CO2 absorption and environmental protection benefits

Drylands cover about 40 percent of the planet’s land area. Deserts at their core are surrounded by vast expanses of grassland, dry forests, and sometimes irrigated fields. They are home to more than a third of the world’s population and are among the most biologically diverse ecosystems in the world, according to the International Union for Conservation of Nature, an international organization of conservation scientists.

Over the past half-century, most drylands have experienced reduced rainfall, along with higher temperatures and higher evaporation rates. Many have also been degraded by poor farming practices and overgrazing. Climate scientists and ecologists have until recently assumed that the combination of increasing meteorological dryness and pressures from human activities would lead to less vegetation. They regularly warn of widespread desertification, which United Nations officials have called “the greatest environmental challenge of our time.”

In most drylands, however, this predicted desertification has not occurred. Instead of withering and dying, vegetation has grown faster and expanded, while deserts have shrunk. This is largely due to excess CO2 in the atmosphere, say researchers who study the world’s carbon and water cycles.

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

Agricultural scientists have long known about the benefits of supplemental CO2 for plant growth. Farmers sometimes inject the gas into the closed atmosphere of greenhouses to boost yields. In fact, we are now doing the same thing with the entire atmosphere.

The negative impacts of a hotter, drier climate aren’t gone yet; but in most drylands, this CO2 pollination effect is proving to be stronger. This boost in plant growth is unlikely to last long if fossil fuel burning keeps atmospheric CO2 levels rising. A new modeling study published last month found that, if anything, it will become more pronounced in the coming decades. “Most drylands globally are projected to see an increase in vegetation productivity,” said Evans, a co-author of the study.

This sudden increase in CO2 will have an impact on the pace of climate change. Instead of drying out ecosystems and causing them to release their CO2, thus accelerating climate change, human emissions into the atmosphere are allowing vegetation to increase its ability to absorb carbon — helping, if only modestly, to reduce it.

For some time now, there has been mounting evidence of global greening across all biomes, not just drylands. Back in 2016, remote sensing expert Ranga Myneni of Boston University, along with a team of 32 others from eight countries, studied NASA satellite imagery to discern vegetation trends. They concluded that between a quarter and half of the planet’s vegetated areas had shown increases in their leaf area index, a standard measure of vegetation richness, since 1980.

Myneni’s further statistical analysis suggests that about 70 percent of this global greening can be attributed to CO2 fertilization. Other factors include local changes in nitrogen deposition from air pollution, rainfall, and land cover.

These findings appear to be confirmed by a 2021 study at the University of California, Berkeley that assessed photosynthesis rates in a range of ecosystems around the world. Using a network of “flux towers” ​​to measure the exchange of gases between vegetation and the air above, carbon cycle researcher Trevor Keenan and colleagues concluded that there had been a 12 percent increase in photosynthesis since 1982, with CO2 fertilization again being the main driver.

The extra CO2 absorbed by faster-growing plants is reducing the buildup of the gas in the atmosphere, Keenan said. “It’s not stopping climate change by any means, but it’s helping us slow it down.”

This global greening is most visible—and has the greatest impact on ecosystems and the lives of the people who depend on them—in drylands. It’s not happening in all drylands. Some places are turning brown. But not many.

A 2020 review by Evans and Arden Burrell, a remote sensing researcher at the Woodwell Climate Research Center in Falmouth, Massachusetts, found that about 6 percent of drylands have experienced desertification since 1982, just a quarter of what was previously estimated based on meteorological conditions. These areas include much of the southwestern United States, drought-prone northeastern Brazil, and parts of Central Asia.

But Evans and Burrell found that significant greening is much more widespread than previously thought—and three times larger than desertification. It covers 41 percent of the world’s drylands, from India to the African Sahel and northern China to southeastern Australia.

Last year, Guolong Zhang and colleagues at Lanzhou University in China reported finding a global divergence between aridity and leaf area in drylands over the past three decades. Zhang said the reason for the “decoupling” lies in the “fertilization effect of CO2.”

Why were previous predictions of widespread desertification so wrong? Evans says one reason is that researchers believed that their standard measure of atmospheric dryness, the aridity index, would reliably predict vegetation growth potential.

The aridity index is the ratio of precipitation to potential moisture loss through evaporation. The lower the ratio, the more arid the conditions. When global CO2 concentrations remain constant, the vegetation cross-reading works well; but as atmospheric CO2 increases, plants use moisture more efficiently and their growth no longer reflects the weather. If rising CO2 levels have a greater impact than falling precipitation, aridity is associated with greening.

