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Two Types of Tree-Ring Response to Atmospheric CO2 Enrichment and Their Implications for Reconstructing Past Trends in Earth's Surface Air Temperature
Volume 3, Number 14: 15 July 2000

In our editorial of 1 July 2000 (There Has Been No Global Warming for the Past 70 Years), we indicated there are two different tree-ring characteristics that have been used to reconstruct histories of past air temperature trends around the world: tree-ring density and tree-ring width.  We also noted that temperature histories derived from the first of these tree-ring properties give no indication of any warming over the latter part of the 20th century, but that those derived from the second factor do.  Finally, we stated that the first of these temperature reconstructions (those based on tree-ring density) are correct, but that temperature reconstructions of the second type (those based on tree-ring width) are incorrect, explaining that the productivity-enhancing effect of the rise in the air's CO2 content that has occurred over the past century or so has dramatically increased tree-ring widths, particularly over the last 70 years of most rapid CO2 rise, and that this tree-ring growth record, although true, has been falsely attributed to a dramatic concomitant increase in air temperature, when, in reality, the earth has experienced no net warming over this period.

Shortly after posting this editorial, we received a communication from a reader who was perplexed by our discussion.  He queried: "How can one of these tree-ring properties be skewed by atmospheric CO2 enrichment, but not the other?"  Since this same question may well have arisen in the minds of additional readers, we here provide the answer.

Actually, both tree-ring density and tree-ring width are positively affected by increasing concentrations of atmospheric CO2.  However, the size of the response is vastly different.  Whereas tree-ring width is greatly increased by atmospheric CO2 enrichment, tree-ring density is but little affected.  In our father's long-term study of elevated CO2 effects on sour orange trees, for example, a 75% (300 ppm) increase in the air's CO2 concentration has increased the overall growth of the trees by 75% or more throughout the entire 13-year history of the experiment; and this growth enhancement is readily apparent in the different thicknesses of the trunks of the trees of the two CO2 treatments and, hence, in the different widths of their annual growth rings.  In studies he has made of the densities of branches removed from the trees over the years, however, the response has been a full order of magnitude less, with the branch densities of the CO2-enriched trees averaging only about 6% more than the branch densities of the ambient-treatment trees.

So what are the implications of these findings?  Over the past 70 years, the air's CO2 concentration has risen about 60 ppm, which increase is only two-tenths of the 300 ppm increase employed in our father's sour orange tree experiment.  Hence, the increase in tree-ring density likely to have been produced in sour orange trees by the historical rise in the air's CO2 content over this period is something on the order of 0.2 x 6%, or just a little over 1%; and within the context of the natural variability of other environmental factors that may influence the density of wood produced in the trunks of trees, this effect of the historical rise in the air's CO2 content is just not large enough to be discerned.  Hence, surface air temperature reconstructions based on long-term tree-ring density chronologies should indeed be correct over the past 70 years.  In fact, they should be correct over the entire expanse of the last millennium.

In the case of tree-ring width increases observed over the last 70 years, however, the productivity enhancement likely to have been induced by the concomitant rise in the air's CO2 content is just too large to be ignored, as those who have reconstructed temperature histories from tree-ring width chronologies have consistently done, without exception.  Two-tenths of a 75% increase in total biomass production, for example, is fully 15%.  Hence, surface air temperature reconstructions derived from long-term tree-ring width chronologies - which vary in response to yearly biomass production (the more wood produced, the thicker the tree ring) - are almost certain to be in error over the last 70 years of the millennium just ended.  As a result, such climate histories, although indeed revealing a dramatic and unprecedented growth increase over the last 70 years, do not imply the existence of a dramatic and unprecedented rise in mean global air temperature.  They are merely reflective of the dramatic and unprecedented increase in atmospheric CO2 concentration experienced over this period.

In light of these facts, we are confident that the highly-hyped global surface air temperature record, which is being used by the IPCC to suggest there is a human influence on climate, is fatally flawed.  The correct story - told by earth's trees and the global satellite and balloon records (see our editorial The Global Surface Air Temperature Record Must Be Wrong) - is one of modest warming in the late 19th and early 20th centuries, as the earth recovered from the global chill of the Little Ice Age, followed by fully 70 years of relative thermal stability.

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