Biogeosciences News

Early Sprouting of Leaves Enhances Northern Hemisphere Warming

As leaf out has been advancing 4–5 days per decade, scientists say the effect of vegetation on climate remains poorly understood.

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Scientists know that climate change is affecting vegetation, but the reverse process—how vegetation change is affecting the climate—is not as well understood. In a new study, researchers have found that the sprouting and opening of leaves earlier than usual are adding to surface warming in the Northern Hemisphere. The phenomenon is having a greater effect in colder regions because of declining snow cover, and it may cause leaf out to occur earlier. In a vicious circle, warming results in earlier leaf out, which in turn enhances warming itself.

Reporting in Nature Climate Change, the researchers from China’s Institute of Atmospheric Physics and Nanjing University of Information Science and Technology, along with Lawrence Berkeley National Laboratory in California, looked at data from more than 40 satellite-derived spring phenology studies across the globe that were conducted since the 1980s. Most showed earlier leaf out resulting in greener spring vegetation, with leaf out advancing 4 to 5 days per decade on average. They then used a global climate model computer simulation, the Community Earth System Model (CESM), which models Earth’s climate in the past, present, and future, to compare the climate consequences with and without the earlier leaf out.

They found that vegetation greening in spring has a profound impact on climate systems because it enhances the release of water vapor. Vapor released during photosynthesis can contribute to cloud formation, which affects the amount of sunlight reaching Earth.

The vapor can also be carried toward the poles and can result in changes in snow and cloud cover in the northern high latitudes; less snow means less surface reflectivity of solar radiation and more surface warming. Researchers think this reduced snow cover explains why there are hot spots in the north. The authors identify warming hot spots in the Canadian Arctic Archipelago of about 0.7°C, the southeastern Tibetan Plateau of about 0.3°C, and the eastern and western edges of Siberia of about 0.4°C.

Great Framework for Follow-Up

The findings follow decades of research into the relationship between vegetation and climate. Despite the enormous complexity of that interaction over larger scales in both area and time, there is growing awareness that vegetation changes can impact global atmosphere circulation.

“It has long been known that vegetation influences climate through altering surface albedo, roughness, canopy conductance, and fluxes of water, energy, and carbon,” said Gensuo Jia, deputy director of the Institute of Atmospheric Physics’ Key Laboratory of Regional Climate-Environment for Temperate East Asia and a coauthor of the study. “Most of our understanding of the role of vegetation on climate is the local impact of water and energy fluxes. Until recent years, with the help with coupled land-atmosphere models, the atmospheric transport of water, energy, and carbon and their nonlocal climate effects have been receiving more attention.”

“This study is very interesting and contributes to our growing body of knowledge that widespread changes in vegetation can affect climate not only locally but globally,” said Park Williams, a research professor at Columbia University’s Lamont-Doherty Earth Observatory who was not involved in the study. Because it’s a model-based study, there will be a wide spread in how the effects are simulated, he added. “So rather than this study giving us the end-all answer to the question of how early green up affects Northern Hemisphere climate, it should be thought of as a first estimate and treated as a great methodological framework for follow-up studies that use alternate climate models.”

The researchers in the study aim to better understand the mechanisms involved with the warming effects of earlier leaf out.

“For the next step, we may further explore field-based land-atmosphere flux measurements and satellite-derived parameters of energy and water budget,” said Xiyan Xu, a research scientist at the Institute of Atmospheric Physicis, Chinese Academy of Sciences, and lead author of the study.

—Tim Hornyak (@robotopia), Science Writer

Citation: Hornyak, T. (2020), Early sprouting of leaves enhances Northern Hemisphere warming , Eos, 101, https://doi.org/10.1029/2020EO141580. Published on 18 March 2020.
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