Water is not the only potential limiting factor in dryland plant growth. The availability of nutrients, especially nitrogen, is another. That raises questions about whether the benefits of CO2 fertilization will continue to increase. But recent climate models suggest that the greening of drylands is unlikely to slow before mid-century and may accelerate.

Last year, Ziwei Liu, a hydrological modeler at Qinghe University in Beijing, concluded that, if the impact of CO2 fertilization on arid lands is taken into account, arid land area will expand by only 5 percent by the end of this century, but vegetation productivity will increase by about 50 percent.

And last month, Burrell, Evans and Xinyue Zhang, also of the University of New South Wales, found the same thing in the most detailed model to date. The new projections “suggest continued increases in aridity due to climate change,” but “less than 4 percent of drylands [will] become desertified ,” they concluded. The exact extent of future greening will depend on how much CO2 accumulates in the atmosphere, Evans said. But in all scenarios, their model predicts that most drylands will become greener.

The impacts of this greening have been profound. Forests in eastern Australia have seen “back-to-back record-breaking droughts and heatwaves” over the past four decades, says Sami Rifai, now at the University of Adelaide. But during that time, “CO2 fertilization has overtaken increasing dryness to drive the greening of Australia’s woodland ecosystems.”

Some researchers believe that other factors, such as how farmers use land, may be important at the local level. Poor land use can often cause desertification, such as when trees are cut down for firewood, poor farming practices cause soil erosion, or too many livestock graze on pastures that cover much of the used dry land. But farmers also sometimes “green” barren land by planting irrigated crops or raising trees on their fields. In some of the most heavily greened areas, there may be multiple forces at play.

A 2019 study by Myneni and others concluded that land-use management “is the primary driver of ‘Green Earth,’ accounting for more than a third, and possibly more, of the observed net increase in green leaf area.”

Take the Sahel region on the southern edge of the Sahara Desert. Vegetation growth there has benefited from the excess CO2 in the atmosphere everywhere. But the region has also seen the return of rain after devastating droughts in the 1970s and 1980s. And in some places, farmers have changed their farming practices, encouraging natural tree regrowth in their fields to provide shade and nutrients for their crops.

Geographer Chris Reij of the World Resources Institute in Washington, DC, has tracked this trend among farmers in Niger. He estimates that there are now about 200 million more trees on about 12.5 million acres of previously nearly treeless land in the south of the country.

“If CO2 fertilization were the key to greening here, it would be happening everywhere in an area, but it’s not,” he said. Instead, the greening stops abruptly at the border with Nigeria, where farmers have shown little interest in nurturing trees.

Evans agrees that the remarkable greening his research found in southern Niger may be related to farmers replanting trees. And he says Indian farmers are also playing a role. In arid states like Gujarat, they are pumping groundwater to irrigate crops on once-barren land. The increase in soil moisture is therefore manifesting as greening, Indra Tripathi, a water resources engineer at the Indian Institute of Technology in Gandhinagar, Gujarat, concluded in a study published in March.

So is this good news? Ecologists warn that it is not. Most obviously, greening fields through irrigated agriculture can devastate scarce water reserves and wipe out valuable dryland ecosystems.

And there are downsides to greening natural ecosystems. “Save the desert” may not be a popular environmental message, but arid ecosystems are important. They are critical habitats for species uniquely adapted to water scarcity, whether plants that can survive decades without rain or desert beetles that have evolved novel body shapes to collect moisture from fog.

Such specialists may be lost as the environment they have evolved to exploit changes. Outsiders may move in. Indeed, the greening of ecosystems may signal the invasion of fast-growing alien plants better adapted to taking advantage of high CO2 levels, growing rapidly and crowding out native species.

Long-term studies by the University of California, Riverside in the Sonoran Desert show that low-lying shrubs better adapted to low rainfall and high temperatures are replacing native plants, creating an impression of greenness that marks ecological degradation.

In some places, excess vegetation in arid environments also increases the risk of wildfires. Four years ago, fires ripped through southeastern Australia, burning an area the size of South Carolina. Foresters blamed the fires on a combination of drought, high temperatures and a buildup of flammable woody vegetation, which analysis suggests is partly due to CO2 fertilization. The symbol of greening has burned.

The world was mistaken in expecting that climate change would cause rapid and widespread desertification in the world’s drylands. In fact, the opposite is happening. But it would be equally foolish to imagine that the astonishing greenness now seen in satellite images of many of these lands is a reason to declare that their troubles are over.