Imperial College London

DrHeatherGraven

Faculty of Natural SciencesDepartment of Physics

Senior Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 5226h.graven

 
 
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Location

 

707Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

36 results found

Cui X, Newman S, Xu X, Andrews AE, Miller J, Lehman S, Jeong S, Zhang J, Priest C, Campos-Pineda M, Gurney KR, Graven H, Southon J, Fischer MLet al., 2019, Atmospheric observation-based estimation of fossil fuel CO2 emissions from regions of central and southern California, SCIENCE OF THE TOTAL ENVIRONMENT, Vol: 664, Pages: 381-391, ISSN: 0048-9697

Journal article

Lavergne A, Graven H, De Kauwe MG, Keenan FT, Medlyn BE, Prentice Iet al., 2019, Observed and modelled historical trends in the water use efficiency of plants and ecosystems, Global Change Biology, ISSN: 1354-1013

Plant water‐use efficiency (WUE, the carbon gained through photosynthesis per unit of water lost through transpiration) is a tracer of the plant physiological controls on the exchange of water and carbon dioxide between terrestrial ecosystems and the atmosphere. At the leaf level, rising CO2 concentrations tend to increase carbon uptake (in the absence of other limitations) and to reduce stomatal conductance, both effects leading to an increase in leaf WUE. At the ecosystem level, indirect effects (e.g. increased leaf area index, soil water savings) may amplify or dampen the direct effect of CO2. Thus, the extent to which changes in leaf WUE translate to changes at the ecosystem scale remains unclear. The differences in the magnitude of increase in leaf versus ecosystem WUE as reported by several studies are much larger than would be expected with current understanding of tree physiology and scaling, indicating unresolved issues. Moreover, current vegetation models produce inconsistent and often unrealistic magnitudes and patterns of variability in leaf and ecosystem WUE, calling for a better assessment of the underlying approaches. Here, we review the causes of variations in observed and modelled historical trends in WUE over the continuum of scales from leaf to ecosystem, including methodological issues, with the aim of elucidating the reasons for discrepancies observed within and across spatial scales. We emphasize that even though physiological responses to changing environmental drivers should be interpreted differently depending on the observational scale, there are large uncertainties in each data set which are often underestimated. Assumptions made by the vegetation models about the main processes influencing WUE strongly impact the modelled historical trends. We provide recommendations for improving long‐term observation‐based estimates of WUE that will better inform the representation of WUE in vegetation models.

Journal article

Brophy K, Graven H, Manning AJ, White E, Arnold T, Fischer ML, Jeong S, Cui X, Rigby Met al., 2019, Characterizing uncertainties in atmospheric inversions of fossil fuel CO2 emissions in California, Atmospheric Chemistry and Physics, Vol: 19, Pages: 2991-3006, ISSN: 1680-7316

Atmospheric inverse modelling has become an increasingly useful tool for evaluating emissions of greenhouse gases including methane, nitrous oxide, and synthetic gases such as hydrofluorocarbons (HFCs). Atmospheric inversions for emissions of CO2 from fossil fuel combustion (ffCO2) are currently being developed. The aim of this paper is to investigate potential errors and uncertainties related to the spatial and temporal prior representation of emissions and modelled atmospheric transport for the inversion of ffCO2 emissions in the US state of California. We perform simulation experiments based on a network of ground-based observations of CO2 concentration and radiocarbon in CO2 (a tracer of ffCO2), combining prior (bottom-up) emission models and transport models currently used in many atmospheric studies. The potential effect of errors in the spatial and temporal distribution of prior emission estimates is investigated in experiments by using perturbed versions of the emission estimates used to create the pseudo-data. The potential effect of transport error was investigated by using three different atmospheric transport models for the prior and pseudo-data simulations. We find that the magnitude of biases in posterior total state emissions arising from errors in the spatial and temporal distribution in prior emissions in these experiments are 1 %–15 % of posterior total state emissions and are generally smaller than the 2σ uncertainty in posterior emissions. Transport error in these experiments introduces biases of −10 % to +6 % into posterior total state emissions. Our results indicate that uncertainties in posterior total state ffCO2 estimates arising from the choice of prior emissions or atmospheric transport model are on the order of 15 % or less for the ground-based network in California we consider. We highlight the need for temporal variations to be included in prior emissions and for continuing efforts to

