The Chicken or the Egg: CO2 and Temperature

The other day, I had a rather civil discussion with a climate change denier. His main argument was that, since the carbon dioxide (CO₂) concentration has historically followed the Earth’s temperature trend, it could not possibly be that man-made CO₂ emissions now dictate our natural climate.

After all, he went on, if CO₂ were responsible for a runaway climate effect, then we would have surely seen such effect during hotter periods in climate history.

When I presented to him science-based counterarguments, he basically ignored them and continued with his narrative. Why people might dismiss climate science, is food for thought for another article.

The conversation with my climate skeptical fellow human being inspired me to descend deeper into the lion’s den. Let us see how far I get. If we are lucky, we might actually find out whether it was indeed the chicken or the egg.

Deep into the Past

First things first: Does atmospheric CO₂ historically lag the Earth’s temperature?

Based on ice core data from the Vostok Station in Antarctica, Georgios Florides and Paul Christodoulides (see section 2.4) provide a positive answer to our question: During the last three glacial-interglacial transitions over the past 420,000 years, an elevation in CO₂-concentration lags global warming.

That is to say, CO₂ cannot have initiated the end of historical glacial periods. What seem to explain the rise and fall of ice ages are the Milankovitch cycles. These cycles describe the effects of deviation in the Earth’s eccentricity, obliquity and precession on how much insolation is reaching the Earth’s surface.

Léa Gest et al. draw the same conclusion when looking specifically at the end of the last deglaciation, which occurred between 18,000 and 11,000 years ago. Going even further back in time to the late Pliocene, i.e. between 2.3 and 3.3 million years ago, Vakulenko Nadezda et al. also reveal a positive answer when considering large timescales of 100,000 to 500,000 years.

In a more recent past, such causal relationship between temperature and CO₂ continues to exist. According to Ole Humlum et al., fluctuations in global temperature exert a significant influence on atmospheric CO₂ variations between January 1980 and December 2011.

Since these findings go against most climate reports in the last 150 years, it is no surprise then that our climate contrarian rushed to bring up this argument. And it looks like he is not the only one.

Physics Is Everywhere

It appears from his reasoning, though, that our climate change denier neglects feedback systems and radiative forcing.

Feedback systems can amplify the initial cause of climate change. As a case in point, the augmented CO₂ levels due to global warming further push the temperature up through the greenhouse effect. Radiative forcing looks at the difference between the radiation absorbed by the Earth and that sent out back into space.

For instance, Jeremy Shakun et al. demonstrate that increased atmospheric CO₂ levels generally precede more elevated temperatures during the course of the last deglaciation. When examining multi-millennial timescales in Vostok Antarctica and Greenland ice cores, Knut Seip et al. identify several periods within the last 400,000 years during which global warming lags CO₂.

Even though he is right in that CO₂ did not cause the onset of deglaciation periods, our climate skeptic forgets about physics, which tells us that CO₂ is a greenhouse gas. This means that CO₂ molecules absorb the longer wavelengths of radiation — infrared radiation — emitted by the Earth.

Absorption takes place because a CO₂ molecule possesses an oscillating dipole by virtue of its bending and asymmetric stretching abilities. The dipole ensures that the vibrating molecule creates an oscillating electric field, which interacts with the electromagnetic field of infrared light.

Subsequently, CO₂ molecules effectively prevent a good portion of these specific electromagnetic waves from escaping into space. Instead, they re-emit that heat back into the atmosphere. Part of that heat finds its way back to the Earth’s surface and warms it. This is in essence the greenhouse effect.

Fig. 2. Annual mean energy fluxes in the Earth’s climate system (in W/m²). (Source: Paper Stephen Schwartz).

One way of measuring the greenhouse effect, according to the related study of Fig. 2, is taking the difference between the emitted longwave energy flux at the surface, i.e. 385 W/m², and the exiting flux at the top of the atmosphere, i.e. 239 W/m². This gives us a greenhouse effect of ±146 W/m².

We have now established that atmospheric CO₂ can indeed shape our natural climate.

