Global Vegetation Growth Has Stalled for the Last 20 Years and Scientists May Now Know Why

Declines in the productivity and growth of land vegetation around the world since the late 1990s are the result of changes in the water content of the atmosphere, according to research which casts new light on the functioning of Earth's ecosystems in the face of climate change.

A team of researchers—led by Wenping Yuan from Peking University—analyzed four global climate datasets which included satellite imagery, revealing that vegetation growth had stalled or reversed over the last two decades or so, according to a study published in the journal Science Advances.

To understand why, the researchers looked at something called vapor pressure deficit (VPD,) which is just one of many variables that scientists can use to assess the functioning of ecosystems. VPD essentially describes the difference between the water vapor pressure at saturation in the air and the actual water vapor pressure for a given temperature. Increases in VPD are often linked to rising air temperatures.

"Two years ago, we noticed global vegetation growth decreases starting from the late 1990s," Yuan said in a statement provided to Newsweek. "We tried to search for the causes [by looking at] temperature, precipitation and radiation. However, we failed to find strong evidence then. One year ago, we are inspired by the linkage between global warming and atmospheric water vapor, and the assumption that VPD will increase due to rising temperatures. Saturation vapor pressure indicates the maximum water vapor that the atmosphere can potentially contain."

VPD is a critical factor when it comes to photosynthesis in plants—the process by which the organisms harness light to fuel themselves. According to the researchers, changes in VPD can have significant affects on ecosystems. For example, higher atmospheric VPD can negatively interfere with photosynthesis in plants, hindering growth and increasing the mortality rates of vegetation and forests.

"In terms of ecological meaning, VPD indicates the water potential of the atmosphere, Yuan said. "Basically, if the water potential is larger in the atmosphere—i.e. VPD is larger—water will dissipate faster from the soil and plants. It's like there's a pump in the air, and the pump extracts the water from the soil and plants via the vascular tissue. When the VPD increases, then the pump extracts the water faster and stronger."

"[This will make] the soil and plants dry, and negatively impact vegetation growth," he said. "In addition, when the VPD increases, the plants will close their stomata in order to reduce water losses. Stomata are like gates between the atmosphere and the plant, through which carbon dioxide enters into the leaves. Carbon dioxide is a raw material for plant photosynthesis. However, stomatal closure with increasing VPD will substantially decrease the rate of CO2 entering into the leaves, and decrease photosynthesis as well as plant growth."

For their study, the researchers found that atmospheric VPD had increased significantly around the world in the late 1990s over land covered in vegetation. This seemingly led to a reduction in vegetation, reversing the growth which the planet had experienced between the years 1982 and 1998.

"We were excited to find substantial increases of VPD over the global scale—which means huge changes of atmospheric water vapor conditions—that has not been found before," Yuan said. "Definitely, these changes will impact vegetation growth. We also used process-based ecosystem models and machine learning methods to quantify and separate the impacts from atmospheric carbon dioxide concentrations and environmental variables on vegetation growth. The results show that increasing VPD since the late 1990s is a dominate cause for decreased vegetation growth."

The results indicate that spikes in VPD in the last few decades may have had a significant role to play in forest mortality where droughts were involved, the researchers say. Once the team had found that VPD was increasing, they tried to understand the reason behind this trend, armed with the knowledge that as global temperatures rise in the face of climate change, VPD is expected to increase.

"First, the rising temperature inland leads to increases in the saturation water vapor," Yuan said. "Second, we found the actual water vapor decreases. These two adverse changes amplify the difference and make VPD increase. It is easy to understand the reason for increasing saturation water vapor pressure. However, the reason for decreased actual water vapor is unexpected and surprising. Finally, we found the reason for this in ocean evaporation.

"The oceans play an important role in regulating global atmospheric water conditions," Yuan said. "Under global warming, the temperature inland increases much more than that of the ocean. This increases saturation water vapor pressure inland. On the other hand, global warming can also decrease the wind speed, which decreases water evaporation from the ocean surface. Previous studies have indicated that the evaporated water from the ocean surface contributes significantly to the water vapor over land. Therefore, decreased water evaporation from the ocean partly decreases the actual water vapor pressure over land and increases VPD."

The researchers say that no previous studies have looked at how VPD will impact the planet on a global level. This makes the latest results significant, because the rises in VPD which are expected to accompany climate change could have severe consequences, according to Yuan.

"Increasing VPD negatively impacts plant growth. Decreased rates of vegetation growth will substantially reduce the ability of plants to absorb atmospheric carbon dioxide," Yuan said. "The CO2 concentration in the atmosphere will increase if other conditions do not change, which will result in a stronger greenhouse gas effect."

Furthermore, "decreased rates of vegetation growth will reduce vegetation biomass inland—including crop yields, herbage yields, forest biomass stock, etc.—which will reduce the supply to human society."

forest mortality, Frazier Park, California
Dead and dying trees are seen in a forest stressed by historic drought conditions in Los Padres National Forest on May 7, 2015 near Frazier Park, California. According to an aerial survey conducted by the U.S. Forest Service in April, about 12 million trees have died in California forestlands in the past year because of extreme drought. David McNew/Getty Images