Journal article

Graven H, Hocking T, Zazzeri G, 2019, Detection of fossil and biogenic methane at regional scales using atmospheric radiocarbon, Earth's Future, Vol: 7, Pages: 283-299, ISSN: 2328-4277

Regional emissions of methane and their attribution to a variety of sources presently have large uncertainties. Measurements of radiocarbon (14C) in methane (CH4) may provide a method for identifying regional CH4 emissions from fossil versus biogenic sources because adding 14C‐free fossil carbon reduces the 14C/C ratio (Δ14CH4) in atmospheric CH4 much more than biogenic carbon does. We describe an approach for estimating fossil and biogenic CH4 at regional scales using atmospheric Δ14CH4 observations. As a case study to demonstrate expected Δ14CH4 and Δ14CH4‐CH4 relationships, we simulate and compare Δ14CH4 at a network of sites in California using two gridded CH4 emissions estimates (Emissions Database for Global Atmospheric Research, EDGAR, and Gridded Environmental Protection Agency, GEPA) and the CarbonTracker‐Lagrange model for 2014, and for 2030 under business‐as‐usual and mitigation scenarios. The fossil fraction of CH4 (F) is closely linked with the simulated Δ14CH4‐CH4 slope and differences of 2–21% in median F are found for EDGAR versus GEPA in 2014, and 7–10% for business‐as‐usual and mitigation scenarios in 2030. Differences of 10% in F for >200 ppb of added CH4 produce differences of >10‰ in Δ14CH4, which are likely detectable from regular observations. Nuclear power plant 14CH4 emissions generally have small simulated median influences on Δ14CH4 (0–7‰), but under certain atmospheric conditions they can be much stronger (>30‰) suggesting they must be considered in applications of Δ14CH4 in California. This study suggests that atmospheric Δ14CH4 measurements could provide powerful constraints on regional CH4 emissions, complementary to other monitoring techniques.

Journal article

Graven H, Zazzeri G, Acuña Yeomans E, 2018, Global and regional emissions of radiocarbon from nuclear power plants from 1972 to 2016, Radiocarbon, Vol: 60, Pages: 1067-1081, ISSN: 0033-8222

CH4 and CO2 emissions from geologic sources, which are devoid of radiocarbon (14C), dilute the atmospheric 14C/C ratio. Observations of 14C/C can be used to estimate fossil fuel-derived CH4 and CO2. However, the atmospheric 14C/C ratio is perturbed by emissions of 14C from nuclear power plants (NPPs) and fuel reprocessing sites, which may affect such 14C/C-based estimation if they are not correctly quantified. We calculate NPP 14C emissions for CO2 and CH4 from 1972–2016 using standard emission factors (14C emitted per unit of power produced) and analyze trends in global and regional emissions. We use available observations of 14C emissions and power generation in Europe to assess emission factors for different reactor types, as well as potential differences related to the age or manufacturer of the NPPs. Globally, nuclear 14C emissions increase until 2005 and then decrease, mostly because of the closure of gas-cooled reactors in the United Kindom and the shutdown of light water reactors after the Fukushima nuclear accident in March 2011. Observed emission factors in Europe show strong variability, spanning values from 0.003 to 2.521 TBq/GWa for PWR and from 0.007 to 1.732 TBq/GWa for BWR reactors, suggesting more information and more sophisticated models are needed to improve estimates of 14C emissions.

Journal article

Khatiwala S, Graven H, Payne S, Heimbach Pet al., 2018, Changes to the air‐sea flux and distribution of radiocarbon in the ocean over the 21st century, Geophysical Research Letters, Vol: 45, Pages: 5617-5626, ISSN: 0094-8276

We investigate the spatiotemporal evolution of radiocarbon (Δ14C) in the ocean over the 21st century under different scenarios for anthropogenic CO2 emissions and atmospheric CO2 and radiocarbon changes using a 3‐D ocean carbon cycle model. Strong decreases in atmospheric Δ14C in the high‐emission scenario result in strong outgassing of 14C over 2050–2100, causing Δ14C spatial gradients in the surface ocean and vertical gradients between the surface and intermediate waters to reverse sign. Surface Δ14C in the subtropical gyres is lower than Δ14C in Pacific Deep Water and Southern Ocean surface water in 2100. In the low‐emission scenario, ocean Δ14C remains slightly higher than in 1950 and relatively constant over 2050–2100. Over the next 20 years we find decadal changes in Δ14C of −30‰ to +5‰ in the upper 2 km of the ocean, which should be detectable with continued hydrographic surveys. Our simulations can help in planning future observations, and they provide a baseline for investigating natural or anthropogenic changes in ocean circulation using ocean Δ14C observations and models.

