Long-lived Greenhouse Gases Research Articles

Three decades of global methane sources and sinks

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. A compilation of observations and results from chemical transport, ecosystem and climate chemistry models suggests that a rise in wetland and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006. Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios — which differ in fossil fuel and microbial emissions — to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain.

The climate impact of travel behavior: A German case study with illustrative mitigation options

Abstract Global greenhouse gas mitigation should include the growing share of emissions from transportation. To help understand the mitigation potential of changing travel behavior requires disaggregating the climate impacts of transportation by transport mode, distance, and travel behavior. Here we use disaggregated data on travel behavior to calculate the climate impact of Germans traveling nationally and internationally in 2008 and develop some illustrative mitigation options. We include all relevant long-lived greenhouse gases and short-lived climate forcers and use global temperature change for 50 years of sustained emissions as the emission metric. The total climate impact is determined almost entirely by car (∼46%) and air travel (∼45%), with smaller contributions from public transportation. The climate impact from the highest income group is 250% larger than from the lowest income group. However, the middle classes account for more than two thirds of the total impact. The relatively few trips beyond 100 km contribute more than half of the total impact because of the trip distance and use of aircraft. Individual behavioral changes, like shifting transport modes or reducing distance and frequency, can lead to useful emission reductions. However, a comprehensive package of mitigation options is necessary for deep and sustained emission reductions.

Ocean circulation response to anthropogenic aerosol and greenhouse gas forcing in the CSIRO-Mk3.6 climate model

We use the CSIRO-Mk3.6 coupled climate model to examine the impact of anthropogenic aerosols (AAs) and long-lived greenhouse gases (GHGs) on aspects of the global ocean circulation. Focusing on the second half of the twentieth century, we compare multiple ten member ensembles of historical climate change, which are forced by different combinations of forcing agents; these different simulations enable us to separately diagnose the effects of changes in AAs and GHGs. We also compare ten member 21st century ensembles driven by Representative Concentration Pathways 4.5 (RCP4.5) and 8.5 (RCP8.5). To a large degree the pattern of change in the oceans due to the impact of AAs is similar to the effect of increasing GHGs, but of opposite sign. The Atlantic Meridional Overturning Circulation (and associated North Atlantic Deep Water formation) strengthens in response to historical changes in AAs but weakens in response to increasing GHGs. Similarly, the Indonesian Throughflow strengthens in response to AAs, and weakens in response to increasing GHGs. The Drake Passage Transport, however, shows a small weakening (strengthening) due to historical changes in AAs (GHGs) in the ensemble mean. The change in the Drake Passage Transport is much clearer in the 21st century, in which it increases strongly in response to increasing GHGs and decreasing AAs in both RCP4.5 and RCP8.5. The results suggests that without the influence of AAs, changes in ocean circulation would have already followed a path much more like one dominated by increasing GHGs. Considering that the AA levels are expected to decrease during the next few decades, the effects of increasing GHGs on ocean circulation will be amplified accordingly.

Anthropogenic effects on the subtropical jet in the Southern Hemisphere: aerosols versus long-lived greenhouse gases

We use single-forcing historical simulations with a coupled atmosphere–ocean global climate model to compare the effects of anthropogenic aerosols (AAs) and increasing long-lived greenhouse gases (LLGHGs) on simulated winter circulation in the Southern Hemisphere (SH). Our primary focus is on the subtropical jet, which is an important source of baroclinic instability, especially in the Australasian region, where the speed of the jet is largest. For the period 1950 to 2005, our simulations suggest that AAs weaken the jet, whereas increasing LLGHGs strengthen the jet. The different responses are explained in terms of thermal wind balance: increasing LLGHGs preferentially warm the tropical mid-troposphere and upper troposphere, whereas AAs have a similar effect of opposite sign. In the mid-troposphere, the warming (cooling) effect of LLGHGs (AAs) is maximal between 20S and 30S; this coincides with the descending branch of the Hadley circulation, which may advect temperature changes from the tropical upper troposphere to the subtropics of the SH. It follows that LLGHGs (AAs) increase (decrease) the mid-tropospheric temperature gradient between low latitudes and the SH mid-latitudes. The strongest effects are seen at longitudes where the southward branches of the Hadley cell in the upper troposphere are strongest, notably at those that correspond to Asia and the western Pacific warm pool.

