Increased levels of carbon dioxide in the atmosphere are changing the pastoral landscape around the world, turning grasslands into shrublands unsuitable as grazing grounds for domestic livestock, according to a study published online Monday.

A group of U.S. researchers found that doubling carbon dioxide levels in the atmosphere in an area of shortgrass steppe led to changes in the lands from grasslands to shrublands.

Researchers say rising carbon dioxide levels over the past 200 years may be contributing to shrubland expansion, which adversely affects cattle grazing.

"This … enhancement of plant growth, among the highest yet reported, provides evidence from a native grassland suggesting that rising atmospheric [carbon dioxide] may be contributing to the shrubland expansions of the past 200 years," the authors said in an article published online Monday in advance of print in the Proceedings of the National Academy of Sciences.

A report from the U.N.-led Intergovernmental Panel on Climate Change released earlier this year said global atmospheric concentrations of carbon dioxide were 379 parts per million in 2005, far above the natural range of between 180 to 300 ppm over the past 65,000 years and significantly higher than in the pre-industrial era, and that levels of the gas in the atmosphere have continued to rise in the past 15 years.

While the rise in carbon dioxide has been connected to rising temperatures across the globe, its impact on rangelands — which account for over 40 per cent of the Earth's land surface — has been largely anecdotal, the authors said.

And while they found the increased carbon dioxide levels didn't affect overall plant diversity over a five-year study period, it did change the makeup of the region.

The experimenters, led by Jack Morgan at the Rangeland Resources Research Unit at the U.S. Department of Agriculture, say the most telling example of the changing landscape was the emergence of a common small woody shrub found in North America and Asia called fringed sagebrush, which experienced an approximately 20-fold increase in production compared with its growth at normal carbon dioxide levels.

"Encroachment of shrubs into grasslands is an important problem facing rangeland managers and ranchers," the authors said. "This process replaces grasses, the preferred forage of domestic livestock, with species that are unsuitable for domestic livestock foraging."