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  • Author or Editor: Trevor F. Keenan x
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Peng Zhu
,
Qianlai Zhuang
,
Lisa Welp
,
Philippe Ciais
,
Martin Heimann
,
Bin Peng
,
Wenyu Li
,
Carl Bernacchi
,
Christian Roedenbeck
, and
Trevor F. Keenan

Abstract

Carbon balance of terrestrial ecosystems in the northern high latitudes (NHL) is sensitive to climate change. It remains uncertain whether current regional carbon uptake capacity can be sustained under future warming. Here the atmospheric CO2 drawdown rate (CDR) between 1974 and 2014, defined as the CO2 decrease in ppm over the number of days in spring or summer, is estimated using atmospheric CO2 observations at Barrow (now known as Utqiaġvik), Alaska. We found that the sensitivity of CDR to interannual seasonal air temperature anomalies has trended toward less carbon uptake for a given amount of warming over this period. Changes in interannual temperature sensitivity of CDR suggest that relatively warm springs now result in less of a carbon uptake enhancement. Similarly, relatively warm summers now result in greater carbon release. These results generally agree with the sensitivity of net carbon exchange (NCE) estimated by atmospheric CO2 inversion. When NCE was aggregated over North America (NA) and Eurasia (EA), separately, the temperature sensitivity of NCE in NA has changed more than in EA. To explore potential mechanisms of this signal, we also examine trends in interannual variability of other climate variables (soil temperature and precipitation), satellite-derived gross primary production (GPP), and Trends in Net Land–Atmosphere Carbon Exchanges (TRENDY) model ensemble results. Our analysis suggests that the weakened spring sensitivity of CDR may be related to the slowdown in seasonal soil thawing rate, while the summer sensitivity change may be caused by the temporally coincident decrease in temperature sensitivity of photosynthesis. This study suggests that the current NHL carbon sink may become unsustainable as temperatures warm further. We also found that current carbon cycle models do not represent the decrease in temperature sensitivity of net carbon flux. We argue that current carbon–climate models misrepresent important aspect of the carbon–climate feedback and bias the estimation of warming influence on NHL carbon balance.

Full access
Mahmoud Osman
,
Benjamin F. Zaitchik
,
Hamada S. Badr
,
Jason Otkin
,
Yafang Zhong
,
David Lorenz
,
Martha Anderson
,
Trevor F. Keenan
,
David L. Miller
,
Christopher Hain
, and
Thomas Holmes

Abstract

Recent years have seen growing appreciation that rapidly intensifying flash droughts are significant climate hazards with major economic and ecological impacts. This has motivated efforts to inventory, monitor, and forecast flash drought events. Here we consider the question of whether the term “flash drought” comprises multiple distinct classes of event, which would imply that understanding and forecasting flash droughts might require more than one framework. To do this, we first extend and evaluate a soil moisture volatility–based flash drought definition that we introduced in previous work and use it to inventory the onset dates and severity of flash droughts across the contiguous United States (CONUS) for the period 1979–2018. Using this inventory, we examine meteorological and land surface conditions associated with flash drought onset and recovery. These same meteorological and land surface conditions are then used to classify the flash droughts based on precursor conditions that may represent predictable drivers of the event. We find that distinct classes of flash drought can be diagnosed in the event inventory. Specifically, we describe three classes of flash drought: “dry and demanding” events for which antecedent evaporative demand is high and soil moisture is low, “evaporative” events with more modest antecedent evaporative demand and soil moisture anomalies, but positive antecedent evaporative anomalies, and “stealth” flash droughts, which are different from the other two classes in that precursor meteorological anomalies are modest relative to the other classes. The three classes exhibit somewhat different geographic and seasonal distributions. We conclude that soil moisture flash droughts are indeed a composite of distinct types of rapidly intensifying droughts, and that flash drought analyses and forecasts would benefit from approaches that recognize the existence of multiple phenomenological pathways.

Open access
Sara H. Knox
,
Robert B. Jackson
,
Benjamin Poulter
,
Gavin McNicol
,
Etienne Fluet-Chouinard
,
Zhen Zhang
,
Gustaf Hugelius
,
Philippe Bousquet
,
Josep G. Canadell
,
Marielle Saunois
,
Dario Papale
,
Housen Chu
,
Trevor F. Keenan
,
Dennis Baldocchi
,
Margaret S. Torn
,
Ivan Mammarella
,
Carlo Trotta
,
Mika Aurela
,
Gil Bohrer
,
David I. Campbell
,
Alessandro Cescatti
,
Samuel Chamberlain
,
Jiquan Chen
,
Weinan Chen
,
Sigrid Dengel
,
Ankur R. Desai
,
Eugenie Euskirchen
,
Thomas Friborg
,
Daniele Gasbarra
,
Ignacio Goded
,
Mathias Goeckede
,
Martin Heimann
,
Manuel Helbig
,
Takashi Hirano
,
David Y. Hollinger
,
Hiroki Iwata
,
Minseok Kang
,
Janina Klatt
,
Ken W. Krauss
,
Lars Kutzbach
,
Annalea Lohila
,
Bhaskar Mitra
,
Timothy H. Morin
,
Mats B. Nilsson
,
Shuli Niu
,
Asko Noormets
,
Walter C. Oechel
,
Matthias Peichl
,
Olli Peltola
,
Michele L. Reba
,
Andrew D. Richardson
,
Benjamin R. K. Runkle
,
Youngryel Ryu
,
Torsten Sachs
,
Karina V. R. Schäfer
,
Hans Peter Schmid
,
Narasinha Shurpali
,
Oliver Sonnentag
,
Angela C. I. Tang
,
Masahito Ueyama
,
Rodrigo Vargas
,
Timo Vesala
,
Eric J. Ward
,
Lisamarie Windham-Myers
,
Georg Wohlfahrt
, and
Donatella Zona

Abstract

This paper describes the formation of, and initial results for, a new FLUXNET coordination network for ecosystem-scale methane (CH4) measurements at 60 sites globally, organized by the Global Carbon Project in partnership with other initiatives and regional flux tower networks. The objectives of the effort are presented along with an overview of the coverage of eddy covariance (EC) CH4 flux measurements globally, initial results comparing CH4 fluxes across the sites, and future research directions and needs. Annual estimates of net CH4 fluxes across sites ranged from −0.2 ± 0.02 g C m–2 yr–1 for an upland forest site to 114.9 ± 13.4 g C m–2 yr–1 for an estuarine freshwater marsh, with fluxes exceeding 40 g C m–2 yr–1 at multiple sites. Average annual soil and air temperatures were found to be the strongest predictor of annual CH4 flux across wetland sites globally. Water table position was positively correlated with annual CH4 emissions, although only for wetland sites that were not consistently inundated throughout the year. The ratio of annual CH4 fluxes to ecosystem respiration increased significantly with mean site temperature. Uncertainties in annual CH4 estimates due to gap-filling and random errors were on average ±1.6 g C m–2 yr–1 at 95% confidence, with the relative error decreasing exponentially with increasing flux magnitude across sites. Through the analysis and synthesis of a growing EC CH4 flux database, the controls on ecosystem CH4 fluxes can be better understood, used to inform and validate Earth system models, and reconcile differences between land surface model- and atmospheric-based estimates of CH4 emissions.

Full access