This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Dryland ecosystems cover 40% of the Earth's land surface and support more than 2 billion people. According to a new study published in Nature Climate Change, they're becoming increasingly unstable.
Satellite records spanning more than four decades show that rising levels of atmospheric carbon dioxide have stimulated dryland plant growth during wet years, causing a persistent "greening" trend over time. But University of Arizona researchers discovered that this trend masks severe year-to-year swings that leave ecosystems especially vulnerable during dry years. That volatility creates unpredictability for rain-fed agriculture and reveals that major global vegetation models are overestimating dryland stability.
Led by Wen Zhang, a doctoral student in the School of Natural Resources and the Environment, the research team analyzed 40 years of satellite data to track changes in vegetation leaf area index—a measure closely linked to vegetation activity and productivity. They used their results to evaluate the performance of a collection of the best available global vegetation models. A key finding of the study was that roughly 80% of global drylands are experiencing escalating instability.
"The upper and lower extremes are getting farther and farther apart as time goes by," Zhang said. "Vegetation activity is increasing during wet years, but dry years are hitting plants harder. It's a bit like the nursery rhyme about the little girl with the curl: When it's good, it's very good, but when it's bad, it's awful." She said it's possible this boom-and-bust dynamic is the result of increasing atmospheric carbon dioxide combined with rainfall variability, but more data is needed to determine the precise mechanisms causing the instability.
"There's some evidence to suggest that under high atmospheric CO2, plants can become more efficient with their water use, which allows them to grow more leaves," she said. "But larger vegetation requires more resources to maintain, so when a moderate drought hits the following year, these larger plant structures need more resources than are available, which leaves them far more sensitive and vulnerable." In rain-fed farming regions like the American Southwest, higher year-to-year volatility may force heavier reliance on artificial irrigation to maintain productivity. It also means pasture forage production will be more unpredictable, with implications for livestock production and rangeland management.
"Higher variability in forage production presents a significant challenge for rangeland managers," said Bill Smith, senior author of the study and associate professor specializing in land, water and climate change geospatial analysis in the School of Natural Resources and the Environment. "Ranchers depend on stable forage production so they can accurately plan out their land needs each growing season. Less predictable forage production can thus disrupt their plans with potential detrimental consequences to livelihoods." The swings in plant productivity may also be an early warning sign of more significant ecological change, according to David Moore, a study co-author and professor in the School of Natural Resources and the Environment and watershed management and ecohydrology program chair.
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