Carbon cycle extremes during the 21st century in CMIP5 models: Future evolution and attribution to climatic drivers

Zscheischler, Jakob; Reichstein, Markus; Buttlar, J. von; Mu, M.; Randerson, James T.; Mahecha, Miguel D.

doi:10.1002/2014gl062409

Cited by 49 publications

(44 citation statements)

References 59 publications

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“…Several studies have found a lower drought sensitivity of R eco compared to GPP (Ciais et al, 2005;Schwalm et al, 2010Schwalm et al, , 2012Rambal et al, 2014;Zscheischler et al, 2014d) whereas we observed comparable or even slightly greater reductions NEP is the sum of the opposing fluxes of GPP and R eco and hence, the direction and amplitude of its change is always determined by the sum of the extreme event impacts on the gross fluxes. For heat extremes, the general increase in R eco adds 5 to slight decreases (or no change) of GPP, leading to a generally reduced rate of net carbon uptake.…”

contrasting

confidence: 50%

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Buttlar¹,

Zscheischler²,

Rammig³

et al. 2017

Preprint

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Abstract.Extreme climatic events, such as droughts and heat stress induce anomalies in ecosystem-atmosphere CO 2 fluxes, such as gross primary production (GPP) and ecosystem respiration (R eco ), and, hence, can change the net ecosystem carbon balance.However, despite our increasing understanding of the underlying mechanisms, the magnitudes of the impacts of different types of extremes on GPP and R eco within and between ecosystems remain poorly predicted. Here we aim to identify the major 5 factors controlling the amplitude of extreme event impacts on GPP, R eco , and the resulting net ecosystem production (NEP).We focus on the impacts of heat and drought and their combination. We identified hydrometeorological extreme events in consistently downscaled water availability and temperature measurements over a 30 year time period. We then used FLUXNET eddy-covariance flux measurements to estimate the CO 2 flux anomalies during these extreme events across dominant vegetation types and climate zones. Overall, our results indicate that short-term heat extremes increased respiration more strongly than 10 they down-regulated GPP, resulting in a moderate reduction of the ecosystem's carbon sink potential. In the absence of heat stress, droughts tended to have smaller and similarly dampening effects on both GPP and R eco , and, hence, often resulted in neutral NEP responses. The combination of drought and heat typically led to a strong decrease in GPP, whereas heat and drought impacts on respiration partially offset each other. Taken together, compound heat and drought events led to the strongest C sink reduction compared to any single-factor extreme. A key insight of this paper, however, is that duration matters most: 15 for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of R eco over time turned from an initial increase to a down-regulation after about two weeks.This confirms earlier theories that not only the magnitude but also the duration of an extreme event determines its impact. Our study corroborates the results of several local site-level case studies, but as a novelty generalizes these findings at the global scale. Specifically, we find that the different response functions of the two antipodal land-atmosphere fluxes GPP and R eco can 20 also result in increasing NEP during certain extreme conditions. Apparently counterintuitive findings of this kind bear great potential for scrutinizing the mechanisms implemented in state-of-the-art terrestrial biosphere models and provide a benchmark for future model development and testing.

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confidence: 50%

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Buttlar¹,

Zscheischler²,

Rammig³

et al. 2017

Preprint

Self Cite

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“…To our knowledge, only a single experiment has so far addressed the consequence of combined eCO 2 and elevated temperature on the ecosystem C budget (39) and found that these combined treatments increased C loss (in one of the four years of study), a result in contradiction to model simulations (40,41). The most notable effect in that experiment was the increased loss of C under elevated CO 2 (through increased heterotrophic soil respiration) in two of the four years.…”

Section: Resultsmentioning

confidence: 88%

Elevated CO₂maintains grassland net carbon uptake under a future heat and drought extreme