Journal article

Graven H, Fischer ML, Lueker T, Jeong S, Guilderson TP, Keeling RF, Bambha R, Brophy K, Callahan W, Cui X, Frankenberg C, Gurney K, LaFranchi BW, Lehman SJ, Michelson H, Miller JB, Newman S, Paplawsky W, Parazoo NC, Sloop C, Walker SJet al., 2018, Assessing fossil fuel CO₂ emissions in California using atmospheric observations and models, Environmental Research Letters, Vol: 13, ISSN: 1748-9326

Analysis systems incorporating atmospheric observations could provide a powerful tool for validating fossil fuel CO2 (ffCO2) emissions reported for individual regions, provided that fossil fuel sources can be separated from other CO2 sources or sinks and atmospheric transport can be accurately accounted for. We quantified ffCO2 by measuring radiocarbon (14C) in CO2, an accurate fossil-carbon tracer, at nine observation sites in California for three months in 2014–15. There is strong agreement between the measurements and ffCO2 simulated using a high-resolution atmospheric model and a spatiotemporally-resolved fossil fuel flux estimate. Inverse estimates of total in-state ffCO2 emissions are consistent with the California Air Resources Board's reported ffCO2 emissions, providing tentative validation of California's reported ffCO2 emissions in 2014–15. Continuing this prototype analysis system could provide critical independent evaluation of reported ffCO2 emissions and emissions reductions in California, and the system could be expanded to other, more data-poor regions.

Journal article

Brophy K, Graven H, Manning AJ, White E, Arnold T, Fischer ML, Jeong S, Cui X, Rigby Met al., 2018, Characterizing uncertainties in atmospheric inversions of fossil fuel CO2 emissions in California, Atmospheric Chemistry and Physics Discussions, Pages: 1-44, ISSN: 1680-7367

Atmospheric inverse modelling has become an increasingly useful tool for evaluating emissions of greenhouse gases including methane, nitrous oxide, and synthetic gases such as hydrofluorocarbons (HFCs). Atmospheric inversions for emissions of CO2 from fossil fuel combustion (ffCO2) are currently being developed. The aim of this paper is to investigate potential errors and uncertainties related to the spatial and temporal prior representation of emissions and modelled atmospheric transport for the inversion of ffCO2 emissions in the US state of California. We perform simulation experiments based on a network of ground-based observations of CO2 concentration and radiocarbon in CO2 (a tracer of ffCO2), combining prior (bottom-up) emission models and transport models currently used in many atmospheric studies. The potential effect of errors in the spatial and temporal distribution of prior emission estimates is investigated in experiments by using perturbed versions of the emission estimates used to create the pseudo-data. The potential effect of transport error was investigated by using three different atmospheric transport models for the prior and pseudo-data simulations. We find that the magnitude of biases in posterior total state emissions arising from errors in the spatial and temporal distribution in prior emissions in these experiments are 1 %–15 % of posterior total state emissions and are generally smaller than the 2σ uncertainty in posterior emissions. Transport error in these experiments introduces biases of −10 % to +6 % into posterior total state emissions. Our results indicate that uncertainties in posterior total state ffCO2 estimates arising from the choice of prior emissions or atmospheric transport model are on the order of 15 % or less for the ground-based network in California we consider. We highlight the need for temporal variations to be included in prior emissions and for continuing efforts to

Journal article

Graven H, Allison CE, Etheridge DM, Hammer S, Keeling RF, Levin I, Meijer HAJ, Rubino M, Tans PP, Trudinger CM, Vaughn BH, White JWCet al., 2017, Compiled records of carbon isotopes in atmospheric CO2 for historical simulations in CMIP6, Geoscientific Model Development, Vol: 10, Pages: 4405-4417, ISSN: 1991-959X

The isotopic composition of carbon (Δ14C and δ13C) in atmospheric CO2 and in oceanic and terrestrial carbon reservoirs is influenced by anthropogenic emissions and by natural carbon exchanges, which can respond to and drive changes in climate. Simulations of 14C and 13C in the ocean and terrestrial components of Earth system models (ESMs) present opportunities for model evaluation and for investigation of carbon cycling, including anthropogenic CO2 emissions and uptake. The use of carbon isotopes in novel evaluation of the ESMs' component ocean and terrestrial biosphere models and in new analyses of historical changes may improve predictions of future changes in the carbon cycle and climate system. We compile existing data to produce records of Δ14C and δ13C in atmospheric CO2 for the historical period 1850–2015. The primary motivation for this compilation is to provide the atmospheric boundary condition for historical simulations in the Coupled Model Intercomparison Project 6 (CMIP6) for models simulating carbon isotopes in the ocean or terrestrial biosphere. The data may also be useful for other carbon cycle modelling activities.

