Methane Growth Rate Research Articles

Effects of the stratospheric quasi‐biennial oscillation on long‐lived greenhouse gases in the troposphere

An analysis is presented of results of an extended integration with a global general circulation model that includes treatment of a long‐lived tropospheric trace constituent as well as a momentum source that forces a realistic stratospheric quasibiennial oscillation (QBO). It is shown that the dynamical QBO may modulate stratosphere‐troposphere exchange in such a manner as to produce a QBO in global‐mean tropospheric tracer mixing ratio. For the growth rate of tropospheric methane this transport‐induced QBO is expected to have a peak‐to‐peak amplitude of 1–2 ppbv yr−1, which, while modest compared with the full range of observed variability in growth rate, is still significant. The observed methane growth rate time series during 1983–1999 is shown to be consistent with the predicted QBO effect, although the record is definitely dominated by other interannual variations.

Reductions in the anthropogenic emissions of CO and their effect on CH4

Using a time dependent chemical “112D” radiative convective model which treats the hemispheres separately but allows for exchange of trace gases between the hemispheres and heat transfer to the ocean, we show that observed reductions in global CO concentrations can cause a significant increase in mean tropospheric OH and a decrease in the methane growth rate. Because tropospheric ozone may also decrease, the result is a reduction in the rate of greenhouse warming. Reductions in anthropogenic CO emissions significantly counteract the growth of CH4 in the atmosphere. This possibility should be taken into account in international climate negotiations.

An inverse modeling approach to investigate the global atmospheric methane cycle

Estimates of the global magnitude of atmospheric methane sources are currently mainly based on direct flux measurements in source regions. Their extrapolation to the entire globe often involves large uncertainties. In this paper, we present an inverse modeling approach which can be used to deduce information on methane sources and sinks from the temporal and spatial variations of atmospheric methane mixing ratios. Our approach is based on a three‐dimensional atmospheric transport model which, combined with a tropospheric background chemistry module, is also employed to calculate the global distribution of OH radicals which provide the main sink for atmospheric methane. The global mean concentration of OH radicals is validated with methyl chloroform (CH3CCl3) observations. The inverse modeling method optimizes the agreement between model‐calculated and observed methane mixing ratios by adjusting the magnitudes of the various methane sources and sinks. The adjustment is constrained by specified a priori estimates and uncertainties of the source and sink magnitudes. We also include data on the 13C/12C isotope ratio of atmospheric methane and its sources in the model. Focusing on the 1980s, two scenarios of global methane sources are constructed which reproduce the main features seen in the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) methane observations. Differences between these two scenarios may probably be attributed to underestimated a priori uncertainties of wetland emissions. Applying the inverse model, the average uncertainty of methane source magnitudes could be reduced by at least one third. We also examined the decrease in the atmospheric methane growth rate during the early 1990s but could not uniquely associate it with changes in particular sources.

Atmospheric methane over the North Pacific from 1987 to 1993.

Atmospheric methane mixing ratios were measured over the North Pacific during the winter season from 1987 to 1993 to extend our methane record since 1978. The latitudinal distribution of methane mixing ratio showed a north-to-south gradient from mid-latitudes to the equator every year. A sharp mixing ratio gradient often appeared at the boundary between the winter monsoon and the trade wind regions around 20°N. No significant longitudinal gradient was found during the winter season, although methane levels along the equator showed a large difference between the western and eastern Pacific. The overall methane increase rate in the western Pacific was estimated to be 13 ppb/yr on the basis of the long-term record for 15 years from 1978 to 1993. This record indicates that the methane growth rate over this Pacific region was gradually slowing down until 1990, followed by no significant increase in the 1990's. The overall deceleration of the growth rate was more rapid in the middle latitudinal zone (20°N–30°N) than in the lower latitudinal zone (3°N–20°N). This latitudinal difference suggests a rapid reduction of methane emissions from the continental regions. The methane growth rate showed an interannual variation with an increasing trend around 1983 and 1987, which was roughly related to the El Nino events. It is suggested that the methane growth rate was affected by a change of interhemispheric transport due to the ENSO events.

