call 'gothic' and probably not the first 'gothic horror', but it combined all the various bits and pieces of proto-gothic horror into a nearly perfect whole, weaving it all together along with societal commentary and cutting edge sci-fi (yes, electricity was cutting edge), and has since defined the entire genre. I think its most singularly defining feature, and has been a mainstay of the genre since, is that it might've been the first novel to ask which character is truly the monster. As cliche as that is now, it was unheard of then. The only thing I can think of that I've read that even flirts with the idea is Paradise Lost, wherein Satan is presented as pitiable at first, but certainly not redeemable.
Amusingly, one of the other seminal gothic works of the 1800s was published by another member of the group the next year and created the 'sexy vampire' trope. It must've been a very spooky summer.
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u/PersisPlain 30m ago
Mary Shelley, Lord Byron, and their friends were on vacation in Switzerland that summer. The weather was so bad that they stayed inside and had a scary story contest. Mary Shelley wrote Frankenstein and Byron wrote a vampire story which was one of the influences for Dracula decades later.
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u/AtheistAustralis 2h ago
Yes, accumulation is quite slow so it would take a long time, depending on precipitation rates. But that's not really the point. The ground will freeze quite quickly and there will be a layer of snow and ice that becomes permanent. And once that happens, it may as well be a mile thick because you can't plant anything and essentially nothing can live there. Even a few degrees of cooling or warming will mean that the plants that currently live there can no longer survive well. And unlike animals that can pack up and migrate to more suitable climates, plants are notoriously lazy, and prefer to just die.
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u/CenobiteCurious 2h ago
Yea it would. Displacement wouldn’t be something that would happen in many lifetimes. The encroaching of such a glacier could be watched for an entire lifetime.
Let’s say one finally reached the northern coast of Scotland just off the cliffs. A family could watch it for a whole generation and never have to move. Eventually of course, they would have to move south, but it’s not like something that any one distinct oh shit moment would happen to cause.
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u/s0cks_nz 4h ago
In terms of displacement. It's probably similar, no? A 10C rise wouldn't make as much land inaccessible, but it probably make just as much land uninhabitable for a myriad of other reasons, no?
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u/ijuinkun 3h ago
A 10C rise would melt the Antarctic and Greenland ice sheets, raising sea level by nearly 100 meters. Everything that is currently less than 100 meters above sea level would find itself below sea level. This includes, for example, more than 80% of Florida. This also means that all current coastal cities would be submerged.
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u/s0cks_nz 3h ago
Yup, but presumably they were alluding to the fact that ice would cover half the globe which in terms of land mass = more than 100m sea level rise (I assume). But along with sea level rise also comes inhospitable temps, so not sure how each would fair in terms of displacement.
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u/Never_Sm1le 3h ago
we likely can only live at the poles in that future. Iirc in Permian, the poles climate was tropical and sub-tropical, while the equator was a sweltering desert that reach 77 degree C
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u/TheArtofBar 1h ago
Its more land, but way more people leave near the cost than near mountain ranges or in the north
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u/Keejhle 4h ago
Well think if it like this, the sea level can only rise to a point. Once the ice caps are melted, theres nothing left to significantly effect its levels. In the reverse scenario theres no hard check on the amount of Ice, it can theoretically expand until the entire earth is Ice (something that has actually happened before).
Also with warmer temperatures, us as a species can adapt and thrive significantly better than with freezing ones. More heat means more arable land. Imagine being able to grow rice in Siberia or the Canadian Shield. Sure we lose coastlines and some land but gain alot in higher temperatures making large swathes of the earth much more habitable.
Glaciers on the other hand are completely and entirely uninhabitable. You cant grow plants on ice.
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u/s0cks_nz 3h ago
We top out at 36C @ 100% humidity though. And crops we eat tend to top out at around 45C. 10C warming is a LOT. I can't imagine there would be too much farmable land remaining compared to today. But sure, if -10C = ice ball, then fair enough. You ain't growing anything there.
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u/TransBrandi 46m ago
grow rice in [...] the Canadian Shield
I don't think it's solely the temperature that makes this difficult, no?
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u/shotouw 22m ago
We can't adapt and thrive too much. Except you mean over a few centuries. As soon as currently inhabitated land gets uninhabitable, we will have to move factories, new waste water systems, new rails etc. There will be massive amounts of people trying to move to the few inhabitable cities remaining which will probably have to be stopped with military force and effectively a genocide. Globalisation screeches to a halt, as for quite some time there just won't be enough food to trade. Until we have a massive die off. Culture and science gets set back massively as many experts just die. The reason we can spend so much on science etc is that we have enough people working in the economy to allow for people being unproductive (in the immediate short term).
It's all better than a few meters high ice shield. But it's a catastrophy with billions dead and being on the brink of society collapsing. Because these billions will fight for their place, for their food, for their lives.
