Temperature & CO₂ move together

The temperature of the Earth’s atmosphere and the carbon dioxide level in the atmosphere have moved together for at least 800,000 years.

Graph: NASA Earth Observatory

• The blue line shows the “carbon dioxide level in the atmosphere” over the 800,000 years before 1950.  (It does not show the rapid rise in CO₂ levels, since 1950, to over 400 parts per million (ppm)).
• The red line shows the “Antarctic air temperature” over this same period.

The two graphs show that the temperature has been high when CO₂ has been high – over the last 800,000 years.  Also, when the level of CO₂ has been low, the temperature has been low. The temperature and carbon dioxide levels tend to move together.

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Ice ages and low CO₂ go together

The most recent ice age ended about 20,000 years ago. On the above graph, this ending is the lowest temperature on the far right of the graph. (The time axis explicitly shows 200,000 and 0 years before the present and the markings on the axis show 200,000, 160,000, 120,000, 80,000, 40,000, and 0 years before the present.)

So, 20,000 years ago, the average temperature was about minus 5 C and the CO₂ level about 180 ppm. This period is an example of a low temperature occurring at a time of low carbon dioxide. During this ice age, half of North America was covered by an ice sheet up to three kilometres deep.

You can see that there are about ten temperature peaks over these 800,000 years. These are inter-glacial periods which are short periods of warmth.  For most of this period, the Earth has been in ice ages.

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How scientists worked this out

Scientists went to Antarctica and drilled deep into the ice. By examining bubbles of air trapped in this ice, they have seen what has happened in Antarctica over the last 800,000 years.  Scientists measure the CO₂ levels in these trapped air bubbles and determine how CO₂ moved over this period.  From these air bubbles, they can also identify the atmospheric temperatures.

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A little CO₂ has a big impact

Some people may think that CO₂ levels, like 300 parts per million, are so small that they cannot be influential.  However, the graph shows otherwise. It shows that at 180 ppm, you have an ice age. Changes in CO₂ level pack a big punch, just like a small pinch of salt can have a big influence on the taste of a meal.

John Tyndall identified the basic science behind this long ago, in 1863. When you increase the concentration of carbon dioxide in the atmosphere, this decreases the heat escaping from our planet into outer space, which tends to raise temperatures on our planet.

“Global temperature” and the “level of carbon dioxide in the atmosphere” tend to move together.  The vast amounts of carbon dioxide released by burning fossil fuels during our industrial age have increased CO₂ levels by a large amount, and this threatens to increase global temperatures by a large amount.  The planet has started to heat, and we need to limit this threatened large change.

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The timing of temperature & CO₂ increases

Antarctic ice core data shows the temperature increases occurring between 200 and 1000 years before the increases in CO₂. Some people incorrectly argue that this means that the current high levels of CO₂ are not causing the current global heating. Here’s an explanation of how the temperature increase can occur first.

Periodic changes in the Earth’s motion around the sun have caused periodic oscillations between ice ages and interglacial periods. As you can see in the above graph, over the last 800,000 years, the Earth has had a warmer interglacial period about every 100,000 years – caused by the Earth’s orbit cycling between a more and less elliptical orbit. This cycle also occurs about every 100,000 years. (Milankovitch orbital cycles and their role in the Earth’s climate, NASA)

When a change in the Earth’s movement around the sun leads to a warming, various amplifying feedbacks detailed on this website can amplify this warming:

• the permafrost feedback
• the ice cover feedback
• The water vapour feedback
• The phytoplankton feedback
• The forest fire feedback
• The ice-darkening feedback
• The glacier altitude feedback

And here’s yet another feedback that can amplify warming – and show how temperature increases can occur before or after carbon dioxide increases.

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The Ocean CO₂ feedback

(1) Higher temperatures		(2) Higher ocean temperatures
(4) More CO2 in the atmosphere		(3) Ocean can hold less CO2

In this feedback loop:

• Increased temperatures tend to
• increase ocean temperatures, which
• reduces the ocean’s ability to hold CO₂, which tends to
• increase carbon dioxide in the atmosphere, which tends to
• increase temperatures, closing the feedback loop.

This amplifying feedback loop can become dominant in two ways:

1. when temperature increases, see node 1 in the diagram. Frequently in the past, the Earth’s temperatures have increased due to changes in the Earth’s movement around the sun, and this has eventually led to CO₂ increases, and
2. when CO₂ levels increase, see node 4 in the diagram. CO₂ increases can then lead to temperature increases.

Note, this feedback loop is very slow. Hot water rises above cold water, so heating water from above is slow. It takes a long time for atmospheric heating to cause ocean heating.

Also note, this feedback loop is reversible, so temperature decreases can occur either before or after CO₂ decreases:

• global cooling tends to
• decrease ocean temperatures, which
• increases the ocean’s ability to hold CO₂, which tends to
• decrease carbon dioxide in the atmosphere, which tends to
• produce global cooling.

Reference: CO2 lags temperature (Skeptical Science)

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Article

The three-minute story of 800,000 years of climate change with a sting in the tail: The Conversation 13 June 2017

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Modified 14 December 2022