Why CO2 is Not the Control Knob of Global Temperature and Observational Proof it is Not Causing Dangerous Warming

There is no debate as to whether or not atmospheric carbon dioxide, or CO2, is a so-called greenhouse gas. When present in the atmosphere, this one-carbon and two-oxygen molecule indeed has the capacity to absorb infrared radiation and warm the planet. There is also no debate as to whether or not the concentration of atmospheric CO2 is rising; over the past two centuries it has increased from a meager 0.028% of the atmosphere by volume to a still-meager 0.041% today. Furthermore, there is no argument that global temperatures are warmer today than they were 50, 100 or even 200 years ago. However, there is much debate on whether or not the modern increase in atmospheric CO2 has caused, or is presently causing, dangerous global warming, warming so severe that it is threatening life all across the planet.

Supporters of the dangerous CO2-induced global warming hypothesis, whom are hereafter referred to as climate alarmists because of their concern or alarm about this issue, typically cite the high degree of correlation that exists between historical proxy records of temperature and atmospheric CO2 as empirical support or proof of their thesis. And to that end they generally show some version of Figure 1, which displays historic temperature and CO2 reconstructions from the Vostok ice core in Antarctica for the past four hundred thousand years.

Figure 1. 400,000 years of historic temperature and CO2 from the Vostok Ice Core, Antarctica. Source: Petit et al. (1999) Nature 399: 429-436.

Given the strong relationship that exists between these two variables showing that higher levels of atmospheric CO2 coincide with warmer temperatures, while lower levels of CO2 correspond with colder ones, climate alarmists make the inference that changes in atmospheric CO2 caused the historic changes in temperature. And because atmospheric CO2 is a greenhouse gas, they thus contend that any rise in CO2 beyond the upper bound of its historic concentration of the recent geologic past, will unequivocally cause unprecedented global warming. Furthermore, they opine, as noted in the recent Paris Climate Treaty and elsewhere, that such warming becomes dangerous when planetary temperatures reach a threshold of 1.5°C above pre-industrial values, due to a host of temperature-induced climate-related catastrophes. And they claim that the only hope to avoid such disaster and destruction, which they say is already underway, is for society to immediately reduce and scale back its CO2 emissions in order to achieve a policy of net-zero emissions by 2050.

But how accurate is this narrative?

In answering this question, one need only examine the historic temperature and CO2 records illustrated in Figure 1 more critically. Certainly, these two variables experience a fairly high degree of correlation over the time period shown. However, it doesn't take a Ph.D. scientist to recognize and understand the fact that correlation among two variables does not prove causation. Every textbook on statistics teaches as much, and they also teach that a hypothesis of causation among two variables can be rejected if there is no statistically significant correlation between them, or if the correlation fails to be maintained in a consistent and expected manner across time.

By applying such principles to the case being considered here, it can confidently be argued that if carbon dioxide is indeed the all-important control knob of temperature that climate alarmists claim it to be, then changes in atmospheric CO2 should always precede changes in temperature. And, because CO2 is a greenhouse gas, to prove causation those changes must always be such that a rise in CO2 induces a corresponding rise in temperature, whereas a decline in CO2 must always induce a corresponding drop in temperature. Consistent observations to the contrary, if present in the historic record, would therefore serve to invalidate a causation claim, as well as demonstrate that atmospheric CO2 is nothing more than a bit player among the many factors that drive climate change.

So, what do the records show?

Many of you may be surprised to learn that the historic temperature and atmospheric CO2 records do indeed violate the afore-mentioned principles of causation, and they do so more often than they maintain them. Multiple peer-reviewed scientific studies (see CO2-Temperature Correlations), for example, have demonstrated that, following the termination of each of the past several global ice ages, air temperatures have always risen well in advance of the increase in atmospheric CO2. In fact, during these glacial terminations, which represent the most dramatic warming events experienced on Earth over the past million years, the air's CO2 content does not even begin to rise until some 400 to 2,8000 years after planetary warming starts.

Figure 2 illustrates the observed temporal lag in CO2, as produced by a team of researchers examining Glacial Termination III, 245,000 years ago. In particular, note the temperature proxy shown in red, which has been shifted by the authors to the left so as to match the rapid rise in atmospheric CO2 that followed 800 years later. And remember, this leading rise in temperature and subsequent lag in CO2 increase, which relationship is opposite climate alarmist expectations, is a consistent and proven feature at the termination of all ice ages in Earth's recent geologic history.

Figure 2. The temperature/CO2 offset during Glacial Termination III. Source: Callion et al. (2003) Science 299: 1728-1731.