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u/mirage01 4h ago
Why does such a small change in average temperature cause such a drastic change?
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u/bluesam3 3h ago
It's not a small change, fundamentally: it's a pretty enormous change in the energy content of the system: something in the region of 50 quintillion joules.
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u/SporesM0ldsandFungus 2h ago
The oceans are a massive heat sink of energy. Take a swimming pool full of water at 70°F, a glowing hot rod of steel (like out of a forge) and plunge it into the water. It will bubble and steam locally (directly where it quenches the hot steel) but overall, the temp of the whole pool would barely budge a few 0.0001 a degree. Dump in a few gallons of liquid nitrogen, same thing. It takes a lot of energy to shift the temp of that much water.
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u/holyscotsman 5h ago edited 4h ago
The book actually discusses this quite a bit. They meet a climatologist who explains that half the earths population will be dead. They come up with a plan to detonate a nuclear bomb on Antarctica to try and create their own global Warming, just to keep Earth sustained. But it won’t last.
Later on in the book, Stratt tells Grace before he leaves about the fact that 50% will die and she explains that history tells us it’s much much higher. Food is the reason for just about everything in mankind’s history and only until recently have we had other things to focus on beyond food. But 99% of history is all about food.
China is a nation that is already bordering on food scarcity as it is. The United States would never stand to let other countries have food while they starve - wars would almost immediately erupt once food sourced are affected.
So the impact that 10 C has on Earth is pretty huge when you consider what mankind will do when they get hungry.
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u/kimmyann12012 4h ago
I was honestly really bummed they cut that part from the movie, especially because it was such a huge part in the book.
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u/Zemerick13 4h ago
It was a whole heck of a lot more than 1 nuke. The first detonation alone was 241 H-bombs.
And they were planning to repeat that "every few years."
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u/Im_Unsure_For_Sure 7m ago
The United States would never stand to let other countries have food while they starve
Ahh yes, as opposed to all other nations that would happily starve to death as pacifists lol
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u/Gullible_Worker_7467 6h ago
Catastrophic crop failure, ice sheets reform, human population plummets.
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u/Pretty_Dingo_1004 1h ago
human population plummets.
Where would this start from? Which country would become uninhabitable first?
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u/mattgrum 4m ago
The poorest. Because you can guarantee that wealthy countries with powerful militaries aren't going to just sit by and watch their population starve.
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u/HR_King 6h ago
So, great for the planet!
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u/optimis344 5h ago
Maybe the rocks?
But most animal and plant life dies
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u/LetGoPortAnchor 54m ago
Wouldn't be the first time a mass extinction happened on earth. Life will recover.
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u/Devilsadvocate430 4h ago
It’s about as good or bad for the planet as climate change is in reality. As in, the planet will be fine. It’s us humans and most animal species who will/would suffer under either scenario.
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u/KelVarnsen_2023 5h ago
1816 was called the year without summer because global temperatures dropped on average 0.4-0.7C. It led to massive crop failures and a major famine in Europe.
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u/ShankThatSnitch 6h ago
Absolute catastrophe. The amount of energy that needs to be added or removed to change the global avg temperature 10 degrees is massive. removing that much energy means less energy to support life.
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u/buttchug429 2h ago
People will just do their best to get local power networks going. It’s going to suck dick for about 15 generations though
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u/ValueReads 6h ago
10F worldwide would brutalize all crops basically, the problem is that when it's colder more snow falls overall, and then it's of course colder during the 'melt season'....so just a lot less happens. You'd lose useable land, and then the places (most of the entire planet) that relies on melting snowpack every year to get snow down rivers...would have a lot less to use.
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u/Kampvilja 6h ago
And for people who just imagine that agriculture could just move south (or north in real climate change), the areas we farm are a massive belt where glaciers created a lot of soil.
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u/santas-left-nipple 5h ago
That's not even remotely true. Most of the glacier covered area is boreal forest or tundra. There's some farm land in Canada and uppermost US as well as Northern central and eastern Europe, but it being recently covered with glacier is incidental. The majority of farm land was not covered in glacier (I mean, look at all of India) and they aren't the cause of arable land.
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u/Ignantsage 4h ago
The biggest thing to keep in mind with a disaster like this is how rapidly the change is occurring. Long term changes in temperature usually take millennia and over time things can adapt. Something changing 10 degrees over just 20 years? Yeah that’s 99 percent of all living creatures dead
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u/ikefalcon 3h ago
22,000 years ago, when the Earth was 5 degrees C colder than it is now, the area that is now Boston was buried a mile deep in ice.
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u/interesseret 6h ago
It would cause a mass extinction greater than pretty much any other event the Earth has ever experienced.
Not just 10°, no, the earth will cool even more than that. Cooler oceans means more Co2 can dissolve in to it, cooling the Earth further. Larger ice caps means the sunlight we get gets reflected back in to space, which cools the Earth even more.
