According to a recent study published in PNAS and involving CREAF and the Spanish National Research Council (CSIC), the Tibetan Plateau’s permafrost, one of the world’s largest perennial ice reserves, is thawing at an accelerated rate in spring. The study’s findings indicate that solar radiation is the primary cause of the thaw, as sunlight penetrates the ground and melts the snow. That, in turn, generates soil moisture, which plants take up through their roots, enabling them to grow more and extend their territory. The second leading cause of the thaw is the warming brought about by climate change. “Intriguingly, we’ve seen temperature’s effect on the onset of the spring freeze-thaw cycle diminishing over the last 40 years, whereas the influence of solar radiation has grown,” states CREAF-based CSIC researcher Josep Peñuelas, one of the study’s authors. One potential cause is the dwindling snow cover. That entails a reduction in albedo (the amount of sunlight the snow is able to reflect), resulting in the ground absorbing more solar energy and soil temperatures rising. “Solar radiation and albedo play a crucial role in the transition from winter to spring, but here we can see how the warmth of the last few years has strengthened their impact,” explains Peñuelas.
The Tibetan Plateau is also known as the “Third Pole” because its ice fields contain Earth’s largest reserve of fresh water outside the polar regions. This reservoir of frozen water feeds the Ganges, Indus, Mekong, Yangtze and Yellow rivers, among others, providing much of Asia with water resources for irrigation, consumption and hydropower. Such is its importance that the authors regard understanding the mechanisms behind the thaw as essential. Their study, they say, represents progress in that direction.
“It is curious because the trend we have observed over the last 40 years is that the effect of temperature has been decreasing and, in contrast, solar radiation has gained weight at the beginning of the spring freeze-thaw cycle.”.
JOSEP PEÑUELAS, CSIC researcher at CREAF.
To arrive at their findings, the researchers examined data spanning the last four decades (1980-2018) on the factors that trigger the spring freeze-thaw cycle. Their analysis involved satellite measurements of variables such as temperature, precipitation, snow depth, solar radiation and leaf area from 64 sites across the Tibetan Plateau.
Greener is not always better
On the subject of the region becoming greener, Peñuelas cautions that an increase in vegetation is not necessarily positive, even if it does mean a slight fall in atmospheric CO2 concentration owing to plant photosynthesis. The problem is that the region’s new vegetation absorbs much more water and competes for resources. Furthermore, its growth will eventually be curtailed by the lack of other nutrients — such as potassium and, in particular, phosphorus — in the soil. Peñuelas points out that as temperatures and water availability increase, the plant and microorganism respiration rate rises, reducing the time for which atmospheric carbon is retained in plants and soil.
The reason is that this new vegetation also absorbs much more water and competes for resources. Moreover, the lack of other nutrients in the soil, such as potassium and, especially, phosphorus, will limit its growth.
The study was led by Nanjing University’s International Institute for Earth System Sciences. Along with CREAF and the CSIC, it also involved Zhengzhou University and Beijing’s Chinese Academy of Sciences. “This research not only broadens our understanding of the causes of the thaw, but also helps us project future scenarios and establish a roadmap for mitigation,” concludes Peñuelas.
Article: L. Jialing et al. Weakening warming on spring freeze–thaw cycle caused greening Earth’s third pole. PNAS 121 (8) e231958112. DOI: https://doi.org/10.1073/pnas.2319581121