Determination of a lower bound on Earth's climate sensitivity

Transient and equilibrium sensitivity of Earth's climate has been calculated using global temperature, forcing and heating rate data for the period 1970–2010. We have assumed increased long-wave radiative forcing in the period due to the increase of the long-lived greenhouse gases. By assuming the change in aerosol forcing in the period to be zero, we calculate what we consider to be lower bounds to these sensitivities, as the magnitude of the negative aerosol forcing is unlikely to have diminished in this period. The radiation imbalance necessary to calculate equilibrium sensitivity is estimated from the rate of ocean heat accumulation as 0.37±0.03 W m−2 (all uncertainty estimates are 1−σ). With these data, we obtain best estimates for transient climate sensitivity 0.39±0.07 K (W m−2)−1 and equilibrium climate sensitivity 0.54±0.14 K (W m−2)−1, equivalent to 1.5±0.3 and 2.0±0.5 K (3.7 W m−2)−1, respectively. The latter quantity is equal to the lower bound of the ‘likely’ range for this quantity given by the 2007 IPCC Assessment Report. The uncertainty attached to the lower-bound equilibrium sensitivity permits us to state, within the assumptions of this analysis, that the equilibrium sensitivity is greater than 0.31 K (W m−2)−1, equivalent to 1.16 K (3.7 W m−2)−1, at the 95% confidence level.

Air quality and climate – synergies and trade-offs

Air quality and climate are often treated as separate science and policy areas. Air quality encompasses the here-and-now of pollutant emissions, atmospheric transformations and their direct effect on human and ecosystem health. Climate change deals with the drivers leading to a warmer world and the consequences of that. These two science and policy issues are inexorably linked via common pollutants, such as ozone (methane) and black carbon. This short review looks at the new scientific evidence around so-called "short-lived climate forcers" and the growing realisation that a way to meet short-term climate change targets may be through the control of "air quality" pollutants. None of the options discussed here can replace reduction of long-lived greenhouse gases, such as CO2, which is required for any long-term climate change mitigation strategy. An overview is given of the underlying science, remaining uncertainties, and some of the synergies and trade-offs for addressing air quality and climate in the science and policy context.

Rate coefficients for the reaction of O(<sup>1</sup>D) with the atmospherically long-lived greenhouse gases NF<sub>3</sub>, SF<sub>5</sub>CF<sub>3</sub>, CHF<sub>3</sub>, C<sub>2</sub>F<sub>6</sub>, c-C<sub>4</sub>F<sub>8</sub>, <i>n</i>-C<sub>5</sub>F<sub>12</sub>, and <i>n</i>-C<sub>6</sub>F<sub>14</sub>

