1379: 4.5 Degrees

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4.5 Degrees
The good news is that according to the latest IPCC report, if we enact aggressive emissions limits now, we could hold the warming to 2°C. That's only HALF an ice age unit, which is probably no big deal.
Title text: The good news is that according to the latest IPCC report, if we enact aggressive emissions limits now, we could hold the warming to 2°C. That's only HALF an ice age unit, which is probably no big deal.


This comic represents the impacts due to climate change by demonstrating the changes in climate that should be expected with a given change in global temperature. This is done by detailing the world's climate in geologic periods where the global average temperature has changed by one or more "Ice Age Units," or IAU. The comic defines an IAU as the difference in global temperature between today and the last ice age, about 4.5 °C. An IAU of 0 represents modern global temperature. It was later followed with a similar but much more elaborate chart in 1732: Earth Temperature Timeline.

One IAU unit happens to be the expected increase in global temperature the world will see by the end of year 2100. The prediction of 4-5 degrees Celsius of warming may not appear significant, but is easy to see as a substantial difference when comparing today to the last ice age.

An IAU of -4 is associated with Snowball Earth. Snowball earth is a near-total freezing of the entire surface around 650 million years ago, in the Cryogenian. This may have been the greatest ice age known to have occurred on Earth.
An IAU of -1 is associated with the last ice age. During this time Randall's neighborhood was buried under an ice sheet.
An IAU of +1 is the predicted global temperature by the end of year 2100. While it makes sense to assume it's just as drastic a difference as -1 IAU, we still don't know the actual nature of what it would be, which is why it is represented by a question mark in the comic.
An IAU of +2 is associated with the "Hothouse Earth" of the early Cretaceous period. At this time there were "palm trees at the poles" as there were polar forests during Cretaceous summers. (Average temperature of North Pole during the summer is 0 °C or 32 °F. 0+2*4.5 = 9 °C = 48.2 °F, hot enough for trees to grow at the North Pole under hypothetical 2 IAU scenarios)

An increase of 4.5 °C (+1 IAU) seems like a small change in temperature, but the changes it would cause are likely very large as it can also be described as halfway to palm trees at the poles.

The topic of ice coverage over various cities has previously been covered in 1225: Ice Sheets. The image of Boston from that comic is reused at the top of the huge chart in 1732: Earth Temperature Timeline.

This comic shows the extreme extent to which global warming can (and will) change our environment. Randall presented this view earlier in 164: Playing Devil's Advocate to Win. Climate change, especially global warming, is a recurring theme in xkcd. This is because many still believe the conspiracy theory that global warming is a hoax.

The title text expands, demonstrating that the potential impacts of an increase by the IPCC report's best case scenario of 2 °C, about half an ice age unit, makes controlling climate change seem more urgent. The figure of 2 °C is the most commonly agreed temperature target that assumes the creation of aggressive emissions limits at the time of the publishing of the comic.

A1F1 Scenario[edit]

The 4.5 degree increase is predicted by the bern2.5cc simulation (a moderate simulation) of the A1FI scenario. In the A1FI scenario the world has a high dependence on fossil fuels, experiences "very rapid economic growth", a declining world population by 2050, as well as a high rate of increase in energy efficiency after 2050.


Without prompt, aggressive limits on CO2 emissions, the Earth will likely warm by an average of 4°-5°C by the century's end.
[A ruler chart is drawn inside a frame.]
In the coldest part of the last ice age, Earth's average temperature was 4.5°C below the 20th century norm.
Let's call a 4.5°C difference one "Ice Age Unit."
[A ruler with five main divisions — each again with 3 smaller quarter division markers. Above it the five main divisions are marked as follows with 0 in the middle:]
-2 IAU -1 IAU 0 +1 IAU +2 IAU
[Next to the 0 marking a black arrow points toward 0.25 on the scale and above it is written:]
Where we are today
[The rest of the text is below the ruler.]
[To the far left below -2 IAU a curved arrow points to the left. Below it is written:]
Snowball earth (-4 IAU)
[Below -1 IAU a black arrow point toward this division. Below the arrow is written:]
20,000 years ago
[Below this an image of a glacier. At the top of the image is written:]
My neighborhood:
[At the bottom of the image is an arrow pointing to the glacier:]
Half a mile of ice
[Below 0 IAU a black arrow point toward this division. Below the arrow is written:]
Average during modern times
[Below this an image of Cueball standing on a green field with a city skyline in the background. At the top of the image is written:]
My neighborhood:
Cueball: Hi!
[Below +1 IAU a black arrow point toward this division. Below the arrow is written:]
Where we'll be in 86 years
[Below this a white image. At the top of the image is written:]
My neighborhood:
[Below this is a very large:]
[Below +2 IAU a black arrow point toward this division. Below the arrow is written:]
Cretaceous hothouse
+200m sea level rise
No glaciers
Palm trees at the poles


The oldest known animal fossils (sponges) are from the Snowball Earth, while flowering plants became the dominant plant species during the Cretaceous period. It is believed that the entire Earth was frozen for the first time about 2,400 to 2,100 million years ago, which could have been a result of the Great Oxygenation Event.

