The 2012 drought was among the most intense and widespread drought events in the United States (US) since the 'Dust Bowl' period in the 1930s and caused substantial economic damage, in particular for agricultural production. Precipitation deficits started to evolve in the Central US during May and by July, 62% of the contiguous US (CONUS) experienced moderate to exceptional drought, covering an area of about 4.7 million square kilometers (larger than the EU). In addition, the year 2012 was among the warmest on record (+1.8 °C), with the the warmest spring (+2.9 °C) and second warmest summer (+1.4 °C) compared to the average of 1895-2012.
We analyzed the impact of the concurrent warmer spring and summer drought on land-atmosphere carbon, water and energy exchange across CONUS in 2012 using direct measurements of biosphere-atmosphere exchange with the eddy covariance technique (EC) at 22 sites, remote sensing observations from MODIS, and carbon uptake estimates from the atmospheric inverse model CarbonTracker.
The results of this large-scale synthesis study were presented at several international conferences and some of the results were recently published in PNAS (Web, PDF, SI Appendix). Among others, this paper was covered by a PNAS Article Highlight, PNAS Commentary (PDF), Nature News & Views (PDF), ETH News, Wageningen UR News, Lawrence Berkeley National Laboratory, EnvironmentalResearchWeb, and by EU Horizon Magazin (PDF).
We found that that warming-induced earlier vegetation activity increased spring carbon uptake, and thus compensated for reduced carbon uptake during the summer drought. The contiguous US thus remained a carbon sink in 2012, in contrast to Europe during the drought of 2003 (see Ciais et al. 2005). However, the enhanced spring carbon uptake came at the cost of soils drying out earlier and thus, further contributed to water limitations, likely enhancing summer heating through reduced evaporative cooling and biosphere-atmosphere feedbacks.
This large-scale synthesis study was based on a collaboration of an international research team from the University of California - Berkeley (USA), ETH Zurich (Switzerland), Macquarie University (Australia), Lawrence Berkeley National Laboratory (USA), NASA's Jet Propulsion Laboratory (USA), the University of Wisconsin - Madison (USA), Harvard University (USA), USDA-ARS (USA), Oregon State University (USA), the University of New Mexico (USA), the University of Kansas (USA), and Wageningen University (The Netherlands). Additional support is acknowledged to all data contributors, the Carbon Dioxide Information Analysis Center at the Oak Ridge National Laboratory, particularly Bai Yang and Dan Ricciuto, and the AmeriFlux Management Project. The research was supported by the European Commission's FP7 with a Marie Curie International Outgoing Fellowship to Sebastian Wolf, and various other funding sources (e.g. DOE, NSF, NASA, NWO) to the colleagues and institutions involved.

This project was 'Evaluating the variability of ecosystem water vapour fluxes and spatiotemporal effects of extreme events' using a synthesis dataset of the global measurement network FLUXNET in combination with a novel biophysical process model (BESS - Breathing Earth System Simulator) to upscale ecosystem fluxes globally.
Water vapour fluxes are a key variable in the biosphere-atmosphere exchange of water in terrestrial ecosystems and have strong interactions with the climate system. However, research on the effects of climate change has been largely focused on carbon cycling and therefore, many open questions remain, in particular with respect to the long-term variability of water vapour fluxes and to spatiotemporal patterns in ecosystem responses to extreme events. The ECOWAX project intended to close this knowledge gap and contribute to a better understanding of ecosystem water cycling, thus assisting in the development of mitigation options to secure water supply for human society in a changing climate.
ECOWAX was funded by the European Commission through a Marie Curie International Outgoing Fellowship from November 2012 to October 2015 in the Biometeorology Lab at UC Berkeley (outgoing phase) and the Grassland Sciences Group at ETH Zurich (return phase).
The ECOWAX project found that (1) the seasonal timing of drought largely determines the impact on ecosystem water and carbon fluxes, i.e. drought impacts are different during spring and summer, and differ among grasslands and forests. (2) Climate change induced warmer temperatures during spring will increases carbon uptake but at the cost of depleting water resources earlier, which results in a heating feedback during summer that could enhance the intensity of heatwaves. (3) Tropical ecosystems play the most important role for the global hydrological cycle, by having the highest absolute rates of water vapour fluxes and the largest proportion of available energy converted to latent heat. The outcome of the ECOWAX project increased the understanding of ecosystem water cycling, especially during extreme events such as large-scale drought. This information will help to develop mitigation strategies to secure water supply for human needs in the face of a changing climate (see ECOWAX Report Summary).
The results of ECOWAX were presented at several international conferences and publications with the results include Wolf et al. 2013 (ERL; Web, PDF), Parazoo et al. 2015 (GBC; Web, PDF), Wolf et al. 2016 (PNAS; Web, PDF), Yao et al. 2016 (AFM; Web, PDF), Ma et al. 2016 (AFM; Web, PDF), Tramontana et al. 2016 (BG; Web, PDF), Verma et al. 2016 (RS; Web, PDF), Zscheischler et al. 2016 (JGR-B; Web, PDF), and van Gorsel et al. 2016 (BG; Web, PDF).
Public outreach for the results of ECOWAX included articles in the EU Horizon Magazin (PDF), CORDIS Result in Brief, ETH News, Wageningen UR News, Lawrence Berkeley National Laboratory, and EnvironmentalResearchWeb.