Abstract. The contribution of atmospherically persistent (long-lived) greenhouse gases to the radiative forcing of Earth has increased over the past several decades. The impact of highly fluorinated, saturated compounds, in particular perfluorinated compounds, on climate change is a concern because of their long atmospheric lifetimes, which are primarily determined by stratospheric loss processes, as well as their strong absorption in the infrared "window" region. A potentially key stratospheric loss process for these compounds is their gas-phase reaction with electronically excited oxygen atoms, O(1D). Therefore, accurate reaction rate coefficient data is desired for input to climate change models. In this work, rate coefficients, k, were measured for the reaction of O(1D) with several key long-lived greenhouse gases, namely NF3, SF5CF3, CHF3 (HFC-23), C2F6, c-C4F8, n-C5F12, and n-C6F14. Room temperature rate coefficients for the total reaction, kTot, corresponding to loss of O(1D), and reactive channel, kR, corresponding to the loss of the reactant compound, were measured for NF3 and SF5CF3 using competitive reaction and relative rate methods, respectively. kR was measured for the CHF3 reaction and improved upper-limits were determined for the perfluorinated compounds included in this study. For NF3, kTot was determined to be (2.55 ± 0.38) × 10−11 cm3 molecule−1 s−1 and kR, which was measured using CF3Cl, N2O, CF2ClCF2Cl (CFC-114), and CF3CFCl2 (CFC-114a) as reference compounds, was determined to be (2.21 ± 0.33) × 10−11 cm3 molecule−1 s−1. For SF5CF3, kTot = (3.24 ± 0.50) × 10−13 cm3 molecule−1 s−1 and kR < 5.8 × 10×14 cm3 molecule−1 s−1 were measured, where kR is a factor of three lower than the current recommendation of kTot for use in atmospheric modeling. For CHF3 kR was determined to be (2.35 ± 0.35) × 10−12 cm3 molecule−1 s−1, which corresponds to a reactive channel yield of 0.26 ± 0.04, and resolves a large discrepancy among previously reported values. The quoted uncertainties are 2σ and include estimated systematic errors. Upper-limits for kR for the C2F6, c-C4F8, n-C5F12, and n-C6F14 reactions were determined to be 3.0, 3.5, 5.0, and 16 (in units of 10−14 cm3 molecule−1 s−1), respectively. The results from this work are compared with results from previous studies. As part of this work, infrared absorption band strengths for NF3 and SF5CF3 were measured and found to be in good agreement with recently reported values.

Global radiative forcing and megacities

Emissions of greenhouse gases and air pollutants from megacities impact the climate. The long-lived greenhouse gases CO2, CH4 and N2O as well as climate-active pollutants such as NOx, VOC and particulate matter (PM) are all emitted from megacities. NOx and VOC contribute to tropospheric ozone formation and affect the lifetime of long-lived greenhouse gases. Anthropogenic aerosols include sulphate, black carbon (BC) and particulate organic matter (POM). Aerosols impact climate directly (absorption, backscattering) and also have indirect (cloud) effects. We assess the climate impact of megacity emissions with the Met Office Hadley Centre Earth System Model HadGEM2 applying an “annihilation” scenario in which the emissions at megacities are entirely removed. Generally, the contribution of megacities to global pollutant emissions is on the order of 2–5% of the total global annual anthropogenic base emission flux. The impact of megacity climate-active pollutants is assessed via an annual mean top-of-atmosphere direct radiative forcing (AMTOA-DRF) from long-lived GHG as well as ozone, methane and aerosols. In this simulations the long-lived component (CO2, CH4 and N2O) contributes a positive TOA-DRF of +120.0, +28.4 and +3.3mWm−2, respectively, under present-day conditions. Climate-active pollutants (NOx, VOC) contribute an AMTOA-DRF of +5.7±0.02mWm−2 from an increase in the ozone burden −1.9±0.04mWm−2, −6.1±0.21mWm−2 from the aerosol AMTOA-DRF in the short-wave spectrum and +1.5±0.01mWm−2 from aerosol in the long-wave spectrum. The combined AMTOA-DRF from all climate-active pollutants is slightly negative at −0.8±0.24mWm−2 and the total AMTOA-DRF amounts to +150.9±0.24mWm−2. Under future conditions (2050s) the total AMTOA-DRF from long-lived GHG is found to profoundly increase to +322.6mWm−2 while the total AMTOA-DRF from climate-active pollutants turns positive and decreases slightly to +0.5±0.09mWm−2 yielding a combined AMTOA-DRF of +323.1±0.09mWm−2 in the future. It is apparent that under the given emission scenarios the radiative forcing from long-lived GHG, particularly CO2, by far dominates the impact of megacities on climate.

