Mann et al. (1999) present a tree-ring-and-ice-core-derived proxy reconstruction of Northern Hemispheric temperatures over the past thousand years that shows relatively warm temperatures early in the millennium but a prolonged cooling trend following the 14th century. This temperature decline reverses itself when the planet begins to recover from the Little Ice Age, however; and the subsequent warming prompts the authors to declare the last decade of the past millennium "the warmest for the Northern Hemisphere this millennium."
Because the air's CO2 content has also risen significantly over the past couple of centuries, this temperature history has led many people - especially certain journalists and politicians - to declare that the ongoing rise in atmospheric CO2 concentration has been the cause of the concomitant rise in air temperature. However, as we have noted previously (see our editorial CO2 and Temperature: The Great Geophysical Waltz), (1) correlation does not prove causation, (2) cause must precede effect, and (3) when attempting to evaluate claims of causal relationships between different parameters, it is important to have as much data as possible in order to weed out spurious correlations. Hence, when we look at data in addition to that presented by Mann et al., such as what has been provided by the scientific studies reviewed in this section of our web site, we find that the increase in atmospheric CO2 concentration over the past century or so is very unlikely to have been the cause of the warming experienced near the end of the past millennium.
Consider, for example, the study of Fischer et al. (1999), who examined trends of atmospheric CO2 and air temperature derived from Antarctic ice core data that extended back in time a quarter of a million years. Over this extended period, the three most dramatic warming events experienced on earth were those associated with the terminations of the last three ice ages; and for each of these climatic transitions, earth's air temperature rose well in advance of any increase in atmospheric CO2. In fact, the air's CO2 content did not begin to rise until 400 to 1,000 years after the planet began to warm. Such findings have been corroborated by Mudelsee (2001), who examined the leads/lags of atmospheric CO2 concentration and air temperature over an even longer time period, finding that variations in atmospheric CO2 concentration lagged behind variations in air temperature by 1,300 to 5,000 years over the past 420,000 years.
Other studies have also documented a fundamental violation of the cause-must-precede-effect principle in the climate alarmist hypothesis of CO2-induced global warming. From a high-resolution temperature and atmospheric CO2 record spanning the period 60 to 20 thousand years ago, Indermuhle et al. (2000) examined the CO2/temperature relationship at four distinct periods when temperatures rose by approximately 2°C and CO2 by about 20 ppm. One type of statistical test performed on the data suggested that the shifts in the air's CO2 content during these intervals lagged those in air temperature by approximately 900 years; while a second statistical test yielded a mean lag time of 1200 years.
Focusing on the transition from glacial to interglacial conditions during the period between 22,000 and 9,000 years ago, Monnin et al. (2001) found that the start of the CO2 increase lagged the start of the temperature increase by 800 years. An additional analysis of this most recent glacial/interglacial transition by Yokoyama et al. (2000), which has also been discussed by Clark and Mix (2000), revealed that a rapid rise in sea level, caused by the melting of land-based ice that began approximately 19,000 years ago, preceded the post-glacial rise in atmospheric CO2 concentration by about 3,000 years. Then, when the CO2 finally began to rise, it had to race to make up the difference; but it still took it a couple more thousand years to catch up with the sea level rise.
Lastly, Petit et al. (1999) have shown that during all of the glacial inceptions of the past half million years, temperature always dropped before the air's CO2 concentration declined; and their data indicate, in their own words, that "the CO2 decrease lags the temperature decrease by several thousand years." Clearly, therefore, changes in the air's CO2 content cannot be responsible for these major climate changes, for it would be a strange cause indeed that followed its effect!
Somewhat smaller, but still large, environmental changes during the last glacial period also demonstrate the weak coupling of CO2 and air temperature. During certain climatic transitions characterized by rapid warmings of several degrees Centigrade, which were followed by slower coolings that returned the climate to full glacial conditions, Staufer et al. (1998) observed the atmospheric CO2 concentration derived from ice core records to typically vary by less than 10 ppm. And here, too, they considered these environmental perturbations to have been caused by changes in climate, rather than by changes in CO2.
