March 7, 2014
Carbon Dioxide and Air Temperature: Who Leads and Who Follows?
by David Legates, Ph.D.
December 10, 2012
On the Evangelical Environmental Network’s Creation Care Blog, Mitch Hescox shows a slide purporting to show the relationship between air temperature and carbon dioxide (CO2) over the past 800,000 years (http://www.creationcare.org/view.php?id=716). His slide is taken from Jouzel et al. (2007) and Lüthi et al. (2008) and he asserts that “carbon dioxide levels and temperature track identically” (emphasis added) with the implication that rising carbon dioxide levels increase the Earth’s temperature. While few deny the existence of the misnamed ‘greenhouse gas’ effect where the Earth’s surface is warmer due to the presence of an atmosphere, most scientists also agree that it is water vapor, and not carbon dioxide that is responsible for most of the warming. But what about this graph; in particular, did changes in CO2 lead to changes in air temperature or is it the other way around?
Part of this confusion comes from the graph itself in that the X-axis appears to be backwards. Time increases to the right and, indeed, that is the case here. But the bigger numbers are on the left, indicating years before present. This can become somewhat confusing to the casual reader who may think they must watch the curves proceed forward in time from right to left. This, coupled with the wide separation between the curves, makes it difficult to visualize whether changes in air temperature or CO2 came first.
More confusion has been raised through books, publications, and blogs that use this graph to argue that, since “carbon dioxide levels and temperature track identically” as Mr. Hescox put it, CO2 must be the predominant, if not sole, cause of changes in air temperature. The assumption, expressed both implicitly and explicitly, is that changes in CO2 come first and the air temperature responds accordingly – a very strong and persuasive argument, if true. Consider, for example, the publication of a widely-read children’s book that purports to teach youngsters about global warming. In the book, The Down-to-Earth Guide to Global Warming
authors Laurie David and Cambria Gordon discuss the relationship between carbon dioxide in the atmosphere and changes in air temperature. They write:
“Deep down in the Antarctic ice are atmosphere samples from the past, trapped tiny air bubbles. These bubbles, formed when snowflakes fell on the ice, are the key to figuring out two things about climate history: what the temperatures were in the past and which greenhouse gases were present in the atmosphere at that time.
The more carbon dioxide in the atmosphere, the higher the temperature climbed.
The less carbon dioxide in the atmosphere, the lower the temperature fell. You can see this relationship for yourself by looking at the graph on your left, which actually combines measurements from three different places in Antarctica. What makes this graph so amazing is that by connecting rising CO2 to rising temperature, scientists have discovered the link between greenhouse gas pollution and global warming.”
To the reader’s left is a flip-up covering a graph that reads “Lift to see how well CO2 and temperature go together.” When the child lifts the flap, the following graph is revealed:
￼The graph is quite impressive, showing air temperature and carbon dioxide mimicking each other. But if one notes that time proceeds to the left on this graph, carbon dioxide moves first and is subsequently followed by an air temperature response. This is what an astute student is supposed to glean from the graph and indeed what the accompanying text suggests is the case.
But is this correct? In a scientific article by Fischer et al
. (1999), ice core records of atmospheric CO2 were examined using records from Antarctic ice cores dating back to 270,000 years ago. They concluded that “high-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 ± 400 years after the warming
of the last three deglaciations” (emphasis added). They went on to argue that “despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations,” indicating that the link was not as strong as either David and Gordon (2007) or Mr. Hescox had suggested.
In a later article by Monnin et al
. (2001), an ice core from Dome Concordia in Antarctica was examined. Their work includes a graph that clearly shows CO2 concentrations following air temperature by a period of less than 1000 years (see below). On this graph, air temperature is the solid line at the top while solid circles represent CO2; note that as before, time progresses to the left. Thus, this graph is consistent with the results of Fischer et al
. (1999) as the rise in air temperature precedes the rise in CO2 by up to 1000 years at the beginning of the Glacial Terminations I, III, and IV.
