Carbon cycle research after Kyoto

Recent progress in research of the global carbon cycle is reviewed and research needs for the immediate future are discussed, in light of the challenge posed to society to come to grips with the problem of man-made climate change. The carbon cycle in the oceans and on the land is reviewed, and how the atmosphere functions to couple them together. Major uncertainties still exist for any projection of the future atmospheric burden of carbon dioxide resulting from postulated emission scenarios of CO2. We present some ideas on how future policies designed to limit emissions or to sequester carbon can possibly be supported by scientific evidence of their effectiveness

Global Carbon Budget 2018

Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere – the “global carbon budget” – is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFF) are based on energy statistics and cement production data, while emissions from land use and land-use change (ELUC), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) and terrestrial CO2 sink (SLAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the last decade available (2008–2017), EFF was 9.4±0.5 GtC yr−1, ELUC 1.5±0.7 GtC yr−1, GATM 4.7±0.02 GtC yr−1, SOCEAN 2.4±0.5 GtC yr−1, and SLAND 3.2±0.8 GtC yr−1, with a budget imbalance BIM of 0.5 GtC yr−1 indicating overestimated emissions and/or underestimated sinks. For the year 2017 alone, the growth in EFF was about 1.6 % and emissions increased to 9.9±0.5 GtC yr−1. Also for 2017, ELUC was 1.4±0.7 GtC yr−1, GATM was 4.6±0.2 GtC yr−1, SOCEAN was 2.5±0.5 GtC yr−1, and SLAND was 3.8±0.8 GtC yr−1, with a BIM of 0.3 GtC. The global atmospheric CO2 concentration reached 405.0±0.1 ppm averaged over 2017. For 2018, preliminary data for the first 6–9 months indicate a renewed growth in EFF of +2.7 % (range of 1.8 % to 3.7 %) based on national emission projections for China, the US, the EU, and India and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. The analysis presented here shows that the mean and trend in the five components of the global carbon budget are consistently estimated over the period of 1959–2017, but discrepancies of up to 1 GtC yr−1 persist for the representation of semi-decadal variability in CO2 fluxes. A detailed comparison among individual estimates and the introduction of a broad range of observations show (1) no consensus in the mean and trend in land-use change emissions, (2) a persistent low agreement among the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) an apparent underestimation of the CO2 variability by ocean models, originating outside the tropics. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding the global carbon cycle compared with previous publications of this data set (Le Quéré et al., 2018, 2016, 2015a, b, 2014, 2013). All results presented here can be downloaded from https://doi.org/10.18160/GCP-2018

An improved Kalman Smoother for atmospheric inversions

We explore the use of a fixed-lag Kalman smoother for sequential estimation of atmospheric carbon dioxide fluxes. This technique takes advantage of the fact that most of the information about the spatial distribution of sources and sinks is observable within a few months to half of a year of emission. After this period, the spatial structure of sources is diluted by transport and cannot significantly constrain flux estimates. We therefore describe an estimation technique that steps through the observations sequentially, using only the subset of observations and modeled transport fields that most strongly constrain the fluxes at a particular time step. Estimates of each set of fluxes are sequentially updated multiple times, using measurements taken at different times, and the estimates and their uncertainties are shown to quickly converge. Final flux estimates are incorporated into the background state of CO2 and transported forward in time, and the final flux uncertainties and covariances are taken into account when estimating the covariances of the fluxes still being estimated. The computational demands of this technique are greatly reduced in comparison to the standard Bayesian synthesis technique where all observations are used at once with transport fields spanning the entire period of the observations. It therefore becomes possible to solve larger inverse problems with more observations and for fluxes discretized at finer spatial scales. We also discuss the differences between running the inversion simultaneously with the transport model and running it entirely off-line with pre-calculated transport fields. We find that the latter can be done with minimal error if time series of transport fields of adequate length are pre-calculated

New directions : watching over tropospheric hydroxyl (OH)