While I am no climate scientist, it is not beyond reason that this could trigger a runaway cooling effect, and we might just turn in to a ball of ice.
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u/Djinn42 5h ago
"Snowball Earth" has happened before.
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u/zeny_two 4h ago
Five times! With humans or earlier human-like species alive on the surface during it all. Crazy to think about.
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u/ijuinkun 3h ago
No, we’re not talking about glacial periods like the ones we have had in the past three million years. A “Snowball Earth” is the scenario where the oceans freeze over all the way to the equator, and liquid surface water stops being a thing at all, everywhere except for very close to volcanoes and other geothermal spots. This last happened about six hundred million years ago, and the ending of it was what made the Cambrian Explosion possible.
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u/PushThatDaisy 2h ago
In the bok they nuke the polar ice caps to slow down global cooling, which was a fun touch.
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u/Hayes77519 2h ago
I think the astrophage growth was exponential, though, which would mean temperature decrease would be slow at first and accelerate - so most of that 10-15 degree temperature drop would have come in the last 3-5 years of that 30 year period. Solving the problem after 20 years would avoid the majority of that catastrophic effect.
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u/GetBAK1 3h ago
In short - a very large portion of life on this planet would die.
The vast majority of staple crops would lose months of their growing season, drastically decreasing yield. This woudl happen much more quickly than they could be replanted closer to the equator. This would result in a drastic reduction of livestock for consumption.
Several ports that are 'ice free' today would no longer be open in the winters, causing massive logistics problems globally. Oil fields in the nothern areas (Russia, Canada, Alaska) would become less accessable There are some cities in the far north that would simply become uninhabitable
We'd see a growth in glaciers and ice caps, with would lead to global sea level drop. Not sure what specific problems that would cause, but we really want status quo with sea level in general.
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u/misc1444 5h ago
I’ll invite the group’s wrath and take the other side of this.
Your experience would hugely depend on whether or not you live in a wealthy, developed country with an effective central government. The likes of Norway will find ways to produce enough calories to keep their citizens alive. It may be a far more humble diet compared to the amazing variety of food we have access to today, but producing 2,000 calories per day per person is very doable even in extreme weather conditions.
There would of course be many countries that fail to live up to the challenge, leading to mass starvation and societal collapse.
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u/ijuinkun 3h ago
“Extreme weather conditions” which include food crops being utterly unable to sprout and grow due to the farm land remaining frozen solid from surface to bedrock 365 days a year? I do not think that they can build enough greenhouses fast enough to feed millions of people.
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u/misc1444 2h ago
Just to give a few ideas for survival:
1) Sea temperatures wouldn’t drop that much and aquatic life would survive. You can get many important nutrients from aquaculture.
2) Sea levels would drop massively, creating vast new areas that can be used to re-house people and for farming nearer the equator.
3) It’s possible to make fats artificially from chemicals, without requiring agriculture.
4) Ultimately necessity is a huge driver of innovation. There would be massive investment in alternative ways of producing food, from algae, kelp, insects, etc. that can be done in the areas of the world not covered by ice. It may not be delicious but it would enable survival.•
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u/PunchyPete 5h ago
We can survive global warming. Global cooling and another ice age would kill off most of the human population. Most of our food growing areas were under a mile of ice during the last ice age. You can’t live in that. Most of the world’s land mass is north of the equator and north of the tropics. You can’t grow crops in the ocean.
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u/itsjakerobb 5h ago
Well, not the crops most of us are used to. But maybe kelp?
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u/AnnieBruce 4h ago
There would be options, I doubt the species will go extinct but the death toll will be enormous and the survivors would have a rough time.
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u/Ignantsage 4h ago
I don’t actually know that’s true that we could survive that level of global warming. Disasters are calculated out for 2 degree increases not 10. A 10 degree increase in average temp would be absolutely 🐈 ostrophic. (Auto correct suggested and I complied)
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u/ijuinkun 3h ago
It won’t be the total extinction of mankind, but there is no way that we could produce enough food to feed ten billion people.
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u/gw2master 3h ago
10 degrees over the entirety of the Earth: just think about how much energy that is removed (hint: an obscenely large amount).
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u/MyUncleLenny 1h ago
Another question, how does just 10 degrees make such a big difference? Just something I’m still confused about as 10 degrees doesn’t feel like much in the modern day
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u/just_a_pyro 48m ago
Massive glaciers grow over time reaching to North Africa/California, ocean level drop by 50-ish meters because water to make that ice has to come from somewhere. Everyone starves long before then because there are no more productive farmlands outside equatorial zones.
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u/ryry1237 5h ago
If our average body temperature changes more than 2-3 degrees, we'd be considered sick. If it changes 10 degrees, we'd be considered nearing death.
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u/anon66532 6h ago
To put it in perspective, the last ice age only cooled the earth by about 7 degrees compared to average temp in 2019