Comment on ‘A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992' by E. J. Dlugokencky et al.

The carefully measured decrease in the growth rate of atmospheric methane (CH4) in 1992 reported by Dlugokencky et al. (1994) is an impressive accomplishment, and testimony for the importance of maintaining high-quality, long-term monitoring of atmospheric composition. The changing growth rate of atmospheric CH4 has important implications for assessing and understanding the potential magnitude and rates of a future greenhouse gas-induced climate change. Furthermore, the CH4 data from the current Climate Monitoring and Diagnostics Laboratory (CMDL) globally-distributed network of cooperative air sampling sites are clearly the best record of global CH4 trends and distribution currently available. However, we argue briefly here that the speculation by Dlugokencky et al. (1994) on possible mechanisms for the decreased growth rate in 1992 is only one scenario of many that could possibly fit with the constraints imposed by the reported data. Our comments are to (1) illustrate the difficulties of deducing small changes in complex, poorly understood, geographically diverse natural and anthropogenic sources of CH4 from measurements at the remotely-located CMDL sampling sites and (2) emphasize that detailed bottoms-up analyses are necessary to really advance the understanding of changes in source strengths; we are not promoting alternative mechanisms to explain the 1992 decrease in atmospheric CH4.

Reply to “Comments on ‘A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992'”

Reply to “Comments on ‘A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992'”

The growth rate and distribution of atmospheric methane

Methane was measured in air samples collected approximately weekly from a globally distributed network of sites from 1983 to 1992. Sites range in latitude from 90°S to 82°N. All samples were analyzed by gas chromatography, with flame ionization detection at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado, and the measurements were referenced against a single calibration scale. The estimated precision of the measurements is ±0.2%. Samples which had clear sampling or analytical errors, or which appeared to be contaminated by local CH4 sources, were identified and excluded from the data analysis. The data reveal a strong north‐south gradient in methane with an annual mean difference of about 140 ppb between the northernmost and southernmost sampling sites. Methane time series from the high southern latitude sites have a relatively simple seasonal cycle with a minimum during late summer‐early fall, almost certainly dominated by the seasonality in its photochemical destruction. Typical seasonal cycle amplitudes there are about 30 ppb. Seasonal cycles at sites in the northern hemisphere are complex when compared to sites in the southern hemisphere due to the interaction among CH4 sources and sinks, and atmospheric transport. Seasonal cycle amplitudes in the high north are about twice those observed in the high southern hemisphere. Annual mean methane mixing ratios were ∼1% lower at 3397 m than at sea level on the island of Hawaii. Trends were determined at each site in the network and globally. The average increase in the globally averaged methane mixing ratio over the period of these measurements is (11.1±0.2) ppb yr−1. Globally, the growth rate for methane decreased from approximately 13.5 ppb yr−1 in 1983 to about 9.3 ppb yr−1 in 1991. The growth rate of methane in the northern hemisphere during 1992 was near zero. Various possibilities for the long‐term, slow decrease in the methane growth rate over the last decade and the rapid change in growth rate in the northern hemisphere in 1992 are given. The most likely explanation is a change in a methane source influenced directly by human activities, such as fossil fuel production.

Concentration and 13C records of atmospheric methane in New Zealand and Antarctica: Evidence for changes in methane sources

Measurements of 13C in atmospheric methane made at Baring Head, New Zealand (41°S), over the 4‐year period, 1989–1993, display a persistent but highly variable seasonal cycle. Values for δ13C peak in summer at about −46.9‰ and drop to around −47.5‰ in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78°S. While the phase of the δ13C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in 13C in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements of the effect. We interpret the origin of this cycle and its inter‐annual variability to be due to episodic southward transport of isotopically heavy methane from large‐scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring Head 13C data show no significant secular trend from 1989 to mid‐1991, followed by a rapid trend toward methane less enriched in 13C. This indicates a major shift in the balance of the sources of atmospheric methane and precludes an increased sink strength. The trend in 13C since mid‐1991 coincided with significant changes to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr−1 thereafter. A 2‐box model of atmospheric methane (one box per hemispheric reservoir) suggests that (1) the recent decline in 13C in methane observed at Baring Head and Scott Base cannot have a solely northern hemispheric origin and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere.

