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==Explanation==
 
==Explanation==
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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]].
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{{Incomplete|Do a more careful comparison of Randall's presented predictions with the most recent IPCC predictions}}
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This comic is a way to visualize changes in climate over the next century. The prediction presented, 4-5 degrees Celsius of warming, doesn't seem like a very large change, but [[Randall]] points out that 4.5 °C is the difference between the {{w|Last glacial period|last ice age}} and today, which is quite a substantial difference. So, to give context to the number, he measures the temperature in "Ice Age Units," or IAU. 1 IAU is defined as the change in average global temperature by 4.5 degrees Celsius (8 °F). The last ice age was 1 IAU colder than the average "modern" {{w|global temperature}}, and Randall's neighborhood was buried under an ice sheet. The predicted change by the year 2100 is +1 IAU, and while we don't know what its effects will be exactly (represented by a large question mark in the comic) it will probably be huge. For reference he notes that a change of +2 IAU created the {{w|Cretaceous Thermal Maximum|"Hothouse Earth"}} of the early {{w|Cretaceous period}}. In short, while 4.5 °C seems like a small change in temperature, it seems quite a lot bigger if you phrase it as "halfway to having {{w|Arecaceae|palm trees}} at the poles." There were {{w|Polar forests of the Cretaceous|polar forests}} during the Cretaceous that grew in latitudes up to 85° in both Northern and Southern hemispheres. Of course there could not be trees in the ocean directly over the North Pole, but at the closest land masses ({{w|Greenland}} for instance) there could be trees.
  
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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.  
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On the colder side, -4 IAU is associated with {{w|Snowball Earth}}, a near-total freezing of the entire surface. How much of the planet was actually frozen in the {{w|Cryogenian}} period is disputed but it could have been the greatest ice age known to have occurred on Earth. The mean temperature must have lowered to a level of 15 to 20 °C below actual values roughly a billion years in the past.
  
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: An IAU of -4 is associated with {{w|Snowball Earth}}. Snowball earth is a near-total freezing of the entire surface around 650 million years ago, in the {{w|Cryogenian}}. This may have been the greatest ice age known to have occurred on Earth.
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The oldest known animal fossils ({{w|Sponges#Fossil_record|sponges}}) are from the Snowball Earth, while {{w|Flowering_plant#Evolution|flowering plants}} became the dominant plant species during the Cretaceous period.
  
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: An IAU of -1 is associated with the last ice age. During this time [[Randall|Randall's]] neighborhood was buried under an ice sheet.
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The 200 m {{w|Current sea level rise|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 60-80 m, according to {{w|Antarctica}}, [http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/ IPCC Third Assessment Report] (section 11.2.3 on Greenland and Antarctic Ice Sheets) and [http://water.usgs.gov/edu/sealevel.html 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 CO<sub>2</sub>, at {{w|Current_sea_level_rise#IPCC_Third_Assessment|expansion of the sea}}, is for an additional 2 m due to thermal expansion, with a decreasing rate of growth over time.
  
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: 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.
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The 5th and most recent {{w|Intergovernmental Panel on Climate Change}} (IPCC AR5) presents four alternative trajectories for future concentrations of greenhouse gasses, termed {{w|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 by 2100 ([http://www.climate2013.org/images/report/WG1AR5_SPM_FINAL.pdf Working Group I Summary for Policymakers]). This seems at odds with the 4-5 °C warming presented by Randall.
  
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: An IAU of +2 is associated with the {{w|Cretaceous Thermal Maximum|"Hothouse Earth"}} of the early {{w|Cretaceous period}}. At this time there were "{{w|Arecaceae|palm trees}} at the poles" as there were {{w|Polar forests of the Cretaceous|polar forests}} during Cretaceous summers. (Average temperature of North Pole during the summer is 0&nbsp;°C or 32&nbsp;°F. 0+2*4.5 = 9&nbsp;°C = 48.2&nbsp;°F, hot enough for trees to grow at the North Pole under hypothetical 2 IAU scenarios)
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The title text says that even with instant and aggressive emissions reduction, the temperature will still rise by roughly half an IAU (2 °C). While it says it's ''probably no big deal'', this is a joke, because even the equivalent of half an Ice Age Unit of warming would cause a huge climate change. But it is unclear where his figure of 2 °C comes from. The coolest of the most recent IPCC trajectories (RCP2.6) forecasts warming of only 0.3 °C to 1.7 °C in the year 2100.
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An increase of 4.5&nbsp;°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.
 
