The authors of the ground-breaking study begin by citing a number of recent peer-reviewed scientific research papers - each of which is equally as ignored by the press as their own - that demonstrate the likelihood of what they call "a crucial role for biospheric feedbacks on climate," including processes driven by CO2-induced changes in land surface albedo, leaf stomatal conductance, plant rooting profile, fractional coverage of the land by vegetation, plant roughness length and displacement height, vegetation phenology, time of planting and harvesting (in the case of agricultural crops), and plant growth.  Next, they validate the model against real-world meteorological and plant growth data obtained for the 1989 growing season for the area located between approximately 35° and 48° N latitude and 96° and 110° W longitude.  Last of all, they investigate how the climate of the region changes when (1) only the radiative effects of a doubling of the air's CO2 concentration are considered, (2) only the biological effects of a doubling of the air's CO2 concentration are considered, and (3) the radiative and biological effects of a doubling of the air's CO2 concentration occur simultaneously.

With respect to the area-averaged and seasonally-averaged daily maximum air temperature, the radiative effects of a doubling of the atmospheric CO2 concentration lead to a warming of only 0.014°C, while the biological effects of the extra CO2 produce a cooling of fully 0.747°C.  Considered together - and including a nonlinear interaction term - the simultaneously-occurring radiative and biological effects of a doubling of the air's CO2 content thus produce a net cooling of 0.715°C.

With respect to the area-averaged and seasonally-averaged daily minimum air temperature, on the other hand, the radiative effects of a doubling of the atmospheric CO2 concentration lead to a warming of 0.097°C, while the biological effects of the extra CO2 produce a warming of 0.261°C.  Considered together - and again including the nonlinear interaction term - the simultaneously-occurring radiative and biological effects of a doubling of the air's CO2 content thus produce a net warming of 0.354°C.

During the day, then, when high air temperatures can be detrimental to both plant and animal life, the combined effect of the simultaneous radiative and biological impacts of an increase in the air's CO2 content acts to decrease daily maximum air temperature, which results in an alleviation of potential heat stress.  Likewise, during the night, when low temperatures can be detrimental to plant and animal life, the combined effect of the simultaneous radiative and biological impacts of an increase in the air's CO2 content acts to increase daily minimum air temperature, which results in an alleviation of potential cold stress.  In addition, when considering the day and night air temperature changes together, the mean daily air temperature range is reduced by approximately 1.069°C, leading to a considerably less thermally-variable - and, hence, more thermally-stable - environment, which in this particular case is also about 0.180°C cooler in the mean.

With such good things happening climatically, one would also expect good things to be happening biologically; and the model results do not disappoint in this regard.  The CO2-induced change in area-averaged and seasonally-averaged leaf area index, for example, is an increase of 0.581 (21.8%) for the case of the simultaneous expression of the radiative and biological effects of a doubling of the air's CO2 content.

In summarizing their findings, the authors say "it is clear" that the radiative effects of a doubling of the air's CO2 content have "little effect on anything," and that they play but a "minor role in the coupled biological and atmospheric system."  Sensing the need to pay homage to political correctness, however, they note that their analysis is "a regional-scale sensitivity study," whose results "cannot be linearly scaled up to global scales."

Nevertheless, and in spite of this strained caveat, the authors immediately thereafter state the obvious truth, when they say (but again in somewhat veiled language) that their results "suggest that the regional response could be on the order of global climate sensitivities."  Hence, they rightly conclude, as we have long argued, that "climate change that results from anthropogenic increases of CO2 must consider [our italics] the biological effects of enriched CO2 as well as its radiative effect."

And why is that?

Because biology rules, even when it comes to climate.

References
Eastman, J.L., Coughenour, M.B. and Pielke Sr., R.A.  2001.  The regional effects of CO2 and landscape change using a coupled plant and meteorological model.  Global Change Biology 7: 797-815.