First, there is the growing-season temperature history of the entire northern boreal forest region, which has been published most recently in the review of Briffa (2000) and is referred to by him as "the best overall indicator to date of long-term temperature changes over the higher northern land areas." Derived from a large number of tree-ring density chronologies obtained from some 400 sites in the western United States, Canada, Europe, Fennoscandia and northern Siberia, this temperature record shows a dramatic departure from the instrumental temperature record over the last 70 years, with the instrumental record depicting unprecedented 20th century warming, but with the tree-ring record showing nothing of the sort. And the reason for the discrepancy? In the words of Briffa, "the reason is not known." We, however, believe that the reason should be obvious: the instrumental temperature record is simply wrong.
Second, there is the somewhat contradictory story told by a number of temperature reconstructions derived from tree-ring width chronologies. As Briffa (2000) recounts it, tree-growth, as represented in various standardized tree-ring chronologies in various parts of the world, often seems anomalous in the 20th century as compared to earlier centuries. "This widespread anomaly is extremely important, for he notes that "the recent high growth rates . . . provide major pieces of evidence being used to assemble a case for anomalous global warming, interpreted by many as evidence of anthropogenic activity," specifically mentioning Mann et al. (1998, 1999) in this regard. But as Briffa further notes, the empirically derived regression equations upon which the temperature reconstructions are based may be compromised if the growth rates of earth's trees have been substantially enhanced over the past century or so by some other global environmental influence that has increasingly manifested itself over the same time period.
What might this influence - if it exists - be? Briffa cites a number of possibilities, including the historical rise in the air's CO2 content over this period and a number of plant physiological processes that become increasingly more efficient in response to this phenomenon; and he explains how this influence could act in opposition to the declining tree-ring density phenomenon described in the preceding paragraph. Indeed, LaMarche et al. (1984) and Graybill and Idso (1993) demonstrated several years ago that the historical rise in the air's CO2 content could readily explain the anomalous 20th century growth spurt in tree-ring width expansion; and Briffa states that "widespread evidence is accumulating of 'enhanced' productivity (ring-width, basal area and wood mass) in the 19th and 20th centuries, similar to positive growth trends observed in earlier studies," that is, in the studies of LaMarche et al. and Graybill and Idso.
It's essentially a no-brainer. Enhanced tree growth induced by the historical rise in the air's CO2 content - possibly augmented by enhanced nitrogen deposition (Idso, 1995) - has been increasing the growth rates of trees all around the world for over a century or more (see, for example, our editorials of 15 April 1999 and 1 April 2000: Atmospheric CO2 Enrichment: Boon or Bane of the Biosphere? and The Future of Forests). Furthermore, this growth enhancement has been accelerating over time (Phillips and Gentry, 1994); and it is this ever-intensifying biological phenomenon that some are using to bolster their claim that the climate is warming at an ever-increasing rate. Nothing, however, could be further from the truth. For the past two decades at least, and possibly for the past seven decades, earth's true surface air temperature has likely experienced no net change.
This latter conclusion may sound incredulous to some; but it follows directly from the likely fact that there has been no net warming from 1979 to 2000, as we indicated in our editorial of 15 June 2000, plus the fact that even the contaminated surface air temperature records depict no warming (they actually show a cooling!) from 1930 to 1979, as can be verified by visiting the World Temperatures section of our website and computing the surface air temperature trend from 1930 to 1979 from both the Global Historical Climatology Network and the Jones et al. data bases.
In view of the extreme likelihood that there has thus been no net warming of the planet over the past 70 years, during which time the vast majority of all anthropogenically-produced CO2 has been emitted to the atmosphere, we conclude that since there should have been a sizeable CO2-induced increase in atmospheric radiative forcing over this period, there must have been a suite of compensatory negative feedbacks that totally overwhelmed the standard "greenhouse" impetus for warming (see our Position Paper on Carbon Dioxide and Global Warming: Where We Stand on the Issue). Hence, there would appear to be absolutely no foundation in factual data of any sort for supposing that any further man-induced increases in the air's CO2 content would warm the planet either.
Dr. Craig D. Idso
President Dr. Keith E. Idso
Vice President
References
Briffa, K.R. 2000. Annual climate variability in the Holocene: Interpreting the message of ancient trees. Quaternary Science Reviews 19: 87-105.
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. 1995. CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution. Third Annual Kuehnast Lecture. Department of Soil, Water and Climate, University of Minnesota, St. Paul, MN.
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.
Phillips, O.L. and Gentry, A.H. 1994. Increasing turnover through time in tropical forests. Science 263: 954-958.
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