What was done
The authors "used the CEVSA (Carbon Exchanges in the Vegetation-Soil-Atmosphere system) model (Cao and Woodward, 1998; Cao et al., 2002), forced by observed variations in climate and atmospheric CO2, to quantify changes in net primary production (NPP), soil heterotrophic respiration (HR) and net ecosystem production (NEP) from 1981 to 1998."  As an independent check on the NPP estimate of CEVSA, they also "estimated 10-day NPP from 1981-2000 using the GLObal Production Efficiency Model (GLO-PEM) that uses data almost entirely from remote sensing, including both the normalized difference vegetation index (NDVI) and meteorological variables (Prince and Goward, 1995; Cao et al., 2004)," which remote sensing inputs were obtained from "Pathfinder AVHRR Land (PAL) data at resolutions of 8 km and 10 days derived from channels 1, 2, 4 and 5 of the AVHRR sensors aboard the NOAA-7, 9, 11 and 14 satellites."

What was learned
Cao et al. report that "global terrestrial temperature increased by 0.21°C from the 1980s to the 1990s, and this alone increased HR more than NPP and hence reduced global annual NEP."  However, they found that "combined changes in temperature and precipitation increased global NEP significantly," and that "increases in atmospheric CO2 produced further increases in NPP and NEP."  With respect to this latter forcing factor, they discovered that "the CO2 fertilization effect [was] particularly strong in the tropics, compensating for the negative effect of warming on NPP."  Enlarging on this point, they note that "the response of photosynthetic biochemical reactions to increases in atmospheric CO2 is greater in warmer conditions, so the CO2 fertilization effect will increase with warming in cool regions and be high in warm environments."  The end result of the application of these models and measurements was their finding that global NEP increased "from 0.25 Pg C yr^-1 in the 1980s to 1.36 Pg C yr^-1 in the 1990s."

What it means
Cao et al. note that "the NEP that was induced by CO2 fertilization and climatic variation accounted for 30% of the total terrestrial carbon sink implied by the atmospheric carbon budget (Schimel et al., 2001), and the fraction changed from 13% in the 1980s to 49% in the 1990s," which indicates the growing importance of the CO2 fertilization effect.  Also, they say that "the increase in the terrestrial carbon sink from the 1980s to the 1990s was a continuation of the trend since the middle of the twentieth century, rather than merely a consequence of short-term climate variability," which suggests that as long as the air's CO2 content continues its upward course, so too will its stimulation of the terrestrial biosphere likely continue its upward course.

Reference
Cao, M.K., Prince, S.D. and Shugart, H.H.  2002.  Increasing terrestrial carbon uptake from the 1980s to the 1990s with changes in climate and atmospheric CO2.  Global Biogeochemical Cycles 16: 10.1029/2001GB001553.

Cao, M.K., Prince, S.D., Small, J. and Goetz, S.  2004.  Satellite remotely sensed interannual variability in terrestrial net primary productivity from 1980 to 2000.  Ecosystems 7: 233-242.

Cao, M.K. and Woodward, F.I.  1998.  Dynamic responses of terrestrial ecosystem carbon cycling to global climate change.  Nature 393: 249-252.

Prince, S.D. and Goward, S.N.  1995.  Global primary production: a remote sensing approach.  Journal of Biogeography 22: 815-835.

Schimel, D.S., House, J.I., Hibbard, J.I., Bousquet, P., Ciais, P., Peylin, P., Braswell, B.H., Apps, M.J., Baker, D., Bondeau, A., Canadell, J., Churkina, G., Cramer, W., Denning, A.S., Field, C.B., Friedlingstein, P., Goodale, C., Heimann, M., Houghton, R.A., Melillo, J.M., Moore III, B., Murdiyarso, D., Noble, I., Pacala, S.W., Prentice, I.C., Raupach, M.R., Rayner, P.J., Scholes, R.J., Steffen, W.L. and Wirth, C.  2001.  Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems.  Nature 414: 169-172.