The original farmer probably did not have an outsized impact on the world. Scattering some seeds, guarding them and perhaps clearing a few other species of plants, this proto-agrarian would have been the first to harness the power of photosynthesis for humanity's benefit. Now, thousands of years later, modern agrarians—along with engineers, foresters and consumers—directly control 23.8 percent of all the world's photosynthesis, according to a new analysis.

Using Food and Agriculture Organization statistics through the year 2000 on areas farmed, crops harvested and animals grazed—as well as models of the photosynthetic production of vegetation worldwide and global data on forested areas—ecologist Helmut Haberl of Klagenfurt University in Austria and his colleagues calculated the difference between the energy produced by plants in the absence of humans and the actual amount of photosynthetic energy available to ecosystems after humans have taken their share.

"We found that, due to human activities, in particular past and present land use, 23.8 percent less [photosynthetic energy] remains currently in ecosystems than would be available without human activities," Haberl says.

This affects everything from the diversity of life on Earth to desertification. More than half of the human share of photosynthetic energy comes from farming, but other forms of land use contribute 40 percent and human-caused forest fires a full 7 percent.

And the burden is not evenly spread. The human share, for example, is as high as 63 percent in southern Asia and as low as 11 percent in central Asia and Russia. Technologies such as industrial fertilizers have helped to lower the human burden by increasing harvests while limiting land area, but this increasing agricultural output has been confined to certain regions of the globe, Haberl notes.

As the human population continues to rise in coming years, Haberl and his colleagues expect the human share to eat up more and more of the available biomass. "On a global scale, our preliminary—so far unpublished—results strongly indicate, in any case, that [the human share] is on the rise and has been so [for] the past 300 years," Haberl says.

And that share could rise explosively should a shift to biofuels occur on a broad scale. Such a shift might result in a doubling of crops harvested in coming decades, Haberl adds, "something that could have massive impacts on ecosystems, above all their resilience and biodiversity" as energy is shifted from feeding animals to fueling cars. Already Haberl has shown in other papers how land-use changes on a local scale undercut the complex web of life in Austria—from breeding bird species to crickets to the plants themselves—and endangered its ability to thrive.

That relationship will be explored in the much larger North and South American context next, Haberl says. Nevertheless, it is already clear that "a remarkable share of global [photosynthetic production] is used to satisfy the needs and wants of just one species on earth," as the researchers write in Proceedings of the National Academy of Sciences USA