Motivation of CARBO-Extreme
Currently European terrestrial ecosystems are estimated to act as a net carbon sink soaking up 7-12% of fossil fuel emissions (Janssens et al., 2003). Also globally the terrestrial biosphere is predicted to sequester carbon at least during the first half of the 21st century (IPCC AR4, 2007). Hence, the terrestrial biosphere potentially contributes to the EU climate protection goal of stabilizing CO2 greenhouse gas concentrations such that “dangerous climate change” is avoided, currently defined by a 2°C global warming ‘guardrail’. However, both the duration and future magnitude of the terrestrial carbon sink are highly uncertain (Friedlingstein et al., 2006). While uncertainties exist regarding the response of the terrestrial carbon cycle to gradual increases in greenhouse gas concentrations and changes in climate, so far the impacts of climate variability and weather extremes have not been accounted for in these considerations of the future evolution, and vulnerability, of terrestrial carbon sinks. This is a very critical gap in our assessment, since studies have clearly shown the climate variability and weather extremes may severely affect the terrestrial carbon cycle and undo several years of carbon sink (Knapp et al., 2002; Ciais et al., 2005; Reichstein et al., 2007). The importance of this topic is enhanced by recent evidence and regional climate model applications that predict more intense and frequent extreme weather events such as droughts, heatwaves and heavy precipitation events over Europe during the 21st century (Christensen and Christensen, 2003; Beniston, 2004; Schär et al., 2004; Frei et al., 2006; Seneviratne et al., 2006), indicating that the climate system no longer is in the assumed stationary mode (Milly et al., 2008).
Secondly, but no less important, another crucial unknown in the terrestrial carbon cycle is the response of soil carbon to increasing temperature. There now are strong indications that temperature may not be the most important factor when other factors become limiting, such as water and substrate availability (Davidson and Janssens, 2006; Fontaine et al., 2007). The limited understanding of these relationships has provoked controversial discussions in the literature (Fang et al., 2005; Knorr et al., 2005). Since modelled responses of respiration temperature sensitivity to drought at ecosystem level appear opposite to observations at hourly to annual time-scales (Reichstein et al., 2007b) there clearly is need for more research. This is even more important since, considerable changes in soil carbon stock over last decades have been reported, which may be related to warming (Bellamy et al., 2005; Smith et al., 2007).
It is evident that a plethora of processes at various time scales are involved in the terrestrial carbon response to climate variability. Our understanding and thus ability to predict effects of climate variability and extreme events on the terrestrial C cycle has so far been hampered by too little integration of experimental data. In particular it is important to integrate different observational quantities that yield information at different time-scales of variability, since single data sets do not contain enough information to constrain models across temporal (and spatial) scales (Sacks et al., 2006). In addition, models are usually parameterized with data under normal conditions, resulting in parameterization that may not be valid under extreme conditions. Hence, there is strong need for integrated multi-data-model fusion approaches in the context of carbon cycling from short-term to centennial scales including extreme conditions.
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Schär C, Vidale PL, Lüthi D, et al., The role of increasing temperature variability in European summer heatwaves. Nature 427, 332-336, 2004. link to publisher \\\
Seneviratne SI, Lüthi D, Litschi M, et al., Land-atmosphere coupling and climate change in Europe. Nature 443, 205-209, 2006. link to publisher