This project was validating an ecosystem flux partitioning approach using correlation analysis of high-frequency data from a Californian oak savanna (Tonzi Ranch).
The eddy covariance technique (EC) directly measures turbulent net ecosystem fluxes of trace gases between terrestrial ecosystem and the atmosphere. However, the component fluxes of carbon dioxide (photosynthesis and respiration) and water vapor (transpiration and evaporation) cannot be inferred directly from EC measurements. Scanlon & Kustas (2010, AFM) proposed a new theory-based partitioning approach that is using correlation analysis of high-frequency time series and requires only standard EC measurements. This project is evaluating the approach by using simultaneous below and above canopy EC measurements from a Californian oak savanna (Tonzi Ranch) during different climatic and phenological conditions.
The project was funded by the Swiss National Science Foundation (SNF) through a Fellowship for Prospective Researchers from November 2011 to October 2012 in the Biometeorology Lab at UC Berkeley.
The results of the project were presented at several international conferences and publications with the results are in preparation.

This project investigated the effects of spring drought on ecosystem carbon and water dynamics. Spring 2011 was the warmest and among the driest since start of meteorological measurements in Switzerland. This combination resulted in a pronounced spring drought with severe effects on ecosystem phenology, agricultural production and water supply. Therefore, the project synthesized data from the regional eddy covariance network Swiss FluxNet to assess the impact of this spring drought on ecosystem carbon dioxide and water vapour fluxes.
The results from this project showed positive carbon cycle effects of warmer spring linked to an earlier phenological development, but also indicated a forward shift of the annual soil moisture minimum from summer into spring. Grasslands and forests responded very differently to this spring drought: forests conserved their limited water ressources and increased the water use efficiency (WUE, i.e. carbon gain per water loss). In contrast, grasslands did not adjust their WUE and depleted soil moisture limited regrowth after the grass was cut. These results were published in Wolf et al. 2013 (ERL; Web, PDF) and the public outreach article Wolf 2013 (EnvironmentalResearchWeb; Web, PDF).
The Swiss FluxNet initiative combines all ecosystem-scale carbon dioxide and water vapour flux measurement sites in Switzerland. It currently encompasses eight long-term ecosystem sites, covering the major land-use types in Switzerland: forest (deciduous: Lägeren; coniferous: Davos), cropland (Oensingen), grassland (Oensingen, Chamau, Früebüel, Alp Weissenstein, Rietholzbach) and an urban area ( Basel). Further sites with short-term measurements such as Dischma (grassland-winter) provide additional information. Besides the Grassland Sciences Group of ETH Zurich, three further research groups contribute to Swiss Fluxnet: Sonia Seneviratne (IAC, ETH Zurich), Jürg Fuhrer (ART, Agroscope), Eberhard Parlow (MCR, University of Basel). Further details on Swiss FluxNet can be found on the project website and poster.

