Desertification was supposed to be the ‘greatest environmental challenge of our time.’ Why are experts now worried about greening?

 

Southeast Australia has been getting hotter and drier. Droughts have lengthened, and temperatures regularly soar above 95 degrees F (35 degrees C). Bush fires abound. But somehow, its woodlands keep growing. One of the more extreme and volatile ecosystems on the planet is defying meteorology and becoming greener.

And Australia is far from alone. From Africa’s Sahel to arid western India, and the deserts of northern China to southern Africa, the story is the same. “Greening is happening in most of the drylands globally, despite increasing aridity,” says Jason Evans, a water-cycle researcher at the Climate Change Research Centre of the University of New South Wales in Sydney, Australia.


What is going on? The primary reason, most recent studies conclude, is the 50-percent rise in carbon dioxide concentrations in the atmosphere since preindustrial times. This increased C02 is not just driving climate change, but also fast-tracking photosynthesis in plants. By allowing them to use scarce water more efficiently, the CO2-rich air fertilizes vegetation growth in even some of the driest places.


As we pump yet more CO2 into the air, arid-land greening seems set to continue, according to two recent modeling studies. But ecologists warn that, despite appearances, going green may have downsides for arid ecosystems and for the people who depend on them. Desert plants and animals will often lose out, and the extra vegetation may soak up scarce water supplies.

Drylands cover roughly 40 percent of the planet’s land surface. The deserts at their core are surrounded by wide expanses of savanna grass, dry woodlands, and sometimes irrigated fields. They are home to more than a third of the world’s population and are among the most biodiverse ecosystems in the world, according to the International Union for Conservation of Nature, the international organization for conservation scientists.

Over the past half-century, most drylands have been experiencing a decline in rainfall, along with higher temperatures and greater rates of evaporation. Many have also been degraded by poor farming practices and overgrazing of livestock. Climate scientists and ecologists alike have until recently presumed that this combination of growing meteorological aridity and pressure from human activities would lead to less vegetation. They have routinely warned of widespread desertification, which U.N. officials have called “the greatest environmental challenge of our time.”

Yet in most drylands, this anticipated desertification has not happened. Rather than shriveling and dying, vegetation is usually growing faster and expanding its terrain, while deserts are retreating. This, researchers of the world’s carbon and water cycles say, is largely due to the extra CO2 in the atmosphere.

Photosynthesis is the process by which plants grow by absorbing CO2 through stomata in their leaves and converting it into plant matter. That process requires water, which in arid regions is often the limiting factor for plant growth. Higher concentrations of CO2 in the air both allow easier photosynthesis and enable plants to use less water in the process.

Agricultural scientists have long known about the benefits of additional CO2 for plant growth. Farmers sometimes dose the enclosed atmospheres of greenhouses with the gas to boost yields. In effect, we are now doing the same thing to the entire atmosphere.

The negative impacts of hotter, drier climates have not gone away; but in most arid lands this CO2 fertilization effect is proving more powerful. This supercharging of plant growth seems unlikely to be short-lived if fossil-fuel burning causes atmospheric concentrations of CO2 to continue rising. A new modeling study published last month found that it will, if anything, become more marked in the coming decades. “Most of the global drylands are projected to see an increase in vegetation productivity,” says Evans, a coauthor of the study.

This unexpected upside to CO2 will have implications for the pace of climate change itself. Instead of desiccating ecosystems and causing the release of their CO2, thus accelerating climate change, anthropogenic releases of the gas into the atmosphere are allowing vegetation to increase its capture of carbon — helping, if only modestly, to reduce it.

For some time, there has been growing evidence of global greening in all biomes, not just arid lands. Back in 2016, remote-sensing specialist Ranga Myneni of Boston University, with a team of 32 others from eight countries, studied NASA satellite images to discern trends in vegetation. They concluded that between a quarter and a half of the planet’s vegetated areas had since 1980 shown an increase in their leaf area index, a standard measure of the abundance of vegetation.

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