But what our climate skeptic categorically repudiates is the role that human-produced CO₂ emissions play in global warming. In other words, he denies the enhanced greenhouse effect by fossil fuel emissions.

Let us now sink a little deeper in the lion’s climate change denying den.

The Contested Relevance of Anthropogenic CO₂ Emissions

The entire controversy in the human-caused climate change debate largely evolves around the following reasoning. Due to fossil fuel-driven industrialization processes, higher levels of anthropogenic CO₂ emissions, especially since the 1960s and 1970s, propel the atmospheric CO₂ concentration upwards, which, successively, brings about global warming. More elevated temperatures may then inflict a devastating impact upon our climate and ecosystem.

Fig. 3. Top: CO₂ Atmospheric Concentration (Source: University of California). Middle and Bottom: Log scale of fossil fuel CO₂ emissions and global surface temperature relative to 1880–1920. (Source: Paper by Hansen, J. et al.).

As a reminder, the skeptical claim at the beginning of this article holds that, because of the historical evidence that atmospheric CO₂ lags temperature, man-made CO₂ emissions cannot alter our climate system.

First off, we already confirmed in the previous section that CO₂ can actually precede global warming.

What we need to look at now, is how man-made CO₂ emissions relate to CO₂ concentration.

Our Human Fingerprint

In science, the causal link between fossil fuel emissions and atmospheric CO₂ is shown via the isotopic signature δ¹³C, which involves the ratio ¹³C/¹²C. As the process of burning fossil fuels shrinks the relative amount of isotope ¹³C, we expect to see a declining tendency in δ¹³C, given the recent surge in global emissions (Fig. 3). Indeed, referring to Fig. 4, R.J. Francey et al. state that, “The overall decrease during the last 1–2 centuries is attributed to anthropogenic emissions.”

Interestingly, already back in 1976, Charles Keeling et al. put forward that, “the observed long term trend of rising CO₂ appears clearly to be in response to increasing amounts of industrial CO₂ in the air on a global scale.”

In lending further support to the link between fossil fuel emissions and atmospheric CO₂, David Hendry and Felix Pretis recognize a significant human contribution of industrial production processes to the recent burgeoning atmospheric CO₂ levels between 1982 and 2003. Michael Raupauch et al., too, corroborate the essential role of anthropogenic drivers in CO₂ build-up for the period 1959–2006.

Fig. 4. The complete record of CO₂ and the isotopic signature δ¹³C. (Source: Paper R.J. Francey et al.).

The Crucial Point of Intersection

But I already hear our climate contrarian asking: Does all that we spew out of CO₂ emissions in the air really stays in the air? The CO₂ airborne fraction displays precisely that and helps us answer this question.

The airborne fraction is important as “it provides the gateway between the anthropogenic forcing and the atmospheric response of the carbon cycle”, according to Raupauch et al.

More technically, it is defined as “the rate of increase of atmospheric CO₂ concentration divided by the rate of released CO₂ by anthropogenic emissions”.

Currently, about 45% of our total human-produced CO₂ emissions stays in the atmosphere, while the Earth’s land, i.e. the land sink, and oceans, i.e. the ocean sink, absorb 31% and 24%, respectively.

Fig. 5. Fossil fuel CO₂ emissions (1959–2015) together with the El Niño–Southern Oscillation cycle (lower) and the airborne fraction (upper inset). Variability in the land uptake is correlated with the El Niño cycle with more fossil fuel CO₂ remaining in the atmosphere during El Niño periods as a result of reduced land uptake and the opposite in La Niñas. (Source: Paper Piers Sellers et al.).

The controversy is partly reflected in the debate on whether the airborne fraction is moving up or has remained constant.

On the one hand, Wolfgang Knorr provides the claim that the fraction of anthropogenic CO₂ emissions remained constant since 1850. Given a higher amount of fossil fuel CO₂ emissions in the last few decades (see Fig. 3), this could imply that the Earth’s carbon cycles absorb with enhanced efficiency large chunks of what we put out there. Although Mikkel Bennedsen et al. encounter the same result for the period 1959–2016, the research also demonstrates evidence for a dwindling rate at which ocean and land take up CO₂. In contrast, Margreet van Marle et al. affirm an improved efficiency in that same rate, while Emanuel Gloor et al. argue against a deteriorating rate.

