Four Decades of Satellite Data Reveal Growing Instability in Global Drylands

Dryland ecosystems, which cover 40 percent of Earth's land surface and support more than two billion people, are becoming progressively less stable, according to research published in Nature Climate Change. The study analyzed four decades of satellite vegetation records and found that year-to-year swings in plant growth are widening - a trend that global climate models have so far failed to capture.

The research was led by doctoral student Wen Zhang from the University of Arizona's School of Natural Resources and the Environment. The team tracked the vegetation leaf area index - a measure of plant canopy density - across global drylands from the early satellite era through the present.

Their analysis confirms a widely reported greening trend in drylands: rising atmospheric CO2 has boosted plant growth during wet years by enhancing photosynthetic efficiency. However, the same conditions that amplify growth in good years appear to worsen the decline in dry years. The result is that the range between peak and trough vegetation levels is expanding over time.

"The upper and lower extremes are getting farther and farther apart as time goes by," Zhang said. The pattern affects approximately 80 percent of global drylands.

The team evaluated 13 leading global vegetation models against the satellite record and found that none of them reproduced the observed increase in year-to-year variability. "The models assume drylands are still stable," Zhang noted, identifying a significant gap in current tools for projecting future ecosystem conditions and their societal effects.

For rain-fed farming regions such as the American Southwest, the increased volatility has direct practical consequences. Higher variance in annual rainfall and plant growth makes crop planning less predictable and may push more producers toward artificial irrigation. Senior author Bill Smith noted that "higher variability in forage production presents a significant challenge for rangeland managers" who depend on relatively consistent seasonal yields.

The findings point to a divergence between the headline greening trend frequently cited in assessments of dryland vegetation and the underlying stability - or lack of it - that determines whether those ecosystems can support the populations and agricultural systems that rely on them.

CONTACT: https://news.arizona.edu/news/satellite-data-exposes-escalating-boom-and-bust-dynamic-greening-drylands