Journal article

Keeling RF, Graven HD, Welp LR, Resplandy L, Bi J, Piper SC, Sun Y, Bollenbacher A, Meijer HAJet al., 2017, Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, Vol: 114, Pages: 10361-10366, ISSN: 0027-8424

A decrease in the 13C/12C ratio of atmospheric CO2 has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the 13C-Suess effect, is driven primarily by the input of fossil fuel-derived CO2 but is also sensitive to land and ocean carbon cycling and uptake. Using updated records, we show that no plausible combination of sources and sinks of CO2 from fossil fuel, land, and oceans can explain the observed 13C-Suess effect unless an increase has occurred in the 13C/12C isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO2 levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C3 plants at times of altered atmospheric CO2, but increasing discrimination has not previously been included in studies of long-term atmospheric 13C/12C measurements. We further show that the inferred discrimination increase of 0.014 ± 0.007‰ ppm−1 is largely explained by photorespiratory and mesophyll effects. This result implies that, at the global scale, land plants have regulated their stomatal conductance so as to allow the CO2 partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO2 concentration.

Journal article

Orr JC, Najjar RG, Aumont O, Bopp L, Bullister JL, Danabasoglu G, Doney SC, Dunne JP, Dutay J-C, Graven H, Griffies SM, John JG, Joos F, Levin I, Lindsay K, Matear RJ, McKinley GA, Mouchet A, Oschlies A, Romanou A, Schlitzer R, Tagliabue A, Tanhua T, Yool Aet al., 2017, Biogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP), Geoscientific Model Development, Vol: 10, Pages: 2169-2199, ISSN: 1991-959X

The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth system models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) vs. when integrated within fully coupled Earth system models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled to ocean circulation models, initialized with observational data or output from a model spin-up, and forced by repeating the 1948–2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF6) and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin-up, preferably for 2000 years or more, and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols

Journal article

Fischer ML, Parazoo N, Brophy K, Cui X, Jeong S, Liu J, Keeling R, Taylor TE, Gurney K, Oda T, Graven Het al., 2017, Simulating estimation of California fossil fuel and biosphere carbon dioxide exchanges combining in situ tower and satellite column observations, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 122, Pages: 3653-3671, ISSN: 2169-897X

Journal article

Jeong S, Newman S, Zhang J, Andrews AE, Bianco L, Bagley J, Cui X, Graven H, Kim J, Salameh P, LaFranchi BW, Priest C, Campos-Pineda M, Novakovskaia E, Sloop CD, Michelsen HA, Bambha RP, Weiss RF, Keeling R, Fischer MLet al., 2016, Estimating methane emissions in California's urban and rural regions using multitower observations, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, Vol: 121, Pages: 13031-13049, ISSN: 2169-897X

Journal article

Thomas RT, Prentice IC, Graven H, Ciais P, Fisher JB, Hayes DJ, Huang M, Huntzinger DN, Ito A, Jain A, Mao J, Michalak AM, Peng S, Poulter B, Ricciuto DM, Shi X, Schwalm C, Tian H, Zeng Net al., 2016, Increased light-use efficiency in northern terrestrial ecosystems indicated by CO2 and greening observations, Geophysical Research Letters, Vol: 43, Pages: 11339-11349, ISSN: 1944-8007

Observations show an increasing amplitude in the seasonal cycle of CO2 (ASC) north of 45°N of 56 ± 9.8% over the last 50 years and an increase in vegetation greenness of 7.5–15% in high northern latitudes since the 1980s. However, the causes of these changes remain uncertain. Historical simulations from terrestrial biosphere models in the Multiscale Synthesis and Terrestrial Model Intercomparison Project are compared to the ASC and greenness observations, using the TM3 atmospheric transport model to translate surface fluxes into CO2 concentrations. We find that the modeled change in ASC is too small but the mean greening trend is generally captured. Modeled increases in greenness are primarily driven by warming, whereas ASC changes are primarily driven by increasing CO2. We suggest that increases in ecosystem-scale light use efficiency (LUE) have contributed to the observed ASC increase but are underestimated by current models. We highlight potential mechanisms that could increase modeled LUE.