Aerosol- and greenhouse gas-induced changes in summer rainfall and circulation in the Australasian region: a study using single-forcing climate simulations

Abstract. We use a coupled atmosphere-ocean global climate model (CSIRO-Mk3.6) to investigate the drivers of trends in summer rainfall and circulation in the vicinity of northern Australia. As part of the Coupled Model Intercomparison Project Phase 5 (CMIP5), we perform a 10-member 21st century ensemble driven by Representative Concentration Pathway 4.5 (RCP4.5). To investigate the roles of different forcing agents, we also perform multiple 10-member ensembles of historical climate change, which are analysed for the period 1951–2010. The historical runs include ensembles driven by "all forcings" (HIST), all forcings except anthropogenic aerosols (NO_AA) and forcing only from long-lived greenhouse gases (GHGAS). Anthropogenic aerosol-induced effects in a warming climate are calculated from the difference of HIST minus NO_AA. CSIRO-Mk3.6 simulates a strong summer rainfall decrease over north-western Australia (NWA) in RCP4.5, whereas simulated trends in HIST are weakly positive (but insignificant) during 1951–2010. The weak rainfall trends in HIST are due to compensating effects of different forcing agents: there is a significant decrease in GHGAS, offset by an aerosol-induced increase. Observations show a significant increase of summer rainfall over NWA during the last few decades. The large magnitude of the observed NWA rainfall trend is not captured by 440 unforced 60-yr trends calculated from a 500-yr pre-industrial control run, even though the model's decadal variability appears to be realistic. This suggests that the observed trend includes a forced component, despite the fact that the model does not simulate the magnitude of the observed rainfall increase in response to "all forcings" (HIST). We investigate the mechanism of simulated and observed NWA rainfall changes by exploring changes in circulation over the Indo-Pacific region. The key circulation feature associated with the rainfall increase in reanalyses is a lower-tropospheric cyclonic circulation trend off the coast of NWA, which enhances the monsoonal flow. The model shows an aerosol-induced cyclonic circulation trend off the coast of NWA in HIST minus NO_AA, whereas GHGAS shows an anticyclonic circulation trend. This explains why the aerosol-induced effect is an increase of rainfall over NWA, and the greenhouse gas-induced effect is of opposite sign. Possible explanations for the cyclonic (anticyclonic) circulation trend in HIST minus NO_AA (GHGAS) involve changes in the Walker circulation or the local Hadley circulation. In either case, a plausible atmospheric mechanism is that the circulation anomaly is a Rossby wave response to convective heating anomalies south of the Equator. We also discuss the possible role of air-sea interactions, e.g. an increase (decrease) of sea-surface temperatures off the coast of NWA in HIST minus NO_AA (GHGAS). Further research is needed to better understand the mechanisms and the extent to which these are model-dependent. In summary, our results suggest that anthropogenic aerosols may have "masked" greenhouse gas-induced changes in rainfall over NWA and in circulation over the wider Indo-Pacific region. Due to the opposing effects of greenhouse gases and anthropogenic aerosols, future trends may be very different from trends observed over the last few decades.

Greenhouse Gases, Aerosols and Reducing Future Climate Uncertainties

The past record of global earth surface temperatures is unable to offer quantitative evidence about the amplitude of climate sensitivity, due to the competing effects of long-lived greenhouse gases and short-lived aerosols. This factor constitutes one of the reasons why uncertainties about climate sensitivity have remained almost unchanged for more than 30 years, and it is also limiting our current capacity to propose reliable climate projections for the coming century. This paper offers a short review of the studies that have dealt with this issue. A number of approaches aim at a process-oriented diagnostic of current models. These studies are in constant progress since the launch of remote-sensing instruments, such as those from the A-train satellite constellation. Past climate fluctuations may also offer some limited possibilities to discriminate the effects of greenhouse gases and aerosols. There is therefore a real hope that climate projections may eventually become more accurate, which would be extremely useful in monitoring global warming during the next decades.

