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Vegetation, Climate and CO2: Their Intertwined Relationship
Volume 11, Number 36: 3 September 2008

In a paper recently published in Quaternary Science Reviews, Gonzales et al. (2008) say "there has been a growing awareness that past variations in CO2 may have globally influenced plant physiology, vegetation composition, and vegetation structure (Idso, 1989a; Cowling, 1999; Cowling and Sykes, 1999; Wu et al., 2007a,b)," and that "this information has spurred a debate over the relative importance of CO2 versus climate as drivers of Quaternary vegetation change." Likewise, as also first indicated back in 1989, Idso (1989b) wrote in Quaternary Research that "since atmospheric CO2 has varied so dramatically in the past, it would seem to be an almost unavoidable conclusion that the effects of atmospheric CO2 on plant water use efficiency would significantly influence the floristic (i.e., species) composition of plant communities, as well as their distributions in space and time, and that this phenomenon, largely disregarded in the reconstruction of past climates, may be introducing errors into interpretations of several paleoclimatic indicators." Hence, it is gratifying to see that steps are finally beginning to be taken to address both of these closely related subjects.

In the paper that prompted our editorial, Gonzales et al. compare simulated and pollen-inferred leaf area index (LAI) values with regional vegetation histories of northern and eastern North America "to assess both data and model accuracy and to examine the relative influences of CO2 and climate on vegetation structure over the past 21,000 years." In doing so, they make use of BIOME4, "a biogeochemistry-biogeography equilibrium vegetation model (Kaplan, 2001)" that "was designed in part for paleo-vegetation applications, and has been widely used to simulate vegetation responses to late-Quaternary CO2 and climates." Concurrently, and "to provide paleo-climate scenarios for the BIOME4 simulations," the three researchers say they "used surface temperature, precipitation and cloudiness values from a series of Hadley Centre Unified Model simulations."

In discussing their results, Gonzales et al. note that this is the first study "to use both BIOME4 simulations and pollen-based reconstructions to develop detailed Quaternary LAI histories for North America," and they report that their "BIOME4 sensitivity experiments indicated that climate was the primary driver of late-Quaternary changes in LAI in northern and eastern North America, with CO2 a secondary factor."

Even more important than this specific conclusion, however -- which could well be modified somewhat as subsequent related studies are conducted -- is Gonzales et al.'s observation that their work "emphasizes the need for models to incorporate the effects of both CO2 and climate on [the reconstruction of] late-Quaternary vegetation dynamics and structure." In like manner, we would further note that their work also emphasizes the need for vegetation-based climate reconstructions to incorporate the effects of changes in atmospheric CO2 concentration, especially when attempting to compare late 20th-century reconstructed temperatures with reconstructed temperatures of the Roman and Medieval Warm Periods and the Holocene Climatic Optimum. Until this common deficiency is corrected, truly valid comparisons between these earlier times and the present cannot be made, for without properly adjusting for the growth- and water use efficiency-enhancing effects of the historical increase in the air's CO2 content, reconstructed 20th-century temperatures -- which must be used in place of actual measured values when making comparisons with earlier reconstructed temperatures -- will be artificially inflated.

Sherwood, Keith and Craig Idso

Cowling, S.A. 1999. Simulated effects of low atmospheric CO2 on structure and composition of North American vegetation at the Last Glacial Maxdimum. Global Ecology and Biogeography Letters 8: 81-93.

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

Gonzales, L.M., Williams, J.W. and Kaplan, J.O. 2008. Variations in leaf area index in northern and eastern North America over the past 21,000 years: a data-model comparison. Quaternary Science Reviews 27: 1453-1466.

Idso, S.B. 1989a. Carbon Dioxide and Global Change: Earth in Transition. IBR Press, Tempe, Arizona, USA.

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

Kaplan, J.O. 2001. Geophysical Applications of Vegetation Modeling. Ph.D. Thesis, Lund University, Sweden.

Wu, H., Guiot, J., Brewer, S. and Guo, Z. 2007a. Climatic changes in Eurasia and Africa at the last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modeling. Climate Dynamics 10.1007/s00382-007-0231-3.

Wu, H., Guiot, J., Brewer, S., Guo, Z. and Peng, C. 2007b. Dominant factors controlling glacial and interglacial variations in the treeline elevation in tropical Africa. Proceedings of the National Academies of Science, USA 104: 9720-9725.