Journal article

Graven HD, 2016, The carbon cycle in a changing climate, Physics Today, Vol: 69, Pages: 48-54, ISSN: 0031-9228

Journal article

Thomas R, Graven H, Hoskins B, Prentice Iet al., 2016, What is meant by ‘balancing sources and sinks of greenhouse gases’ to limit global temperature rise?, Grantham Institute Briefing Note, Imperial College London, 3

In an effort to limit global temperature rise to well below 2˚C, the COP21 Paris Agreement stipulates that a ‘balance’ between anthropogenic (man-made) sources and sinks of greenhouse gases must be reached by 2050-2100. An overall greenhouse gas ‘balance’ must consider individual gases in terms of how strongly they absorb solar infrared radiation, their concentration in the atmosphere, and their lifetime in the atmosphere.• Long-lived greenhouse gases, including carbon dioxide (CO2), accumulate in the atmosphere and continue to affect the climate for many centuries. To stabilise the concentrations of these long-lived gases, and thereby their effect on the climate, their sources must be progressively reduced towards zero. • For short-lived greenhouse gases that remain in the atmosphere for less than 100 years, including methane, stable or decreasing concentrations could be achieved within decades if emissions were stabilised or decreased. However, these gases currently only contribute about 20% of the total warming from greenhouse gases, so their reduction alone cannot successfully stabilise global temperature.• An overall ‘balance’ of sources and sinks of greenhouse gases could be facilitated by deliberate removal of CO2 from the atmosphere, for example, by combining biomass energy production with carbon capture and storage. Most current greenhouse gas emission scenarios that keep global temperature rise below 2˚C include some deliberate removal of CO2 to compensate for continued emissions of CO2 and other greenhouse gases

Report

Jones CD, Arora V, Friedlingstein P, Bopp L, Brovkin V, Dunne J, Graven H, Hoffman F, Ilyina T, John JG, Jung M, Kawamiya M, Koven C, Pongratz J, Raddatz T, Randerson J, Zaehle Set al., 2016, C4MIP - The Coupled Climate-Carbon Cycle Model Intercomparison Project: experimental protocol for CMIP6, Geoscientific Model Development, Vol: 9, Pages: 2853-2880, ISSN: 1991-9603

Coordinated experimental design and implementation has become a cornerstone of global climate modelling. Model Intercomparison Projects (MIPs) enable systematic and robust analysis of results across many models, by reducing the influence of ad hoc differences in model set-up or experimental boundary conditions. As it enters its 6th phase, the Coupled Model Intercomparison Project (CMIP6) has grown significantly in scope with the design and documentation of individual simulations delegated to individual climate science communities. The Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP) takes responsibility for design, documentation, and analysis of carbon cycle feedbacks and interactions in climate simulations. These feedbacks are potentially large and play a leading-order contribution in determining the atmospheric composition in response to human emissions of CO2 and in the setting of emissions targets to stabilize climate or avoid dangerous climate change. For over a decade, C4MIP has coordinated coupled climate–carbon cycle simulations, and in this paper we describe the C4MIP simulations that will be formally part of CMIP6. While the climate–carbon cycle community has created this experimental design, the simulations also fit within the wider CMIP activity, conform to some common standards including documentation and diagnostic requests, and are designed to complement the CMIP core experiments known as the Diagnostic, Evaluation and Characterization of Klima (DECK). C4MIP has three key strands of scientific motivation and the requested simulations are designed to satisfy their needs: (1) pre-industrial and historical simulations (formally part of the common set of CMIP6 experiments) to enable model evaluation, (2) idealized coupled and partially coupled simulations with 1 % per year increases in CO2 to enable diagnosis of feedback strength and its components, (3) future scenario simulations to project how the Earth system will re

Journal article

Jones CD, Arora V, Friedlingstein P, Bopp L, Brovkin V, Dunne J, Graven H, Hoffman F, Ilyina T, John JG, Jung M, Kawamiya M, Koven C, Pongratz J, Raddatz T, Randerson J, Zaehle Set al., 2016, The C4MIP experimental protocol for CMIP6, Geoscientific Model Development Discussions, Vol: 9, Pages: 2853-2880, ISSN: 1991-962X

