Really interesting (and freely available!) article on Quantifying and mapping the human appropriation of net primary production in earth's terrestrial ecosystems over at PNAS.
Human appropriation of net primary production (HANPP), the aggregate impact of land use on biomass available each year in ecosystems, is a prominent measure of the human domination of the biosphere. We found an aggregate global HANPP value of 15.6 Pg C/yr or 23.8% of potential net primary productivity, of which 53% was contributed by harvest, 40% by land-use-induced productivity changes, and 7% by human-induced fires. This is a remarkable impact on the biosphere caused by just one species.
I agree that it really is amazing that one species has appropriated almost 1/4 of all NPP on the globe for their use. Much of that is completely wasted as almost 1/2 of HANPP is forest fires and land-use-induced productivity changes. If we got rid of those loses then HANPP would go down to 12% of total NPP. It should also be noted that of actual vegetation, human appropriation is 15% of total NPP.
Also interesting is the fact that almost half (46%) of total potential NPP is below ground, but 90% of HANPP from human harvest and fires are above ground. On the other hand, 71% of NPP loses due to human induced alteration are below ground.
This map gives you a good feel for where humans are appropriating the highest percentage of NPP. India is highest, with Europe and parts of the US, Canada and China also using a large fraction of potential NPP.