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Where Have All the Flowers Gone?
Volume 6, Number 27: 2 July 2003

On 17 June 2003, the Independent Digital (UK) Ltd published a story entitled "Global Warming May Wipe Out a Fifth of Wild Flower Species," stating in its opening paragraph that "one in every five species of wild flower could die out over the next century if levels of carbon dioxide in the atmosphere double."  Other news agencies ran similar stories; and the world was once again reminded of the devastating effects of CO2-induced global warming ? or, more accurately, of the predictions of such effects.

The source of the news stories was an article published in the Proceedings of the U.S. National Academy of Sciences one day earlier, in which Zavaleta et al. (2003) described the results of a three-year study conducted on a California grassland at the Jasper Ridge Biological Preserve in the San Francisco Bay area that dealt with essentially three groups of plants -- annual grasses, perennial grasses and forbs -- where for three years they had exposed a number of experimental plots to various combinations of increased atmospheric CO2 concentration (an extra 300 ppm), increased temperature (an extra 80 Wm-2 of ground-directed thermal radiation), increased precipitation (50% above normal, which extended the growing season by about 20 days), and enhanced nitrogen deposition (an extra 7 g N per square meter per year).

What did the scientists find?  With respect to the core concern of climate alarmists, i.e., rising temperatures, Zavaleta et al. determined that their imposed warming actually tended to increase total plant diversity, by a non-significant 6%.  With respect to the "wild flowers" of the media headlines (the forbs), however, there was a decline; but it too was non-significant and of only about 2%, which is a far cry from the claimed 20% reduction.

So where did the 20% figure originate?  It came from what was observed in the plots where both temperature and CO2 concentration were increased together -- which for a presumed CO2-induced warming would appear to be a reasonable thing to do -- and under these circumstances, forb diversity did indeed decrease as reported.  However, if one is going to take this approach, one should actually look at the combination of all of the predicted climatic consequences of elevated CO2, which includes increased precipitation; and when the experimental increase in precipitation was added to the mix, the decrease in forb diversity was reduced to only about 10%.  Furthermore, with respect to total plant diversity, there was actually no change under this scenario.

Adding enhanced nitrogen deposition to the other three environmental perturbations ultimately swung things back in the other direction, to an approximate 28% decrease in forb diversity.  However, there was still only a non-significant 4% decrease in total plant diversity.  In addition, it must be remembered that enhanced nitrogen deposition is not a climatic consequence -- or any other type of consequence -- of either atmospheric CO2 enrichment or global warming; and, hence, its effects should not have been construed to be due to either of these two phenomena, as was done in the various news reports.

Another problem with the news stories was their expansive generalizing of the results of but a single experiment conducted in a single location.  The Independent/UK, for example, declared that "one in every five species of wild flower could die out over the next century if levels of carbon dioxide in the atmosphere double," while the Stanford Report (18 June 2003) claimed that "doubling the amount of carbon dioxide in the air significantly reduces the number of plant species that grow in the wild."  In actuality, one cannot deduce anything about "the number of plant species that grow in the wild" from this particular experiment, for the ecosystem studied had some unique characteristics that clearly preclude such a vast extrapolation.

Zavaleta et al. make this point perfectly clear by stating "there is no rule of thumb for understanding combined global change responses in natural ecosystems," and by noting that their study is but one of "many tests that will be required for a general picture of ecological response to multiple global changes."  In fact, their study does not even imply that the species that disappeared from their experimental plots are ultimately headed for extinction; for those particular plants could well begin appearing in other places that become more conducive to their well-being, as climatic zones gradually shift in response to global environmental change.

When all is said and done, therefore, the study of Zavaleta et al. actually tells us nothing about the future well-being of wild flowers, or any other types of plants, in a potentially CO2-enriched world of the future that might, in the mean, become both warmer and wetter.  For that assessment we must look to real-world studies of how plants migrate from one location to another as various climatic parameters change; and when this is done, there are no significant signs that any of earth's plants are teetering on the edge of a CO2-induced destruction.

Sherwood, Keith and Craig Idso

Zavaleta, E.S., Shaw, M.R., Chiariello, N.R., Mooney, H.A. and Field, C.B.  2003.  Additive effects of simulated climate changes, elevated CO2, and nitrogen deposition on grassland diversity.  Proceedings of the National Academy of Sciences, USA 100: 7650-7654.