Yes, CO2 Leads Temperature

GMST Follows CO2 Forcings, not Solar Forcings

One of the more common mantras I hear from contrarians is that "CO2 always lags temperature," and usually this is accompanied with "by hundreds of years."  This idea goes back to the late 1990s, when the Vostok ice core was drilled, and plots of CO2 and Antarctic temperature (AT) showed what appeared to be a multi-century lag between CO2 and AT proxy data. From the observation that CO2 lagged Antarctic temperature during the last four glacial cycles, contrarians predictably began to expand and modify this observation by dropping "Antarctic" and adding "always," giving us the familiar refrain, "CO2 always lags temperature." Is there any sense in which this mantra is true? Not really.

Paleoclimate Evidence

That mantra was never technically correct, but developments since 1999 have shed more light on what is actually going on.
  1. In 2012, Shakun et al[1] showed that while CO2 lags AT, it leads changes in GMST. That is, global temperatures follow CO2.
  2. In 2013, Parrenin et al[2] re-evaluated the dating of the proxies for AT and CO2 and found that, after correcting the dating of these proxies, CO2 and AT actually rise concurrently with each other.
I go into a lot more detail about this in another post. Ever since 2013, we've understood that during these glacial cycles orbital forcings triggered regional warming, which caused the outgassing of CO2 from the oceans as ocean waters warm (CO2 is less soluble in warm water). CO2 amplifies this warming signal (as do changes in dust and ice-albedo) and GMST follows the increase in CO2. In the proxy evidence, this looks like AT and CO2 increase at the same time and GMST follows. It's been well understood that CO2 can and does operate as a feedback when orbital cycles trigger warming. Since CO2 changes by ~100 ppm during the glacial cycles, and GMST changes by ~6°C, we can estimate that CO2 increases by ~17 ppm for every 1°C of warming. So it's true that CO2 can lag AT, but generally speaking, GMST follows CO2.

Historical Evidence

And it's not always true that CO2 lags AT. That is, sometimes CO2 is the driver of global warming. It's pretty easy to demonstrate this to be the case since 1750 with data from the 2025 Global Carbon Budget. We know from historical data that CO2 was about 278 ppm in 1750, which is the equivalent carbon mass of 591 GtC. Human emissions from fossil fuel and industry (FFI) and land use change (LUC) has totaled 752 GtC, which would be enough to increase CO2 concentrations by 353 ppm to 631 ppm. However, CO2 concentrations today are only 424 ppm. This means that human carbon emissions is responsible for 100% of the increase 147 ppm increase in CO2 and that the land and oceans have been carbon sinks, taking up about 440 GtC of our emissions. 

This is due to Henry's Law, which states that the amount of CO2 dissolved in the oceans is proportional to the partial pressure of CO2 in the atmosphere above the oceans. As CO2 increases from human emissions, the land takes up a fraction of our emissions (greening) and the oceans take up another fraction of our emissions (ocean acidification). In the oceans, there are actually two dynamics at play:
  1. Human carbon emissions generates an atmosphere-to-ocean CO2 flux.
  2. Ocean warming from CO2 causes an ocean-to-atmosphere CO2 flux
Both of these occur at the same time, but as I demonstrate in another post the the CO2 flux to the oceans is about 10x greater than the flux to the atmosphere. And this should make intuitive sense. Ocean warming has been slower than land warming. The oceans have warmed by about 1°C, so if CO2 was increasing as it was during the glacial cycles, we should see only about a 17 ppm increase in CO2 (about 36 GtC). But CO2 has increased by 147 ppm (312 GtC), and and the land/ocean sinks have taken up 440 GtC. Since both the land and oceans are net carbon sinks, they cannot be the source of the increase in CO2. This means necessarily that CO2 is not following the increase temperature as the oceans outgas CO2 in response to warming. The oceans are not contributing CO2 to the atmosphere; they are taking up a significant fraction of our emissions. So we can conclusively rule out the possibility that CO2 has lagged temperature since 1750.

These conclusions are also supported by the Stips et al 2016[4] analysis of CO2 and T causality, which found the direction of causation in the direction of AT -> CO2 using ice core data and in the direction of CO2 -> GMST using historical data, consistent with the theoretical predictions of climate science.

In fact, it's long been documented that GMST increases linearly with cumulative emissions. A simple regression between GMST and cumulative emissions shows that global warming has been occurring at a rate of about 0.018°C for every 10 GtC emissions. This is of course approximate, since it doesn't account for emissions of other GHGs or changes in aerosol pollution. However, this value is within the CI for the IPCC's value of 0.0165°C for 10 GtC.

