SNF Post-doctoral Scholar at the California Institute of Technology, Pasadena, United States
I am a climate scientist working on the interactions between the atmosphere, land hydrology and terrestrial ecosystems in the context of recent climate change. I try to focus on scientific problems that have a strong potential to impact and advance the work of the community.
Water-Carbon interactions in the Earth System
Interactions between the water and the carbon cycles play an important role in regulating carbon uptake by the biosphere, however they are still poorly represented in current Earth system models, partly due to the lack of observational constraints.
Funded by the Swiss National Science foundation, this research project will make use of recent and innovative satellite observations in order to improve our understanding and model representation of the interactions between photosynthetic activity and water limitation.
The project will run from January 2019 to December 2020 at the California Institute of Technology, and will be conducted in the group of Prof. Christian Frankenberg. First results were presented in April 2019 at the General Assembly of the European Geoscience Union (EGU) in Vienna.
Reconstructing past changes in water storage
The GRACE-REC dataset is a reconstruction of historical climate-driven changes in water resources over the last century, covering the entire world.
This was done by combining recent satellite observations of terrestrial water storage changes with historical precipitation and temperature datasets. Our estimates are validated against long-term in-situ observations of river discharge taken at more than 10'000 stations.
Droughts enhance atmospheric CO₂ growth
Humphrey, V., Zscheischler, J., Ciais, P., Gudmundsson, L., Sitch, S., & Seneviratne, S. I. (2018). Sensitivity of atmospheric CO 2 growth rate to observed changes in terrestrial water storage. Nature, 560(7720), 628.
We have found that during global droughts the concentration of carbon dioxide in the atmosphere rises faster because stressed ecosystems absorb less carbon. This global effect is stronger than previously thought and appears to be underestimated in current climate models.
For the first time, we were able to quantify a yearly sensitivity of -1.33 Gigaton of carbon per Teraton of water (95% confidence interval −1.85 to −1.07 GtC per Tt H₂O). In other words, on average, for each kilogram of water that is missing, there is 1.33 additional grams of carbon in the atmopshere.
Land ecosystems absorb on average 30% of anthropogenic CO2 emissions, thereby slowing the increase of CO2 concentration in the atmosphere. But plants need water to grow. When a drought occurs and soils dry out, plants reduce photosynthesis and breathe less in order to save water and preserve their tissues. As a consequence, they are no longer able to capture carbon dioxide from the surrounding air and more CO2 remains in the air.
Why is this important? As human CO2 emissions continue to increase, current models disagree on whether more droughts will limit the ability of ecosystems to take up CO2, which would accelerate global warming. We have to make sure models get this right!
Our results suggest that current carbon cycle models tend to underestimate the global effect of droughts on terrestrial carbon uptake. Some known limitations of current models could explain why this happens. For instance, the representation of hydrological processes in those models might not be entirely adequate. Current parameterizations of the sensitivity of plants to water stress might need improvement. Finally, some important carbon processes that are strongly linked to the water cycle (i.e. carbon transport and mineralization in inland waters) are only just starting to be implemented in the next generation of models.
Terrestrial water storage trends in GRACE and CMIP5
Jensen, L., Eicker, A., Dobslaw, H., Stacke, T., & Humphrey, V. (2019). Long‐term wetting and drying trends in land water storage derived from GRACE and CMIP5 models. Journal of Geophysical Research: Atmospheres.
Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) mission have been measuring trends in water storage from anomalies of the Earth's gravity field. In her recent paper, Laura Jensen showed how comparing these estimates with climate models is not straightforward because water storage trends computed over less than 15 years are often obscured by inter-annual and decadal natural variability.
Disentangling natural and anthropogenic changes in water resources
In many regions of the world, water resources are affected by changes due to both human activities (e.g. groundwater extraction for irrigation) and climate variability (droughts, climate change).
Our paper illustrates how these two effects could be disentangled by using statistically generated ensemble estimates of the climate-driven variability in terrestrial water storage.
Global assessment of water storage variability
Humphrey, V., Gudmundsson, L., & Seneviratne, S. I. (2016). Assessing global water storage variability from GRACE: Trends, seasonal cycle, subseasonal anomalies and extremes. Surveys in Geophysics, 37(2), 357-395.
This review synthesizes our current knowledge on the dominant temporal modes of variability in terrestrial water storage. We show how water storage is related to the seasonal cycles of precipitation and temperature and identify which regions are most affected by long-term changes. We also introduce a filtering approach for relating high-frequency changes in water storage to antecedent precipitation anomalies.
California Institute of Technology
1200 E. California Blvd., MC 131-24
Pasadena, CA 91125
vincent.humphrey [- at -] caltech.edu