On the other hand, Michael Raupauch et al. register an upwards trend between 1959 and 2006 in the total airborne fraction, which comprises man-made emissions from both fossil fuels and land-use change. According to the authors, a rising fraction asserts that the growth in total emissions surpasses that of carbon sinks. And more authors arrive at similar conclusions, including Josep Canadell et al., Corinne Le Quéré et al. and Peter Rayner et al.

When we turn our attention to the existing climate models, Chris Jones et al. indicate that, “All models agree that the future airborne fraction depends strongly on the emissions profile with higher airborne fraction for higher emissions scenarios.”

If we conservatively assume that the airborne fraction has remained fairly constant since the mid-nineteenth century, it may be tempting for our climate change denier to think that it will also stay this way in the future. As a result, he might conclude that, regardless of the amount of fossil fuel CO₂ emissions that we pump into the atmosphere, the natural carbon cycle will keep the climate system in check.

But such conclusion would be impetuous. Per definition, the airborne fraction depends on the land and ocean sinks. In turn, these carbon sinks are themselves dependent on climate. Such climate-carbon cycle feedback suggests that, “An increase in atmospheric CO₂, which results in a climate change via the greenhouse effect, may therefore change the carbon storage in the ocean and on land and therefore modify the CO₂ concentration of the atmosphere producing a further change in climate.”

In other words, a constant airborne fraction does not imply that fossil fuel emissions cannot cause an alteration in the carbon cycle. And this goes to the heart of the climate change controversy.

Fig. 6. Schematic view of a climate-carbon cycle feedback. (Source: Paper Richard Williams et al.).

Rising Temperatures

Perhaps all the above does not impress much our climate skeptic. After all, he might argue, the atmospheric CO₂ concentration has been higher in the past as well as the variation in temperature. And the planet is still here, right?

I come back to higher CO₂ levels in the next section. Let us first have a closer look at the historical record of the Earth’s temperature.

Christopher Field indeed points out that the Earth has gone through comparable and even greater temperature fluctuations in the past 65 million years. But the catch is that they spread out over tens of thousands and even millions of years and did not materialize within a timespan of several decades.

What is unprecedented in the past 100 years is the rate of change in temperature. As Field elucidates, “We find periods of Earth’s history where the global temperature change was of similar magnitude, but the rate was an order of magnitude slower.”

This unparalleled rate of global warming is reflected in what scientists have coined the “hockey stick” effect. That phenomenon refers to the temperature’s evolution whereby it was gradually receding in the past thousands of years to only take a drastic upturn in the twentieth century. The corresponding graph bears the resemblance of a hockey stick (see Fig. 7).

Fig. 7. Estimated average global temperatures as anomalies relative to 1880–1920 for the Holocene period. (Source: Paper by Hansen, J. et al.).

Let us now bring back feedback systems and radiative forcing into the discussion.

What is equally unique about the recent past is that CO₂ and other greenhouse gasses now seem to act as an anthropogenic radiative forcing mechanism on the global temperature instead of a feedback system to external natural forcing.

External natural forcing mechanisms can produce disruptions in the Earth’s energy balance, eliciting climate change. Examples of such external forcing include solar radiation variation, the Milankovitch cycles, cosmic rays or the upper atmosphere’s absorption of ultraviolet light.

Usually, atmospheric CO₂ acts as a feedback mechanism to climate change, as we have seen previously. However, in light of the temperature’s unmatched behaviour in the recent past, the intricate relationship between fossil fuel emissions and CO₂ concentration as well as the knowledge that CO₂ can precede global warming, atmospheric CO₂ has taken on the role of external radiative forcing whereby man-made emissions are reinforcing the greenhouse effect.

In fact, the smoking gun for human-driven global warming appears to be the following. The empirical evidence of the rising temperature records since the second half of the twentieth century can only be explained by incorporating anthropogenic CO₂ emissions into the climate models. David Archer of the University of Chicago clarifies this to us in greater detail in below video.

