What was done
The authors used the Joint UK Land Environment Simulator (JULES) -- which is a land surface and carbon cycle model -- to simulate the biospheric carbon balance of Europe and its sensitivity to rising CO2 and changes in climate experienced over the period 1948-2005.

What was learned
Over the 57-year period of their study, Harrison et al. report that "the impact of climate changes since 1948 has been to decrease the ability of Europe to store carbon by 97 TgC year^-1," but that "the effect of rising atmospheric CO2 has been to stimulate [our italics] increased uptake and storage." In fact, this latter phenomenon is apparently so strong that it leads, in their words, to "a net increase in stored carbon of 114 TgC year^-1."

What it means
The three researchers from the UK's Met Office Hadley Centre note that "Davi et al. (2006) made a similar attempt to assess the relative impacts of climate change and rising CO2 on European carbon storage," and that they too "found decreased storage as a result of climate changes and increased uptake due to fertilization from rising CO2." Also like them, they report that Davi et al. found the CO2-induced uptake of carbon to dominate over the warming-induced loss of carbon, but less so than in their more recent simulation, which difference they explain by noting that the period modeled by Davi et al. stretched all the way from 1960 to the end of this century (AD 2100), when climate models project air temperature to be much higher than it is currently. However, in an attempt to be at least halfway politically correct, they state that this dominance of the physiological effects of the anticipated increase in the air's CO2 content over the climatic effects predicted for the remainder of this century "may not continue indefinitely." Yet even if this speculation were to prove correct, it would still give the nations of the earth adequate time to naturally transition from our current large dependence upon coal and oil to a more evenly distributed dependence upon a much larger mix of energy sources, without the need to artificially force such a change and risk the dangers that are always associated with a "rush job" where haste makes waste.

Reference
Davi, H.l, Dufrene, E., Francois, C., Le Maire, G., Loustau, D., Bose, A., Rambal, S., Granier, A. and Moors, E. 2006. Sensitivity of water and carbon fluxes to climate changes from 1960-2100 in European forest ecosystems. Agricultural and Forest Meteorology 141: 35-56.