GHG-Europe is a large-scale integrating research project that aims to improve our understanding and capacity for predicting the European terrestrial carbon and greenhouse gas budget. GHG-Europe aims to integrate the results from various national and international data sources in a comprehensive assessment. GHG-Europe will elucidate the contribution of different land-use types to the emissions of the three most important GHGs carbon dioxide, nitrous oxide and methane. Measurements from more than one hundred continental stations distributed across all European climatic regions and ecosystems provide the basis for the integrated assessment. The measurements from this network of stations will be used in computer models to project future GHG budgets under changing climate conditions. The models will also include socio-economic effects to address interactions between economic development, land use and GHG emissions.
The GHG-Europe project is funded by the EU's 7th Framework Programme for Research and Technological Development for a period of 3.5 years (2010-2013). The project involves more than 40 research partners from all over Europe and is coordinated by the Johann Heinrich von Thuenen Institute for Agricultural Climate Research (vTI-AK) in Braunschweig, Germany. Further details on GHG-Europe can be found at the project website.

Within my PhD project at the Grassland Sciences Group at ETH Zurich, I investigated the carbon dioxide and water vapour fluxes of two tropical ecosystems in Panama with micrometeorological measurements using the eddy covariance technique. The project was funded by the North-South Centre (former Swiss Centre for International Agriculture) of ETH Zurich and my supervisors were Nina Buchmann, Werner Eugster and Nicolas Gruber.
Tropical forest ecosystems play an important role in regulating the global climate, yet deforestation and land-use change mean that the tropical carbon sink is increasingly influenced by agroecosystems and pastures. Despite this, it is not yet fully understood how carbon and water cycling in the tropics respond to land-use change, particularly for pasture and afforestation. In collaboration with the Smithsonian Tropical Research Institute (STRI), we were running two flux towers in Sardinilla (Panama) between February 2007 and January 2010. Sardinilla is located about 40 km north of Panama City, at 9.19° N, 79.38° W at 70 m a.s.l. (see Map). The site has a semi-humid tropical climate with a mean annual temperature of 25.2 °C, 2289 mm annual precipitation and a pronounced dry season from January to April. One flux tower was installed in a native tree species afforestation and a second one in an adjacent pasture (see Photos).
The land-use change from pasture to afforestation reduced the seasonal variability of carbon dioxide and water vapour fluxes. The shallow rooting depth of grasses compared to trees resulted in a higher sensitivity of the pasture ecosystem to water limitation and seasonal drought. Radiation and soil moisture were the main environmental controls of ecosystem fluxes. Substantial carbon sequestration was observed in the afforestation whereas the pasture ecosystem was a strong carbon source, associated with seasonal drought and overgrazing.
The results of the project were presented at several conferences, e.g. at AGU 2010 (PDF) and the 3rd iLEAPS Science Conference 2011 (PDF), and published among others in Wolf et al. 2011 (AFM; Web, PDF), Wolf et al. 2011 (GCB; Web, PDF), Wolf et al. 2011 (Ecosystems; Web, PDF), Potvin et al. 2011 (FEM; Web, PDF), and Schneebeli et al. 2011 (RSE; Web, PDF). Public outreach included articles in FluxLetter (PDF), STRI News (Web, PDF), STRI Headline News (Web, PDF), and ETH Life (Web). My PhD thesis entitled "Carbon dioxide and water vapour fluxes of tropical pasture and afforestation: seasonal variations of net ecosystem exchange and carbon sequestration potentials" is available as PDF.

This project (MSc/Diploma thesis) at Dresden University of Technology, in collaboration with the Leibniz Institute of Ecological and Regional Development, investigated the effects of changing land-use patterns on soil erosion. My supervisors for this project were Arno Kleber and Ulrich Walz.
Soil erosion is largely triggered by surface runoff that results from intense convective precipitation. Changes in land-cover patterns influence slope lengths and, thus, erosional displacement. Based on historical maps and physical factors, mainly soil properties and precipitation, we utilized modeling techniques and Geographic Information Systems (GIS) to assess these effects. After developing a method, a case study was conducted for the Saxon Switzerland National Park Region (see Map) during the past century. Our results indicate that changes in land-cover patterns affected soil erosion to a greater extent than estimated so far, and thus need to be considered more precisely in erosion modeling.
The results of this project were presented at several conferences, e.g. Nature Protection GIS 2006 (PDF) and AGIT 2007 (PDF), and were published in Walz et al. 2008 (PDF) and Wolf et al. 2009 (PDF). Furthermore, my thesis (in German) can be downloaded as PDF.