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If a climate contrarian works out an alternative explanation for the recent climate change, they have two tasks: firstly, to disprove why anthropogenic CO₂ emissions explicate so well the ascending temperature trend in the models and, secondly, to prove how the provided explanation fits the data.

As a reassuring side note on the robustness of climate modelling, Zeke Hausfather et al. highlight that, “[the] climate models published over the past five decades were generally quite accurate in predicting global warming in the years after publication, particularly when accounting for differences between modeled and actual changes in atmospheric CO₂ and other climate drivers.”

The Climate Is Not Going Anywhere. Or Is It?

Let us move on to the second part of the climate skeptical statement: if CO₂ is truly responsible for a runaway climate effect, then we would have witnessed such effect on various historical occasions, especially during times of much higher CO₂ levels than today.

Runaway Climate: Not So Much

What is essentially being misunderstood here is that atmospheric CO₂ is not the only driver of climate change. And to some extent, the physics of the greenhouse effect is also being challenged.

As a matter of fact, a runaway climate effect did not occur at any moment in history, but not because the greenhouse effect is not physically real.

It is good to bear in mind that higher CO₂ concentrations in the past in absence of such runaway effect do not imply that CO₂ does not cause global warming.

For the record, at January 18 2020, the Mauna Loa Observatory registered a CO₂ concentration of 413.39 parts per million (ppm). The last time that this level was attained or exceeded happened well before the human species roamed the Earth. Yet, it is no sinecure to determine an exact time window. For instance, Greta Bartoli et al. demarcate a time period within the Pliocene, i.e. between 2.0 and 4.6 million years ago, whereas Aradhna et al. tie that timeframe to the Middle Miocene, i.e. 11.6 to 16.0 million years ago.

The greenhouse effect was in full swing also back then. But the point that our climate contrarian is overlooking is that the climate conditions were very different at different times in history.

Around 3 million years ago during mid-Pliocene, with CO₂ levels oscillating between 365 and 415 ppm for several thousands of years, the global temperatures even outstripped those of the pre-industrial era by 2–3°C, according to Adam Csank et al. And the Arctic temperatures were superior to current measurements by no less than approximately 11–16°C. Harry Dowsett et al. contend that greenhouse forcing was indeed one of the main drivers of this global warming. Nonetheless, a runaway climate effect did not ensue because of time-specific climate conditions that were primarily dominated by a larger obliquity signal from the Milankovitch forcing.

Fig. 8. Estimates of Pliocene atmospheric CO₂ with pre-industrial and present day levels (horizontal dashed lines) for comparison; dark blue dots, δ¹³C; green band, δ¹¹B; pink band, alkenone; red band, alkenone; orange band, stomata; yellow band, δ¹¹B; blue band, Ba/Ca. (Source: Paper Alan Haywood et al.).

At the Middle Miocene Climate Optimum (MMCO) around 15 million years ago, CO₂ concentration measured roughly between 430 ppm and 800 ppm, with global temperatures 3°C above current levels. Yougui Song et al. suggest that also now higher CO₂ levels may be responsible for this global warming. Again, a runaway climate effect was not observed, in view of other more potent climate change drivers, such as a shift in solar insolation caused by a transition from an obliquity-driven to a eccentricity-driven forcing, deep oceanic circulation variations and a CO₂ drawdown by organic carbon burial in the eastern equatorial Pacific. The MMCO is then followed by the cooler Middle Miocene Climate Transition (MMCT).

Fig. 9. Proxy-based CO₂ records during the Miocene. (Source: Paper Shunchuan Ji et al.).

Lastly, we stumble upon a more extreme example during the Late Ordovician, circa 444 million years ago. It is estimated that the atmospheric CO₂ reached astounding levels between 2240 and 5600 ppm. Despite the elevated CO₂ concentrations, a runaway climate effect did not come to pass. At the contrary, a period of glaciation surprisingly arose around 440 million years ago for which possible explanations include a 4.5% reduced solar luminosity relative to present levels, the combination of reduced volcanic activity and continuing silicate rock weathering and enhanced oceanic circulation.

