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In the very compelling graph from "An Inconvenient Truth" shown below in figure 1, Al Gore shows the close correlation between CO2 and temperature. The implication is clear: the temperature increase is controlled by the CO2 level. His simple model is illustrated in the block diagram in figure 2, but one is left wondering what controlled the level of CO2 before humans started burning fossil fuels.
Figure 1. Al Gore showing temperature and CO2. The present is on the right near his head and the past is on the left.
Figure 2. Gore's simple model is: CO2 controls temperature. Nothing more need be said. But what controlled the CO2 before humans burned fossil fuels?
Gore is an advocate for a certain point of view, not an objective scientist, so perhaps he can be forgiven for leaving out a fundamental point concerning this relationship. That point is illustrated below in figure 3, which shows part of page 431 from the journal Nature, 3 June, 1999. The article is "Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica" by J. R. Petit, et. al., (scientists from France, Russia, and the United States). The graph in figure 3 shows the same data that Gore is standing in front of in figure 1, plus several other things that Gore doesn't show. Please note that Gore's graph shows the present at the right and the Petit paper shows the present at the left. On Petit's graph CO2 level is circled on top, temperature is circled in the middle of the graph, and the insolation at 65 degrees latitude is circled at the bottom.
Figure 3. Graph from Petit paper showing CO2 and temperature (same as Gore is showing in figure 1) and insolation (which Gore apparently forgot to show). Note that the time direction is reverse between Gore's graph and Petit's graph.
The most important thing that Gore does not show is the changing insolation pattern. Insolation is the amount of light from the sun that shines on a particular place. It is important to understand that neither the temperature nor CO2 level can influence the insolation. According to Andre Berger, in the Journal of Atmospheric Sciences, the source for Petit's insolation data, the insolation is primarily governed by "the eccentricity, the longitude of the perihelion, the precessional parameter and the obliquity" of the Earth's orbit. If there is any correlation between the insolation and the temperature, or the insolation and the CO2 level, then the insolation must be driving the correlation, not the temperature or CO2.
Figure 4, below shows my digitized version of the temperature and CO2 level over the last 150,000 years from the Petit (1999) paper. This time period covers the current interglacial (warm)period, the previous ice age, and the interglacial prior to the ice age. Compare the data in figure 3 with Gore's data in figure 1. They are, in fact, the same (although I show the unsmoothed and smoothed temperature, while Gore shows only his smoothed temperature). Figures 5 through 7 show the temperature, CO2 level, and relative ice volume each compared with the insolation for the last 150,000 years.
Figure 4. Digitized version of data from figure 3 or Petit, et. al. (1999). I have included the unsmoothed version of temperature data overlaid by a smoothed version. Gore shows only a smoothed version. See text below for explanation of relationship between deuterium isotope ratio and temperature.
Figure 5. Temperature at Vostok, Antarctica and insolation at 65 degrees north over the last glacial - interglacial cycle. Insolation, the magnitude of solar radiation, shown here as a deviation from an average. Both sets of data are digitized from figure 3 of Petit's (1999) paper.
Figure 6. CO2 at Vostok, Antarctica and insolation at 65 degrees north over the last glacial - interglacial cycle. Both sets of data are digitized from figure 3 of Petit's(1999) paper.
Figure 7. Ice Volume and insolation at 65 degrees north over the last glacial-interglacial cycle. Ice volume is digitized from figure 2 of Petit's(1999) paper.
Early in Gore's movie and book he relates an anecdote about his sixth grade classmate who, upon seeing a map of the Earth, wondered whether or not South America and Africa once fit together. The teacher responded "Of course not! That's the most ridiculous thing I've ever heard!" Gore says "That sixth-grade teacher had an assumption in his mind that he didn't bother to question: Continents are so big, obviously they don't move." The moral of the story being, of course, that the CO2 and temperature graphs do fit together and an unprejudiced child can see it. I wonder what Gore's classmate would say if he saw the two side by side graphs of figure 8, below.
Figure 8. Would Gore's sixth grade classmate only see the relationship between CO2 and temperature in the graph on the left? Or would he also see the relationship between insolation and temperature in the graph on the right? Was Gore talking about himself when he quoted Mark Twain: "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."
It is clear that in the relationship between CO2, temperature, and insolation only insolation can be the primary driver. According to Andre Berger, in the Journal of the Atmospheric Sciences, Insolation is controlled by the "eccentricity, the longitude of the perihelion, the processional parameter and the obliquity" of the Earth's orbit.
Hubertus Fischer (1999)(Scripps Institution of Oceanography) wrote about Ice core records of Atmospheric CO2 Around the Last Three Glacial Terminations, in the Journal Science and concluded "High resolution records from Antarctic ice cores sow that carbon dioxide concentrations increased... 600 ± 400 years after the warming..." That is, as the last three ice ages ended and temperatures started rising, the CO2 lagged behind the temperature rise, indicating that CO2 was not the primary driver of temperature rise.
The timing between CO2 and temperature rise for climate transitions was also studied by Manfred Mudelsee of the Institute of Meteorology at the University of Leipzig. He reported his findings in the Quaternary Science Reviews in 2001. He used a "lagged, generalized least-squares regression" technique to conclude that the Vostok ice records show "CO2 variations lag behind atmospheric temperature changes in the Southern Hemisphere by 1.3 ± 1.0 ka."
More recently Lowell Stott (2007) from the department of Earth Sciences at the University of Southern California wrote about the end of the last ice age in the Journal Science. He points out that temperature led "the rise of in atmospheric CO2 and tropical surface ocean warming by ~1000 years." He explains the following sequence of events: 1. "The trigger for the initial deglacial warming around Antarctica was the change in solar insolation over the Southern Ocean during austral spring that influenced the retreat of sea ice." 2. "Retreating sea-ice would have led to enhanced Ekman transport in the Southern Ocean and decreased stratification due to stronger air-sea fluxes. 3. "These forcings promoted enhanced ventilation of the deep sea and subsequent rise in atmospheric CO2."
My interpretation of the papers by Fischer, Mudelsee, and Stott, as well as the correlation between insolaton, temperature and CO2 suggest that Gore's simple model is better replaced by the more realistic model shown in figure 9, below.
Figure 9. This model is more realistic than Gore's. It does not start with a mystery about what controlled CO2 levels before humans started burning fossil fuels. Insolation is the primary controller of temperature. Temperature controls CO2 with a small feedback.
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