Another violation of the principles of causation in the CO2/temperature relationship is witnessed at the onset of Ice Ages. Here, scientists report that temperatures always drop first at the start of these glacial periods, and they do so well before the air's CO2 concentration begins its decline. What is more, similar to glacial terminations, these data also indicate that the CO2 decreases observed at the beginning of the Ice Ages lag behind the temperature decreases, often by several thousand years.

Other equally problematic findings in the CO2/temperature relationship have been discovered by scientists examining periods other than the onset or termination of the most recent Ice Ages, including (1) times when CO2 rises and temperatures fall, (2) times when CO2 falls and temperatures rise, or (3) times when a change in either of these two parameters evokes no change in the other. And once again, such changes in CO2 are typically observed to follow changes in temperature from hundreds to thousands of years.

A more recent example of such contrary behavior is illustrated in Figure 3, which depicts atmospheric CO2 and temperature during the Holocene. As seen there, for 7,000 of the past 10,000 years atmospheric CO2 concentrations rose while air temperatures declined. Now, think about that. If CO2 was indeed the potent, climate-controlling greenhouse gas that climate alarmists claim it to be, global temperatures would not have declined 1.3°C in response to the concomitant 25 ppm increase in atmospheric CO2 that occurred over this period. Rather, this rise in CO2 should have warmed planetary temperatures. But it didn't. That harsh reality, in conjunction with all the other conflicting and contrary observations previously discussed regarding the historical relationship between atmospheric CO2 and temperature, leaves little doubt that forces other than CO2 play a much greater role in controlling and driving changes in global temperature.

Figure 3. The reverse coupling of the Holocene temperature and CO2 record. Sources: Global temperature (Marcott et al. 2013. Science 339: 1198-1201) and CO2 (GRIP2 Dome C ice core).

Additional supporting evidence that CO2 plays only a minor role in global climate change can be seen when examining the most recent 150 years of the instrumental temperature and CO2 record. Over this period, it is estimated that carbon dioxide concentrations increased by around 40% (120 ppm) and global temperatures rose between 0.8-1.0°C. According to climate alarmists, some three-fourths to 100% of this observed temperature rise was of anthropogenic origin, most all of which warming they ascribe to the concomitant increase in atmospheric CO2. This is because, notwithstanding the plethora of observational data that demonstrate otherwise, the only narrative climate alarmists are willing to embrace regarding the CO2/temperature relationship is one in which atmospheric CO2 is a powerful greenhouse gas whose increase will always result in climate warming. And based upon that narrative, they have constructed multiple highly sophisticated, though imperfect, computer climate models, which not surprisingly project dangerous global warming is already underway in response to the modern and still-ongoing rise in the air's CO2 content. Nevertheless, while the historic record of the CO2/temperature relationship over the past half million years certainly proves problematic to this thesis, in actuality, one need only examine the most recent 150 years to recognize there is little or no rigorous evidence that rising concentrations of atmospheric CO2 are causing dangerous global warming.

One red flag that immediately pops up when focusing on this time period is the observation that, despite a consistent and near-exponential rise in atmospheric CO2 over the past 150 years, temperatures fluctuated between periods of both global warming and global cooling (see Figure 4). In addition, it is difficult -- if not impossible -- for climate alarmists to adequately reconcile the fact that the two largest warming events that occurred in this record (the first between 1910 and 1945 and the second between 1975 and 2005), experienced nearly identical rates and magnitudes of warming, despite an atmospheric CO2 increase in the latter event that was five times that which occurred during the former event. Nor is it easy for climate alarmists to adequately explain without abandoning their CO2-induced global warming thesis how a three-decade-long cooling event can follow a three-and-a-half-decade-long warming event, when the CO2 increase during the cooling event was twice that which occurred during the preceding warming event. The only way to properly reconcile each of these contrary observations is for climate alarmists to admit they have overestimated the warming power of atmospheric CO2. And if you haven't yet been convinced of this fact, consider the following additional evidences.

Figure 4. CO2/temperature relationship violations in the modern record. Sources: Global temperature (HadCRUT4) and CO2 (Law Dome ice core and Mauna Loa observations).

Figure 5 presents a record of historic temperatures over the past four ice age cycles. As shown there, the peak warmth of the preceding four interglacial periods was between 1 and 2°C higher than that observed during the current interglacial in which we now live, despite there being 45 percent more CO2 in the present atmosphere. Thus, even if temperatures were to warm another 1 or 2°C above their current values in the near future, there is no way such warming can definitively be attributed to the additional CO2 humans have added to the atmosphere in the modern era, because the higher temperatures in each of the past four interglacials occurred under CO2 concentrations that are about half the value that they are today.

