PhD defence: Zhongxiang Fang
Zhongxiang Fang defends his thesis,
Earth Observation of Vegetation Change in a Warming World
Supervisors:
Professor Rasmus Fensholt, IGN
Associate Professor Martin Stefan Brandt, IGN
Assessment Committee:
Professor Lars Eklundh, University of Lund – Sweden
Associate Professor Trevor Keenan, University of California, Berkeley – US
Associate Professor Stéphanie Horion (chair), IGN
Summary:
Global warming, mainly caused by the rapid increase of anthropogenic greenhouse gas emissions, has been in the spotlight in recent years and has profoundly affected sea level, global and regional weather, food security, and ecosystem structure and biodiversity on the Earth. Vegetation, which plays an essential role in the carbon and water cycles of global terrestrial ecosystems, is changing rapidly in response to the globally rising temperature reported by both field and satellite observations. In return, vegetation can also mitigate global warming through increased carbon sequestration via photosynthesis.
However, a deep global understanding of the impact of warming and/or human land management-induced changes in vegetation composition on the vegetation growth cycle (phenology) and the adaptation of vegetation to global warming is still insufficient. Based on these research gaps, this thesis focuses on three research questions: 1) What historical and projected atmospheric aridity changes may threaten the global terrestrial ecosystems? 2) What is the role of change in vegetation composition in the response of land surface phenology to climate change? 3) Is the adaptation of vegetation to increasing temperature a global phenomenon? These three questions are proposed to be answered in this thesis, which can improve our understanding of the response of the global carbon cycle to climate warming.
The first study evaluated the quality of the vapour pressure deficit (VPD) products derived from three gridded climate datasets (ERA5, MERRA2, and CRU) against the in situ observations. Historical and projected changes in VPD were also estimated from the mean of three gridded climate datasets and CMIP6 simulations under different scenarios, respectively. The results showed that the estimated VPD from three gridded datasets (ERA5, MERRA2, and CRU) is consistent with the VPD derived from in situ observations (R2 = 0.915–0.954). Lower consistency was found mainly in areas with few station observations. Higher agreements were found mainly in the summer and lower in the winter, related to the temperature level. Furthermore, global VPD has increased significantly (p < 0.05) from 1981 to 2020 at a rate of 0.025 hPa year-1, and the increase in VPD derived from CMIP6 will continue in the future, particularly under high emission scenarios. This increase in VPD indicates an increase in atmospheric aridity in the future, which could threaten global terrestrial ecosystems, especially crops in drylands.
A digital version of the PhD thesis can be obtained from the PhD secretary Mikala Heckscher at mikala@ign.ku.dk