What was done
The authors modeled potential future changes in the geographic distribution of the Joshua Tree (Yucca brevifolia Engelm.), whose current distribution is roughly coincident with the Mojave Desert of the southwestern United States (southern California, southern Nevada, and western Arizona), based on (1) climate changes predicted to accompany a doubling of the air's CO2 content and (2) the experimental observation that a doubling of the atmosphere's CO2 concentration, in the words of the authors, "enhances the low-temperature tolerance of Y. brevifolia seedlings, such that the lethal low temperature is lowered by 1.6°C," as reported by Loik et al. (2000).

What was learned
The authors report that "the increase in freezing tolerance caused by doubled CO2 would increase the potential habitat of this species by 14%, independent of any climate change."  On the other hand, if only the predicted warming effect of the doubled CO2 is considered, the total area occupied by Y. brevifolia would decline by 25%.  Nevertheless, the authors indicate that when both effects are considered together, "the model predicts a different and slightly larger future distribution."

What it means
The authors say their results indicate "the importance of including the physiological effects of CO2 in studies that try to predict the biological effects of climate change."  This admonition is most appropriate, for many global-warming scare stories of impending biological catastrophe are based on studies that fail to comply with this important and necessary condition.  Also, they note that an "interesting implication of this study is that anthropogenic CO2 increases will drive ecosystem change even in the absence of significant climate change."

Reference
Loik, M.E., Huxman, T.E., Hamerlynck, E.P. and Smith, S.D.  2000.  Low temperature tolerance and cold acclimation for seedlings of three Mojave Desert Yucca species exposed to elevated CO2.  Journal of Arid Environments 46: 43-56.