Figure 5. 400,000 years of historic temperature from the Vostok Ice Core, Antarctica. Source: Petit et al. (1999) Nature 399: 429-436.

Another problematic issue for climate alarmists is the recent global warming pause or temperature hiatus. Despite an 11% increase in atmospheric CO2 over the past two decades (which increase represents one-fourth of the total increase in CO2 during the modern era), global temperatures have shown little, if any, warming (see Figure 6). Not surprisingly, not one of their climate models predicted this temperature plateau. Expecting warming, they all failed to see it coming.

Figure 6. The global warming pause of the past two decades. Data Source: Version 6 of the UAH MSU/AMSU global satellite temperature dataset.

And speaking of the models, according to theory inherent in all climate models, CO2-induced global warming over the past 40 years should show a unique fingerprint in the form of a warming trend that increases with altitude in the tropical troposphere, as indicated by the red and orange colors presented in the center of Figure 7 and outlined in blue. Climate changes due to solar variability or other known natural factors do not yield this pattern. However, as shown in Figure 8, real-world observations do not match this model-expected theory.

Figure 7. The supposed “fingerprint” of CO2-induced global warming. Source: Canadian Model Run 3 temperature trends over the period 1979-2017 (°C per decade).

Each of the red bars depicted in Figure 8 shows warming that should have occurred in the tropical upper troposphere over the period 1979-2017, as predicted by simulations from 102 different climate models. The average predicted warming rate over this nearly four-decade-long period, as shown by the black horizontal dashed line, is 0.44°C per decade. In contrast, radiosonde temperature measurements observed in this portion of the atmosphere, shown in blue, reveal that the actual warming rate is three times smaller than that predicted by the models.

Figure 8. Climate model predictions of tropical tropospheric warming over the period 1979-2017 obtained from simulations from 102 different climate models. Source: Christy and McNider (2017) Asia-Pacific Journal of Atmospheric Sciences 53: 511-518.

This divergence between model projections and observational data is even more evident in Figure 9, which plots both predicted and observed temperature anomalies of the upper tropical troposphere. Once again observational data reveal that the model-derived projections of CO2-induced warming are running far too hot, so much so, in fact, that mathematical analyses confirm a statistically significant difference between the two temperature series. This key fact alone, is sufficient to provide more than enough of a credible, scientific basis for invalidating all of the climate models and their associated predictions.

Figure 9. The model-based temperature trend of the tropical troposphere is significantly different from the observation-derived trend. Data Source: Christy and McNider (2017) Asia-Pacific Journal of Atmospheric Sciences 53: 511-518.

Consequently, considering each of the several evidences presented above, it is truly disingenuous for climate alarmists to claim with any degree of certitude that the modern temperature increase bears an anthropogenic fingerprint or that rising atmospheric CO2 is causing dangerous global warming. There exists far too much data to the contrary. And because the models all fail in this regard, the global warming debate should really end here, for pretty much all of the additional ancillary climate-related predictions made by climate alarmists rely on the unrealistic temperature rise predicted by their invalidated models.

Although atmospheric CO2 is a greenhouse gas, it is not the control knob that drives global temperature and it is certainly not causing dangerous global warming. The historic record is clear: a control knob does not consistently lag several hundred to thousands of years behind changes in the parameter it is supposedly governing and a control knob most certainly does not produce multiple occurrences of a response that is opposite its expectation. In debating this issue one cannot ignore such contrary empirical behaviors, especially when they are far more characteristic of the historic relationship observed between atmospheric CO2 and temperature than they are not.

Further proof that the planet is not in danger of imminent peril as a result of increasing atmospheric CO2 concentrations is witnessed in scientific studies examining the health and vitality of the terrestrial biosphere over the past several decades. In fact, such studies reveal the truth is pretty much the opposite; rising concentrations of atmospheric CO2 are actually benefitting humanity and the biosphere in a great number of ways.

In one recent example of such benefits, a team of researchers employed a series of globally-distributed data sets to estimate changes in planetary gross primary productivity over the period 1982-2015. Yet, before describing their findings, consider that since 1980 the Earth has weathered 3 of the warmest decades in the modern instrumental temperature record, as well as a handful of intense and persistent El Niño events, large-scale deforestation, supposedly "unprecedented" forest fires, and episodes of persistent, widespread and severe weather events, including hurricanes, droughts and floods. At the same time, the air's CO2 content has increased by more than 16% and the human population has grown by over 55%. To global warming alarmists, the Earth has been in the throes of a veritable climate Armageddon!. So just how bad has the planet suffered in response to these much-feared events?

To answer frankly, hardly at all! Despite the many real (and imagined) assaults on vegetation by humanity and nature alike (including perceived dangerous CO2-induced global warming), the terrestrial biosphere has met and overcome these challenges. As indicated in Figure 10, the spatial distribution of the linear trends in gross primary productivity over the past 34 years are overwhelmingly positive, revealing a persistent and widespread vegetative enhancement has occurred in more than 75% of the planet's land area. And as icing on this cake of good findings, the researchers who conducted this analysis determined that the principal factor responsible for inducing these positive trends just so happens to be the same factor climate alarmists claim should be obliterating them -- rising concentrations of atmospheric CO2. How ironic, even more so when one learns that the next most significant factor contributing to this planetary-wide growth stimulation was rising temperatures, albeit its influence was more than five times less than the positive influence exerted by CO2.

It would thus seem that climate alarmists have missed Nature's memo; recent warming has not been dangerous and rising atmospheric CO2 is benefitting the biosphere.

Figure 10. Spatial pattern of trends in gross primary production (1982-2015). Source: Sun et al. 2018. Ecological Informatics 46: 156-165.

In summation, the underlying question in the climate change debate is whether or not atmospheric CO2 is a powerful greenhouse gas capable of causing dangerous global warming that will threaten life on the planet. In testing this hypothesis, this primer has presented multiple paths of evidence based on real-world observations that it is not. More often than alarmists are willing to admit, the historical record defines CO2 as the dependent variable, following changes in temperature with a lag-time that varies from hundreds to thousands of years. How can a cause, perceived to be so powerful, so often trail is supposed effect? And how can a cause elicit responses that are directly opposite of its hypothesized effect?

The answer is quite simple, and it doesn't take a Ph.D. scientist to figure it out. It is because atmospheric CO2 is not the all-important greenhouse gas that climate alarmists claim it to be. Sufficient proof is documented in the historic records. Sufficient proof is also found in the missing fingerprint of CO2-induced warming in the tropical upper troposphere that observations fail to capture. And sufficient proof is noted in a vast array of real-world data that fail to match model projections for a host of subordinate temperature-related climate catastrophes. Consequently, there is little or no rigorous evidence that rising concentrations of carbon dioxide are causing dangerous global warming.

Using another common-sense approach to recognize this fact, consider once again Figure 5 (seen here as Figure 11), which shows that all four of the preceding interglacial periods in Earth's temperature history reached values that were 1 to 2°C warmer than the present, and they did so naturally despite 45 percent less CO2 in the air then there is now. It is likely that no one on the planet would take the extreme position that temperatures during those interglacials were dangerous. Yet, recall from the beginning of this primer that the Paris Climate Treaty actually defines dangerous warming as temperatures that cross a threshold of 1.5°C above pre-industrial values. Since global temperatures have already warmed between 0.8 and 1.0°C since pre-industrial times, using the Paris Climate Treaty's standard, which is echoed by climate alarmists everywhere, the planet cannot warm another 0.5 to 0.7°C or dangerous warming/climate Armageddon will ensue. Of course this logic is preposterous. Such warming, if it occurs, would still place global temperatures some 0.3 to 1.3°C below the peak warmth of the previous interglacial warm periods, which were by no means were periods of planetary disaster.

Figure 11. 400,000 years of historic temperature from the Vostok Ice Core, Antarctica. Source: Petit et al. (1999) Nature 399: 429-436.

Clearly, the model-based projections of the consequences of rising atmospheric CO2 are way off the mark. Dangerous CO2-induced global warming is not presently occurring, nor will it likely ever occur in the future. Likewise, pessimistic forecasts of ecosystem degradation and collapse fail to match observed trends showing widespread enhancement of planetary vegetation. These and other phantom pillars that form the foundation of all climate alarmist claims are merely the conceptual constructs of woefully-inadequate climate models. Real-world observational data, in contrast, do not support a catastrophic -- or even problematic -- view of global warming. Consequently, efforts to restrict CO2 emissions and/or fossil fuel use based on the presumption that atmospheric CO2 is causing dangerous warming and threatening life on the planet should be abandoned.

For supporting information on this topic, click on the several links below.

CO2-Temperature Correlations:
Scroll down this page and click on the numerous links listed there to read numerous reviews of peer-reviewed science articles demonstrating the historic relationship between atmospheric CO2 concentrations and temperature. These papers show atmospheric CO2 is only a minor player in global temperature.

CO2 Enhancements to the Terrestrial Biosphere:
Scroll down this page and click on the numerous links under the heading of Biospheric Productivity to read numerous reviews of peer-reviewed science articles linking higher levels of atmospheric CO2 to increased plant productivity throughout the terrestrial biosphere.