For at least a century, air, ocean, and land temperatures on Earth have been steadily rising. For at least the past forty years, the planet has been growing a bit greener. Now researchers have found that the greening of the planet can change the movement of air near the land surface in ways that offset at least some warming. Essentially, global warming would be even worse were it not for extra greenery changing how and where heat builds up across the landscape.
In 2019, remote sensing scientists Chi Chen, Ranga Myneni, and colleagues at Boston University used satellite observations to show that vegetation cover had increased globally by 5 percent since the early 2000s. In 2020, the research group linked that increase in greenness to a slight offset in global temperatures.
Now, in a new study, Chen and colleagues have worked to decipher how that greening could affect land temperatures. Using satellite data and advanced computer models, they found that increased vegetation has a cooling effect that comes from an increased efficiency in the vertical movement of heat and water vapor between the land surface and atmosphere.
There are several ways vegetation can alter temperatures at the surface. Changing leaf area can change albedo, or how much sunlight is absorbed or reflected by a landscape. More greenery can also change land surface resistance, or how well water can penetrate and be retained by soil and leaves. And it can change emissivity, or how the surface emits or reflects longwave radiation.
But according to the new study, the strongest cooling effect comes from the way increasing leaf cover leads to less aerodynamic resistance, or how features on the ground increase or decrease drag and turbulence in the air above. In many environments, extra leaves can enhance the efficiency of vertical air mixing, allowing more heat and water vapor to rise into the atmosphere. Extra leaves may also increase the amount of water transpired (exhaled) by plants, allowing even more water to be transferred. That extra moisture can carry away a significant amount of heat from the ground level and lead to cooler surfaces.
In their studies, Chen and colleagues have found that most vegetated areas on Earth (about 93 percent) see their land surfaces cool when leaf area increases. Since 2000, at least 30 percent of areas with more leaf coverage have been cooled by it, while 5 percent have grown warmer.
The map at the top left of this page shows the trends in the “leaf area index” around the world from 2000 to 2014. Leaf area index (LAI) is a measure of the amount of leaf area relative to ground area during the growing season. LAI is computed from data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA’s Terra and Aqua satellites. Note that the map does not show overall greenness, but how greenness has changed since 2000.
The second map (above right) shows global changes in land surface temperatures over the same period due to the increased leaf area. It is derived from satellite observations and from the Community Land Model. Note that the map depicts land surface temperatures (LSTs), not air temperatures. LSTs reflect how hot the surface of the Earth would feel to the touch in a particular location, and they can sometimes be significantly hotter or cooler than air temperatures.
“In the fight against climate change, plants are the lonely-only defenders,” said Chen, now a postdoctoral researcher at the Lawrence Berkeley National Lab. “Stopping deforestation and ecologically sensible large-scale tree-planting could be one simple, but not sufficient, defense against climate change.”
The study authors noted, however, that the cooling effect from extra vegetation is large from an energy dissipation perspective, but it is small compared to the pace and intensity of global warming.
NASA Earth Observatory images by Joshua Stevens, using data from Chen, C., et al. (2020). Story by Michael Carlowicz, with Kassie Perlongo, NASA Ames Research Center.