Shifting grasslands: Why overgrazing isn't the biggest threat

A new study analyzing 41 years of nationwide data from Mongolia finds that warming temperatures and shifting weather patterns explain most long-term shifts in grasslands, while herd size matters mainly in the short term. That flips a common assumption about what is driving change on the steppe.

More than half of Earth’s land is rangeland, which supports about half of global livestock and the livelihoods of over 2 billion people. Understanding what truly limits these grasslands is not an academic debate, it shapes real decisions that affect food, jobs, and ecosystems.

Climate drives grassland changes

The analysis was led by Chris Barrett of Cornell University’s SC Johnson College of Business (SCJCB). His team disentangled the effects of climate, year to year weather, and herd size on primary productivity, a measure of how much plant matter the land produces.

“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 Barrett.

Herd size had a modest negative effect on productivity from one year to the next. Over 10 year spans, that effect disappeared, while heat exposure dominated the results.

The data also showed differences across ecological zones. Cooler and more productive zones felt short term stocking impacts, while hotter and drier zones were hit hardest by warming itself.

Testing the cause and effect

The researchers used a quasi-experimental approach rather than simple correlations. This is significant because herders adjust herds to local conditions, which can blur cause and effect if not handled carefully.

The experts used Mongolia’s severe dzud winters as a case study. These events bring bitter cold, strong winds, and heavy snow after dry summers, cutting off animals from food and leading to widespread mortality.

They paired official herd counts with satellite measures of plant growth using NDVI, the normalized difference vegetation index. NDVI tracks vegetation greenness from space and provides a consistent way to compare grassland productivity over large areas.

The team predicted early summer herd sizes based on where animals grazed in winter and how harsh those winters were. They then estimated how those predicted herds and weather conditions affected summer plant growth.

Impact of climate on grasslands

Mongolia has already warmed sharply. The country’s official climate INDC reports a 2.07 °C (3.73 °F) rise in annual mean temperature from 1940 to 2014, alongside shifts in precipitation that increase aridity in many regions.

The trends help explain why heat exposure overshadowed grazing over longer periods in the new analysis. Herders can and do adapt stocking and movement in the short run, but persistent warming lowers the ceiling on what grasslands can produce.

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

What this means for policy

Policies that tax herds or cap numbers may deliver limited gains at multiyear scales. The study’s long run estimates suggest those measures do not change the trajectory when the climate signal keeps strengthening.

Targeted, local management still has value. For example, protecting higher elevation summer refuges and improving pasture rotation can reduce short term pressure on sensitive zones.

The heavier lift sits outside herders’ control. Cutting greenhouse emissions remains central to stabilizing the heat exposure that the analysis links to long term productivity declines.

Climate change and grasslands

At annual scales, doubling herd size reduced the greenness metric by a few percent in the average soum, a district roughly comparable to a county. That matters for a given season, especially in cooler, more productive zones.

Across decades, temperature exposure explained a much larger share of variation in grass growth than herd size. That pattern held across steppe, semidesert, and desert zones that dominate Mongolia’s vast interior.

The team was careful to control for factors fixed in place. They used location and year indicators so that unchanging features and national shifts did not bias the results.

Changing the rangeland story

The new estimates help reconcile mixed findings from past work. Correlation based studies often pointed to overgrazing, while small plots sometimes showed heavy grazing effects that did not generalize well across space and time.

Causal inference at national scale adds a missing piece. It shows that local choices matter in the short run, but the background climate signal becomes the main driver as years pile up.

That result also clarifies fairness concerns. Mongolia contributes little to global emissions, yet its grasslands absorb the impact of warming driven elsewhere.

Future research directions

Remote sensing is improving quickly, which will allow finer tracking of plant communities, soil moisture, and heat stress. Combining those data with herder movement records can reveal which adaptations work best as conditions shift.

Researchers can also study thresholds. Identifying the temperatures at which productivity drops fastest would help managers set early warning triggers.

Finally, linking grass growth to livelihoods can guide support through volatile years. That means connecting productivity forecasts to animal weight, milk yields, and income risk.

Herders adapting to climate change

Herders know their lands well and respond quickly to seasons. The evidence here shows that their adjustments can offset short term grazing pressure, but only up to the limits set by heat.

Stronger climate information services can help families move earlier and avoid bottlenecks. Better forage reserves and access to weather safe shelters can buffer animals during harsh winters.

Public choices matter too. Investments that reduce emissions and fund climate resilience will do more to protect these grasslands over time than blanket herd taxes.

The study is published in the journal Science.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–