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.