Background
Over the past several decades, the diurnal temperature range (daily maximum minus daily minimum) has declined significantly over many parts of the globe (Easterling et al., 1997).  Although many people have attributed this phenomenon to CO2-induced global warming, Hansen et al. (1995) note that climate model simulations do not indicate a diurnal temperature range (DTR) decline of the magnitude that has been observed.  Hence, the authors of this study search for another explanation for this well-documented phenomenon.

What was done
The authors employed a simple land surface subroutine in a general circulation model of the atmosphere that included parameterizations of canopy physiological responses to various environmental changes.  By running the model with and without the vegetation subroutine, they were able to determine the degree of influence the planet's plant life may have on near-surface air temperature in a world of rising temperature and atmospheric CO2 concentration.

What was learned
It was determined that realistic changes in the amount and physiological activity of earth's plant life can produce changes in the DTR of the order observed in the real world.  In addition, the authors explicitly state that their results "suggest that reported increases in vegetation cover in the Northern Hemisphere during the 1980s [Myneni et al., 1997] could have contributed to the lowered DTR."

What it means
Whereas it has long been believed that the declining diurnal temperature range near the surface of the earth is a "fingerprint" of deleterious CO2-induced global warming, it now appears that the declining DTR may be an indication of beneficial CO2-induced "global greening," which by its very observation - as per Myneni et al. (1997) - is by definition known to be helping both natural and agro-ecosystems become more productive, which is, of course, the definition of "greening."

References
Easterling, D.R., Horton, B., Jones, P.D., Peterson, T.C., Karl, T.R., Parker, D.E., Salinger, M.J., Razuvayev, V., Plummer, N., Jamason, P. and Folland, C.K.  1997.  Maximum and minimum temperature trends for the globe.  Science 277: 364-367.

Hansen, J., Sato, M. and Ruedy, R.  1995.  Long-term changes of the diurnal temperature cycle: Implications about mechanisms of global climate change.  Atmospheric Research 37: 175-209.

Myneni, R.C., Keeling, C.D., Tucker, C.J., Asrar, G. and Nemani, R.R.  1997.  Increased plant growth in the northern high latitudes from 1981 to 1991.  Nature 386: 698-702.