Equivalence of greenhouse-gas emissions for peak temperature limits

A study using a newly developed framework shows how future peak temperature is related to cumulative emissions of long-lived greenhouse gases such as carbon dioxide and sustained emissions of shorter-lived species such as methane, and suggests an approach for limiting future warming to 2 °C above pre-industrial levels. Climate policies address emissions of many greenhouse gases including carbon dioxide, methane, nitrous oxide and various halogen-containing compounds. These are aggregated and traded on a CO2-equivalent basis using the 100-year global warming potential (GWP100); however, the GWP100 has received scientific and economic criticism as a tool for policy1,2,3,4. In particular, given international agreement to limit global average warming to 2 °C, the GWP100 does not measure temperature and does not clearly signal the need to limit cumulative CO2 emissions5,6,7. Here, we show that future peak temperature is constrained by cumulative emissions of several long-lived gases (including CO2 and N2O) and emission rates of a separate basket of shorter-lived species (including CH4). For each basket we develop an emissions-equivalence metric allowing peak temperature to be estimated directly for any emissions scenario. Today’s emissions of shorter-lived species have a lesser impact on ultimate peak temperature than those nearer the time of peaking. The 2 °C limit could therefore be met by setting a limit to cumulative long-lived CO2-equivalent emissions while setting a maximum future rate for shorter-lived emissions.

Emissions halted of the potent greenhouse gas SF<sub>5</sub>CF<sub>3</sub>

Abstract. Long term measurements in background air (Cape Grim, Tasmania) and firn air (NEEM, Greenland) of the potent long-lived greenhouse gas SF5CF3 show that emissions declined after the late 1990s, having grown since the 1950s, and became indistinguishable from zero after 2003. The timing of this decline suggests that emissions of this gas may have been related to the production of certain fluorochemicals; production of which have been recently phased out. An earlier observation of closely correlated atmospheric abundances of SF5CF3 and SF6 are shown here to have likely been purely coincidental, as their respective trends diverged after 2002. Due to its long lifetime (ca. 900 yr), atmospheric concentrations of SF5CF3 have not declined, and it is now well mixed between hemispheres, as is also shown here from interhemispheric aircraft measurements. Total cumulative emissions of SF5CF3 amount to around 5 kT.

The link between a global 2 °C warming threshold and emissions in years 2020, 2050 and beyond

In the Copenhagen Accord, nations agreed on the need to limit global warming to two degrees to avoid potentially dangerous climate change, while in policy circles negotiations have placed a particular emphasis on emissions in years 2020 and 2050. We investigate the link between the probability of global warming remaining below two degrees (above pre-industrial levels) right through to year 2500 and what this implies for emissions in years 2020 and 2050, and any long-term emissions floor. This is achieved by mapping out the consequences of alternative emissions trajectories, all in a probabilistic framework and with results placed in a simple-to-use set of graphics. The options available for carbon dioxide-equivalent (CO2e) emissions in years 2020 and 2050 are narrow if society wishes to stay, with a chance of more likely than not, below the 2 °C target. Since cumulative emissions of long-lived greenhouse gases, and particularly CO2, are a key determinant of peak warming, the consequence of being near the top of emissions in the allowable range for 2020 is reduced flexibility in emissions in 2050 and higher required rates of societal decarbonization. Alternatively, higher 2020 emissions can be considered as reducing the probability of limiting warming to 2 °C. We find that the level of the long-term emissions floor has a strong influence on allowed 2020 and 2050 emissions for two degrees of global warming at a given probability. We place our analysis in the context of emissions pledges for year 2020 made at the end of and since the 2009 COP15 negotiations in Copenhagen.