Coordinated experimental design and implemen-tation has become a cornerstone of global climate modelling.Model Intercomparison Projects (MIPs) enable systematicand robust analysis of results across many models, by reduc-ing the influence of ad hoc differences in model set-up or ex-perimental boundary conditions. As it enters its 6th phase,the Coupled Model Intercomparison Project (CMIP6) hasgrown significantly in scope with the design and documenta-tion of individual simulations delegated to individual climatescience communities.The Coupled Climate–Carbon Cycle Model Intercompar-ison Project (C4MIP) takes responsibility for design, docu-mentation, and analysis of carbon cycle feedbacks and in-teractions in climate simulations. These feedbacks are poten-tially large and play a leading-order contribution in determin-ing the atmospheric composition in response to human emis-sions of CO2and in the setting of emissions targets to sta-bilize climate or avoid dangerous climate change. For overa decade, C4MIP has coordinated coupled climate–carboncycle simulations, and in this paper we describe the C4MIPsimulations that will be formally part of CMIP6. While theclimate–carbon cycle community has created this experimen-tal design, the simulations also fit within the wider CMIP ac-tivity, conform to some common standards including docu-mentation and diagnostic requests, and are designed to com-plement the CMIP core experiments known as the Diagnos-tic, Evaluation and Characterization of Klima (DECK).C4MIP has three key strands of scientific motivation andthe requested simulations are designed to satisfy their needs:(1) pre-industrial and historical simulations (formally partof the common set of CMIP6 experiments) to enable modelevaluation, (2) idealized coupled and partially coupled sim-ulations with 1 % per year increases in CO2to enable di-agnosis of feedback strength and its components, (3) futurescenario simulations

Journal article

Graven HD, 2015, Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century., Proceedings of the National Academy of Sciences of the United States of America, Vol: 112, Pages: 9542-9545, ISSN: 1091-6490

Radiocarbon analyses are commonly used in a broad range of fields, including earth science, archaeology, forgery detection, isotope forensics, and physiology. Many applications are sensitive to the radiocarbon ((14)C) content of atmospheric CO2, which has varied since 1890 as a result of nuclear weapons testing, fossil fuel emissions, and CO2 cycling between atmospheric, oceanic, and terrestrial carbon reservoirs. Over this century, the ratio (14)C/C in atmospheric CO2 (Δ(14)CO2) will be determined by the amount of fossil fuel combustion, which decreases Δ(14)CO2 because fossil fuels have lost all (14)C from radioactive decay. Simulations of Δ(14)CO2 using the emission scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report, the Representative Concentration Pathways, indicate that ambitious emission reductions could sustain Δ(14)CO2 near the preindustrial level of 0‰ through 2100, whereas "business-as-usual" emissions will reduce Δ(14)CO2 to -250‰, equivalent to the depletion expected from over 2,000 y of radioactive decay. Given current emissions trends, fossil fuel emission-driven artificial "aging" of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected. This finding has strong and as yet unrecognized implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.

Journal article

Lucas DD, Kwok CY, Cameron-Smith P, Graven H, Bergmann D, Guilderson TP, Weiss R, Keeling Ret al., 2015, Designing optimal greenhouse gas observing networks that consider performance and cost, GEOSCIENTIFIC INSTRUMENTATION METHODS AND DATA SYSTEMS, Vol: 4, Pages: 705-749, ISSN: 2193-0856

Journal article

Lucas DD, Kwok CY, Cameron-Smith P, Graven H, Bergmann D, Guilderson TP, Weiss R, Keeling Ret al., 2015, Designing optimal greenhouse gas observing networks that consider performance and cost, Geoscientific Instrumentation, Methods and Data Systems, Vol: 4, Pages: 121-137, ISSN: 2193-0864

