Heather Graven

Enhanced seasonal exchange of CO2 by northern ecosystems since 1960.

Downs and Ups Every spring, the concentration of CO2 in the atmosphere of the Northern Hemisphere decreases as terrestrial vegetation grows, and every fall, CO2 rises as vegetation dies and rots. Climate change has destabilized the seasonal cycle of atmospheric CO2 such that Graven et al. (p. 1085, published online 8 August; see the Perspective by Fung) have found that the amplitude of the seasonal cycle has exceeded 50% at some latitudes. The only way to explain this increase is if extratropical land ecosystems are growing and shrinking more than they did half a century ago, as a result of changes in the structure and composition of northern ecosystems. The amplitude of the seasonal cycle of carbon dioxide in high northern latitudes has increased by 50% since 1960. [Also see Perspective by Fung] Seasonal variations of atmospheric carbon dioxide (CO2) in the Northern Hemisphere have increased since the 1950s, but sparse observations have prevented a clear assessment of the patterns of long-term change and the underlying mechanisms. We compare recent aircraft-based observations of CO2 above the North Pacific and Arctic Oceans to earlier data from 1958 to 1961 and find that the seasonal amplitude at altitudes of 3 to 6 km increased by 50% for 45° to 90°N but by less than 25% for 10° to 45°N. An increase of 30 to 60% in the seasonal exchange of CO2 by northern extratropical land ecosystems, focused on boreal forests, is implicated, substantially more than simulated by current land ecosystem models. The observations appear to signal large ecological changes in northern forests and a major shift in the global carbon cycle.

Methods for High-Precision 14C AMS Measurement of Atmospheric CO2 at LLNL

Development of radiocarbon analysis with precision better than 2‰ has the potential to expand the utility of 14CO2 measurements for carbon cycle investigations as atmospheric gradients currently approach the typical measurement precision of 2-5‰. The accelerator mass spectrometer at Lawrence Livermore National Laboratory (LLNL) produces high and stable beam currents that enable efficient acquisition times for large numbers of 14C counts. One million 14C atoms can be detected in approximately 25 min, suggesting that near 1‰ counting precision is economically feasible at LLNL. The overall uncertainty in measured values is ultimately determined by the variation between measured ratios in several sputtering periods of the same sample and by the reproducibility of replicate samples. Experiments on the collection of 1 million counts on replicate samples of CO2 extracted from a whole air cylinder show a standard deviation of 1.7‰ in 36 samples measured over several wheels. This precision may be limited by the reproducibility of oxalic acid I standard samples, which is consid- erably poorer. We outline the procedures for high-precision sample handling and analysis that have enabled reproducibility in the cylinder extraction samples at the <2‰ level and describe future directions to continue increasing measurement precision at LLNL.

Vertical profiles of biospheric and fossil fuel-derived CO2 and fossil fuel CO2 :CO ratios from airborne measurements of Δ14C, CO2 and CO above Colorado, USA

Measurements of Δ14C in atmospheric CO2 are an effective method of separating CO2 additions from fossil fuel and biospheric sources or sinks of CO2. We illustrate this technique with vertical profiles of CO2 and Δ14C analysed in whole air flask samples collected above Colorado, USA in May and July 2004. Comparison of lower tropospheric composition to cleaner air at higher altitudes (>5 km) revealed considerable additions from respiration in the morning in both urban and rural locations. Afternoon concentrations were mainly governed by fossil fuel emissions and boundary layer depth, also showing net biospheric CO2 uptake in some cases. We estimate local industrial CO2:CO emission ratios using in situ measurements of CO concentration. Ratios are found to vary by 100% and average 57 mole CO2:1 mole CO, higher than expected from emissions inventories. Uncertainty in CO2 from different sources was ±1.1 to ±4.1 ppm for addition or uptake of −4.6 to 55.8 ppm, limited by Δ14C measurement precision and uncertainty in background Δ14C and CO2 levels.

Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century

Significance A wide array of scientific disciplines and industries use radiocarbon analyses; for example, it is used in dating of archaeological specimens and in forensic identification of human and wildlife tissues, including traded ivory. Over the next century, fossil fuel emissions will produce a large amount of CO2 with no 14C because fossil fuels have lost all 14C over millions of years of radioactive decay. Atmospheric CO2, and therefore newly produced organic material, will appear as though it has “aged,” or lost 14C by decay. By 2050, fresh organic material could have the same 14C/C ratio as samples from 1050, and thus be indistinguishable by radiocarbon dating. Some current applications for 14C may cease to be viable, and other applications will be strongly affected. 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 (14C) 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 14C/C in atmospheric CO2 (Δ14CO2) will be determined by the amount of fossil fuel combustion, which decreases Δ14CO2 because fossil fuels have lost all 14C from radioactive decay. Simulations of Δ14CO2 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 Δ14CO2 near the preindustrial level of 0‰ through 2100, whereas “business-as-usual” emissions will reduce Δ14CO2 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.

Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis

Significance Climate change and rising CO2 are altering the behavior of land plants in ways that influence how much biomass they produce relative to how much water they need for growth. This study shows that it is possible to detect changes occurring in plants using long-term measurements of the isotopic composition of atmospheric CO2. These measurements imply that plants have globally increased their water use efficiency at the leaf level in proportion to the rise in atmospheric CO2 over the past few decades. While the full implications remain to be explored, the results help to quantify the extent to which the biosphere has become less constrained by water stress globally. 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.

