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Uncertainty in the future atmospheric burden of methane—a potent greenhouse gas—represents an important challenge to the development of realistic climate projections. Airborne observations of methane suggest that the remote Arctic Ocean could prove to be a potentially important methane source.

The rising abundance of greenhouse gases in the atmosphere has been associated with a change in the stratospheric circulation and a decrease in the mean age of stratospheric air. Balloon-borne measurements of trace gases over 30 years suggest that, in contrast to model predictions, there have been no significant trends (at a 90% confidence limit) in the age of stratospheric air. The rising abundances of greenhouse gases in the atmosphere is associated with an increase in radiative forcing that leads to warming of the troposphere, the lower portion of the Earth’s atmosphere, and cooling of the stratosphere above1. A secondary effect of increasing levels of greenhouse gases is a possible change in the stratospheric circulation2,3, which could significantly affect chlorofluorocarbon lifetimes4, ozone levels5,6 and the climate system more generally7. Model simulations have shown that the mean age of stratospheric air8 is a good indicator of the strength of the residual circulation9, and that this mean age is expected to decrease with rising levels of greenhouse gases in the atmosphere10. Here we use balloon-borne measurements of stratospheric trace gases over the past 30 years to derive the mean age of air from sulphur hexafluoride (SF6) and CO2 mixing ratios. In contrast to the models, these observations do not show a decrease in mean age with time. If models are to make valid predictions of future stratospheric ozone levels, and of the coupling between ozone and climate change, a correct description of stratospheric transport and possible changes in the transport pathways are necessary.

The background concentrations of atmospheric CFC-11 and CFC-12were monitored to assess their impact on stratospheric ozone depletion and global warming from September 1995 to March 1999 at Kosan, Korea, located at eastern margin of the Asian Continent. The concentrations of atmospheric CFC-11 at Kosan have decreased slightly, at a rate of –2.5 pptv yr-1, over the period in response to the Montreal Protocol. The CFC-12 mixing ratio at Kosan continues to increase in the atmosphere at a rate of 5.7 pptv yr-1 despite international regulations, because of its extreme atmosphere persistence. Recent trends ofthese two chlorofluorocarbons at Kosan, Korea were concordant with those of the northern hemispheric background monitored unitat Mauna Loa, Hawaii. The maximum seasonal mean mixing ratios of CFC-11 and CFC-12 at Kosan, Korea, were 270±4 pptv inthe spring and 538±9 pptv in the winter, and the corresponding seasonal minima were 267±7 and 529±12 pptv. This occurred in the summer and was due to southeasterlywinds from the northwestern Pacific Ocean. By performing a three-day isentropic backward trajectory analysis, it was shownthat air masses at Kosan, and with the exception of summer, mainly originated from central and northern China. In particular, the mixing ratios of these two contaminant speciesare closely related with their air mass trajectories.

Measurements of trace gases in air trapped in polar firn (unconsolidated snow) demonstrate that natural sources of chlorofluorocarbons, halons, persistent chlorocarbon solvents and sulphur hexafluoride to the atmosphere are minimal or non-existent. Atmospheric concentrations of these gases, reconstructed back to the late nineteenth century, are consistent with atmospheric histories derived from anthropogenic emission rates and known atmospheric lifetimes. The measurements confirm the predominance of human activity in the atmospheric budget of organic chlorine, and allow the estimation of atmospheric histories of halogenated gases of combined anthropogenic and natural origin. The pre-twentieth-century burden of methyl chloride was close to that at present, while the burden of methyl bromide was probably over half of today's value.

The burden of ozone-depleting chemicals in the lower atmosphere has been decreasing since 1994 as a result of the Montreal Protocol1,3. Here we show how individual chemicals have influenced this decline, in order to estimate how the burden could change in the near future. Our measurements of atmospheric concentrations of the persistent, anthropogenic chemicals that account for most ozone-depleting halogens in today's stratosphere show that the decline stems predominantly from the decrease in the atmospheric load of trichloroethane (CH3CCl3), a previously common cleaning solvent. The influence of this chemical on the decline has now peaked, however, and will become much smaller over the next five to ten years. As this influence lessens, a decrease in theburden of ozone-depleting halogen will be sustained only if emissions of other halocarbons fall. Although emissions of most gases regulated by the Montreal Protocol have decreased substantially over the past ten years (refs 4), emissions of the potent ozone-depleting gas CBrClF2 (halon-1211) have remained fairly constant during this period12,29, despite stringent limits on production in developed countries since 1994. The consequent atmospheric accumulation of this halon is retarding the decline of ozone-depleting halogens in the atmosphere more than any other persistent gas.

