A recent report in the journal Science (Crimmins et al. 2011) concluded that (1) over the last 70 years, plant species in northern California had migrated downward by an average of about 90 m, (2) the downward migration was caused by a precipitation-induced decline in climatic water deficit, and (3) similar downhill species shifts can be expected where future increases in precipitation outpace temperature-induced increases in evaporative demand.

In sum, the paper concluded that increasing precipitation can counteract the effects of increasing temperature. But in a technical response to Science, USGS scientists Nate Stephenson and Adrian Das demonstrate the conclusions of Crimmins et al. are incorrect.

First, Stephenson and Das show that the historical and contemporary vegetation plots compared by Crimmins et al. were geographically biased relative to one another. For example, the median latitude of their contemporary plots was about 140 km farther north than that of historical plots. Because a northward shift of 100 km is generally accompanied by a roughly 100 m decline in species’ elevations, this and other geographic biases likely account for the lower species’ elevations.

Second, Crimmins et al. miscalculated climatic water deficit. Corrected calculations show that the majority of extra precipitation in the contemporary period fell in winter and became biologically unusable surplus water.

Most importantly, increasing precipitation will not counteract the effects of increasing temperature. Increasing temperature and precipitation will lead to warmer, wetter environments, which are fundamentally different from cooler, drier environments. Other research also suggests that under a warming climate, most plant species will shift upward, not downward in elevation, regardless of changes in precipitation.