Estimating methane emissions in California's urban and rural regions using multitower observations

We present an analysis of methane (CH_4) emissions using atmospheric observations from 13 sites in California during June 2013 to May 2014. A hierarchical Bayesian inversion method is used to estimate CH_4 emissions for spatial regions (0.3° pixels for major regions) by comparing measured CH_4 mixing ratios with transport model (Weather Research and Forecasting and Stochastic Time-Inverted Lagrangian Transport) predictions based on seasonally varying California-specific CH_4 prior emission models. The transport model is assessed using a combination of meteorological and carbon monoxide (CO) measurements coupled with the gridded California Air Resources Board (CARB) CO emission inventory. The hierarchical Bayesian inversion suggests that state annual anthropogenic CH_4 emissions are 2.42 ± 0.49 Tg CH_4/yr (at 95% confidence), higher (1.2–1.8 times) than the current CARB inventory (1.64 Tg CH_4/yr in 2013). It should be noted that undiagnosed sources of errors or uncaptured errors in the model-measurement mismatch covariance may increase these uncertainty bounds beyond that indicated here. The CH_4 emissions from the Central Valley and urban regions (San Francisco Bay and South Coast Air Basins) account for ~58% and 26% of the total posterior emissions, respectively. This study suggests that the livestock sector is likely the major contributor to the state total CH_4 emissions, in agreement with CARBs inventory. Attribution to source sectors for subregions of California using additional trace gas species would further improve the quantification of Californias CH_4 emissions and mitigation efforts toward the California Global Warming Solutions Act of 2006 (Assembly Bill 32).

Refractory dissolved organic nitrogen accumulation in high-elevation lakes

The role of dissolved organic matter (DOM) as either a sink for inorganic nutrients or an additional nutrient source is an often-neglected component of nutrient budgets in aquatic environments. Here, we examined the role of DOM in reactive nitrogen (N) storage in Sierra Nevada (California, USA) lakes where atmospheric deposition of N has shifted the lakes toward seasonal phosphorus (P)-limitation. Nuclear magnetic resonance (NMR) spectroscopy and isotope analyses performed on DOM isolated from Lake Tahoe reveal the accumulation of refractory proteinaceous material with a 100-200-year residence time. In contrast, smaller lakes in the same watershed contain DOM with typical terrestrial characteristics, indicating that proteins in Lake Tahoe are autochthonously produced. These data support the role of DOM as a possible sink for reactive N in these lake ecosystems and identify a potential role for DOM in affecting the inorganic nutrient stoichiometry of these environments.

Simulating Estimation of California Fossil Fuel and Biosphere Carbon Dioxide Exchanges Combining In-situ Tower and Satellite Column Observations:

Author(s): Fischer, Marc L.; Parazoo, Nicholas; Brophy, Kieran; Cui, Xinguang; Jeong, Seongeun; Liu, Junjie; Kelling, Ralph; Taylor, Thomas E.; Gurney, Kevin; Oda, Tomohiro; Graven, Heather | Abstract: We report simulation experiments estimating the uncertainties in California regional fossil fuel 36 and biosphere CO2 exchanges that might be obtained using an atmospheric inverse modeling 37 system driven by the combination of ground-based observations of radiocarbon and total CO2, 38 together with column-mean CO2 observations from NASA’s Orbiting Carbon Observatory 39 (OCO-2). The work includes an initial examination of statistical uncertainties in prior models for 40 CO2 exchange, in radiocarbon-based fossil fuel CO2 measurements, in OCO-2 measurements, 41 and in a regional atmospheric transport modeling system. Using these nominal assumptions for 42 measurement and model uncertainties, we find that flask measurements of radiocarbon and total 43 CO2 at 10 towers can be used to distinguish between different fossil fuel emissions data products 44 for major urban regions of California. We then show that the combination of flask and OCO-2 45 observations yield posterior uncertainties in monthly-mean fossil fuel emissions of ~ 5-10%, 46 levels likely useful for policy relevant evaluation of bottom-up fossil fuel emission estimates. 47 Similarly, we find that inversions yield uncertainties in monthly biosphere CO2 exchange of ~ 48 6%-12%, depending on season, providing useful information on net carbon uptake in 49 California’s forests and agricultural lands. Finally, initial sensitivity analysis suggests that 50 obtaining the above results requires control of systematic biases below approximately 0.5 ppm, 51 placing requirements on accuracy of the atmospheric measurements, background subtraction, and 52 atmospheric transport modeling.

The carbon cycle in a changing climate

As temperatures rise, precipitation patterns change, and land- and sea-ice extents shrink, scientists are learning how the exchanges of carbon between Earth’s atmosphere, ocean, and land ecosystems respond to and feed back on climate change.

Changes to the Air‐Sea Flux and Distribution of Radiocarbon in the Ocean Over the 21st Century

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. Plain Language Summary The carbon content and acidity of the ocean are increasing as the ocean has absorbed roughly a third of the CO2 emitted by the burning of fossil fuels. Human activities are also changing the isotopic composition of carbon in the atmosphere and ocean. In the midtwentieth century, nuclear weapons testing produced a large amount of 14C (radiocarbon), the heavy radioactive isotope of carbon that is extensively used for archeological dating. At the same time, the combustion of fossil fuels reduces the amount of 14C relative to the more common isotope 12C, since fossil fuels have lost all their radiocarbon through radioactive decay. As time passes since the bomb testing, fossil fuel emissions are becoming an increasingly dominant influence on the carbon isotope composition of the atmosphere and ocean. Here we use a computer model to simulate how this composition is likely to evolve over the coming century in response to continued fossil fuel burning. Our results show that by the end of the century surface ocean waters will be more depleted in radiocarbon than deeper waters, a complete reversal of the pattern that prevailed before humans started changing the environment.