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Is the long-term trend in CO2 caused by warming of the oceans?

What the science says...

Hocker is claiming that his model shows that the long-term upward trend in CO2 is explained by temperature, when his methods actually removed the long-term trend. In today's world, the greatly increased partial pressure of CO2 from fossil fuel emissions causes a flux of CO2 from the atmosphere to the oceans. Observations show the oceans are a "sink" rather than a source of CO2 in the atmosphere

Climate Myth...

Warming causes CO2 rise

'Differentiating the CO2 measurements over the last thirty years produces a pattern that matches the temperature anomaly measured by satellites in extreme detail.    That this correlation includes El Niño years, and shows that the temperature rise is causing the rise in CO2, rather than the other way around.' (Lon Hocker)

A new article by Lon Hocker at the website "Watts Up With That?" examines the relationship between global temperature and CO2 over the last three decades.  The article's conclusion is concisely summarised in its title:  The temperature rise has caused the CO2 increase, not the other way around. This conclusion would be rather startling if it were true, since the scientific consensus is that CO2 is currently acting as a "forcing" that warms the climate.  How does Hocker reach this conclusion, and is it reasonable?

The data used in Hocker's analysis are monthly atmospheric CO2 measurements at Mauna Loa (obtained from NOAA) and satellite-measured temperature data for the lower troposphere (from UAH, apparently using a subset of the global data over the oceans only).  The temperature data are recorded as anomalies, or differences between the actual temperature and the long-term mean. 

The Mauna Loa CO2 data show a long-term increase in atmospheric CO2 concentration.  The Mauna Loa data are the longest high-quality CO2 record, dating back to 1958.  While one might think that the side of a volcano might not be the best place to measure CO2, in fact the procedures used at Mauna Loa compensate for any contamination by volcanic gases.  As shown in Figure 1, since 1980 we have had global CO2 data from a network of stations, and these data show that the Mauna Loa trend is very representative of the global trend in CO2.


 Figure 1: Global atmospheric CO2 (NOAA) versus Mauna Loa CO2 (NOAA).

Hocker begins his analysis by calculating the first derivative of the CO2 data.  He does this using the difference between the CO2 measurement six months after a given month and the measurement six months before.  (Calculating this difference over a 12-month interval effectively removes the seasonal variation in atmospheric CO2 concentration.)

At this point, alert readers may begin to glimpse the flaw in Hocker's methods.  However, let's follow Hocker through to his conclusion. 

He derives a simple model to estimate the temperature anomaly as a function of the derivative of CO2 concentration:

 Temperature Anomaly = (CO2[n+6] – CO2[n-6])/(12*0.22) – 0.58

 Hocker's Figure 2 shows a comparison of the observed and modeled global ocean temperature anomaly:

Figure 2.  Comparison of global lower troposphere temperature anomaly over the oceans (blue line) to a model based on the first derivative of atmospheric CO2 concentration at Mauna Loa (red line).  From Hocker 2010.

Looking at this figure, Hocker notes "There is a strong correlation between the measured anomaly and the Derivative model.  It shows the strong El Niño of 1997-1998 very clearly, and also shows the other El Niño events during the plotted time period about as well as the satellite data does."  He does not quantify the correlation between the two, but the squared correlation coefficient (r2) for the two time series is 0.36.

Let's pause here to consider the actual effect of Hocker's methods to this point.  Taking the first derivative of the CO2 data removes the long-term trend in CO2 concentration, and shows the effect of short-term variability around that trend.  Thus, it would be appropriate to conclude from this that short-term fluctuations in the overall upward CO2 trend are moderately well correlated with temperatures in the lower troposphere over oceans.

What Hocker actually concludes is quite different:  "Using two well accepted data sets, a simple model can be used to show that the rise in CO2 is a result of the temperature anomaly, not the other way around.  This is the exact opposite of the IPCC model that claims that rising CO2 causes the temperature anomaly." 

In other words, Hocker is claiming that his model shows that the long-term upward trend in CO2 is explained by temperature, when his methods actually removed the long-term trend.

This is where the previously-mentioned alert readers will be nodding their heads and saying "Yes!  We knew it!"  The error that Hocker makes - taking the derivative of a time series to remove its long-term trend, then correlating a second data set with this derivative, and finally claiming the second data set explains the long-term trend - is exactly the same error that was recently discovered in a prominent "skeptical" paper by McLean 2009.  McLean correlated an index of the El Niño/Southern Oscillation with the first derivative of temperature, while Hocker correlates temperature with the first derivative of CO2 concentration.  Perhaps if Hocker were an avid reader of Skeptical Science, he would have been familiar with this error in McLean's analysis and would have avoided repeating it!