The 200m sea level rise given in the last panel for a "Cretaceous Hothouse" (i.e. if all ice on earth melted, including the Antarctic ice cap) could not be explained by this melt-off alone. If all the ice melted the water level would only increase by about 60-80m, according to Antarctica, IPCC Third Assessment Report (section 11.2.3 on Greenland and Antarctic Ice Sheets) and Sea Level and Climate: USGS Water-Science School. Additional sea level rise can be expected from thermal expansion of seawater, and indeed the main reason for rising sea level at the moment is actually caused by this expansion of the sea due to increasing temperature. But the high-end 500-year projection for a 4x increase in CO2, at expansion of the sea, is for an additional 2m due to thermal expansion, with a decreasing rate of growth over time. (Some of the sea level change in the Cretaceous are due to changes in bathymetry.)

The 5th and most recent Intergovernmental Panel on Climate Change (IPCC AR5) presents four alternative trajectories for future concentrations of greenhouse gasses, termed Representative Concentration Pathways (RCPs): RCP2.6, RCP4.5, RCP6, and RCP8.5. They are named after possible ranges of radiative forcing values in the year 2100 relative to pre-industrial values (+2.6, +4.5, +6.0, and +8.5 W/m2, respectively). The hottest of these, RCP8.5, is predicted to result in a warming of 2.6 °C to 4.8 °C for the 2081−2100 period, and between 3 and 5.5 by the year 2100 (Working Group I Summary for Policymakers).

The lack of internationally binding agreements makes breaching an increase of 2 °C more and more likely.

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Scary thoughts there... Kynde (talk) 05:11, 9 June 2014 (UTC)

I imagine the Earth's axial tilt wouldn't change even if the temperature changed by +2 IAU. So, would palm trees survive the extreme day/night lengths at the poles? 05:31, 9 June 2014 (UTC) P.S. Also, wouldn't the North Pole be underwater, so incapable of supporting palm trees? Also, regarding the IAU, is it a reference to the IAU that named an asteroid after Randall?

"While it says it's "probably no big deal," this is probably a joke, because even half of an Ice Age would be a lot of ice." The article has it wrong. It's a 2 degree increase, not decrease. Ice would melt. 07:33, 9 June 2014 (UTC)

-- Fixed (talk) (please sign your comments with ~~~~)

To prevent global warming, act yesterday! ... or, well, since we already failed to do it, maybe ... just maybe ... we should invest some resources to ADAPTING to the change. Because the USSR communist party wanted to command “wind and rain” and how it worked? ... of course, we SHOULD be trying to lower the CO2 emissions ... not like Germany, which replaced it's nuclear power plants with coal ones ... -- Hkmaly (talk) 10:03, 9 June 2014 (UTC)

While it is true that we have build more coal plants, the majority part that replace the nuclear power is from renewable energy, see diagram on wikipedia. -- 15:51, 9 June 2014 (UTC)
... note that burning biomass, while renewable, also adds CO2. Not speaking about oil. You shouldn't be closing nuclear plants, you should be closing coal ones if you have exceed energy. -- Hkmaly (talk) 10:02, 10 June 2014 (UTC)
While burning biomass adds CO2, the whole point of "burning a biologically-sourced fuel" like biodiesel is that you are merely returning to the atmosphere CO2 that was sucked out of the atmosphere by the biological material in the first place. So you grow an acre of plant material, and that acre of plant material sucks a certain amount of CO2 out of the atmosphere. When you then burn that plant material, you are releasing that CO2 back in to the atmosphere. Thus it is a "net zero" operation. While yes, it would be better to do a "net negative" operation (plant more plants while NOT releasing ANY CO2,) a net zero operation is still better than what we're doing now - releasing massive amounts of CO2 that have been locked up for geological-scale lengths of time, all in a VERY short timeframe. If you were to replace all work-generation power sources with "net zero" sources like biodiesel production and biomass generation, the levels of CO2 in the atmosphere would stop rising immediately. (Well, once they have reached equilibrium from other sources, anyway.) But of course, the difficulty is growing sufficient biological fuel material fast enough to create enough fuel for our needs. (The famous "it would take more farmland than currently exists on all of planet Earth, all of it dedicated to growing corn, to grow enough corn to make enough corn-derived ethanol to fuel every vehicle on the planet" problem.) So obviously energy efficiency and non-bio-fuel renewable energy methods are also needed. But biofuel (burning biomass, ethanol, biodiesel, etc,) is still a SIGNIFICANT improvement over oil/natural gas/coal. 07:31, 20 June 2014 (UTC)

