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Tree-Based Climate Reconstructions in CO2-Accreting Air
Volume 12, Number 9: 4 March 2009

Two decades ago, one of us (Idso, 1989) rhetorically asked if the ongoing rise in the air's CO2 content was "a problem for paleoclimatology," noting that LaMarche et al. (1984) had already made "a good case for the existence of a significant CO2-induced growth enhancement in subalpine conifers growing in the western United States," which had led them to suggest -- with respect to tree-ring data -- that "changing atmospheric CO2 could introduce non-climatic growth fluctuations that could interfere with calibration of climate and its reconstruction."

Graybill and Idso (1993) also demonstrated the reality of the problem. Based on analyses of tree-ring chronologies of four species of pine tree -- Pinus aristata, P. balfouriana, P. flexilis and P. longaeva -- stretching back in time a thousand years and more, they found a large increase in radial growth over the 20th century that far surpassed anything seen in earlier centuries. They then explored the possibility that "changes in climate during the past century might be responsible for the unusual increases in ring-width growth of [the] subalpine conifers," including temperature, precipitation and various drought indices; but they found that "trends of the magnitude observed in 20th century ring-width growth are conspicuously lacking in all of the time series of instrumented climatic variables that might reasonably be considered growth-forcing in nature." Hence, they concluded that "atmospheric CO2 fertilization of natural tree growth has been occurring from at least the mid- to late-19th century."

Unfortunately, these warnings were not heeded; and Mann et al. (1998, 1999), and others thereafter, continued to interpret the large increases in 20th-century radial growth rates of these and other trees as evidence of unprecedented CO2-induced global warming, when in reality they were more likely the result of unprecedented CO2-induced increases in water use efficiency -- or tree growth per unit of water incorporated into tree biomass plus that lost to the atmosphere in the process -- which parameter rises in response to both the aerial fertilization effect of rising atmospheric CO2 concentrations and the transpiration-reducing effect of elevated CO2. But now we have a new dataset and a new analysis of the subject, which adds to the growing body of evidence that demonstrates the difficulty of deriving a valid long-term temperature history from tree-ring data that are not adjusted for these latter two growth-enhancing phenomena during the time of huge anthropogenic CO2 emissions to the atmosphere.

The authors of the new study -- Knapp and Soule (2008) -- examined recent radial growth increases in western juniper trees (Juniperus occidentalis var. occidentalis Hook.) based on their analysis of a master tree-ring chronology dating from AD 1000-2006, which they developed from eleven semi-arid sites in the interior U.S. Pacific Northwest that had experienced minimal anthropogenic influence, other than that provided by the historical increase in the air's CO2 content that is everywhere present.

Their first step after developing the chronology was to use measured climate data for the period 1907-2006 to determine to which climatic parameter tree radial growth was most responsive: temperature, precipitation or drought severity, as represented by the Palmer Drought Severity Index (PDSI) for the month of June. This exercise revealed June PDSI to be the most important factor, explaining fully 54% of annual radial growth variability; and when they added CO2 as a second predictive factor, they found it "accounted for a 14% increase in explanatory power." In addition, they report that "use of the PDSI-only regression model produced almost exclusively positive residuals since 1977," but that "the +CO2 model has a greater balance of positive (53%) and negative residuals over the same period." As a result of these findings, plus those of other tests they performed with the data, they concluded that "climatic reconstructions based on pre-1980 data would not be significantly influenced by rising CO2 levels," but that reconstructions produced after that time would be.

These observations suggest that the late-20th-century/early-21st-century radial growth of the western juniper trees -- which was 27% greater than the long-term (AD 1000-2006) average during the period 1977-2006 that Knapp and Soule describe as being "unlike any other period during the last millennium," i.e., truly unprecedented -- was likely due to the increase in the air's CO2 content over the latter period. It for sure could not have been due to any increase in air temperature; for they report that "western juniper responds negatively to temperature, negating any linkages to regional warming." Neither could the anomalous growth increase have been due to anomalous nitrogen fertilization, for the two researchers note that "the eleven chronology sites do not fall under any of the criteria used to identify ecosystems significantly impacted by N-deposition," citing the work of Fenn et al. (2003).

In conclusion, therefore, Knapp and Soule were left with about the only remaining alternative explanation, i.e., that the truly unprecedented increase in the radial growth rates of western juniper trees throughout the interior U.S. Pacific Northwest over the last few decades of the 20th century, as well as throughout the initial years of the 21st century, was likely the result of the positive impact of the recent large increase in the air's CO2 concentration on the water use efficiency of the trees. Furthermore, since the water use efficiency of nearly all trees exhibits a significant positive response to atmospheric CO2 enrichment, it follows that nearly all radial growth chronologies likely possess a recent positive component that was not caused by rising temperatures or anomalous nitrogen fertilization, but by the sizable concomitant increase in the air's CO2 content. And this likelihood must be factored into all analyses designed to reconstruct temperature histories from long-term tree-ring chronologies, especially those that are developed for the purpose of comparing the degree of recent warmth with that of the Medieval Warm Period of a thousand years ago, when the air's CO2 concentration was more than a hundred parts per million less than it is today.

Sherwood, Keith and Craig Idso

References
Fenn, M.E., Baron, J.S., Allen, E.B., Rueth, H.M., Nydick, K.R., Geiser, L., Bowman, W.D., Sickman, J.O., Meixner, T., Johnson, D.W. and Neitlich, P. 2003. Ecological effects of nitrogen deposition in the western United States. BioScience 53: 404-420.

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 paleoclimatology? Quaternary Research 31: 433-434.

Knapp, P.A. and Soule, P.T. 2008. Use of atmospheric CO2-sensitive trees may influence dendroclimatic reconstructions. Geophysical Research Letters 35: 10.1029/2008GL035664.

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.