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Temperature Reconstructions Based on Plant-Climate Interactions Are Inaccurate If Atmospheric CO2 Varied Over the Period of Reconstruction
Volume 3, Number 16: 2 August 2000

In their recent review of plant-based methods for reconstructing earth's temperature history, Cowling and Sykes (1999) note that many tried-and-(supposedly)-true methods of palaeoclimate reconstruction "are built upon the assumption that plant-climate interactions remain the same through time or that these interactions are independent of changes in atmospheric CO2."  This assumption was challenged nearly a dozen years ago by Idso (1989) and a few years later by Polley et al. (1993).  Since that time, a number of other scientists have come to the same conclusion they espoused; and there is now a sufficient volume of published research on the topic to conduct a review of it, which is precisely what Cowling and Sykes did.  And their conclusion?  "A growing number of physiological and palaeoecological studies indicate that plant-climate interactions are likely not [our italics] the same through time because of sensitivity to atmospheric CO2."

So why aren't they the same?  Cowling and Sykes describe three major reasons.  First, "C3-plant physiological research shows that the processes that determine growth optima in plants (photosynthesis, mitochondrial respiration, photorespiration) are all highly CO2-dependent."  Second, "the ratio of carbon assimilation per unit transpiration (called water-use efficiency) is sensitive to changes in atmospheric CO2."  Third, "leaf gas-exchange experiments indicate that the response of plants to carbon-depleting environmental stresses are strengthened under low CO2 relative to today."

All of these phenomena combine to produce dramatic increases in plant growth and water use efficiency with increases in the air's CO2 content; and such increases in vegetative vigor have - to the date of this writing - been interpreted, almost exclusively, not in terms of atmospheric CO2 variations, but in terms of changes in air temperature and/or precipitation.  Clearly, therefore, all such plant-based climate reconstructions that have been made for a period of time over which the air's CO2 content has experienced a significant change must be wrong, unless they account for the growth-enhancing effects of the CO2 increase, which none have yet done, except for the studies of LaMarche et al. (1984) and Graybill and Idso (1993).

Why do we make such a point about this fact?  We do so because there are people using long-term tree-ring chronologies - specifically, Mann et al. (1998, 1999) - to create a climatic history of the earth that exhibits dramatic late-20th century warming that never occurred!  (For more on this subject, see our Editorial: There Has Been No Global Warming for the Past 70 Years.)  As Briffa (2000) explains, the recent high growth rates of the trees in these chronologies "provide major pieces of evidence being used to assemble a case for anomalous global warming, interpreted by many as evidence of anthropogenic activity."  Yet, as he further notes, "the empirically derived regression equations upon which our reconstructions are based may be compromised if the balance between photosynthesis and respiration is changed" by anything other than air temperature.  And as Cowling and Sykes have now concluded as a consequence of their literature review, "implicit in the assumption that plant-climate relationships remain the same through time is the notion that temperature-plant interactions are independent of changes in atmospheric CO2, which is not [our italics again] supported by physiological data."

Unfortunately - and, as shown here, in a manner that is totally unwarranted - the flawed studies of Mann et al. are fast becoming the centerpiece (Crowley, 2000; Mann, 2000) of the IPCC's misguided, but seemingly intentional, effort to rewrite earth's climatic history in an attempt to prod national governments to adopt Kyoto-type measures to combat imaginary global warming (see our Editorials of 15 June, 1 July and 15 July 2000).  It is of utmost importance, therefore, that the facts of this case be widely publicized, so that legislative bodies the world over are not stampeded into taking action on important matters of international energy policy on the basis of manifestly inaccurate global temperature reconstructions of the past millennium.

Dr. Craig D. Idso
Dr. Keith E. Idso
Vice President

Briffa, K.R.  2000.  Annual climate variability in the Holocene: Interpreting the message of ancient trees.  Quaternary Science Reviews 19: 87-105.

Cowling, S.A. and Sykes, M.T.  1999.  Physiological significance of low atmospheric CO2 for plant-climate interactions.  Quaternary Research 52: 237-242.

Crowley, T.J.  2000.  Causes of climate change over the past 1000 years. Science 289: 270-276.

Graybill, D.A. and Idso, S.B.  1993.  Detecting the aerial fertilization effect of atmospheric CO2 enrichment in tree-ring chronologies.  Global Biogeochemical Cycles 7: 81-95.

Idso, S.B.  1989.  A problem for palaeoclimatology?  Quaternary Research 31: 433-434.

LaMarche Jr., V.C., Graybill, D.A., Fritts, H.C. and Rose, M.R.  1984.  Increasing atmospheric carbon dioxide: Tree ring evidence for growth enhancement in natural vegetation.  Science 225: 1019-1021.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1998.  Global scale temperature patterns and climate forcing over the past six centuries.  Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K.  1999.  Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties and limitations.  Geophysical Research Letters 26: 759-762.

Polley, H.W., Johnson, H.B., Marino, B.D. and Mayeux, H.S.  1993.  Increases in C3 plant water-use efficiency and biomass over glacial to present CO2 concentrations.  Nature 361: 61-64.