Correction to “A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992” by E. J. Dlugokencky, K. A. Masarie, P. M. Lang, P. P. Tans, L. P. Steele, and E. G. Nisbet

Correction to “A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992” by E. J. Dlugokencky, K. A. Masarie, P. M. Lang, P. P. Tans, L. P. Steele, and E. G. Nisbet

Effects of reductions in stratospheric ozone on tropospheric chemistry through changes in photolysis rates

The observed reductions in stratospheric ozone since the late 1970s are likely to have affected the penetration of UV radiation into the troposphere. We have examined the sensitivity and the response of the tropospheric chemistry to such changes in UV radiation. Based on observations of ozone column densities and model calculations of changes in the ozone column densities after 1970, photodissociation rates for selected years (1970, 1980, 1990, 2000 and 2050) are calculated. These calculations give significant changes in the dissociation rates of some gases, particularly in the dissociation rate for O 3 yielding O( 1 D). This increase in photodissociation rates initiates increases in tropospheric OH. However, the model studies show that the OH changes are somewhat damped compared to the changes in short wave radiation due to strong interactions between key species in the troposphere. Ozone is reduced in most areas when the UV radiation increases, but the percentage reductions in ozone are significantly smaller than the percentage UV increases. However, during spring, O 3 increases in regions where NO x is enhanced. The calculations give increased levels of H 2 O 2 , although the magnitude of the response varies considerably with time of year and with region, and they are largest at high latitudes during spring. The relative changes in methane are found to be less than the relative changes in global average of OH since the largest relative OH changes take place outside the region of most importance for the methane oxidation (low latitudes and low altitudes). In addition, the effect is delayed in accordance with the chemical lifetime of methane (∼ 10 years). The results indicate that increased fluxes of UV due to reduced ozone columns may have contributed to the reduction in the growth rate of methane, and the magnitude of this effect is estimated to ∼ 1 3 of the reduction. It is therefore unlikely that changes in UV fluxes is the main cause of the reduction in the growth rate of methane which has been observed during the 1980s. The mechanism constitutes a negative indirect chemical effect on climate change from the ozone depleting substances, mainly the CFCs. DOI: 10.1034/j.1600-0889.1992.t01-3-00001.x-i1

A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992

Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983–1991 was (11.6±0.2) ppbv yr−1, but the increase in 1992 was only (1.8±1.6) ppbv. In the Southern Hemisphere, the average increase (1983–1991) was (11.1±0.2) ppbv yr−1, and the 1992 increase was (7.7±1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man's activities.

The global distribution of methane in the troposphere

Methane has been measured in air samples collected at approximately weekly intervals at 23 globally distributed sites in the NOAA/GMCC cooperative flask sampling network. Sites range in latitude from 90° S to 76° N, and at most of these we report 2 years of data beginning in early 1983. All measurements have been made by gas chromatography with a flame ionization detector at the NOAA/GMCC laboratory in Boulder, Colorado. All air samples have been referenced to a single secondary standard of methane-in-air, ensuring a high degree of internal consistency in the data. The precision of measurements is estimated from replicate determinations on each sample as 0.2%. The latitudinal distribution of methane and the seasonal variation of this distribution in the marine boundary layer has been defined in great detail, including a remarkable uniformity in background levels of methane in the Southern Hemisphere. We report for the first time the observation of a complete seasonal cycle of methane at the South Pole. A significant vertical gradient is observed between a sea level and a high altitude site in Hawaii. Globally averaged background concentrations in the marine boundary layer have been calculated for the 2 year-period May 1983-April 1985 inclusive, from which we find an average increase of 12.8 ppb per year, or 0.78% per year when referenced to the globally averaged concentration (1625 ppb) at the mid-point of this period. We present evidence that there has been a slowing down in the methane growth rate.