Similarly, after evaluating air bubbles from ice cores in the Vostok core in Antarctica, Caillon et al
. (2003) concluded “the sequence of events during Termination III [see figure for Monnin et al
. 2001] suggests that the CO2 increase lagged Antarctic deglacial warming by 800 ± 200 years
”. These three research groups, examining ice core data from Antarctica, came up with essentially the same conclusion – and one that is in diametric opposition to the suggestion in The Down-to-Earth Guide to Global Warming
Furthermore, Soon (2007) has reviewed much of the literature on air temperature-CO2 relationships. He concludes, “there is no quantitative evidence that varying levels of minor greenhouse gases like CO2 and CH4 have accounted for even as much as half of the reconstructed glacial-interglacial temperature changes or, more importantly, for the large variations in global ice volume on both land and sea over the past 650,000 years …changes in solar insolation at climatically sensitive latitudes and zones exceed the global radiative forcings of CO2 and CH4 by severalfold, and that regional responses to solar insolation forcing will decide the primary climatic feedbacks and changes.”
So where does the graph shown by David and Gordon (2007) originate? It appears to be a smoothed representation of one of the two research efforts cited by the figure in Mr. Hescox’s blog. But when you go back to the original article, a completely different picture emerges. Despite claims to the contrary, the article, by Jouzel et al
. (2007), concludes that “our [East Antarctica, Dome C] ice core shows no indication that greenhouse gases have played a key role in such a coupling [with radiative forcing]”. They go on to state, “not only does the obliquity component of the radiative forcing – calculated accounting for both CO2 and CH4 changes – have a small amplitude over the past 650,000 years (~0.5 Wm-2) but it also seems to lag Antarctic and tropical temperature changes.” So their argument is consistent with the results of the previous studies in that changes in CO2 lag air temperature and is diametrically opposite to the claims made by David and Gordon (2007).
The other citation, of Lüthi et al
. (2008), further shows the relationship between air temperature and CO2 for the Antarctic Dome C, Vostok, and Taylor Dome ice cores dating back 800,000 years (see below – note that here, time advances to the left). The figure shown in Mr. Hescox’s blog posting is a cleaner version of this figure. But what is evident in this graph is that CO2 varies with air temperature but it follows air temperature rather than leading it. This, therefore, is consistent with all the previous research cited and is a consistent interpretation of the graph presented by Mr. Hescox.
But now compare this graph closely with that presented by David and Gordon (2007). Although David and Gordon have drawn only the last 650,000 years (from Jouzel et al
. 2007), their curve labeled as “climate temperature” matches the Lüthi et al
. (2008) curve for carbon dioxide! And, not surprisingly, their curve labeled “CO2 Concentration in the Atmosphere” matches with the Lüthi et al
. (2008) curve for air temperature anomaly! Clearly, in their haste to make a convincing argument that air temperature follows changes in CO2, David and Gordon (2007) have reversed the two curves. Thus, we can correct the David and Gordon graph on their page 18 to read:
Now, it is clear from both curves and the science that backs them, that air temperature is the leading variable and that CO2 follows air temperature by about 600 to 800 years.
Alas, convincing illusions die hard. When presented with this information, Dr. Michael Oppenheimer of Princeton University wrote (November 19, 2007):
“I have reviewed the figure on page 18 of The Down-to-Earth Guide to Global Warming. It appears that the labeling of the axes has been reversed. As a result, the curve labeled ‘carbon dioxide concentration’ should be labeled ‘climate temperature’, and vice versa.”
Dr. Oppenheimer thus agrees that the science shows CO2 lags air temperature and the David and Gordon book makes an erroneous claim. But surprisingly and despite the scientific evidence, Oppenheimer went on to argue:
“However, the description of the figure in the accompanying text is accurate, and it fairly represents the current state of scientific knowledge, in terms that would be comprehensible to children 8-years of age or older.”
So despite the error in the graph, it can be concluded “the more carbon dioxide in the atmosphere, the higher the temperature climbed” and “by connecting rising CO2 to rising temperature, scientists have discovered the link between greenhouse gas pollution and global warming”? The average student would be expected to reject such a claim based on the correct representation of the graph and the scientific evidence available to one who only took the time to examine the source. Dr. Oppenheimer is wrong to suggest that we can continue to argue ‘greenhouse gas pollution leads to global warming’ when the facts and the scientific research prove otherwise.
The curve cited by both David and Gordon (2007) and by Mr. Hescox in his blog cannot legitimately be used to argue that CO2 is the predominant cause of air temperature changes. If anything, it shows that CO2 follows air temperature changes by half a millennium or more and that the changes we see in air temperature must have another cause.
