CO2 lags temperature - what does it mean?

Background of the myth:

Earth’s climate has varied widely over its multi-billion year history - from ice ages characterized by large ice sheets covering many land areas, to warm periods with no ice at the poles. Several factors have affected past climate change, including solar variability, the tilt and wobble of the Earth's orbit relative to the sun, volcanic activity, and changes in the composition of the atmosphere. Using data from Antarctic ice cores, we can explore what climate cycles have looked like over the past 800,000 years (Figure 1). Over this time period, CO₂ and temperature are closely correlated, which means they rise and fall together. However, based on some Antarctic ice core data, changes in CO₂ appear to follow changes in temperatures by about 600 to 1000 years. That is to say that changes in CO₂ lag, or come after, changes in temperature. This has led some to incorrectly conclude that CO₂ cannot be responsible for the current rise in Earth’s temperature.

Figure 1. The EPICA Dome C ice core from East Antarctica provides the current longest ice core record, going back 800,000 years. From the core, scientists can measure past levels of CO₂ in the atmosphere and estimate temperature changes over the period of the record. The modern level of CO₂ in the atmosphere is plotted with the * to provide a reference.

This myth misses the mark for a number of reasons:

(see Intermediate Rebuttal for a full breakdown)

1. It presents a false dichotomy by claiming that if atmospheric CO₂ increased before temperature, then an increase in temperature cannot cause an increase in CO₂. In fact, both of these causal relationships are true.
2. It doesn’t distinguish between Antarctic and global temperature. Antarctic ice cores give a measure of Antarctic temperature, which increased before global CO₂ levels. However, global temperature, as estimated from records all over the planet, increased after rises in CO₂.
3. It conflates past climate changes with present climate change. Though it’s true that we can learn a lot about Earth’s climate from studying the past, we cannot assume that changes that occurred in the past are the same as changes occurring today. In fact, ice core data shows us that present climate change is fundamentally different from past climate changes. Past changes were driven by small changes in Earth’s orbit, while current climate change is driven by human emissions of CO₂.
4. It is based on old data. While the old data aren’t wrong, newer ice cores with higher-resolution data show that the changes in CO₂ and Antarctic temperature occurred so close together, that we cannot fully distinguish which happened first.

Each of these four issues with the myth relates to past and present changes in temperature and atmospheric CO₂. To fully understand these changes, we need to breakdown the processes within Earth’s climate system as a whole.

Climate System Components

Within the Earth's climate system, there are many complex relationships between the processes that occur in different components of the system. Some of the main components of the system are the atmosphere, ocean, and ice sheets. Each component and the timescale over which it changes play important roles in impacting the state of Earth’s climate at any given time. 

Earth’s atmosphere covers the entire planet and can quickly circulate between the northern and southern hemispheres (over 10s of years). Gases like CO₂ persist in the atmosphere for long enough that their concentration is uniform across the planet. Temperature and pressure of the atmosphere impact the circulation patterns of the atmosphere across the planet.

Over much of the Earth, the atmosphere is in direct contact with the surface of the ocean. The temperature and CO₂ level of the atmosphere is directly related to those of the ocean surface. However, the ocean is very deep (12,000 feet on average), and the depths of the ocean are isolated from the surface. This large body of water circulates much more slowly than the thin, fast-moving atmosphere and thus is slow to move water between the hemispheres (over 100s of years).

Ice sheets build up over thousands of years on land areas that are cold enough that snow doesn’t melt in the summer. The presence of an ice sheet can affect the ocean circulation by providing a source of cold, fresh water melting off the ice and into the adjacent ocean. This input water has a different density than warmer, saltier water in the ocean and can thus affect how water sinks and mixes, in some cases driving ocean circulation on a global scale. A tall ice sheet can also affect atmospheric circulation simply because it pushes air higher in the atmosphere and can effectively sit in the way of moving air masses. In turn, the atmosphere and ocean can affect ice sheets by causing them to grow (with cold temperatures and lots of snow fall) or to retreat (with warm temperatures). Of these three components, ice sheets change over the slowest timescales, taking 1000s of years in some instances to respond to changes in climate conditions.