Other studies periodically demonstrate a complete uncoupling of CO2 and temperature (Cheddadi et al., 1998; Gagan et al., 1998; Raymo et al., 1998). Steig (1999) for example, demonstrated that between 7,000 and 5,000 years ago, atmospheric CO2 concentrations increased by just over 10 ppm at a time when temperatures in both hemispheres cooled. Such findings were echoed by Indermuhle et al. (1999), who demonstrated that after the termination of the last great ice age, the CO2 content of the air gradually rose by approximately 25 ppm in almost linear fashion between 8,200 and 1,200 years ago, also during a period of time that saw a slow but steady decline in mean global air temperature, which results are obviously just the opposite of what would be expected if changes in atmospheric CO2 drove climate change in the way claimed by the popular CO2-greenhouse effect theory.
Going back even further in time, Pagani et al. (1999), working with sediment cores from three deep-sea drilling sites, found the air's CO2 concentration to be uniformly low (180 to 290 ppm) throughout the early to late Miocene (25 to 9 million years ago), at a time when deep-water and high-latitude surface water temperatures were as much as 6° C warmer than they are today, leading them to state that what they found "appears in conflict with greenhouse theories of climate change." Furthermore, they noted that the air's CO2 concentration seemed to rise following the expansion of the East Antarctic Ice Sheet, which is also in conflict with greenhouse theories of climate change.
With respect to the middle Eocene climate of 43 million years ago, Pearson and Palmer (1999) report the planet then may well have been as much as 5°C warmer than today; but the mean CO2 concentration of the atmosphere, as determined by pH data inferred from boron isotope composition in planktonic foraminifera, was only on the order of 385 ppm.
Much the same thing was found by these authors one year later in an analysis of atmospheric CO2 and temperature over the past 60 million years (Pearson and Palmer, 2000). Starting 60 million years before present (BP), for example, the authors note the atmosphere's CO2 concentration is approximately 3600 ppm and the oxygen isotope ratio is about 0.3 per mil. Thirteen million years later, however, the air's CO2 concentration dropped all the way down to 500 ppm; but the oxygen isotope ratio dropped (implying a rise in temperature) to zero, which is, of course, just the opposite of what one would expect were CO2 the all-important driver of climate change that the climate alarmist make it out to be.
Next comes a large spike in the air's CO2 content, all the way up to a value of 2400 ppm. And what does the oxygen isotope ratio do? It rises slightly (implying temperature falls slightly) to about 0.4 per mil, which is again just the opposite of what one would expect under the CO2-induced global warming hypothesis. After the spike in CO2, of course, the air's CO2 concentration drops dramatically, declining to a minimum value of close to what it is today. And the oxygen isotope ratio? It barely changes at all, defying once again the common assumption of the CO2-induced global warming hypothesis. Between this point and the break in the record at 40 million years BP, the air's CO2 concentration rises again to approximately 1000 ppm; and - need we say? - the oxygen isotope ratio rises slightly (implying a slight cooling) to 0.6 per mil. And once again, well, you get the picture: the common assumption of the CO2-induced global warming hypothesis, i.e., that changes in atmospheric CO2 drive changes in air temperature, fails miserably.
Picking up the record at 24 million years BP, there are but relatively tiny variations in atmospheric CO2 concentration up to the present; but, of course, there are large variations in oxygen isotope values, both up and down, again in clear contradiction of the CO2-induced global warming hypothesis. The most interesting of these last oxygen isotope changes is the dramatic increase (implying a dramatic cooling) over the most recent two million years, when, of course, the air's CO2 concentration has actually risen slightly.
Considered in their entirety, these several results present a truly chaotic picture with respect to any possible effect that variations in atmospheric CO2 concentration may have on global temperature. Clearly, atmospheric CO2 is not the all-important driver of global climate change the climate alarmists make it out to be.
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