The calculated 110-year California temperature increases were fairly evenly distributed between 1.5 and 5.0°C; and as best we can judge from Lobell et al.'s graphical results, the median temperature-induced yield decreases between the base period of 2000-2003 and the year 2050 were 2, 11, 11, 4, 7, and 31% for wine grapes, almonds, table grapes, oranges, walnuts and avocados, respectively, when the yield anomalies were constrained to not exceed historical extremes, but they were 5, 14, 20, 4, 10, and 49% when yields were allowed to exceed historical extremes.

Noting that "climate change resulting from human activity [our italics] has the potential to substantially alter agricultural systems," Lobell et al. concluded from these exercises that "the unambiguous effect of warming from climate change will be to reduce yields for several major perennials," stating more specifically that "climate change in California is very likely to put downward pressure on yields of almonds, walnuts, avocados and table grapes by 2050." The more important question, however, is what will be the net change in yield produced by the direct positive effects of the projected human-induced rise in the air's CO2 content, i.e., its aerial fertilization effect, and the negative effects of the change in temperature Lobell et al. project to be concurrent with it.

Between the base period of 2000-2003 and the year 2050, the mean increase in the air's CO2 concentration for the lowest and highest CO2 emission scenarios employed in Lobell et al.'s temperature calculations is approximately 170 ppm, which just happens to be the approximate mean increase in the air's CO2 concentration employed in the several FACE experiments included in the meta-analysis of Ainsworth and Long (2005), who found mean yield increases on the order of 17% for many C3 herbaceous crops when they were exposed to such an increase in atmospheric CO2 concentration, as actually noted - but not thoroughly enough discussed - by Lobell et al. For tree crops, however, Ainsworth and Long observed a mean dry weight increase on the order of 28%; and this positive effect would more than compensate for the negative temperature effect calculated by Lobell et al. for all of the woody crops they studied with the lone exception of avocados, which experienced a calculated maximum warming-induced yield decrease of 49%.

But there is still other pertinent information that was totally ignored by Lobell et al. that could well end up saving even poor avocados.

For five of the six crops they studied - the exception being almonds - their statistical crop-yield models revealed the existence of an optimum temperature, above which yields declined in response to further increases in temperature. The form of this relationship also holds in CO2-enriched air. However, it has been experimentally demonstrated for a number of different plants that as the atmosphere's CO2 concentration rises, so too does plant optimum temperature tend to rise, so that the negative effect of warming on plant productivity does not "kick in" until a somewhat higher temperature is reached in CO2-enriched air.

From theoretical considerations alone, Long (1991) calculated that a 300-ppm increase in atmospheric CO2 should raise the optimum temperatures of most C3 plants by about 5°C; and in an analysis of pertinent experimental studies of seven different species, Idso and Idso (1994) reported a mean optimum temperature increase of 5.9°C for such an increase in the air's CO2 concentration, while in an analysis of this phenomenon in eleven species, Idso and Idso (2003) calculated a mean optimum temperature increase of 4.6 ± 1.2°C, which latter result suggests that the air temperature would have to raise anywhere from 3.4 to 5.8°C before there would be a negative effect of warming on plant productivity that was concurrent with a 300-ppm increase in atmospheric CO2 concentration, which for a 170-ppm increase in atmospheric CO2 translates into a 1.9 to 3.3°C rise in temperature before warming would begin to have a negative impact on yield. Hence, it is likely that for the situation analyzed by Lobell et al. - where the approximate 50-year increase in temperature from 2000-2003 to 2050 would have been only about half of the 110-year range of values they calculated for the period 1960-1990 to 2070-2100 (half of 1.5 to 5.0°C = 0.75 to 2.5°C) - the net impact on crop yield of the concomitant increases in air temperature and atmospheric CO2 concentration that have been predicted to occur by the year 2050 could well end up being positive ... even for avocados!

Sherwood, Keith and Craig Idso

References
Ainsworth, E.A. and Long, S.P. 2005. What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2. New Phytologist 165: 351-372.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.

Idso, S.B., Idso, C.D. and Idso, K.E. 2003. The Specter of Species Extinction: Will Global Warming Decimate Earth's Biosphere? Center for the Study of Carbon Dioxide and Global Change. Tempe, Arizona, USA.

Lobell, D.B., Field, C.B., Cahill, K.N. and Bonfils, C. 2006. Impacts of future climate change on California perennial crop yields: Model projections with climate and crop uncertainties. Agricultural and Forest Meteorology 141: 208-218.

Long, S.P. 1991. Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated? Plant, Cell and Environment 14: 729-739.