Some 800 million people around the world depend on livestock that graze on natural vegetation for their livelihoods and food security. In a good season, grasses and other plants flourish, supporting robust herds. In a bad season, the system suffers -- as do the people who rely on it. The difference between a good and bad year? One significant and increasingly volatile factor is precipitation.

A new study in Nature Climate Change reveals that over the past century year-to-year precipitation variability has increased significantly on 49 percent of the world's grazing lands, affecting vegetation and constraining its ability to support livestock. The study's authors, led by a team from the UMN Institute on the Environment, used climate data from 1901 to 2014 to create global maps of precipitation variability trends. While some grazing lands showed decreases in rainfall variability, the overall trend is an increase in fluctuation, both within and between years.

"Visualizing precipitation variability trends allows us to identify grazing lands that have undergone large changes -- and to learn from those places where people have managed to adapt well despite increased variability," says lead author Lindsey Sloat, a postdoctoral research associate with IonE's Global Landscapes Initiative.

This insight is important, because grazing lands are already typically marginal: unsuitable for crops, either too dry or with poor soils. "Even small changes in rainfall put them at more risk," says Paul West, co-director of GLI. Furthermore, some grazing lands are even more inhospitable than others. Changes in precipitation variability especially affect these more vulnerable lands, which -- adding to global risk -- also tend to be home to the smallholder farmers and pastoralists who most depend on livestock for food. The researchers found:

- Global grazing lands already experience 25 percent more year-to-year variability in precipitation than the average global surface land area - Regions with high year-to-year precipitation variability support lower livestock densities than less variable regions - Overall precipitation variability has increased the most in areas where grazing is predicted to be important for local food access

"This study is showing us that grazing is potentially highly vulnerable to climate change, right across the world, from Australia to Central Asia, sub-Saharan Africa and the Americas," says co-author Mario Herrero of Australia's Commonwealth Scientific and Industrial Research Organisation.