Bayesian Learning of Climate Sensitivity I: Synthetic Observations

The instrumental temperature records are affected by both external climate forcings—in particular, the increase of long-lived greenhouse gas emissions—and natural, internal variability. Estimates of the value of equilibrium climate sensitivity—the change in global-mean equilibrium near-surface temperature due to a doubling of the pre-industrial CO2 concentration—and other climate parameters using these observational records are affected by the presence of the internal variability. A different realization of the natural variability will result in different estimates of the values of these climate parameters. In this study we apply Bayesian estimation to simulated temperature and ocean heat-uptake records generated by our Climate Research Group’s Simple Climate Model for known values of equilibrium climate sensitivity, T2x direct sulfate aerosol forcing in reference year 2000, FASA, and oceanic heat diffusivity, ΔT2x. We choose the simulated records for one choice of values of the climate parameters to serve as the synthetic observations. To each of the simulated temperature records we add a number of draws of the quasi-periodic oscillations and stochastic noise, determined from the observed temperature record. For cases considering only values of ΔT2x and/or κ, the Bayesian estimation converges to the value(s) of ΔT2x and/or κ used to generate the synthetic observations. However, for cases studying FASA, the Bayesian analysis does not converge to the “true” value used to generate the synthetic observations. We show that this is a problem of low signal-to-noise ratio: by substituting an artificial, continuously increasing sulfate record, we greatly improve the value obtained through Bayesian estimation. Our results indicate Bayesian learning techniques will be useful tools in constraining the values of ΔT2x and κ but not FASA In our Group’s future work we will extend the methods used here to the observed, instrumental records of global-mean temperature increase and ocean heat uptake.

Anthropogenic radiative forcing time series from pre-industrial times until 2010

Abstract. In order to use knowledge of past climate change to improve our understanding of the sensitivity of the climate system, detailed knowledge about the time development of radiative forcing (RF) of the earth atmosphere system is crucial. In this study, time series of anthropogenic forcing of climate from pre-industrial times until 2010, for all well established forcing agents, are estimated. This includes presentation of RF histories of well mixed greenhouse gases, tropospheric ozone, direct- and indirect aerosol effects, surface albedo changes, stratospheric ozone and stratospheric water vapour. For long lived greenhouse gases, standard methods are used for calculating RF, based on global mean concentration changes. For short lived climate forcers, detailed chemical transport modelling and radiative transfer modelling using historical emission inventories is performed. For the direct aerosol effect, sulphate, black carbon, organic carbon, nitrate and secondary organic aerosols are considered. For aerosol indirect effects, time series of both the cloud lifetime effect and the cloud albedo effect are presented. Radiative forcing time series due to surface albedo changes are calculated based on prescribed changes in land use and radiative transfer modelling. For the stratospheric components, simple scaling methods are used. Long lived greenhouse gases (LLGHGs) are the most important radiative forcing agent with a RF of 2.83±0.28 W m−2 in year 2010 relative to 1750. The two main aerosol components contributing to the direct aerosol effect are black carbon and sulphate, but their contributions are of opposite sign. The total direct aerosol effect was −0.48±0.32 W m−2 in year 2010. Since pre-industrial times the positive RF (LLGHGs and tropospheric O3) has been offset mainly by the direct and indirect aerosol effects, especially in the second half of the 20th century, which possibly lead to a decrease in the total anthropogenic RF in the middle of the century. We find a total anthropogenic RF in year 2010 of 1.4 W m−2. However, the uncertainties in the negative RF from aerosols are large, especially for the cloud lifetime effect.

Quantifying the Invisible: Getting a Handle on Methane's Climate Impact

Earlier this year, a Cornell University professor made quite a splash publishing a paper asserting that emissions from shale gas rivaled those from coal. A July 2011 study issued by the Post Carbon Institute underscored this conclusion. Not so, say five separate recent reports—from Carnegie Mellon University, IHS Cambridge Energy Research Associates (CERA), the US National Energy Technology Lab-oratory (NETL), Argonne Laboratory, and Deutsche Bank Climate Change Advisors (coauthored by individuals from Worldwatch Institute and ICF International). At heart are issues related to measuring and quantifying emissions of an odorless, colorless gas—methane (CH4)—considered the second-most prevalent long-lived greenhouse gas after carbon dioxide (CO2). The conventional wisdom is that natural gas is “cleaner” than coal. This is indisputable when considering its combustion. For an equivalent amount of heat, natural gas combustion produces about 30% less CO2 than petroleum combustion and about 45% less CO2 than coal combustion. However, the contention these studies wrestle with is that the release of methane during the life cycle associated with natural gas extraction—including drilling, hydraulic fracturing, completions, production, processing, and transport—could be great enough to outweigh this benefit. Why Atmospheric Methane Matters According to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), issued in 2007, “over the last 650,000 years, ice core records indicate that the abundance of CH4 in the Earth’s atmosphere has varied from lows of about 400 ppb during glacial periods to highs of about 700 ppb during interglacials.” In 2005, the report states, the global average abundance of CH4, measured at a network of 40 surface air flask sampling sites in both hemispheres, was 1,774.62 ±1.22 ppb. It would thus appear that present atmospheric levels of CH4 are unprecedented in at least the last 650,000 years. According to figures released by the US National Oceanic & Atmospheric Administration, CH4 is responsible for 18% of global warming since the beginning of the Industrial Revolution, while CO2 is responsible for 64%. According to Martin Heimann in Enigma of the Recent Methane Budget, published in the 11 August 2011 issue of Nature, “About 60% of global methane stems from human activities. This includes emissions associated with energy production and usage—such as coal mining, incomplete combustion of fossil fuels, and gas leaks. These fossil-fuel-derived emissions, together with methane from landfills, organic waste, cattle raising, rice agriculture, and bio-mass burning, were probably the main cause of the increasing concentrations after pre-industrial times.”

