Water as the hidden driver of the Earth’s carbon cycle

  • February 6th, 2017
Fotograma Gustau Camps-Valls

An international research involving the Universitat de València just provided a new vision in the quest to determine whether temperature or water availability is the most influential factor in determining the success of global, land-based carbon sinks. The results determined that the water is so important at local and regional scales as temperature is at a global planetary scale. The study, recently published in Nature, is led by the Max Planck Institute for Biogeochemistry in Jena.

The study, carried out by an international interdisciplinary team of scientists, including the participation of the physicist at the University of Valencia Gustau Camps-Valls, revealed new clues on how land carbon sinks are regulated at both local and global scales. The current climate change is characterized by rising atmospheric carbon dioxide (CO2) concentrations and concomitant atmospheric warming. However, the annual growth rate of CO2 measured in the atmosphere during  decades varies largely from year to year. 

In this context, discussions on whether temperature or water availability drive these year-to-year changes have been very controversial. Different studies have linked these variability to the annual carbon balance with global or tropical temperatures, while others have found that the greater variability of carbon stock is in water-limited regions.

«Explaining that interannual variability has been an unresolved problem so far; and this article has reconciled the two conflicting visions» says Gustau Camps-Valls, professor in the Department of Electronics Engineering of the University of Valencia and researcher at the Image Processing Laboratory (IPL) in the Parc Científic. «For the study we applied two types of independent models; on the one hand, models based on machine learning -such as neural networks-, and on the other hand physical models. Then we analyzed the local and global effects of changes in temperature and water availability throughout all the carbon cycle,» says Camps-Valls, who participated in the first approximation.

The team found that, locally, water availability provides the most dominant cause of the year-to-year variability of both CO2 uptake in plants by photosynthesis, and CO2 release from plants and microbes respiration. However, on a global scale variability is mostly driven by temperature fluctuations, the research showed.

«What looks quite paradox at a first view, can be illustrated by looking close at the different spatial and temporal variations of the biosphere-atmosphere interactions», explains Dr. Martin Jung, lead author of the Nature publication. «There are two compensatory effects of water availability: first, at the local scale, temporal water-driven photosynthesis and respiration variations compensate each other. In addition, on a global scale, anomalies of water availability also compensate in space. If it is very dry in one part of the world, it is often very wet in another region, thus globally water-controlled anomalies in net carbon exchange outweigh in space,» adds Jung.

The research, led by the Max Planck Institute in Jena (Germany) and conducted by an international team of 24 scientists, not only sheds light on the contradictory findings, but the result also points to the need to investigate the relative relevance of climate change variables across different spatial and temporal scales, especially in the current conditions of global warming.

Martin Jung, Markus Reichstein, Christopher R. Schwalm, Chris Huntingford, Stephen Sitch, Anders Ahlström, Almut Arneth, Gustau Camps-Valls, Philippe Ciais, Pierre Friedlingstein, Fabian Gans, Kazuhito Ichii, Atul K. Jain, Etsushi Kato, Darío Papale , Ben Poulter, Botond Raduly, Christian Rodenbeck, Gianluca Tramuntana, Nicolas Viovy, Ying-Tennis Wang, Ulrich Weber, Sönke Zaehle, Ning Zeng. Compensatory water effects link yearly global land CO2 sink changes to temperature. Nature 2017; DOI: 10.1038/nature20780


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