4.5 Degrees
4.5 Degrees
Without prompt, aggressive limits on CO 2 emissions, the Earth will likely warm by an average of 4°-5°C by the century's end. HOW BIG A CHANGE IS THAT?
[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 CO 2 , 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.

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.
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 CO 2 emissions, the Earth will likely warm by an average of 4°-5°C by the century's end. HOW BIG A CHANGE IS THAT?
[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 CO 2 , 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.