Well, this seems like a topic that could generate heated comments. 10:09, 9 June 2014 (UTC)

Would anyone care to comment on the +200 meter sea rise? I googled "how much would sea level rise" a bit, and I seem to bump into 60 to 70 meters repeatedly for all glaciers melting. I found nothing direct from IPCC. I wonder if Randall really has another view on this. (talk) (please sign your comments with ~~~~)

Cretaceous sea levels are generally accepted to have been 200m above the present level - you have large shallow seas (with geological evidence showing depths of 200m) over many of the continents - e.g. the Eromunga Sea in Australia. This is not from the IPCC, it predates that considerably. 11:35, 15 June 2014 (UTC)
I hope the explanation isn't that he made a meter/feet mistake. 13:04, 9 June 2014 (UTC)
I would assert that he rounded for a clean read for a relative scale. Also, the '+' denotes the likelihood of a larger actual amount. (talk) (please sign your comments with ~~~~)
60 meters is indeed the amount the sea would rise if all the glacial ice melted. However, that figure presumably does not take into account have much the sea would rise by expansion due to the increased heat. That is, after all, the main reason for rising sea levels today. So I would guess that the +200 figure is the 60 meters of added water from glacial ice plus the amount it would rise due to warming and expanding. Calebxy (talk)
While that's possible, and desalination of water can also cause it to expand (sea water is more dense than fresh), we shouldn't try to justify the numbers if they are incorrect. If we can find some reliable data to suggest the rise would be 200 ft instead of 200m, we should include that. Or at least include a range of estimates from reliable sources. 15:42, 9 June 2014 (UTC)
Having just re-read the explanation after posting my comment, I can see that the article attempts to do just that. But the link provided says 110 to 770 mm. Isn't the millimeters? 15:44, 9 June 2014 (UTC)
But the sea level would rise more than 60m if the expansion of the sea is taken into account. If the earth became as hot as the graph indicates, then logically the seas would expand considerably. Calebxy (talk) 16:04, 9 June 2014 (UTC)
Sometimes metres and centimetres and millimetres might get confounded, I'm thinking the 200m figure from the cretaceous is a joke, because the 2 ice age units increase isn't really part of the serious discussion, right? cretaceous 200m higher sea level rise prediction chart and also the other wiki's section on future sea level . I hope I did this correctly, as don't have an account and haven't done this before. I realize it's a pretty dead thread.-- 23:48, 20 November 2017 (UTC)
Cretaceous sea levels seem to have been that high, but this tends to be attributed to the shape of the ocean basins, in particular the mid-ocean ridges, rather than to the temperature. 17:01, 9 June 2014 (UTC)

So sad that Randall is pushing the carbon tax agenda long after the AGW myth has been debunked. IGnatius T Foobar (talk) 16:00, 9 June 2014 (UTC)