New estimates of direct N<sub>2</sub>O emissions from Chinese croplands from 1980 to 2007 using localized emission factors

Abstract. Nitrous oxide (N2O) is a long-lived greenhouse gas with a large radiation intensity and it is emitted mainly from agricultural land. Accurate estimates of total direct N2O emissions from croplands on a country scale are important for global budgets of anthropogenic sources of N2O emissions and for the development of effective mitigation strategies. The objectives of this study were to re-estimate direct N2O emissions using localized emission factors and a database of measurements from Chinese croplands. We obtained N2O emission factors for paddy fields (0.41 ± 0.04%) and uplands (1.05 ± 0.02%) from a normalization process through cube root transformation of the original data. After comparing the results of normalization from the original values, Logarithmic and cube root transformations were used because the frequency of the original data was not normally distributed. Direct N2O emissions from Chinese croplands from 1980 to 2007 were estimated using IPCC (2006) guidelines combined with separate localized emission factors for paddy fields and upland areas. Direct N2O emissions from paddy fields showed little change, increasing by 10.5% with an annual rate of increase of 0.4% from 32.3 Gg N2O-N in 1980 to 35.7 Gg N2O-N in 2007. In contrast, emissions from uplands changed dramatically, increasing by 308% with an annual rate of 11% from 68.0 Gg N2O-N in 1980 to 278 Gg N2O-N in 2007. Total direct N2O emissions from Chinese croplands increased by 213% with an annual rate of 7.6% from 100 Gg N2O-N in 1980 to 313 Gg N2O-N in 2007, and were determined mainly by upland emissions (accounting for 67.8–88.6% of total emissions from 1980 to 2007). Synthetic N fertilizers played a major role in N2O emissions from agricultural land, and the magnitude of the contributions to total direct N2O emissions made by different amendments was synthetic N fertilizer > manure > straw, representing about 78, 15, and 6% of total direct N2O emissions, respectively, between 2000 and 2007. The spatial pattern of total N2O emissions in 2007 in China shows that high direct N2O emissions occurred mainly in the north and in the Sichuan Basin in the southwest. The provinces with the highest emissions were Henan (35.4 Gg) and Shandong (31.6 Gg) and Tibet had the lowest (0.65 Gg). High direct N2O emissions per unit of arable land occurred mainly on the North China Plain and the southeast coast. The mean value nationally was 2.52 kg N ha−1, with 18 provinces above this value and with emissions of >4.0 kg N ha−1 in Beijing, Tianjin and in Jiangsu, Shandong, Fujian and Henan provinces.

