Science Snippets: Climate Change is Destroying My Beloved Sonoran Desert

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The saguaro cactus is the signature species of the Sonoran Desert. A columnar cactus, it grows to more than 12 meters tall and lives for more than 300 years. Found throughout the Sonoran Desert, individuals of this species grow a large network of roots and also have taproots. Characterized by beautiful white flowers, the saguaro cactus is the consummate indicator of the Sonoran Desert.

During the 20 years I lived in Tucson, Arizona, I took walks in the desert each morning and evening, usually with a dog named Savanna Louise Rose O’Malley leading the way. Although she found and chased many skunks, lizards, birds, and small mammals, Savanna was never injured by a saguaro cactus.

If there are species thought to be resistant to global warming, they are drought-resistant species such as cactuses and other succulents. Imagine my dismay when I came across a peer-reviewed paper titled Population decline of the saguaro cactus throughout its distribution is associated with climate change. Published in the Annals of Botany on 1 January 2025, this paper is authored by four scholars.

The saguaro cactus grows slowly, requiring 20-50 years to reach a height of one meter. With a scientific name of Carnegiea gigantea, individuals of the species grow to an expectedly great height. The tallest specimen measured so far was nearly 24 meters tall before it was toppled by a windstorm in 1986.

Like other plants in the cactus family, and other species of succulents, the saguaro cactus relies on an unusual and little-known means of sequestering carbon. Crassulacean acid metabolism is a process that allows plants to fix carbon at night and release it during the day. At night, plants that use crassulacean acid metabolism synthesize malic acid. This allows them to keep their stomates closed during the day. Stomates are the tiny holes in the plants—typically in the leaves—that allow the exchange of gases between the atmosphere and the plant. In most plants, stomates are open during the day, which allows carbon dioxide to enter the plant. However, open stomates also allow moisture to escape. If you live in a desert, water is too useful to release during warm days. As a result, crassulacean acid metabolism separates the processes of sequestration and respiration. Carbon can be captured at night without the loss of precious water. Plant growth and respiration continue throughout the day with stomates closed, thus ensuring that water is preserved.

The peer-reviewed paper in the Annals of Botany provides relevant information about the ongoing terrors of climate change. It begins with a four-part Abstract. The first is called Background and Aims: “Climate change is a global phenomenon affecting species, which in arid regions will translate into more frequent and intense periods of drought. The Sonoran Desert is becoming hotter and drier, and many organisms are rapidly changing in abundance and distribution. These population attributes depend directly on the dynamics of the population, which in turn depends on the vital rates of its individuals; yet few studies have documented the effects of climate change on the population dynamics of keystone species such as the saguaro cactus ... Although saguaros have traits that enable them to withstand present environmental conditions, climate change could make them vulnerable if forced beyond their tolerance limits.”

I skip the Methods section and turn instead to the Key Results section: “We found that all populations will decline, mainly due to future increases in drought, mostly hindering recruitment. However, the decline will be different across populations, since those located near the coast will be affected by harsher drought events than those located further inland.”

The Abstract concludes with a Conclusions section: “Our study demonstrates that climate change and its associated increase in drought pose a significant threat to the saguaro cactus populations in the Sonoran Desert. Our findings indicate that the recruitment of saguaros, vital for establishing new individuals, is particularly vulnerable to intensifying drought conditions. Importantly, regional climate trends will have different impacts on saguaro populations across their distribution range.”

The Introduction of the peer-reviewed paper relies on abundant peer-reviewed evidence in describing the extent and importance of arid regions: “Deserts and semi-deserts are the most extensive biomes on Earth, occupying more than one-third of the global land surface ... These ecosystems are predicted to be highly vulnerable to global warming ... Increasing CO₂, rising temperature, decreasing precipitation, and expanding variance in rainfall and temperature are changing community species compositions, ecosystem processes and biodiversity through their effects on population demography, risking the persistence of key species ... The Sonoran Desert is projected to experience increased aridity in the 21st century due to increasing temperatures and variability in mean annual precipitation ... Regional droughts will be the most common form of disturbance, producing gaps and pulses in population age distributions ... The consequences of the ongoing climatic changes on keystone species in the Sonoran Desert have been modelled using bioclimatic variables and species distribution models ... However, few studies have used demographic data such as growth, survival and fecundity to project population performance under climate change, which provide a mechanistic understanding of the processes driving population dynamics. Demographic data for long-lived desert species are scarce, with few long-term studies …, and even fewer that measure the entire set of vital rates (growth, survival and fecundity) or cover a sizable portion of a species’ geographical range. These long-term demographic datasets could be important assets to inform how climatic variation affects the demography of key species, and how climate change will in turn determine their population viability in the future.

The saguaro cactus … is a keystone and iconic species of the Sonoran Desert, whose distribution spans almost all of the Desert’s mainland extension ... Saguaros are particularly suitable plants for evaluating the effects of climate change because their growth and survival are mainly controlled by temperature and water availability ... More specifically, adult saguaros can persist well in the present desert environment and have fine-tuned their physiological responses to current environmental cues ... However, under a rapid climate change scenario, the selective pressures acting on saguaro vital rates could become more intense. This is particularly important considering that saguaros may already exist close to their physiological limits in some parts of their range.”

I know this is a personal story, the likes of which is rare in this space. However, it’s also a warning as our time winds down. Even the most iconic, rugged species in arid regions are threatened with extinction. If you believe you’ll be fine because you have a little food stored away, I suspect you need to change your belief.