Global vegetation growth has stalled in the past two decades because of a long-term deficit in atmospheric water content, according to a new analysis of global climate datasets published in the August 14 issue of Science Advances.
The findings reveal that atmospheric water vapor is expected to further wane throughout the 21st century due to rising air temperatures and a decline in the evaporation of the world's oceans.
The decrease in atmospheric water cancelled out the effects of higher concentrations of carbon dioxide in the atmosphere, which would normally boost plant growth. The findings also suggest that if this drying continues, plants may not be able to absorb as much carbon dioxide from the atmosphere and forest and crop yields potentially could shrink.
"Our results support [lower atmospheric water] being part of the drivers of the widespread drought-related forest mortality over the past decades, which has been observed in multiple biomes and on all vegetated continents," said the authors.
The health and diversity of the terrestrial ecosystem rests on an intricate web of environmental variables. One of these variables is vapor pressure deficit (VPD) — the difference between the actual pressure exerted by water vapor in the air and the pressure that would be exerted in saturated air, according to the study. In essence, higher levels of VPD indicate the atmosphere holds less water.
VPD plays an important role in ecology and has a significant impact on plant life worldwide, say the study authors. Higher levels of atmospheric VPD push plants to close their stomata — pores that absorb carbon dioxide and release oxygen and water vapor — impairing photosynthesis and slowing the growth of vegetation and forests.
"Basically, if the water potential is larger in the atmosphere, if VPD is larger, water will dissipate faster and stronger from soil and plants," said Wenping Yuan, a professor at Sun Yat-Sen University in China and lead author of the new study.
He likened the influence of VPD to a "pump" in the atmosphere that extracts water from soil and plants. If VPD increases, then the pump extracts more water and dries soil and plants, negatively impacting vegetation growth.
Researchers have begun to more closely look at VPD in addition to factors such as precipitation when studying the health of the world's forests. However, it remains unclear how rapidly VPD is increasing, as well as how any changes could be affecting vegetation growth in ecosystems.
To investigate long-term trends in VPD, Yuan and his research team sifted through four observation-based, globally-gridded climate datasets. Using several statistical methods, they calculated how VPD changed from 1957 to 2015, and identified "turning points" that marked notable shifts in VPD trends.
The researchers discovered that VPD over vegetated land — areas identified using a classification system for land cover — sharply increased near the end of the 20th century. Prior to the late 1990s, VPD had increased only slightly, but they observed that after 1998 the upward trends in VPD dramatically grew, increasing in size by up to 17 times in the four datasets.
Approximately 53% to 64% of vegetated areas worldwide experienced a surge in VPD since 1998, according to the study. Notably, the mean annual VPD during the growing season from 2011 to 2015 was 11% higher than that of 1982 to 1986.
These changes in VPD were linked primarily to rising air temperatures and shifts in the evaporation of the world's oceans — the most important source of water vapor in the atmosphere. The researchers found oceanic evaporation began to decline in 1998 due to global warming after having increased since 1957, and reached a new low in 2015.
Having established that VPD has been steadily climbing, the scientists then set out to determine whether this spike reduced the growth of vegetation around the world. They tracked changes in two satellite-based indices of vegetation and leaf coverage on land as well as a global measure of the amount of biomass produced by plants.
Although both measures of plant growth had been increasing from 1982, they reversed and began to decline after 1998. Specifically, 59% of vegetated areas showed a pronounced decrease in vegetation after 1999, and up to 80% also displayed downward trends in land leaf coverage.
Global plant productivity similarly reversed and began to decline after 1998, in a manner opposite to the observed VPD trends. Two satellite-based models showed global plant productivity was highly sensitive to VPD, whose influence counteracted the growth-supporting effects of higher concentrations of carbon dioxide in the atmosphere.
Yuan and his team also studied a comprehensive dataset of tree-ring width measurements, based on data from 171 locations worldwide. Tree-ring widths were on average smaller after 1998 compared to those before 1998 at 64% of the measurement sites, indicating tree growth was affected by rising VPD.
As VPD rises, plants will not be able to absorb as much carbon dioxide from the atmosphere, limiting their ability to counteract rising carbon dioxide concentrations, he said. Furthermore, vegetation biomass will continue to decline, decreasing crop and forest yields and reducing the supply available to human society.
According to the research, six established climate models have projected that VPD will continue to increase throughout the 21st century. The growing deficit in atmospheric water could end up having very serious consequences for plant growth in the future, said Yuan.