Fig. 10. A: Comparison of the best-guess atmospheric CO₂ predictions through the Phanerozoic of GEOCARB III with a smoothed representation of the proxy record (10 my time-steps are used in both curves). B: Intervals of glacial (dark blue) or cool climates (light blue). (Source: Paper Dana Royer et al.).

Planck to the Rescue

Another point that we have to make is that positive feedback in the context of global warming usually does not equal a runaway effect.

In a positive feedback system, the effect strengthens the cause, which generates a stronger effect. Such unstable systems are more often than not characterized by exponential growth or chaotic behaviour.

Suppose for a moment that we live in a world without any anthropogenic CO₂ emissions, but with climate conditions similar to present times. And now we put a substantial amount of fossil fuel emissions into the atmosphere. A simplified positive climate feedback works as follows. Adding fossil fuel emissions to the atmosphere enhances the greenhouse effect. As a result, the additionally trapped infrared radiation boosts the Earth’s temperature, which warms the oceans and thaws previously frozen permafrost. These events discharge more CO₂, which causes the temperature to climb even further.

However, against the backdrop of global warming, as this positive feedback system advances with time, the incremental effect of every feedback loop diminishes in strength. And partly, we owe it to physics: the Stefan-Boltzmann law, which is derived from Planck’s law, acts as a negative feedback mechanism. It states that, for a given rise in the Earth’s temperature, the equivalent outgoing longwave infrared radiation — heat — increases exponentially. Insomuch as a part of that outgoing heat is siphoned off from the Earth’s atmosphere into space, the Stefan-Boltzmann law partially counters the greenhouse effect.

In other words, under the scenario of a single perturbation of human-produced CO₂ emissions, the entire climate system tends to evolve towards a new unique thermodynamic stable state, which is controlled by negative feedbacks, at a higher temperature.

Does this then mean that the Earth will also progress towards a new steady state regardless of the quantity of anthropogenic emissions that we release into the atmosphere? No, simply because of the fact that we keep on adding more CO₂ to the atmosphere.

That is, we are continually introducing one perturbation after the other into the Earth’s climate system. We are de facto depriving the Earth of the possibility to form a relatively stable state. And this is the crux of the climate change debate.

Fig. 11. A schematic representation of a positive feedback mechanism triggered by anthropogenic CO₂ emissions. (Source: Climate Council).

It is perhaps not unwise to mention that a positive feedback can prompt a runaway scenario, if the reinforcing signal is powerful enough. Put differently, this might occur if the expanding cumulated amount of anthropogenic CO₂ emissions in the atmosphere warms the Earth until a point of no return.

A runaway climate effect is what transpired on planet Venus. With a rising temperature, a greater amount of water vapor enters the atmosphere. In turn, this raises the chances for water vapor to come into contact with ultraviolet radiation, which breaks up the water molecules. Key is that hydrogen atoms move fast enough to escape into space — both Venus and Earth have matching escape velocities. In this way, water was gradually lured away from Venus, leaving it without the means to maintain a biosphere.

How Valid Is the Climate Change Denying Statement?

Yes, there is academic research that does not grant much significance to an anthropogenic contribution to the climate change narrative or even questions the physics of the greenhouse effect. Specific literature includes, among others, Georgios Floridos, Jamal Munshi, Thomas Allmendinger, Hermann Harde or Mike Hulme et al.

Nevertheless, there is also global growing confidence among scientists that a human influence is noteworthy in global warming. As a matter of fact, this meta-study by John Cook et al. even finds a 97% consensus in published climate research on the importance of anthropogenic drivers in climate change.

The authors furthermore pinpoint that this consensus is not widely known. And that is precisely the reason, they argue, why the global population is susceptible to the climate skeptics’ argument that there is no scientific consensus on human-induced global warming.

Is the Knot Untangled?

I am grateful for the conversation that I had with our climate contrarian, for it allowed me to explore the lion’s den and ponder about the debate around human-driven climate change.

In accordance with climate dynamics, it appears that it is not the chicken or the egg, but the chicken and the egg. Notwithstanding, science is telling us that we might run out of chickens and eggs if we do not collectively acknowledge our stake in global warming and act accordingly.

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