Terrestrial water storage: large-scale variability and impacts on the global carbon cycle

Autor: Humphrey, Vincent
Přispěvatelé: Seneviratne, Sonia I., Rodell, Matthew, Gudmundsson, Lukas
Jazyk: angličtina
Rok vydání: 2017
Předmět:
Popis: Terrestrial water storage consists of all water reservoirs of the Earth system, including groundwater, soil moisture, land ice, surface waters, snow cover as well as water stored in the biosphere. Every year, natural variability in climatic conditions worldwide leads to droughts and floods which have significant impacts for ecosystems and societies. All water reservoirs act as natural buffers and balance these fluctuations by providing water supply during dry conditions and by storing water surplus after rain and snow events. However, these changes in terrestrial water storage do not only reflect a response to climate variability, they also feed back to the energy and carbon cycles, introducing a complex picture of inter-annual variability and delayed response mechanisms within the climate system. Since 2002, the Gravity Recovery And Climate Experiment (GRACE) satellites have provided the very first global observations of terrestrial water storage changes, allowing for entirely new analyses on the role of this variable for the Earth system and the degree to which human activities impact freshwater resources. However, while a large number of regional studies have provided a rich overview on individual aspects of water storage changes, there are still few global assessments of the sensitivity of terrestrial water storage to changes in atmospheric conditions. In addition, our understanding of how extremes in water storage feed back to the global energy and carbon cycles is still very uncertain. This is partly because some of these effects occur on inter-annual to decadal time scales and are therefore difficult to identify from the relatively short GRACE record. The overarching goal of this thesis, which is organized in three individual studies, is to resolve these important research gaps. The first study presented in this thesis provides a global survey of the spatial and temporal modes of variability found in GRACE observations of terrestrial water storage changes. An extensive review of the literature shows that these different modes of variability tend to coincide with different hydrological processes and can be partly related to changes in temperature and precipitation. In addition, most of the high-frequency GRACE signal can be explained by precipitation if an adequate averaging filter is applied to the daily precipitation time series. Based on these relationships, the second study makes use of historical precipitation and temperature data to reconstruct terrestrial water storage changes prior to the GRACE mission. The main outcome of this study is an open-access dataset providing estimates of monthly terrestrial water storage changes for the period 1985 to 2015 and at a spatial resolution of 3° (about 330 km). This dataset is shown to be on average superior to hydrological model estimates, which represent the state of the art for estimating past water storage changes. Two potential applications are then illustrated, showing that this dataset can be used to isolate secular trends from climate-driven inter-annual variability and to investigate inter-annual changes in sea level. Finally, the third study is dedicated to the impact of water storage changes on the global carbon cycle. For the first time, a significant negative coupling between changes in terrestrial water storage and year-to-year variations in the atmospheric CO2 growth rate is shown at the global scale. The identified relationship suggests that drier years coincide with a reduction in the assimilation of CO2 by the terrestrial biosphere. In contrast, current dynamic global vegetation models are shown to underestimate the strength of this relationship. These results challenge the prevailing paradigm that tropical temperature is the dominant driver of global fluctuations in the net land carbon flux. They highlight terrestrial water storage as a global driver of changes in carbon cycle-climate feedbacks. Altogether, this thesis demonstrates how fluctuations in terrestrial water storage both influence and respond to climate variability, stressing the importance of an accurate representation of this variable in Earth system models.
Databáze: OpenAIRE
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