But what about the relationship between air temperature and CO2 under current climate conditions? Until now, we have focused largely on century-to-millennial scale changes in CO2 and air temperature but what about the current climate where anthropogenic increases in atmospheric trace gases are possibly leading to increased air temperatures over the next several decades. The traditional presentation is that there is a very strong relationship between air temperature and CO2 with CO2 as the leading variable. Even if CO2 has largely followed air temperature over the past 800,000 years, could it now become the leader in the 21st Century?
To address this question, Soon (2005) compared Arctic-wide surface air temperature anomalies with annual values of CO2 obtained from the climate modeling group at NASA GISS. The Arctic region is particularly important as climatologists widely agree that changes in air temperature are more pronounced in high latitudes due to the ice/snow-albedo feedback (i.e.
, melting ice and snow uncovers a darker underlying land surface that absorbs more solar radiation which increases the air temperature even more), the fact that cold air has very little moisture even when saturated (water vapor has a higher specific heat than dry air), and that it takes more energy to change the temperature of warm air by 1K than cold air (i.e.
, the derivative of the Stefan-Boltzmann radiation law).
The plot below from Soon (2005) shows the graph of yearly CO2 and Arctic-wide surface air temperature anomalies from 1875 to 2000. Changes in atmospheric CO2 fail to explain the warming between 1920 and 1960 and the percent of variance in air temperature explained by CO2 (i.e.
, the square of the correlation) is less than 20%. When smoothed by a decadal filter (to reduce the effects of short-term climate variability), the explained variance increases only to 22%. By contrast, the graph of total solar irradiance (i.e.
, incoming solar energy) and Arctic-wide surface air temperature anomalies versus year (below) shows a much better correspondence with more than 40% of the variance in air temperature explained by total solar irradiance. But when the decadal filter is applied, the explained variance increases to nearly 80%. While correlation certainly does not imply causality, it is very easy to argue on a physical basis that changes in solar irradiance should drive changes in atmospheric surface air temperature. Nevertheless, we can conclude that CO2 is not as good at predicting changes in Arctic-wide surface air temperature as is solar variability.
But finally, let’s put all that aside and focus just on the last half-century (from 1959). A comparison of global data from the Climatic Research Unit of the University of East Anglia (http://www.cru.uea.ac.uk/cru/data/temperature/
) and CO2 data from the Mauna Loa Observatory (ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_annmean_mlo.txt
) yields a fairly impressive plot (below). Certainly, given the rising air temperatures and the rise in CO2 and the understanding that for this time period, changes in CO2 are caused largely by human activity, surely we can make the case that CO2 drives air temperature now. Can’t we?
That is exactly the focus of a recent paper by Humlum et al
. (2013). Their premise was to examine the lags and leads between a number of annually-averaged variables including (1) surface air temperature from the Climatic Research Unit of the University of East Anglia and the Hadley Centre, (2) surface air temperature data from the Goddard Institute for Space Studies, (3) surface air temperature data from the US National Climatic Data Center, (4) sea surface temperature data from the Hadley Centre, (5) lower troposphere air temperature data from the University of Alabama-Huntsville, (6) globally-averaged marine CO2 data, (7) data on anthropogenic releases of CO2 from the Carbon Dioxide Information and Analysis Center, and (8) global warming potential data on volcanic eruptions.
Interestingly, they found that although conventional wisdom suggests that CO2 should lead air temperature, it doesn’t. They concluded that “changes in the amount of atmospheric CO2 always [lag] behind corresponding changes in air temperature.” Overall, the chain of effects proceeds from the oceans to the land surface to the lower troposphere. They go on to conclude “the maximum positive correlation between CO2 and temperature is found for CO2 lagging 11-12 months in relation to global sea surface temperature, 9.5-10 months to global surface air temperature, and about 9 months to global lower troposphere temperature.” Moreover, changes in ocean temperatures are good predictors of the observed changes in atmospheric CO2 while CO2 released from anthropogenic sources are not well correlated with changes in total atmospheric CO2.
Thus, changes in CO2 may be lagging changes in temperature (oceanic, land surface, and lower troposphere) on annual time-scales, consistent with the results from ice cores over longer time periods. Sampling differences are likely the cause for the different lag rates between the Humlum et al
. (2013) study and the ice core records (yearly versus multi-centennial lags) but a clear picture is beginning to emerge: CO2 does not drive changes in global air temperature, it follows them.
In short: Mr. Hescox puts the cart before the horse.
David R. Legates, Ph.D., is Professor of Climatology at the University of Delaware, former Delaware State Climatologist, and a Senior Fellow of The Cornwall Alliance for the Stewardship of Creation.
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