The extraction and analysis of air from the snowpack (firn) at the South Pole provides atmospheric concentration histories of biogenic greenhouse gases since the beginning of the present century which confirm and expand on those derived from studies of air trapped in ice cores. Furthermore, calculations based on the inferred atmospheric concentrations of oxygen and carbon dioxide indicate that–in contrast to the past few years—the terrestrial biosphere was neither a source nor sink of C02 between ˜1977 and 1985.

THE discovery of the Antarctic ozone hole1 in 1985 led to international efforts to reduce emissions of ozone-destroying chlorofluorocarbons2. These efforts culminated in the Montreal Protocol3 and its subsequent amendments, which called for the elimination of CFC production by 1996. Here we focus on CFC-11 (CC13F) and CFC-12 (CC12F2), which are used for refrigeration, air conditioning and the production of aerosols and foams4, and which together make up about half of the total abundance of stratospheric organic chlorine5. We report a significant recent decrease in the atmospheric growth rates of these two species, based on measurements spanning the past 15 years and latitudes ranging from 83° N to 90° S. This is consistent with CFC-producers' own estimates of reduced emissions6,7. If the atmospheric growth rates of these two species continue to slow in line with predicted changes in industrial emissions, global atmospheric mixing ratios will reach a maximum before the turn of the century, and then begin to decline.

HALONS H-1301 (CBrF3) and H-1211 (CBrClF2) have been introduced into the atmosphere, mainly through use in fire extinguishers, for almost three decades. Although each is now present in the troposphere at a concentration of only 2 parts per 1012, these gases have long atmospheric lifetimes (65–77 yr for H-1301 and 11–16 yr for H-1211)1,2 and carry significant amounts of bromine to the stratosphere2,3, where it can destroy ozone catalytically4,5. For this reason, the halons have high ozone depletion potentials6. The manufacture of both gases is to be discontinued globally by the year 2000, according to the Montreal Protocol, and perhaps sooner, as a result of unilateral action by users, manufacturers and producing countries7,8. Here we present a six-year record of tropospheric halon mixing ratios which shows that the growth rates of H-1301 and H-1211 have already begun to decrease substantially. This recent decrease in growth rates is consistent with industry emission estimates (although these have greater uncertainties), and supports current appraisals of atmospheric lifetimes. Our results suggest that, even though these halons are relatively long-lived species, their atmospheric mixing ratios may stabilize or begin to decrease within the next few years.

The N2O content of waters in the Potomac and Merrimack Rivers was measured on a number of occasions over the period April to July 1977. The concentrations of dissolved N2O exceeded those which would apply in equilibrium with air by factors ranging from about 46 in the Potomac to 1.2 in the Merrimack. Highest concentrations of dissolved N2O were associated with sewage discharges from the vicinity of Washington, D. C., and analysis indicates a relatively high yield, 1.3 to 11%, for prompt conversion of waste nitrogen to N2O. Measurements of dissolved N2O in fresh water ponds near Boston demonstrated that aquatic systems provide both strong sources and sinks for atmospheric N2O.

Data are presented which suggest the complexity of the aquatic nitrogen cycle as it affects N2O. The data are from studies made in the central and south-east tropical regions of the Pacific Ocean and in Chesapeake Bay. The data indicate that oxidation of ammonium and amino nitrogen and nitrification form the principle source for marine N2O. It is estimated that the yearly global yield for oceanic N2O is less than about 10 to the 7th power tons. The consumption of atmospheric N2O by the open ocean has not been evidenced, although data from the south-east tropical Pacific and Chesapeake Bay show the consumption of dissolved N2O in low-oxygen conditions. Preliminary observations have also indicated the consumption of atmospheric N2O by aquatic systems such as freshwater pond and a tidal saltmarsh.