So it's technically true that AT can lead CO2 a very small amount of time when local warming is triggered by orbital cycles. In this case, Henry's Law operates to outgas CO2 to the atmosphere, but global warming follows CO2. However, CO2 can also trigger warming independently of orbital cycles. This happens with long-term eruptions of large igneous provinces (like the PETM) and human carbon emissions. During these types of events, volcanic or human activity perturbs the carbon cycle and drives global warming, and the oceans become a carbon sink and acidify. During the PETM, this led to a mass extinction of benthic foraminifera. And the oceans are acidifying now in response to human carbon emissions as well. 


References:

[1] Shakun et al, “Global Warming Preceded by Increasing Carbon Dioxide Concentrations during the Last Deglaciation” Nature 484(7392):49-54 · April 2012
https://www.researchgate.net/publication/223987444_Global_Warming_Preceded_by_Increasing_Carbon_Dioxide_Concentrations_during_the_Last_Deglaciation

[2] Parrenin, F. et al. “Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming.” Science 339, 1060 (2013).
DOI: 10.1126/science.1226368
https://pdfs.semanticscholar.org/d61d/0fbcb5828af1d434d1bd0282ed36e0f00d2a.pdf

[3] Friedlingstein, P., O'Sullivan, M., Jones, M. W., Andrew, R. M., Bakker, D. C. E., Hauck, J., Landschützer, P., Le Quéré, C., Li, H., Luijkx, I. T., Peters, G. P., Peters, W., Pongratz, J., Schwingshackl, C., Sitch, S., Canadell, J. G., Ciais, P., Aas, K., Alin, S. R., Anthoni, P., Barbero, L., Bates, N. R., Bellouin, N., Benoit-Cattin, A., Berghoff, C. F., Bernardello, R., Bopp, L., Brasika, I. B. M., Chamberlain, M. A., Chandra, N., Chevallier, F., Chini, L. P., Collier, N. O., Colligan, T. H., Cronin, M., Djeutchouang, L., Dou, X., Enright, M. P., Enyo, K., Erb, M., Evans, W., Feely, R. A., Feng, L., Ford, D. J., Foster, A., Fransner, F., Gasser, T., Gehlen, M., Gkritzalis, T., Goncalves De Souza, J., Grassi, G., Gregor, L., Gruber, N., Guenet, B., Gürses, Ö., Harrington, K., Harris, I., Heinke, J., Hurtt, G. C., Iida, Y., Ilyina, T., Ito, A., Jacobson, A. R., Jain, A. K., Jarníková, T., Jersild, A., Jiang, F., Jones, S. D., Kato, E., Keeling, R. F., Klein Goldewijk, K., Knauer, J., Kong, Y., Korsbakken, J. I., Koven, C., Kunimitsu, T., Lan, X., Liu, J., Liu, Z., Liu, Z., Lo Monaco, C., Ma, L., Marland, G., McGuire, P. C., McKinley, G. A., Melton, J., Monacci, N., Monier, E., Morgan, E. J., Munro, D. R., Müller, J. D., Nakaoka, S.-I., Nayagam, L. R., Niwa, Y., Nutzel, T., Olsen, A., Omar, A. M., Pan, N., Pandey, S., Pierrot, D., Qin, Z., Regnier, P. A. G., Rehder, G., Resplandy, L., Roobaert, A., Rosan, T. M., Rödenbeck, C., Schwinger, J., Skjelvan, I., Smallman, T. L., Spada, V., Sreeush, M. G., Sun, Q., Sutton, A. J., Sweeney, C., Swingedouw, D., Séférian, R., Takao, S., Tatebe, H., Tian, H., Tian, X., Tilbrook, B., Tsujino, H., Tubiello, F., van Ooijen, E., van der Werf, G., van de Velde, S. J., Walker, A., Wanninkhof, R., Yang, X., Yuan, W., Yue, X., and Zeng, J.: Global Carbon Budget 2025, Earth Syst. Sci. Data Discuss. [preprint], https://doi.org/10.5194/essd-2025-659, in review, 2025.

[4] Stips, A., Macias, D., Coughlan, C. et al. On the causal structure between CO2 and global temperature. Sci Rep 6, 21691 (2016).
https://www.nature.com/articles/srep21691

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