Mean tropospheric hydroxyl radical (OH) abundance is often used as a measure of the oxidation capacity (or “self-cleansing”) of the atmosphere. The primary mechanism by which atmospheric pollutant gases are removed from the atmosphere is initiated by the reaction with OH. As a result, large interannual or decadal variations in OH concentrations, as suggested in recent reports, are of great concern. In addition, an important method for discerning tropospheric OH burdens and variability, the analysis of methyl chloroform (MCF) observations, will soon become less useful as the concentration of this industrial gas approaches zero. With these concerns in mind, a workshop focusing on global OH trends and variability was convened in Boulder, Colorado, on 28–30 November 2005. Although the concept of tropospheric mean OH does not do justice to regional OH differences, and ignores the less significant contributions by other oxidants, global OH changes integrate the response to large-scale atmospheric chemistry forcings. The latter include the tendencies of atmospheric water vapour, solar radiation and notably the human-induced emissions of NOx, CO, CH4 and other hydrocarbons. Analogously, the global mean surface temperature change is an integral climate response to natural and anthropogenic forcings by greenhouse gases, aerosols, etc. Furthermore, the variability of mean OH is an indicator of the sensitivity of atmospheric chemistry to global air pollution and natural events (e.g., large volcano eruptions, El Niño). A large response to a small forcing is typical for a system that is not well buffered and vice versa. The analysis of relatively long-lived gases for which emission magnitudes are well characterized can provide insight into the interannual and decadal variability of tropospheric OH. Analysis of MCF measurements, a tracer with a lifetime of about 5 yr owing to its removal by OH, points to a substantial OH growth in the 1980s, a decline in the 1990s and a recovery after 1998, indicating decadal OH changes of 10–15% (R.G. Prinn et al., 2005, Geophysical Research Letters 32, L07809, doi:10.1029/2004GL022228). MCF analysis furthermore suggests a large interannual OH variability of 8.5±1.0%; however, this may reflect uncertainties in the MCF emission inventory (P. Bousquet et al., 2005, Atmospheric Chemistry and Physics 5, 2635–2656). Using radiocarbon 14CO as a diagnostic for OH gives additional evidence of 10% variability in OH over timescales of less than a year, although the 14CO measurements are only representative of the extratropical southern hemisphere (M.R. Manning et al., 2005, Nature 436, 1001–1004). Even though chemistry–transport models fail to reproduce the large OH variability, many studies point to relatively large OH changes after the 1991 Mt. Pinatubo eruption and during the 1997/8 El Niño event. The likelihood of large OH variability is challenged by CH4 mass balance calculations based on the NOAA network measurements. Emissions of CH4 (E) can be derived from the measured global burden [CH4] and rate of CH4 increase, and an estimate of the CH4 lifetime: E=d[CH4]/dt+[CH4]/τ, where d[CH4]/dt is the observed rate of increase and τ the CH4 lifetime. Since τ is not constant in the real atmosphere, fixing it in the equation means that E includes variability of the sink, i.e. changes in OH. Calculation of E based on yields a mean source of 556±10 Tg CH4 yr−1 for 1984 to 2004, with a trend of 0.1±0.4 Tg yr−1. Maximum deviations from E are 18.4 Tg in 1991 and 27.0 Tg in 1998 (see Fig. 1). Assuming that emissions are uncorrelated with OH, these anomalies provide upper limits of the interannual variability of OH, e.g., 3–5% in 1991 and 1998. In other years the OH variability is typically less than 2%, in agreement with chemistry–transport models. Yet, there is little doubt that global OH decreased immediately after the Mt. Pinatubo eruption, as this is evident in both the CH4 and 14CO measurements. The large anomaly in CH4 in 1998 had contributions by increased emissions from wetlands and biomass burning (S. Morimoto et al., 2006, Geophysical Research Letters 33, L01807, doi:10.1029/2005GL024648), and decreased OH resulting from the Indonesian biomass burning emissions (T.M. Butler et al., 2005, Journal of Geophysical Research 110, D21310, doi:10.1029/2005JD006071)

A three‐dimensional synthesis study of δ18O in atmospheric CO2 1. Surface fluxes

International audienceThe isotope 18O in CO2 is of particular interest in studying the global carbon cycle because it is sensitive to the processes by which the global land biosphere absorbs and respires CO2. Carbon dioxide and water exchange isotopically both in leaves and in soils, and the 18O character of atmospheric CO2 is strongly influenced by the land biota, which should constrain the gross primary productivity and total respiration of land ecosystems. In this study we calculate the global surface fluxes of 18O for vegetation and soils using the SiB2 biosphere model coupled with the Colorado State University general circulation model. This approach makes it possible to use physiological variables that are consistently weighted by the carbon assimilation rate and integrated through the vegetation canopy. We also calculate the air‐sea exchange of 18O and the isotopic character of fossil emissions and biomass burning. Global mean values of the isotopic exchange with each reservoir are used to close the global budget of 18O in CO2. Our results confirm the fact that the land biota exert a dominant control on the δ18O of the atmospheric reservoir. At the global scale, exchange with the canopy produces an isotopic enrichment of CO2, whereas exchange with soils has the opposite effect