Warming climate, not herd size, is biggest threat to rangelands

More than half of Earth's terrestrial surface is rangeland - vast open areas of native vegetation, suitable for grazing. These areas feed 50% of the world's livestock and support the livelihoods of more than 2 billion people. The continental U.S. is about one-third rangeland.

Overgrazing has long been seen as a key factor in rangeland degradation - and is the reason for herd-size restrictions or livestock taxes that in some places can limit herders' ability to make a living. But a new Cornell study points to another variable: climate change.

Using four decades' worth of detailed data from Mongolia, where 70% of the land area is rangeland, researchers in the Cornell SC Johnson College of Business found that while larger herds can slightly reduce rangeland productivity year to year, weather and climate have a much bigger effect.

"When we look really carefully at the equivalent of county scale over the whole country, over 41 years, we find that the longer-run changes in rangeland conditions are entirely attributable to changes in the climate," said Chris Barrett, the Stephen B. and Janice G. Ashley Professor of Applied Economics and Management, in the Cornell SC Johnson College of Business and a professor in the Jeb E. Brooks School of Public Policy.

Barrett's team found that Mongolian rangelands are impacted more by collective greenhouse gas-emitting behaviors around the globe than by local herders. They urge policymakers to focus more attention on global mitigation, as well as on international compensation for climate damages, and less on taxing herders in a nation that contributes little to global greenhouse gas emissions.

Barrett is the senior author of "Climate Rather than Overgrazing Explains Most Rangeland Primary Productivity Change in Mongolia," which published Sept. 18 in Science.

Barrett's co-authors are Tumenkhusel Avirmed '21, M.S. '23, now a research data analyst at Stanford University; Avralt-Od Purevjav, Ph.D. '20, a consultant at the World Bank; and Steven Wilcox, Ph.D. '22, an assistant professor of applied economics at Utah State University.

Avirmed grew up on the rangelands of Mongolia and spearheaded this research, along with Purevjav, who also is from Mongolia. Avirmed approached Barrett, who has done extensive research on rangelands in Africa, and asked if he'd ever studied Mongolia.

"I said no," Barrett said, "and he explained that he was interested in doing research, and he's from Mongolia, and he had access to data."

The Mongolian government does an annual end-of-year census of all livestock in the country, then in June surveys and samples rangeland vegetation to determine conditions. Based on this rich trove of data, in 2021 the government re-instituted a nationwide livestock head tax, aimed at inducing lower herd rates to address perceived adverse rangeland impacts.

Barrett's team used this data along with a sophisticated two-stage statistical analysis method, using herd census data at the "soum" level - a soum is similar to a county - along with "dzud" events (extreme winter storms that cause massive livestock mortality) on winter grazing ranges, to predict variation in June herd size. This analysis covered the whole country over a 41-year period.

In the second stage of the study, the researchers used the predicted June herd size to generate causal estimates of the effects of herd size and climate on summer rangeland productivity. To distinguish between climate and short-term variations in weather, based on year-on-year comparisons, the team constructed multi-year averages of each variable and compared them over 10- and 20-year periods.

Analyzing the vast trove of data, the group found that larger herd sizes have a modest negative effect on rangeland productivity in the short term, but no significant effect over longer periods. Climate, and even year-to-year variations in weather, had a much bigger impact.

The difference was striking, Barrett said.

"I was surprised by the magnitude of the climate effect as compared to herd size effects, even in the short run," he said. "Even just year-on-year changes in weather had about 20 times the effect of herd size."

Barrett thinks this research could spur officials in Mongolia to rethink current methods for preserving rangeland productivity - methods that can put a financial strain on herders.

"It's always struck me as puzzling, why people in suits and ties in capital cities seem to think that the pastoralists don't understand very well how to manage these lands," Barrett said. "And yet, there's this common belief that you have to get them to reduce their herd sizes. That just hurts the herders."