Biodiversity hotspots rich in species and numbers include central Mexico and the Brazilian Atlantic Forest.

To test the notion that cacti will benefit from a warmer and more drought-prone world, researchers led by Michiel Pillet from the University of Arizona examined data on more than 400 species and ran models projecting how they would fare at mid-century and beyond under different greenhouse gas emissions scenarios.

The findings "paint a more pessimistic future," according to the study, published Thursday.

Currently, the main threat to cacti is expanding agriculture, along with land degradation, biodiversity loss and harvesting for various uses.

Even without climate change, cacti "is one of the most endangered groups of organisms on the planet," with more than 30 percent classified as at risk of extinction, the authors note.

Under a moderate emissions scenario in line with current policies, global warming will soon be a significant threat as well.

"Our results suggest that climate change will become a primary driver of cactus extinction risk, with 60 to 90 percent of species assessed negatively impacted" by global warming, the researchers reported.

Within four or five decades, some 25 percent of cacti species could experience unfamiliar climates over a quarter of their current range.

Earlier studies have shown impaired photosynthesis -- the process by which plants use sunlight to make foods from CO2 and water -- with only two degrees Celsius of global warming.

Earth's average surface temperature, including oceans, is already 1.1C warmer than preindustrial times, and about 1.7C warmer over land only.

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