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The Importance of Caveats in Scientific Studies
Volume 8, Number 38: 21 September 2005

Papers published in journals such as Science and Nature typically attract lots of attention, especially when they deal with high-profile subjects such as global warming, which some have described as a threat worse than nuclear warfare or global terrorism.  And when a study describes a phenomenon that could potentially exacerbate that threat, it behooves its authors and the editors of the journal in which it is published to be especially careful in the way they describe what was found and what its implications may be.

A case in point concerns the report of Heath et al. (2005), who studied soil sequestration of root-derived carbon from seedlings of six European tree species and found it to decline in response to atmospheric CO2 enrichment.  This finding led them to write that "should similar processes operate in forest ecosystems, the size of the annual terrestrial carbon sink may be substantially reduced, resulting in a positive feedback on the rate of increase in atmospheric carbon dioxide concentration."  The same ultimate outcome was parroted by the journal's editors, who wrote that the new findings "raise the possibility that the future rise in atmospheric CO2 concentrations could be higher than expected," which is exactly the type of scenario climate-alarmists love to promote.

How reasonable are these speculations?  To answer this question, it is important to determine how closely the experimental setting of Heath et al.'s study mimicked that of real-world forests.  For example, was their study a FACE experiment, such as that being conducted in the Duke University Forest, where multiple 30-m-diameter plots of initially 13-year-old loblolly pine trees have been exposed continuously to ambient and elevated atmospheric CO2 concentrations each growing season since August of 1996?  Or, was it an open-top chamber study, such as the Phoenix, Arizona sour orange tree experiment, where trees were grown from the sapling stage to mature adults over a period of 17 years of continuous atmospheric CO2 enrichment?  No, it was neither of the above; the Heath et al. experiment was but a 15-month study that was conducted in small greenhouses, where seedlings were grown in vertical sections of 16-cm-diameter polyethylene tubes supplied with but 10 liters of soil.

To their credit, Heath et al. readily acknowledge the many deficiencies of their study.  They state, for example, that "young trees, grown in mesocosms in a semicontrolled environment and protected from major herbivores, may respond differently from mature trees growing in a natural forest."  They also state that their experiment "ran for only two growing seasons and the input of leaf litter to the soil was excluded [our italics]."  With respect to these latter two points, they additionally state that "the possibility that longer term increased inputs of leaf litter under elevated CO2 could counteract the effect on the sequestration of root-derived carbon cannot be ruled out."  Furthermore, they say that "although soil microbial respiration increased under elevated CO2, the effect of this on the decomposition of native soil carbon is not known."

That these deficiencies likely preclude the discovery of the truth sought by Heath et al. - within the context of their experiment, at least - is revealed by their acknowledgment that "in contrast to our experiment, CO2 enrichment caused an increase [our italics] in soil carbon sequestration beneath Betula seedlings over the course of one growing season (Ineson et al., 1996)," and that "free-air CO2 enrichment (FACE) also caused an increase [our italics] in the sequestration of new carbon in C4 soil cores transplanted into former agricultural ground beneath 2- to 3-year-old Populus saplings (Hoosbeek et al., 2004)."  Consequently, and after reviewing the results of still other pertinent experiments, they ultimately concluded "there is insufficient evidence to predict with certainty whether plant responses to elevated CO2 will result in increased or decreased sequestration of new carbon in the soils of forest ecosystems."

In light of these many astounding observations, which are made in Heath et al.'s own paper, and which indicate there is no compelling reason to believe their results bear any resemblance whatsoever to what will actually occur in the real world of nature as the air's CO2 content continues to climb, one wonders why their paper was ever accepted for publication in so prestigious a journal as Science.  Could it be that it was not for scientific reasons at all, but for the support it may have been hoped it might provide for the political views of its editor, who strongly supports Kyoto-type regulations of anthropogenic CO2 emissions?  We'll likely never know, but the whole episode certainly reeks of strangeness.

Sherwood, Keith and Craig Idso

Heath, J., Ayres, E., Possell, M., Bardgett, R.D., Black, H.I.J., Grant, H., Ineson, P. and Kerstiens, G.  2005.  Rising atmospheric CO2 reduces sequestration of root-derived soil carbon.  Science 309: 1711-1713.

Hoosbeek, M.R., Lukac, M., van Dam, D., Godbold, D.L., Velthorst, E.J., Biondi, F.A., Peressotti, A., Cotrufo, M.F., de Angelis, P. and Scarascia-Mugnozza, G.  2004.  More new carbon in the mineral soil of a poplar plantation under Free Air Carbon Enrichment (POPFACE): Cause of increased priming effect?  Global Biogeochemical Cycles 18: GB1040.

Ineson, P., Cotrufo, M.F., Bol, R., Harkness, D.D. and Blum, H.  1996.  Quantification of soil carbon inputs under elevated CO2: C3 plants in a C4 soil.  Plant and Soil 187: 345.