Emission rates of greenhouse gases (GHGs) enteringinto the atmosphere can be inferred using mathematicalinverse approaches that combine observations from a networkof stations with forward atmospheric transport models.Some locations for collecting observations are better thanothers for constraining GHG emissions through the inversion,but the best locations for the inversion may be inaccessibleor limited by economic and other non-scientific factors.We present a method to design an optimal GHG observingnetwork in the presence of multiple objectives that may bein conflict with each other. As a demonstration, we use ourmethod to design a prototype network of six stations to monitorsummertime emissions in California of the potent GHG1,1,1,2-tetrafluoroethane (CH2FCF3, HFC-134a). We use amultiobjective genetic algorithm to evolve network configurationsthat seek to jointly maximize the scientific accuracyof the inferred HFC-134a emissions and minimize the associatedcosts of making the measurements. The genetic algorithmeffectively determines a set of “optimal” observingnetworks for HFC-134a that satisfy both objectives (i.e., thePareto frontier). The Pareto frontier is convex, and clearlyshows the tradeoffs between performance and cost, and thediminishing returns in trading one for the other. Without dif-ficulty, our method can be extended to design optimal networksto monitor two or more GHGs with different emissionspatterns, or to incorporate other objectives and constraintsthat are important in the practical design of atmosphericmonitoring networks.

Journal article

Goldberg SJ, Ball GI, Allen BC, Schladow SG, Simpson AJ, Masoom H, Soong R, Graven HD, Aluwihare LIet al., 2015, Refractory dissolved organic nitrogen accumulation in high-elevation lakes, Nature Communications, Vol: 6, ISSN: 2041-1723

The role of dissolved organic matter (DOM) as either a sink for inorganic nutrients or anadditional nutrient source is an often-neglected component of nutrient budgets in aquaticenvironments. Here, we examined the role of DOM in reactive nitrogen (N) storage in SierraNevada (California, USA) lakes where atmospheric deposition of N has shifted the lakestoward seasonal phosphorus (P)-limitation. Nuclear magnetic resonance (NMR) spectroscopyand isotope analyses performed on DOM isolated from Lake Tahoe reveal the accumulationof refractory proteinaceous material with a 100–200-year residence time. Incontrast, smaller lakes in the same watershed contain DOM with typical terrestrial characteristics,indicating that proteins in Lake Tahoe are autochthonously produced. These datasupport the role of DOM as a possible sink for reactive N in these lake ecosystems andidentify a potential role for DOM in affecting the inorganic nutrient stoichiometry of theseenvironments.

Journal article

Sitch S, Friedlingstein P, Gruber N, Jones SD, Murray-Tortarolo G, Ahlstrom A, Doney SC, Graven H, Heinze C, Huntingford C, Levis S, Levy PE, Lomas M, Poulter B, Viovy N, Zaehle S, Zeng N, Arneth A, Bonan G, Bopp L, Canadell JG, Chevallier F, Ciais P, Ellis R, Gloor M, Peylin P, Piao SL, Le Quere C, Smith B, Zhu Z, Myneni Ret al., 2015, Recent trends and drivers of regional sources and sinks of carbon dioxide, BIOGEOSCIENCES, Vol: 12, Pages: 653-679, ISSN: 1726-4170

Journal article

Graven HD, Keeling RF, Piper SC, Patra PK, Stephens BB, Wofsy SC, Welp LR, Sweeney C, Tans PP, Kelley JJ, Daube BC, Kort EA, Santoni GW, Bent JDet al., 2013, Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960, SCIENCE, Vol: 341, Pages: 1085-1089, ISSN: 0036-8075

Journal article

Turnbull J, Graven H, Miller J, Lehman Set al., 2013, ATMOSPHERIC RADIOCARBON WORKSHOP REPORT, RADIOCARBON, Vol: 55, Pages: 1470-1474, ISSN: 0033-8222

Journal article

Wanninkhof R, Park G-H, Takahashi T, Sweeney C, Feely R, Nojiri Y, Gruber N, Doney SC, McKinley GA, Lenton A, Le Quere C, Heinze C, Schwinger J, Graven H, Khatiwala Set al., 2013, Global ocean carbon uptake: magnitude, variability and trends, BIOGEOSCIENCES, Vol: 10, Pages: 1983-2000, ISSN: 1726-4170

Journal article

Graven HD, Xu X, Guilderson TP, Keeling RF, Trumbore SE, Tyler Set al., 2013, COMPARISON OF INDEPENDENT Delta(CO2)-C-14 RECORDS AT POINT BARROW, ALASKA, RADIOCARBON, Vol: 55, Pages: 1541-1545, ISSN: 0033-8222

Journal article

Khatiwala S, Tanhua T, Fletcher SM, Gerber M, Doney SC, Graven HD, Gruber N, McKinley GA, Murata A, Rios AF, Sabine CLet al., 2013, Global ocean storage of anthropogenic carbon, BIOGEOSCIENCES, Vol: 10, Pages: 2169-2191, ISSN: 1726-4170

Journal article

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