In this project (Research assignment) at Dresden University of Technology and the Leibniz Institute of Ecological and Regional Development, we investigated the development in landscape fragmentation in the cross-border region of Saxon-Bohemian Switzerland. My supervisors for this project were Arno Kleber and Ulrich Walz.
Our results indicate an increase of landscape fragmentation since the end of the 19th century, especially in the time after World War II as a result of increasing mobility and growth of settlements. Driveways were paved and thus, the road network was enlarged. Consequently, the size of non-fragmented areas decreased significantly, by more than 50%. In addition, we demonstrated that landscape fragmentation in cross-border regions is often underestimated.
The results of this project were presented at Nature Protection GIS 2006 (PDF), and published in Walz 2005 (PDF) and Wolf & Walz 2005 (PDF).

Within the project "Mountain Pine Beetle Detection and Monitoring" at Simon Fraser University (Canada), in collaboration with the British Columbia Ministry of Forests, we investigated Mountain Pine Beetle (MPB) infestations using airborne remote sensing. My supervisor for this project was Arthur Roberts.
The main motivation for this project was the increasing number and size of MPB infestations in the pine forests of British Columbia due to mild winter temperatures. We investigated the possibility of identifying and mapping early infestation stages (current attack) of MPB in lodgepole pine forests. We used visible and infrared aerial photographic imagery, recorded with our research airplane, in combination with ground truth tree data. Our results indicate a reliable early detection of MPB infestations in spring (mid May to early June at our sites) before the MPBs are able to migrate again.
The results of this project were published in two research reports, Roberts et al. 2003a (PDF) and Roberts et al. 2003b (PDF).

In August and September 2002, I lead and organized a scientific mapping expedition with 13 participants (see photo of expedition team) to the Tien Shan Mountains in Kyrgyzstan (see Map). The objective of this Dresden University of Technology, Institute of Cartography project was to create a new 1:100 000 scale map of the central Tien Shan in the style and quality of European mountaineering maps. The project was headed by Manfred Buchroithner. As part of the International Year of the Mountains in 2002, this was the first western mapping expedition that received permission to map this 'unresolved' border region to China. Being in charge of organizing and leading this expedition was my greatest responsibility up to this point. Working with a team of reliable and motivated participants, unexpected stable weather conditions, and good support from our local partners were indispensible to the success of this expedition.
Public outreach included media coverage by National Geographic Germany (PDF), Scinexx (PDF), regional newspapers (e.g. PDF), and radio stations (e.g. DLF). Publications related to our mapping expedition are Wolf 2002 (PDF), Wolf 2003 (PDF) and the Expedition Report (English, German). The resulting maps based on data from this expedition have been published by the Cartographic Service of the German Alpine Association (see e.g. imprint 2008).

In February and March 2002, I participated in a scientific mapping expedition to Ojos del Salado (6893 m, highest volcano on earth) in the Atacama Desert in Chile (see map). The objective of this Dresden University of Technology, Institute of Cartography project was to create a new 1:100 000 scale map of the area surrounding Ojos del Salado, in the style and quality of European mountaineering maps. We collaborated with the Instituto Geográfico Militar de Chile and received support from SERNATUR (Copiapo) and the German Alpine Association. The project and mapping expedition were lead by Manfred Buchroithner.
Public outreach of this project included several articles in regional newspapers (e.g. PDF). Furthermore, a scientific documentary movie (46 min) has been made on this mapping expedition ("Routensucher in der Atacama"), with short versions telecasted on several TV channels, e.g. ORF and 3sat. Publications related to our mapping expedition are Graf 2002 (PDF), Wolf 2002 (PDF) and Buchroithner 2006 (PDF). The resulting map based on data from this expedition was published by the Cartographic Service of the German Alpine Association in 2004 (see imprint).

In September 2001, I lead and organized a scientific surveying expedition with 15 participants to the Dachstein South Face Cave in Austria (see Map). The objective of this Dresden University of Technology, Institute of Cartography project was to survey the large limestone cave in order to improve understanding of geological formations and to test novel 3D visualization techniques in such an environment. The project was headed by Manfred Buchroithner.
Public outreach of this project included several articles in regional newspapers (e.g. PDF), media coverage by a TV channel (MDR, Biwak), and a scientific documentary movie (36 min) made during the surveying expedition ("Vermessung einer Finsterniss", see imprint). A publication related to this surveying expedition is Wolf 2001 (PDF).