Roy

Picon‐Cochard

Augusti

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

120

110

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Extreme climatic events (ECEs) such as droughts and heat waves are predicted to increase in intensity and frequency and impact the terrestrial carbon balance. However, we lack direct experimental evidence of how the net carbon uptake of ecosystems is affected by ECEs under future elevated atmospheric CO 2 concentrations (eCO 2 ). Taking advantage of an advanced controlled environment facility for ecosystem research (Ecotron), we simulated eCO 2 and extreme cooccurring heat and drought events as projected for the 2050s and analyzed their effects on the ecosystem-level carbon and water fluxes in a C3 grassland. Our results indicate that eCO 2 not only slows down the decline of ecosystem carbon uptake during the ECE but also enhances its recovery after the ECE, as mediated by increases of root growth and plant nitrogen uptake induced by the ECE. These findings indicate that, in the predicted near future climate, eCO 2 could mitigate the effects of extreme droughts and heat waves on ecosystem net carbon uptake.climate change | extreme events | elevated CO 2 | carbon fluxes | grassland ecosystem I ncreased aridity and heat waves are projected to increase in the 21st century for most of Africa, southern and central Europe, the Middle East, and parts of the Americas, Australia, and southeast Asia (1-3). These regions have a large fraction of their land covered by grasslands and rangelands, a biome covering approximately one-quarter of the Earth's land area and contributing to the livelihoods of more than 800 million people (4). There is mounting evidence that extreme climatic events (ECEs) may significantly affect the regional and global carbon (C) fluxes (3, 5-9) and potentially feed back on atmospheric CO 2 concentrations and the climate system (7). However, our knowledge concerning the outcome of the interaction between future ECEs and elevated atmospheric CO 2 concentrations (eCO 2 ) for ecosystem C stocks is equivocal (10-12). Studies focusing on plant physiological responses have shown that eCO 2 has the potential to mitigate future drought-related stress on plant growth by reducing stomatal conductance, thereby increasing water use efficiency (WUE) (13-15) and preserving soil moisture (16)(17)(18). However, to date, little is known on whether and how eCO 2 alters the consequences of ECEs for ecosystem net C uptake. Because the capacity of ecosystems to act as a C sink depends on the relative effects of eCO 2 , ECE, and their potential interaction on both plant and soil processes, an integrated assessment of all C fluxes during and after the ECEs is important if we are to estimate the overall C balance.Using the Montpellier CNRS Ecotron facility (www.ecotron. cnrs.fr), we tested with 12 large controlled environment units (macrocosms, SI Appendix, Fig. S1) whether (i) an ECE (severe drought and heat wave) predicted for the 2050s reduces ecosystem net C uptake by reducing ecosystem photosynthesis relative to ecosystem respiration (R eco ), (ii) eCO 2 buffers the negative effects of the ECE on ecosystem CO 2 fluxes ...

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“…Continued growth modeled through CO 2 fertilization and increased water-use efficiency Cox et al 2013, Zscheischler et al 2014, Sitch et al 2015 PJ 9…”

Section: Observationsmentioning

confidence: 99%

“…There is abundant and strong evidence demonstrating that CO 2 fertilization and associated effects on water-use efficiency are large globally, including v www.esajournals.org substantial increases in water-use efficiency (Leakey 2009, Frank et al 2015a, increased tree growth and NPP (Martı-nez-Vilalta et al 2008, Pan et al 2013, widespread woody plant expansion in dryland ecosystems (Buitenwerf et al 2012, Fensholt et al 2012, Poulter et al 2014, and an overall ''greening'' observed in many regions (Jong et al 2012, Donohue et al 2013, Ahlströ m et al 2015. Thus, with further enrichment of atmospheric CO 2 concentrations at a global scale, forests are projected to continue to grow with enhanced water-use efficiency into the future (Cox et al 2013, Zscheischler et al 2014, Sitch et al 2015, which will constrain the overall importance of tree mortality from hotter drought. Greater vulnerability evidence (Table 2).…”

Section: Co 2 Fertilization and Wuementioning

confidence: 99%

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

2015

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Abstract. Patterns, mechanisms, projections, and consequences of tree mortality and associated broadscale forest die-off due to drought accompanied by warmer temperatures-''hotter drought'', an emerging characteristic of the Anthropocene-are the focus of rapidly expanding literature. Despite recent observational, experimental, and modeling studies suggesting increased vulnerability of trees to hotter drought and associated pests and pathogens, substantial debate remains among research, management and policy-making communities regarding future tree mortality risks. We summarize key mortalityrelevant findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence suggesting lesser vulnerability includes forest benefits of elevated [CO 2 ] and increased water-use efficiency; observed and modeled increases in forest growth and canopy greening; widespread increases in woody-plant biomass, density, and extent; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses and accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates; projected increases in drought frequency, intensity, and duration; limitations of vegetation models such as inadequately represented mortality processes; warming feedbacks from die-off; and wildfire synergies. Grouping these findings we identify ten contrasting perspectives that shape the vulnerability debate but have not been discussed collectively. We also present a set of global vulnerability drivers that are known with high confidence: (1) droughts eventually occur everywhere; (2) warming produces hotter droughts; (3) atmospheric moisture demand increases nonlinearly with temperature during drought; (4) mortality can occur faster in hotter drought, consistent with fundamental physiology; (5) shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly; and (6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, in concert with research supporting greater vulnerability perspectives, support an overall viewpoint of greater forest vulnerability globally. We surmise that mortality vulnerability is being discounted in part due to difficulties in predicting threshold responses to extreme climate events. Given the profound ecological and societal implications of underestimating global vulnerability to hotter drought, we highlight urgent challenges for research, management, and policy-making communities.

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Carbon cycle extremes during the 21st century in CMIP5 models: Future evolution and attribution to climatic drivers

Cited by 49 publications

References 59 publications

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Elevated CO₂maintains grassland net carbon uptake under a future heat and drought extreme

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

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Carbon cycle extremes during the 21st century in CMIP5 models: Future evolution and attribution to climatic drivers

Cited by 49 publications

References 59 publications

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Impacts of droughts and extreme temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Elevated CO2maintains grassland net carbon uptake under a future heat and drought extreme

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

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Product

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Elevated CO₂maintains grassland net carbon uptake under a future heat and drought extreme