External Forcing on Earth's Climate

Earth’s climate cycles in and out of ice ages about every 100,000 years, which we can clearly see in the ice core record (Figure 1). This timing is driven by small changes in the orbit around the sun, known as Milankovitch cycles (Hays 1976). We call this an external forcing because it depends on the timing and distribution of energy received from the sun, a component outside of the Earth system. 

There are three main changes to the earth's orbit. The shape of the Earth's orbit around the sun (eccentricity) varies between an ellipse and a more circular shape. The earth's axis is tilted relative to the sun at around 23°. This tilt oscillates between 22.5° and 24.5° (obliquity). As the earth spins around its axis, the axis wobbles from pointing towards the North Star to pointing at the star Vega (precession).

Figure 2: The three main orbital variations. Eccentricity: changes in the shape of the Earth’s orbit. Obliquity: changes in the tilt of the Earth’s rotational axis. Precession: wobbles in the Earth’s rotational axis.

The combined effect of these orbital cycles causes long term changes in the amount of sunlight, or insolation, hitting the earth at different seasons, particularly at high latitudes. However, on their own, these small changes in Earth’s orbit would not cause very large fluctuations in Earth’s climate. Instead, these small changes are amplified into full-blown ice ages due to reinforcing processes that occur between the components of the atmosphere, ocean, and ice sheets (Cuffey 2016).

Internal Feedbacks on Earth's Climate

The global ice age cycles follow changes in the northern hemisphere insolation because the components of the Earth system amplify the impact in the north over the entire planet. The well-mixed greenhouse gases in the atmosphere play an important role in synchronizing temperature changes in the north and the south due to the direct impact of atmospheric CO₂ on temperature. Additionally, interactions between the components of Earth’s climate system cause further changes in the global temperature and atmospheric CO₂ level (Brook and Buizert 2018). We call these interactions internal feedbacks because they describe changes that depend entirely on processes within the Earth’s climate system.

Among the three components of the climate system described above, there are many internal feedbacks that affect both global temperature and atmospheric CO₂ level. Because there is more land surface in the northern hemisphere, large ice sheets form over much of North America and Eurasia during ice ages. These large piles of ice in the north begin to melt when northern insolation increases. This cold and fresh melt water disrupts the ocean circulation, slowing the mixing of the global ocean and the transport of heat between northern and southern hemispheres in the water, which is important for determining global temperature (Stocker and Johnsen 2003). These changes in ocean circulation also impact how biological processes in the ocean affect atmospheric CO₂ level, for example changes in ocean circulation affect how efficiently plankton move CO₂ from the atmosphere into the ocean via photosynthesis (Sigman 2010). At the same time as these ocean changes affect the transport of heat between hemispheres over 100s of years, changes to atmospheric circulation caused by these feedbacks impact this transport much more quickly, over 10s of years (Markle 2017). These examples give just a taste of the many internal feedbacks that occur between the components of the climate system and affect global temperature and atmospheric CO₂ level on both slow and fast timescales.

Putting It All Together

Given the many internal feedbacks that occur between the components of the climate system in response to external forcings on Earth’s climate, it’s no surprise that the transitions into and out of ice ages are complex. Data from ice cores show us the total effect of these simultaneous processes, all of which we must consider in order to understand what caused the Earth to warm out of the last ice age. The ice core data that show near-simultaneous increase of Antarctic temperature and atmospheric CO₂ level provide important information for scientists studying the interplay of these processes. These data reflect the fact that both increased global temperature leads to increased atmospheric CO₂ level AND increased atmospheric CO₂ level leads to increased global temperature. Scientists use these data alongside computer models of Earth’s climate system to better understand how these interactions have caused climate changes in the past and how they may impact future change. Although the primary drivers of past changes differ from current and future changes (variations in Earth’s orbit around the sun vs. human emissions of greenhouse gases), the internal feedbacks of the climate system remain the same. Understanding these processes is essential for projecting the impacts of current and future climate change.

