Why do CO2 Concentrations Lag Behind Temperature?

February 24, 2010 by jason

Let me start from the beginning. Well, not all the way at the beginning, because this is a topic that has a history extending way further back than I care to remember, but I'll start with Al Gore's famous film "The Inconvenient Truth".

The Debate about Al Gore's film An Inconvenient Truth

If you're reading this, chances are you've seen the film, and you might remember this famous image, cited during the film:

CO2 Concentrations against temperature from Al Gore's film An Inconvenient Truth

What the film does not tell you, something which is the subject of a huge amount of global debate, is that the onsets of the temperature increases, seen to occur approximately every 100,000 years, occur slightly before the onsets in CO2 concentration increases. This can be seen more accurately here:

CO2 concentrations lag temperature for the past 400,000 years (source: Petit et al)

This is the cause of some debate and confusion among the public, especially for those who are new to the science of climate change. You will see many articles on the web from people claiming that global warming is a farce, based on the fact that CO2 concentrations initially lag temperature spikes in the past. I wanted to write this article to tell you that it's just not that simple.

Over the past 400,000 years, our planet has gone through several long and drawn out ice ages, indicated by the cooler periods on the above graph, lasting in the order of 100,000 years. These long periods are regularly broken up by temperature spikes, called Interglacials. Throughout these ups and downs, the existence of a relationship between Carbon Dioxide (CO2) concentrations (blue) and temperature levels (red) is not really debatable. When I first saw this graph (thank you Al), I immediately saw this relationship and assumed for a fact that CO2 always has and always will be the driving force in temperature changes. Evidently that was the aim of the film. Then I saw the graph in more detail, noticed the lags, and immediately assumed I had been wrong. Although it seems logical to conclude that because CO2 concentrations increased only after jumps in temperature they cannot have been responsible for the temperature increases, this is not entirely the case. It turns out that the story is far more complex than that, and although Al Gore was not lying when he said that the CO2 and temperature are related, his presentation of what occurred was intentionally misleading.

The Milankovitch Cycle

Interglacials occur around every 100,000 years, and are brought on by shifts in the earth's orbit. The effect of changes in the Earth's orbit on our climate is called the Milankovitch Cycle. The exact reasons for these changes in regards to these 100,000 year cycles are:

• Eccentricity:The orbital pattern of the Earth around the sun alternates between a circular and elliptical pattern.
• Obliquity:The earth is titled towards the sun at angles alternating between 22.5° and 24.5°.
• Precession:The axis about which the earth spins (giving rise to day and night) alternates between pointing towards the North Star and the star Vega.

The Milankovitch cycle is represented schematically below:

Schematic representation of the Milankovich cycle (source: Wikimedia Commons: Precession and Seasons)

Although there is a little more to the Milankovitch cycle than this, these are the main aspects of the cycle which are considered relevant to major changes in our climate. The accuracy of the predictions has been well documented and accepted as true since the publication "Variations in the Earth's Orbit: Pacemaker of the Ice Ages" by J. D. Hays et. al (1976). The combination of these 3 elements of the Milankovitch cycle give rise to major changes in the amount of sunlight hitting the surface of the earth at higher latitudes.

Why CO2 is a thousand years behind...

The earth's climate is incredibly complex, and any shifts in climate give rise to countless intricate feedback systems (ie. they cause further changes). The one that interests us most in this article, is of course the relationship between changes in temperature and CO2. As oceans undergo warming, the ability of water to hold CO2 decreases, meaning that a global increase in temperature can cause the CO2 to be emitted from the oceans, as was found by Martin et. al (2005).

This claim, also by Martin et. al (2005), illustrates the reasoning behind the theory that a feedback is at work to create the large, otherwise unexplained, rises in temperature we are seeing during the interglacials. For more info on this topic see my article on radiative forcing of CO2.

The exact mechanisms are not fully understood and are somewhat debated among the scientific community, but it has been shown that it is expected to take about 1000 years Mudelsee, 2001. This explains why we see a lag in the onset of CO2 concentration increases (of about 1000 years) following an increase in temperature.

Does this show that CO2 hasn't caused warming in the past?

Basically, no. What it does show, is that CO2 concentrations did not cause the onset of the temperature increases. The Milankovitch cycle itself is rather weak, and it is generally accepted that it could not have been orbital patterns alone that caused the the extensive warming seen during the interglacial period (need reference). The explanation for this, one which is generally (not universally) accepted by the scientific community, is that CO2 emissions from the oceans (cause by an increase in temperature initiated by the Milankovitch Cycle) caused the heating required to push the heating for 5000 years, and to such an extensive degree.

For a more in depth look into the relationship between CO2 and temperature, please read my article on the EPICA Ice Core Records and the relationship between CO2 and global temperature