Emission pathways consistent with a 2 °C global temperature limit

In recent years, international climate policy has increasingly focused on limiting temperature rise, as opposed to achieving greenhouse-gas-concentration-related objectives. The agreements reached at the United Nations Framework Convention on Climate Change conference in Cancun in 2010 recognize that countries should take urgent action to limit the increase in global average temperature to less than 2 C relative to pre-industrial levels 1 . If this is to be achieved, policymakers need robust information about the amounts of future greenhouse-gas emissions that are consistent with such temperature limits. This, in turn, requires an understanding of both the technical and economic implications of reducing emissions and the processes that link emissions to temperature. Here we consider both of these aspects by reanalysing a large set of published emission scenarios from integrated assessment models in a risk-based climate modelling framework. We find that in the set of scenarios with a ‘likely’ (greater than 66%) chance of staying below 2 C, emissions peak between 2010 and 2020 and fall to a median level of 44 Gt of CO2 equivalent in 2020 (compared with estimated median emissions across the scenario set of 48 Gt of CO2 equivalent in 2010). Our analysis confirms that if the mechanisms needed to enable an early peak in global emissions followed by steep reductions are not put in place, there is a significant risk that the 2 C target will not be achieved. Cumulative emissions of long-lived greenhouse gases (GHGs) approximately define the temperature response of the climate system at timescales of centuries to millennia 24 because a significant fraction of CO2 emissions, the dominant anthropogenic GHG, is removed very slowly from the atmosphere 5,6 . The temperature response will therefore continue, even when global emissions return to zero, or when concentrations are stabilized 6,7 . Cumulative emissions provide very little information on the technical feasibility and cost implications of following a particular ‘emissions pathway’, information that is needed for policymakers who are deciding now on emissions goals for the coming decades. Path-dependent assessments, such as the United Nations

Alternative “Global Warming” Metrics in Life Cycle Assessment: A Case Study with Existing Transportation Data

The Life Cycle Assessment (LCA) impact category "global warming" compares emissions of long-lived greenhouse gases (LLGHGs) using Global Warming Potential (GWP) with a 100-year time-horizon as specified in the Kyoto Protocol. Two weaknesses of this approach are (1) the exclusion of short-lived climate forcers (SLCFs) and biophysical factors despite their established importance, and (2) the use of a particular emission metric (GWP) with a choice of specific time-horizons (20, 100, and 500 years). The GWP and the three time-horizons were based on an illustrative example with value judgments and vague interpretations. Here we illustrate, using LCA data of the transportation sector, the importance of SLCFs relative to LLGHGs, different emission metrics, and different treatments of time. We find that both the inclusion of SLCFs and the choice of emission metric can alter results and thereby change mitigation priorities. The explicit inclusion of time, both for emissions and impacts, can remove value-laden assumptions and provide additional information for impact assessments. We believe that our results show that a debate is needed in the LCA community on the impact category "global warming" covering which emissions to include, the emission metric(s) to use, and the treatment of time.

Nitrous Oxide Emissions from Wastewater Treatment and Water Reclamation Plants in Southern California

Nitrous oxide (N₂O) is a long-lived and potent greenhouse gas produced during microbial nitrification and denitrification. In developed countries, centralized water reclamation plants often use these processes for N removal before effluent is used for irrigation or discharged to surface water, thus making this treatment a potentially large source of N₂O in urban areas. In the arid but densely populated southwestern United States, water reclamation for irrigation is an important alternative to long-distance water importation. We measured N₂O concentrations and fluxes from several wastewater treatment processes in urban southern California. We found that N removal during water reclamation may lead to in situ N₂O emission rates that are three or more times greater than traditional treatment processes (C oxidation only). In the water reclamation plants tested, N₂O production was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%). We also measured stable isotope ratios (δN and δO) of emitted N₂O and found distinct δN signatures of N₂O from denitrification (0.0 ± 4.0 ‰) and nitrification reactors (-24.5 ± 2.2 ‰), respectively. These isotope data confirm that both nitrification and denitrification contribute to N₂O emissions within the same treatment plant. Our estimates indicate that N₂O emissions from biological N removal for water reclamation may be several orders of magnitude greater than N₂O emissions from agricultural activities in highly urbanized southern California. Our results suggest that wastewater treatment that includes biological nitrogen removal can significantly increase urban N₂O emissions.