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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]].
 
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This comic shows the extreme extent to which {{w|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 [[:Category:Climate change|recurring theme]] in xkcd. This is because many still believe the {{rw|conspiracy_theory|conspiracy theory}} that {{rw|global_warming|global warming}} is a hoax.
 
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The title text expands, demonstrating that  the potential impacts of an increase by the IPCC report's best case scenario of 2&nbsp;°C, about half an ice age unit, makes controlling climate change seem more urgent. The figure of 2&nbsp;°C is the most commonly agreed {{w|Climate change mitigation#Temperature targets|temperature target}} that assumes the creation of aggressive emissions limits at the time of the publishing of the comic.
 
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===A1F1 Scenario===
 
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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.
 
  
 
==Transcript==
 
==Transcript==
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:Without prompt, aggressive limits on CO<sub>2</sub> emissions, the Earth will likely warm by an average of 4°-5°C by the century's end.
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:Without prompt, aggressive limits on CO<sub>2</sub> emissions, the Earth will likely warm by an average of 4°-5°C by the century’s end.
 
:'''HOW BIG A CHANGE IS THAT?'''
 
:'''HOW BIG A CHANGE IS THAT?'''
  
 
:[A ruler chart is drawn inside a frame.]
 
:[A ruler chart is drawn inside a frame.]
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:In the coldest part of the last ice age, Earth's average temperature was 4.5°C below the 20th century norm.
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:In the coldest part of the last ice age, Earth’s average temperature was 4.5°C below the 20<sup>th</sup> century norm.
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:Let's call a 4.5°C difference one '''"Ice Age Unit."'''
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: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:]
 
:[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
 
:-2 IAU  -1 IAU  0 +1 IAU  +2 IAU
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:[Next to the 0 marking a black arrow points toward 0.25 on the scale and above it is written:]
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:[Next to the 0 marking a black arrow points toward 0.2 on the scale and above it is written:]
 
:Where we are today
 
:Where we are today
  
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:Cueball: Hi!
 
:Cueball: Hi!
 
:[Below +1 IAU a black arrow point toward this division. Below the arrow is written:]
 
:[Below +1 IAU a black arrow point toward this division. Below the arrow is written:]
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:Where we'll be in 86 years
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:Where we’ll be in 86 years
 
:[Below this a white image. At the top of the image is written:]
 
:[Below this a white image. At the top of the image is written:]
 
:My neighborhood:
 
:My neighborhood:
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:No glaciers
 
:No glaciers
 
:Palm trees at the poles
 
:Palm trees at the poles
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==Trivia==
 
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The oldest known animal fossils ({{w|Sponges#Fossil record|sponges}}) are from the Snowball Earth, while {{w|Flowering plant#Evolution|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 {{w|Great Oxygenation Event}}.
 
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The 200m {{w|Current sea level rise|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 {{w|Antarctica}}, [http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/ IPCC Third Assessment Report] (section 11.2.3 on Greenland and Antarctic Ice Sheets) and [http://water.usgs.gov/edu/sealevel.html 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 CO<sub>2</sub>, at {{w|Current sea level rise#IPCC Third Assessment|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.)
 
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The 5th and most recent {{w|Intergovernmental Panel on Climate Change}} (IPCC AR5) presents four alternative trajectories for future concentrations of greenhouse gasses, termed {{w|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&nbsp;W/m2, respectively). The hottest of these, RCP8.5, is predicted to result in a warming of 2.6&nbsp;°C to 4.8&nbsp;°C for the  2081−2100 period, and between 3 and 5.5 by the year 2100 ([http://www.climate2013.org/images/report/WG1AR5_SPM_FINAL.pdf Working Group I Summary for Policymakers]).
 
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The lack of internationally binding agreements makes breaching an increase of 2&nbsp;°C more and more likely.
 
  
 
{{comic discussion}}
 
{{comic discussion}}
  
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[[Category:Comics featuring Cueball]]
 
[[Category:Comics with color]]
 
[[Category:Comics with color]]
 
[[Category:Charts]]
 
[[Category:Charts]]
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[[Category:Climate change]]
 
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[[Category:Comics featuring Cueball]]
 

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