Københavns Universitet
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PhD thesis defense

Jie Wu, IGN, defends her thesis at Geography Section

PhD thesis defense — Jie Wu 13 NOV


Date & Time:

Aud C, Department of Geosciences and Natural Resource Management, Øster Voldgade 10, 1350 Kbh K

Hosted by:
Geography Section


The photo has been designed using assets from Freepik.com

Jie Wu defends her thesis,

Biogeophysical impacts of Earth greening under past and future climate

Associate Professor Guy Schurgers, IGN
Associate Professor Zhenzhong Zeng, SUSTech – China

Assessment committee:
Professor Bart van den Hurk, Deltares – The Netherlands
Professor Justin Sheffield, University of Southampton – UK
Associate Professor Ylva Sjöberg (chair), IGN

Vegetation greenness measured by leaf area index (LAI) has increased during the past 30 years across the globe, a phenomenon known as Earth greening, mainly attributable to the combined effect of rising CO2 concentration ([CO2]) and global warming. Earth greening plays a crucial role in regulating climate and hydrological cycle through biogeochemical and biogeophysical feedbacks. While the influences of historical Earth greening on mean temperatures and hydrological cycle have been extensively documented, a deep understanding of the global-scale biogeophysical impacts of Earth greening, observed in the past and projected for the future, in the context of increasing [CO2] and global warming, is lacking.
To fill in these research gap, this thesis focus on three research questions: (1) How has Earth greening affected global hot temperature extremes during the past three decades? (2) How does the hydrological cycle respond to the projected Earth greening in the 21st century? (3) How will vegetation transpiration (Et) change in the future and what are the underlying drivers?
By performing a pair of transient AMIP simulations prescribed with different LAI conditions in IPSL-CM, Paper I showed that although dampened by the radiative forcing and physiological effects of rising [CO2], Earth greening has led to global reductions in the hot days frequency index (TX90p) and the warm nights frequency index (TN90p) at the rate of -0.26±0.10 days decade-1 and -0.38±0.11 days decade-1, respectively. The mitigation effects offset 4.7% and 5.8% of the observed TX90p and TN90p trends globally. This paper complements previous studies on mean temperature, investigated based on long-term remote sensing observations (Li et al., 2023) or land-atmosphere global climate modeling (Zeng et al., 2017).
By incorporating the LAI change projected by Coupled Model Intercomparison Project Phase (CMIP) 5 into IPSL-CM, Paper II showed that the projected Earth greening in the late 21st century simultaneously intensifies precipitation (P) and evapotranspiration (ET) over land. The response of soil moisture emerges as a DDWW paradigm caused by the spatial difference between the responses of P and ET: soil moisture decreases significantly over dry regions in response to increasing LAI, but does not decrease over wet regions particularly Amazon and Congo rainforests, as greening-induced increase in ET favors more convective P.
Integrating the grid cell level sensitivities from idealized single-forcing experiments with changes projected by Shared Socioeconomic Pathways (SSPs) in a 14-model ensemble of CMIP6 model simulations, Paper III showed that the simulated Et shifted from an increase of 10.42±87.82 mm yr-1 (+3.9%) under SSP1-2.6 to a decrease of 14.33±85.52 mm yr-1 (-5.4%) under SSP5-8.5. The shift in Et is driven primarily by the stronger reduction effect from physiological effect of CO2 than the enhancement driven by increasing air temperature and LAI, especially over the humid tropics where the CO2 effect is strong.
This thesis offers insights into the complex dynamics linking Earth greening, CO2, and their combined influence on the occurrence of hot temperature extremes. It also provides valuable insights into the complex interplay between climate drivers, terrestrial vegetation processes and global water cycle, thereby contributing to a more comprehensive understanding of the potential impacts of Earth greening on global water resources and regional water availability, and enhancing the predictive capability of Earth system models in the context of climate change assessments.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk