Andrew C. Manning

Global oceanic and land biotic carbon sinks from the Scripps atmospheric oxygen flask sampling network

Measurements of atmospheric O2/N2 ratio and CO2 concentration are presented over the period 1989–2003 from the Scripps Institution of Oceanography global flask sampling network.Aformal framework is described for making optimal use of these data to estimate global oceanic and land biotic carbon sinks. For the 10-yr period from 1990 to 2000, the oceanic and land biotic sinks are estimated to be 1.9 ± 0.6 and 1.2 ± 0.8 Pg C yr-1, respectively, while for the 10-yr period from 1993 to 2003, the sinks are estimated to be 2.2 ± 0.6 and 0.5 ± 0.7 Pg C yr-1, respectively. These estimates, which are also compared with earlier results, make allowance for oceanic O2 and N2 outgassing based on observed changes in ocean heat content and estimates of the relative outgassing per unit warming. For example, for the 1993–2003 period we estimate outgassing of 0.45 × 1014 mol O2 yr-1 and 0.20 × 1014 mol N2 yr-1, which results in a correction of 0.5 Pg C yr-1 on the oceanic and land biotic carbon sinks. The basis for this oceanic outgassing correction is reviewed in the context of recent model estimates. The main contributions to the uncertainty in the global sinks averages are from the estimates for oceanic outgassing and the estimates for fossil fuel combustion. The oceanic sink of 2.2 Pg C yr-1 for 1993–2003 is consistent, within the uncertainties, with the integrated accumulation of anthropogenic CO2 in the ocean since 1800 as recently estimated from oceanic observations, assuming the oceanic sink varied over time as predicted by a box-diffusion model.

Rising atmospheric methane: 2007-2014 growth and isotopic shift

From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr−1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short-term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.

Methods for measuring changes in atmospheric O2 concentration and their application in southern hemisphere air

Methods are described for measuring changes in atmospheric O2 concentration with emphasis on gas handling procedures. Cryogenically dried air samples are collected in 5 L glass flasks at ambient pressure and analyzed against reference gases derived from high-pressure aluminum tanks. Fractionation effects are minimized by avoiding pressure and flow variations throughout the gas-handling system. The overall external reproducibility is approximately +/-3.3 per meg, with systematic errors associated with collecting samples and with storing them for 1 year reduced to the level of 3 per meg or smaller. The demonstrated short-term reproducibly of air delivered from high-presure tanks is +/-1.5 per meg, with the composition changing by at most 5 per meg by surface desorption reactions as the tank is depleted to below 3500 kPa. A 9-year survey of a suite of six reference gases showed no systematic long-term trends in relative O2 concentrations to the level of 5 per meg. Results are presented from samples collected at Cape Grim (41 degrees S), Macquarie Island (54 degrees S) and the South Pole Station (90 degrees S). From measurements spanning 1991-1995 it is found that the O2 concentrations at the South Pole are on average 3.6+/-1.2 per meg higher than at Cape Grim. This result runs contrary to the expectation that the air at high southern latitudes should be depleted in O2 as a result of O2 uptake from the Southern Ocean and may require the existence of unknown O2 sources near Antarctica or unexpected atmospheric transport patterns.

Precise atmospheric oxygen measurements with a paramagnetic oxygen analyzer

A methodology has been developed for making continuous, high-precision measurements of atmospheric oxygen concentrations by modifying a commercially available paramagnetic oxygen analyzer. Incorporating several design improvements, an effective precision of 0.2 ppm O-2 from repeated measurements over a 1-hour interval was achieved. This is sufficient to detect background changes in atmospheric O-2 to a level that constrains various aspects of the global carbon cycle. The analyzer was used to measure atmospheric O-2 in a semicontinuous fashion from air sampled from the end of Scripps Pier, La Jolla, California, and data from a 1-week period in August 1996 are shown. The data exhibit strongly anticorrelated changes in O-2 and CO2 caused by local or regional combustion of fossil fuels. During periods of steady background CO2 concentrations, however, we see additional variability in O-2 concentrations, clearly not due to local combustion and presumably due to oceanic sources or sinks of O-2. This variability suggests that in contrast to CO2, higher O-2 sampling rates, such as those provided by continuous measurement programs, may be necessary to define an atmospheric O-2 background and thus aid in validating and interpreting other O-2 data from flask sampling programs. Our results have also demonstrated that this paramagnetic analyzer and gas handling design is well suited for making continuous measurements of atmospheric O-2 and is suitable for placement at remote background air monitoring sites.

On the long‐term stability of reference gases for atmospheric O2/N2 and CO2 measurements

Measurements of changes in the atmospheric O 2 /N 2 ratio have typically relied on compressed air derived from high-pressure tanks as the reference material against which atmospheric changes are assessed. The validity of this procedure is examined here in the context of the history of 18 O 2 /N 2 reference tanks compared over a 12-yr time-frame. By considering differences in tank sizes, material types, and by performing additional tests, the long-term stability of the delivered gas is evaluated with respect to surface reactions, leakage, regulator effects, and thermal diffusion and gravimetric fractionation. Results are also reported for the stability of CO 2 in these tanks. The results emphasize the importance of orienting tanks horizontally within a thermally insulated enclosure to reduce thermal and gravimetric fractionation of both O 2 /N 2 and CO 2 concentrations, and they emphasize the importance of avoiding elastomeric O-rings at the head-valve base. With the procedures documented here, the long-term drift in O 2 /N 2 appears to be zero to within approximately ±0.4 per meg yr -1 , which projects to an uncertainty of ±0.16 Pg C yr -1 (1σ) in O 2 -based global carbon budgets. DOI: 10.1111/j.1600-0889.2006.00228.x

A ship-based methodology for high precision atmospheric oxygen measurements and its application in the Southern Ocean region

Amethod for achieving continuous high precision measurements of atmospheri O2 is presented based on a commercially available fuel-cell instrument, (Sable Systems, Oxzilla FC-II) with a precision of 7 per meg (approximately equivalent to 1.2 ppm) for a 6-min measurement. The Oxzilla was deployed on two voyages in the Western Pacific sector of the Southern Ocean, in February 2003 and in April 2004, making these the second set of continuous O2 measurements ever made from a ship. The results show significant temporal variation in O2, in the order of 10 per meg over 6-hourly time intervals, and substantial spatial variation. Data from both voyages show an O2 maximum centred on 50°S, which is most likely to be the result of biologically driven O2 outgassing in the region of subtropical convergence around New Zealand, and a decreasing O2 trend towards Antarctica. O2 from the ship-based measurements is elevated compared with measurements from the Scripps Institution of Oceanography flask-sampling network, and the O2 maximum is also not captured in the network observations. This preliminary study shows that ship-based continuous measurements are a valuable addition to current fixed site sampling programmes for the understanding of ocean.atmosphere O2 exchange processes.

The atmospheric signature of carbon capture and storage

Compared with other industrial processes, carbon capture and storage (CCS) will have an unusual impact on atmospheric composition by reducing the CO2 released from fossil-fuel combustion plants, but not reducing the associated O2 loss. CO2 that leaks into the air from below-ground CCS sites will also be unusual in lacking the O2 deficit normally associated with typical land CO2 sources, such as from combustion or ecosystem exchanges. CCS may also produce distinct isotopic changes in atmospheric CO2. Using simple models and calculations, we estimate the impact of CCS or leakage on regional atmospheric composition. We also estimate the possible impact on global atmospheric composition, assuming that the technology is widely adopted. Because of its unique signature, CCS may be especially amenable to monitoring, both regionally and globally, using atmospheric observing systems. Measurements of the O2/N2 ratio and the CO2 concentration in the proximity of a CCS site may allow detection of point leaks of the order of 1000 ton CO2 yr−1 from a CCS reservoir up to 1 km from the source. Measurements of O2/N2 and CO2 in background air from a global network may allow quantification of global and hemispheric capture rates from CCS to the order of ±0.4 Pg C yr−1.

A statistical gap-filling method to interpolate global monthly surface ocean carbon dioxide data

We have developed a statistical gap-filling method adapted to the specific coverage and properties of observed fugacity of surface ocean CO2 (fCO2). We have used this method to interpolate the Surface Ocean CO2 Atlas (SOCAT) v2 database on a 2.5°×2.5° global grid (south of 70°N) for 1985–2011 at monthly resolution. The method combines a spatial interpolation based on a “radius of influence” to determine nearby similar fCO2 values with temporal harmonic and cubic spline curve-fitting, and also fits long-term trends and seasonal cycles. Interannual variability is established using deviations of observations from the fitted trends and seasonal cycles. An uncertainty is computed for all interpolated values based on the spatial and temporal range of the interpolation. Tests of the method using model data show that it performs as well as or better than previous regional interpolation methods, but in addition it provides a near-global and interannual coverage.

Greenhouse gases in the Earth system: setting the agenda to 2030

What do we need to know about greenhouse gases? Over the next 20 years, how should scientists study the role of greenhouse gases in the Earth system and the changes that are taking place? These questions were addressed at a Royal Society scientific Discussion Meeting in London on 22–23 February 2010, with over 300 participants.

Iconic CO2 Time Series at Risk

The steady rise in atmospheric long-lived greenhouse gas concentrations is the main driver of contemporary climate change. The Mauna Loa CO2 time series (1, 2), started by C. D. Keeling in 1958 and maintained today by the Scripps Institution of Oceanography and the Earth System Research Laboratory (ESRL) of NOAA, is iconic evidence of the effect of human-caused fossil fuel and land-use change emissions on the atmospheric increase of CO2. The continuity of such records depends critically on having stable funding, which is challenging to maintain in the context of 3- to 4-year research grant funding cycles (3), and is currently threatened by the financial crisis. The ESRL Global Monitoring Division maintains a network of about 100 surface and aircraft sites worldwide at which whole air samples are collected approximately every week for analysis of CO2, CH4, CO, halocarbons, and many other chemical species (4). This is complemented by high-frequency measurements at the Mauna Loa, Barrow, American Samoa, and South Pole observatories, and about 10 North American tall towers. The success of the NOAA program has inspired similar efforts in Europe (5), China (6), India (7), and Brazil (8), with the United Nations World Meteorological Organization providing guidance and precision requirements through the Global Atmosphere Watch program (9), but no funding. The data collected by NOAA and its worldwide partners have been used not only to demonstrate the unassailable rise of atmospheric greenhouse gas concentrations, but also to infer the magnitudes, locations, and times of surface-atmosphere exchange of those gases based on small concentration gradients between sites (10). Important findings from analysis of these records include the detection of a significant terrestrial carbon sink at northern mid-latitudes (11) and subsequent research aimed at identifying the mechanisms by which that sink must operate. Long-term, high-quality, atmospheric measurements are crucial for quantifying trends in greenhouse gas fluxes and attributing them to fossil fuel emissions, changes in land-use and management, or the response of natural land and ocean ecosystems to climate change and elevated CO2 concentrations. Greenhouse gas measurements along tall towers in the interior continents allow quantification of regional sources and sinks, which has a very high relevance for measuring the effectiveness of climate policy. NOAA ESRL provides measurements that are critical for the U.S. national security in that they provide independent verification and early warning of changing greenhouse gas emissions from countries involved in efforts to mitigate greenhouse gases. Dedicated carbon-observing satellites such as GOSAT and OCO-2 are needed to fill in the missing geographical information required for verification of carbon flux mitigation efforts. However, satellite retrievals do not yet provide sufficient information to deliver new constraints on surface fluxes, although quick progress is being made in this direction. In situ observations are crucial for anchoring space-borne measurements, for detecting potential biases of remote sensing techniques, and for providing continuity given the finite lifetime of satellites. Despite the growing importance of greenhouse gas observations to humanity, substantial budget cuts at NOAA have resulted in curtailment of our ability to observe and understand changes to the global carbon cycle. Already, a dozen surface flask-sampling sites have been removed from NOAAs operational network and aircraft profiling sites have been eliminated and reduced in frequency at the remaining NOAA sites. The planned growth in the tall tower program has stopped, and plans for closing some towers are being developed. The U.S. budget process in this election year, with the added risk of mandatory across-the-board cuts due to the 2011 Budget Control Act, foretells more bleak news for greenhouse gas monitoring at NOAA and could cause further retreat from the goal of recording ongoing changes in atmospheric composition. As scientists, we believe that preserving the continuity of these vital time series must remain a priority for U.S. carbon cycle research.