The annual greening of vegetation follows the seasons and therefore occurs staggered in the northern and southern hemispheres. Researchers have now used a new method to investigate how this “green wave” moves across the globe and how it has changed in the last few decades. This shows that the center of greening has shifted further and further north and east. The team sees climate change as the cause: milder winters and longer growing seasons in northern regions are shifting the focus of the green wave further there. What was surprising, however, was that there is no similarly strong southward shift in summer in the southern hemisphere, as the researchers report. Instead, there is also a trend towards the north.
Seen from space, a green wave travels across the globe every year: In autumn and winter in the northern hemisphere, our vegetation loses a large part of its leaves and goes dormant, while at the same time the flora in the southern hemisphere turns green because spring and summer prevail there. If the days become longer and warmer in our country, this trend is reversed: the vegetation in the northern hemisphere develops its greatest size and leaf area, while winter rest sets in in the temperate and higher latitudes of the southern hemisphere. How this “green wave” moves and how strong it is depends on the climate and the seasons, but also on land use and human intervention in forests and other natural ecosystems. The CO2 levels in the atmosphere also influence how lush the vegetation grows in different regions. Conversely, changes in the “green wave” can provide clues to large-scale ecological and climatic trends.
Measuring method shows center of gravity of the green wave
However, despite the importance of this earth-spanning vegetation cycle, there has so far been a lack of a global measurement method that can regularly and quantitatively record changes in the “green wave”. “Unlike the global metrics for temperature, radiation incidence or sea ice area, there is no corresponding method for the dynamic phenology of vegetation,” explain Miguel Mahecha from the University of Leipzig and the Helmholtz Center for Environmental Research (UFZ) and his colleagues. Previous studies have mostly focused on regional shifts in growing seasons or biomass. Mahecha and his team have now presented a new method to track the development of the “green wave” in a standardized way. To do this, they use the concept of the centroid: They determine how the center of gravity of greening shifts compared to the centers of mass of the continents.
To do this, the team evaluated satellite data from 1982 to 2022 on vegetation indices such as the “greenness” of land areas and the leaf area index and then used special formulas to determine where the focus of plant growth lies. “Imagine holding a perfectly round globe in your hands and attaching small weights to it, each representing the green leaves at each point on the Earth’s surface,” explains Mahecha. “If you then carefully place this globe in calm water, the center of mass points downward.” The evaluation showed that the center of gravity of the green wave in northern summer is around 2,390 kilometers north of the equator, approximately at the level of Iceland. In southern summer, however, the focus of vegetation is only around 160 kilometers from the equator at the level of Liberia. “This asymmetry reflects the uneven distribution of plant-covered land masses on Earth,” explains the team.
Shift to the north and east
The analyzes also showed that the “green wave” has changed significantly over the last few decades. Their center of gravity has shifted further and further north since 1982 – moving north by an average of three to 14 kilometers per year. Contrary to expectations, this not only happens during the northern summer, but also continues in the southern summer – when there should actually be a countermovement to the south. “That really surprised us,” says Mahecha. According to common assumptions, climate change should promote plant growth in the higher latitudes of both hemispheres. But this trend seems to be stronger in the north than in the south. “This could be caused by the larger landmasses in the northern hemisphere,” the team writes. If climate change causes more vegetation in northern Eurasia to survive the winter or even continue to grow, its mass and area will outweigh that of the same effect in the southern hemisphere. “As a result, even a slight greening up in the northern winter months can compensate for the corresponding southward shift,” explain Mahecha and his colleagues. Overall, the amplitude of the “green wave” has reduced – it oscillates less pronouncedly to the south.
At the same time, the “green wave” shows another, similarly strong trend: “Its movement is shifting not only to the north, but also to the east,” the researchers report. “This is unexpected but could be related to regional greening hotspots in India, China, Europe and possibly Russia.” Overall, the evaluations suggest that terrestrial vegetation is also showing initial reactions to global climate change in terms of its temporal and geographical dynamics. Using Earth system models, Mahecha and his team also determined that the shift in the focus of vegetation will continue. “Even under the most optimistic future scenarios, the northward movement of the ‘green wave’ will continue,” the researchers write. “This highlights the impact of climate change on Earth’s ecosystems.” The new method now offers scientists a tool to understand how our planet’s biosphere is reorganizing in a warming world.
Source: Miguel Mahecha (University of Leipzig) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2515835123