Waitwhat? a) I saw no mention of tax. b) AGW==Anthropogenic Global Warming==debunked? This may not be the place for this whole discussion (despite the relevance), but it's far from debunked. And even if "there was going to be some Global Warming anyway", you can't dismiss the probability that we're adding something to this effect and making it more extreme. If not pushing it over the edge in some way. (I'm actually more optimistic than that, but I do find "it's a myth!" to be annoyingly naive, so excuse me if I try to balance that out. It's really not worth tying this discussion box up in this debate, however.) 18:36, 9 June 2014 (UTC)
I'm not as sure that it isn't worth it. GCC is fact. GW, might be. AGW, that's where we get into the mythical and unproven range, because it's *really hard* to tell the difference between correlation and causation, and because of other problems I wrote below.Seebert (talk) 19:28, 10 June 2014 (UTC)
Randall is a scientist. He follows scientific consensus. 20:03, 9 June 2014 (UTC)
Randall is a comic artist. While he's a really smart guy, he popularizes science, he doesn't do the experiments himself.Seebert (talk) 19:28, 10 June 2014 (UTC)
No snark intended here, and I am a non-scientist, so I do not speak from a position of authority. However, I thought (one of the) the point(s) of science was that you don't have to do the science yourself in order to understand and interpret the results. In fact, you can read the reports and conclusions of others in order to draw your own. In law, for example, we follow the cases that have been established in similar situations so that we can advise our clients on the best course (and by best, I mean the course that won't land you in court paying outrageous fees) of action. We don't have to experience it ourselves in order to reach the desired outcome. We can draw analogies from similar fact patterns. Right? Orazor (talk) 09:09, 9 October 2014 (UTC)
Wrong. Meta analysis, while useful, is not original scientific research. It is the first order derivative of science. Law is art, not science, and is not related to the truth at all. Analogies are not facts, analogies are designed to hide the facts, and therefore, hide the truth.Seebert (talk) 14:32, 22 April 2015 (UTC)
There is nothing scientific about following consensus. (talk) (please sign your comments with ~~~~)
Of course there is... When 99% of climatologists are reasonably certain (which means "very very sure" for non-scientists) that there is Global Warning and that the primary cause is us (humanity greenhouse gas emissions), I wouldn't say that AGW has been "debunked" and that there is nothing scientific in following this consensus (after having made sure of its existence by reading diverse peer-reviewed studies of the field) ! You may have an agenda to defend but could you at least try to make some sense, please. Note that this doesn't mean that the current political propositions are the right way to go about it and that this comic doesn't say anything about that. Jedaï (talk) 21:47, 9 June 2014 (UTC)
And this is why climatologists playing with models instead of actually examining data from the real world, aren't scientists. It's possible to get so addicted to your models, that you fail to realize that you've fallen into confirmation bias. And consensus, also known as mob-based peer pressure, is only as smart as the lowest IQ in the mob. Which is why climatologists, attempting to top each other's predictions, have a tendency to fall for worst case scenarios, such as Randall's scenario above.Seebert (talk) 02:42, 10 June 2014 (UTC)
There really ISN'T anything scientific about following consensus. Correlation is not causation. The 99% figure will be scientifically relevant if it will be produced by every scientist independently proving it, not by consensus. And even then ... 100% scientists though time is same everywhere ... then Einstein came with theory and models ... and THEN the models were verified. By Sir Arthur Eddington four years later. THAT made Einstein famous. We don't really have the same kind of proof for AGW. We have lot of data which has been tampered with or cherry-picked, even the scientists can't be sure what to believe. What we DO have proof for is that climate is changing (although some of those changes are LOWERING of temperature).
And about the political propositions ... most of them fail to reduce the greenhouse gas emissions itself, not speaking about global temperature - but their economic effect would be huge. -- Hkmaly (talk) 10:02, 10 June 2014 (UTC)
Where is he speaking about carbon tax? "Acting now" does not equal one possible instrument. There are plenty of ways for climate change mitigation.--Ojdo (talk) 07:55, 11 June 2014 (UTC)

I *think* (haven't confirmed) that the 200 m figure is the difference between the peak of the last ice age (sea level low—"-1 IAU" in the strip) and if everything melted. We've already come up 140 m, so we can't go up 200 m from here. 20:16, 9 June 2014 (UTC)

There are several troubling things with this comic (including the sea level figure), but the most basic is the opening statement: "Without prompt, aggressive limits on CO2 emissions, the Earth will likely warm by an average of 4°-5°C by the century’s end." This is probably from the latest IPCC report, but it takes the worst of several proposed scenarios, and claims it to be the likely one. RCP8.5 projects 2.6C-4.8C, and I suppose that's what getting averaged *up* to "4.5C" for the temperature line in the comic. The second most troubling thing is that mouse-over text, regarding the 2C lid if we "enact aggressive emissions limits now"—this is an entirely arbitrary (unscientific) number based on largely unspecified changes to what the world is doing now. It gives me the sense that Randall didn't look too deep... 20:43, 9 June 2014 (UTC)

According to Wikipedia, the polar forests during the Ceretaceous period were temperate, not tropical. Thus Firs in the North and Evergreens in Antartica, not Palm trees. http://en.wikipedia.org/wiki/Polar_forests_of_the_Cretaceous Seebert (talk) 21:17, 9 June 2014 (UTC)

Oh wait, did he really say "Palm trees at the poles"? The north pole is already 4,261 meters under water. The nearest land is 700 km away. 05:14, 10 June 2014 (UTC)