References and Related Reading

Hays, J. D., Imbrie, J., & Shackleton, N. J. (1976). Variations in the Earth’s orbit: pacemaker of the ice ages. science, 194(4270), 1121-1132.

Broecker, W. S., & Denton, G. H. (1990). The role of ocean-atmosphere reorganizations in glacial cycles. Quaternary science reviews, 9(4), 305-341.

Lorius, C., Jouzel, J., Raynaud, D., Hansen, J., & Le Treut, H. (1990). The ice-core record: climate sensitivity and future greenhouse warming. Nature, 347(6289), 139-145.

Stocker, T. F., & Johnsen, S. J. (2003). A minimum thermodynamic model for the bipolar seesaw. Paleoceanography, 18(4).

Anderson, R. F., Ali, S., Bradtmiller, L. I., Nielsen, S. H. H., Fleisher, M. Q., Anderson, B. E., & Burckle, L. H. (2009). Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. science, 323(5920), 1443-1448.

Sigman, D. M., Hain, M. P., & Haug, G. H. (2010). The polar ocean and glacial cycles in atmospheric CO 2 concentration. Nature, 466(7302), 47-55.

Pedro, J. B., Rasmussen, S. O., & van Ommen, T. D. (2012). Tightened constraints on the time-lag between Antarctic temperature and CO2 during the last deglaciation. Climate of the Past, 8(4), 1213.

Shakun, J. D., Clark, P. U., He, F., Marcott, S. A., Mix, A. C., Liu, Z., ... & Bard, E. (2012). Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. Nature, 484(7392), 49-54.

Fudge, T. J., Steig, E. J., Markle, B. R., Schoenemann, S. W., Ding, Q., Taylor, K. C., ... & Alley, R. B. (2013). Onset of deglacial warming in West Antarctica driven by local orbital forcing. Nature, 500(7463), 440-444.

Parrenin, F., Masson-Delmotte, V., Köhler, P., Raynaud, D., Paillard, D., Schwander, J., ... & Jouzel, J. (2013). Synchronous change of atmospheric CO2 and Antarctic temperature during the last deglacial warming. Science, 339(6123), 1060-1063.

Buizert, C., Gkinis, V., Severinghaus, J. P., He, F., Lecavalier, B. S., Kindler, P., ... & White, J. W. (2014). Greenland temperature response to climate forcing during the last deglaciation. Science, 345(6201), 1177-1180.

Marcott, S. A., Bauska, T. K., Buizert, C., Steig, E. J., Rosen, J. L., Cuffey, K. M., ... & McConnell, J. R. (2014). Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature, 514(7524), 616-619.

Buizert, C., Adrian, B., Ahn, J., Albert, M., Alley, R. B., Baggenstos, D., ... & Brook, E. J. (2015). Precise interpolar phasing of abrupt climate change during the last ice age. Nature, 520(7549), 661-665.

Markle, B. R., Steig, E. J., Buizert, C., Schoenemann, S. W., Bitz, C. M., Fudge, T. J., ... & Sowers, T. (2017). Global atmospheric teleconnections during Dansgaard–Oeschger events. Nature Geoscience, 10(1), 36-40.

Buizert, C., Sigl, M., Severi, M., Markle, B. R., Wettstein, J. J., McConnell, J. R., ... & Fujita, S. (2018). Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north. Nature, 563(7733), 681-685.

Brook, E. J., & Buizert, C. (2018). Antarctic and global climate history viewed from ice cores. Nature, 558(7709), 200-208.

Rae, J. W., Burke, A., Robinson, L. F., Adkins, J. F., Chen, T., Cole, C., ... & Stewart, J. A. (2018). CO 2 storage and release in the deep Southern Ocean on millennial to centennial timescales. Nature, 562(7728), 569-573.

Uemura, R., Motoyama, H., Masson-Delmotte, V., Jouzel, J., Kawamura, K., Goto-Azuma, K., ... & Ohno, H. (2018). Asynchrony between Antarctic temperature and CO 2 associated with obliquity over the past 720,000 years. Nature communications, 9(1), 1-11.

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