What else can be said about this subject?  Well, it is true that the solubility of CO2 in seawater is a function of temperature, and all else being equal, as the ocean warms it will give off CO2 to the atmosphere.  And in fact this is the mechanism by which a CO2 feedback amplified the temperature swings during the Pleistocene glacial/interglacial cycles.  But in today's world, the greatly increased partial pressure of CO2 from fossil fuel emissions causes a flux of CO2 from the atmosphere to the oceans.  This is known from decades of oceanographic surveys that show the oceans are a "sink" rather than a source of CO2 in the atmosphere (Takahashi 2009, Sabine 2004).

It's also interesting to note that climate scientists have known for at least three decades that short-term fluctuations in temperature (e.g., those associated with the ENSO cycle) are correlated with short-term fluctuations in the rate of increase of atmospheric CO2 (Bacastow and Keeling 1981).  Section of the IPCC AR4 Working Group 1 report discusses this in some detail.

Intermediate rebuttal written by ned

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 13 July 2015 by pattimer. View Archives

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Comments 26 to 33 out of 33:

  1. johnd, please re-look at the data from Heidelberg you quoted. All of the divergence from natural levels is on the positive side of the curve. This is due to the highly industrialized area. It is not random. It is an excursion above normal levels. In your mind draw a curve at the lowest points in the data, you will see that that curve is very similar to non-industrialize areas. Your large sinks are imaginary. Maybe I was wrong in saying "daily" excursions to >410 ppm but they are, non the less, of very short duration and are due to local production of large quantities of CO2 which is then diluted out by normal air, not absorbed by your "large" sinks. Please do not confuse the issue.
  2. I couldn't bring myself to read the comments. If one wants to argue that recent CO2 levels depend on temperature, one would first attempt to regress CO2 against temperature, rather than temperature against difference in CO2. However ill-conceived, the formula "Temperature Anomaly = (CO2[n+6] – CO2[n-6])/(12*0.22) – 0.58" asserts that temp is determined by CO2, together with some 12 initial conditions. In this formula, CO2 is the independent variable while temp is the dependent variable. How can this model possibly provide evidence that CO2 depends upon temp? Equally as bizarre, the model asserts the temp depends on the CO2 6 months in the future! This model violates causality. The given model also assets that when CO2 levels are constant the temp anomaly decreases by .58 every month. Is that reasonable? In any 15 year period of constant CO2, the anomaly would would decrease by over 100. In this model, the CO2 is obliged to increase, each month, just to maintain a constant temp.
  3. @Ned I think what you need to show is that if his conclusion can be true in short term why can it not be true in long term. (I am not very skeptical about anthropogenic climate change, I just think it's a good debate to have from a scientific perspective.)
  4. This Heidelberg stuff is the same silly argument that Beck used in disputing Callendar. When Callendar started collecting CO2 readings back in the 1930s and 40s he found alot which showed a slow consistent upward curve and then a bunch of outliers... all on the high side of the curve and all near major industrial centers. He reasoned that the outliers were due to local emissions which hadn't dispersed throughout the atmosphere yet. Beck argued that they were instead indicative of global changes (even though contradicted by other readings) and should be factored in to show atmospheric CO2 levels roller coasting up and down by huge margins. Of course, Keeling solved this problem in the late 1950s by taking readings at a location (Mana Loa) far removed from any local emissions... and getting results which proved that Callendar's steady curve conclusion was correct. This has subsequently been duplicated at dozens of other isolated sites around the world showing the same results. Arguing that higher readings in industrial areas are thus indicative of anything except the source of the increasing atmospheric CO2 levels is thus clearly specious. You might as well argue that temperature readings taken inside ovens should be factored into global trends... when the oven is off they display massive cooling! Clearly our global warming fears are misplaced!
  5. meznaric If the natural environment were a net source of CO2 into the atmosphere, then atmospheric CO2 levels would be rising faster than the rate of anthropogenic emissions as both the natural environment and mankind were net sources. However this is not the case, atmospheric levels are only rising at a rate about half that of anthropogenic emissions, which shows that the natural environment is a net carbon sink and takes more carbon out of the atmosphere each year than it puts in. This is one of the relatively few things we know about the climate where we can be certain, beyond reasonable doubt.
  6. @Dikran, OK thanks for that. Having read much of this website, I am quite convinced that what you write is true. But what puzzles me is how do you explain the data in that graph (second from the top)? It's strange that even the El Nino would be in such good agreement.
  7. meznaric, El Nino causes big changes in the terrestrial biosphere around the pacific, so it isn't surprising that ENSO affects CO2. However, the sneaky thing in that plot is that it shows that ENSO changes the rate of change of CO2, not the level of CO2. If you look at a plot of CO2 itself, from Mauna Loa, you see a big exponential increase (due to anthropogenic emissions), an annual oscillation caused by the growth and die back of terrestrial plants (becuase there is more land in the north hemisphere than the south), the variations in the rate of change in CO2 due to ENSO result in the even smaller inter-annual variations on top of that, which are barely perceptable. As the text explains, the differencing operation obliterates the linear trend in the data, which is essentially most of the increase due to anthropogenic emissions.
  8. @Dikran, ah I think I see where you're going with this. Thanks for taking the time to explain this.

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