It's hyperbole. 05:46, 10 June 2014 (UTC)
Not completely. It's refering to a specific time, the ceretaceous period. When there where forests above 85 degrees in both north and south poles. The forests where temperate though, so palm trees are hyperbole. 12:18, 10 June 2014 (UTC)
No, it's not hyperbole at all, actually there were tropical-climate trees in polar latitudes in the northern hemisphere during parts of the Cretaceous. 11:26, 15 June 2014 (UTC)
Citation please- everything I could find was Temperate Rain Forests (kind of like still exist in Washington State and British Columbia).Seebert (talk) 12:28, 16 June 2014 (UTC)
Continental drift. The north pole stays in the same point (relative to the sun and the plane of the planets' orbits), but it doesn't stay in the center of the Arctic Ocean for long (geologically speaking.)Mathmannix (talk) 14:39, 16 August 2018 (UTC)

Independent of everything else, I'm having a tough time reconciling the fact that sea level was apparently 6m or more higher during the Roman era. E.g. the roman settlements and their harbors in places like Caister and Burgh Castle in Norfolk, England? I'm not aware that England has risen 6m. Seems to me that if see levels were to rise as much as 6m we'd just be back to where things were 1600-1700 years ago. (talk) (please sign your comments with ~~~~)

I'd like to research that, so [needs citation]Seebert (talk) 17:22, 11 June 2014 (UTC)
Things can be complicated by the likes of 'rebound' of the local area of the Earth's crust after the removal of the weight of glacial ice from various landmasses (although I'm not sure whether that was still producing such measureable effects to those particular locations in Roman times) and other effects. 11:07, 12 June 2014 (UTC)
1600-1700 years ago there were 6+ billion fewer people (a large proportion with dwellings near shorelines, or economically dependant on them somehow) on the planet! 11:38, 15 June 2014 (UTC)

According to the Scientific Forecasts from 1986, this should have had already happened by the year 2000: http://www.nytimes.com/1986/06/24/movies/earth-s-climatic-crisis-examined-by-nova.html 01:18, 28 June 2014 (UTC)

That link is basically a TV Guide listing for a rerun of a NOVA program which was filmed in 1983. The listing was written by a movie critic who presumably watched the program but may not have quoted it correctly. Anyway, that's popular media, not real science. If you want real science, look at peer-reviewed scientific journals. In the 30+ years since that program was filmed, we have gathered a LOT more data. It's not surprising that our understanding of what's going on is more complete now than it was in 1983. That's how science works. The more data you gather, the more accurate your predictions become (hence older predictions were generally less accurate). 18:53, 23 February 2015 (UTC)

Since I used to live next to Burgh Castle, can I point out that the castle is indeed now c6m higher than the current estuary level. The nearby town of Great Yarmouth is built on land that first appeared above the waves around 1100AD. In Roman times it was possible to sail from Burgh Castle to the castle at Caistor - that's why they were built, to defend the mouth of the estuary between them.If you look at map very roughly all the green was under water circa 300AD -- 19:04, 1 November 2014 (UTC)

All the angry people who like to shout "AGW has been debunked! The models aren't exact! I have a fantasy that because I'm the smartest person in the world I will be rich by 30 and therefore I hate anything related to taxes! I've been trained to growl at liberals!" What do they want to do? Even if they're right that the changes would have happened even without humanity and that the effects will be more chaotic and less straightforward and that the 25-year projection will really take 45 years and so on... Does that mean we should just gleefully accept all the changes? I realize that San Francisco; New York, Brussels, Amsterdam, and Stockholm all being underwater sounds like fun to a right-wing partisan--no more hippies, no more "liberal media", no more UN and EU, no more wildly successfully social democracies that disprove all of their economic theories, etc.--don't they care that most of the world's financial and knowledge industries, every conservative think-tank, and most of Rupert Murdoch's houses will also be gone? 22:01, 24 September 2015 (UTC)

This image is fun to cite!

Munroe, Randall. The Good News Is That According to the Latest IPCC Report, If We Enact Aggressive Emissions Limits Now, We Could Hold the Warming to 2°C. That's Only HALF an Ice Age Unit, Which Is Probably No Big Deal. Digital image. Xkcd.com. Xkcd, 9 June 2014. Web. 8 Dec. 2015. <http://xkcd.com/1379/>.

Not too sure what any teachers will think of that. ( 00:06, 9 December 2015 (UTC))

"Sea level was higher during most of the Cretaceous than at any other time in Earth history, and it was a major factor influencing the paleogeography of the period. In general, world oceans were about 100 to 200 metres (330 to 660 feet) higher in the Early Cretaceous and roughly 200 to 250 metres (660 to 820 feet) higher in the Late Cretaceous than at present. The high Cretaceous sea level is thought to have been primarily the result of water in the ocean basins being displaced by the enlargement of midoceanic ridges." https://www.britannica.com/science/Cretaceous-Period Jubal Harshaw (talk) 03:35, 14 August 2016 (UTC)