Small interannual variability of global atmospheric hydroxyl

The oxidizing capacity of the global atmosphere is largely determined by hydroxyl (OH) radicals and is diagnosed by analyzing methyl chloroform (CH(3)CCl(3)) measurements. Previously, large year-to-year changes in global mean OH concentrations have been inferred from such measurements, suggesting that the atmospheric oxidizing capacity is sensitive to perturbations by widespread air pollution and natural influences. We show how the interannual variability in OH has been more precisely estimated from CH(3)CCl(3) measurements since 1998, when atmospheric gradients of CH(3)CCl(3) had diminished as a result of the Montreal Protocol. We infer a small interannual OH variability as a result, indicating that global OH is generally well buffered against perturbations. This small variability is consistent with measurements of methane and other trace gases oxidized primarily by OH, as well as global photochemical model calculations.     

Atmospheric Trends and Lifetime of CH3CCI3 and Global OH Concentrations

Determination of the atmospheric concentrations and lifetime of trichloroethane (CH(3)CCI(3)) is very important in the context of global change. This halocarbon is involved in depletion of ozone, and the hydroxyl radical (OH) concentrations determined from its lifetime provide estimates of the lifetimes of most other hydrogen-containing gases involved in the ozone layer and climate. Global measurements of trichloroethane indicate rising concentrations before and declining concentrations after late 1991. The lifetime of CH(3)CCI(3) in the total atmosphere is 4.8 +/- 0.3 years, which is substantially lower than previously estimated. The deduced hydroxyl radical concentration, which measures the atmosphere's oxidizing capability, shows little change from 1978 to 1994.     

Elevation change of the southern greenland ice sheet

Seasat and Geosat satellite altimeter measurements for the Greenland ice sheet (south of 72 degreesN latitude) show that surface elevations above 2000 meters increased at an average rate of only 1. 5 +/- 0.5 centimeters per year from 1978 to 1988. In contrast, elevation changes varied regionally from -15 to +18 centimeters per year, seasonally by +/-15 centimeters, and interannually by +/-8 centimeters. The average growth rate is too small to determine if the Greenland ice sheet is undergoing a long-term change due to a warmer polar climate.     

Atmospheric trends in methylchloroform and the global average for the hydroxyl radical

Frequent atmospheric measurements of the anthropogenic compound methylchloroform that were made between 1978 and 1985 indicate that this species is continuing to increase significantly around the world. Reaction with the major atmospheric oxidant, the hydroxyl radical (OH), is the principal sink for this species. The observed mean trends for methylchloroform are 4.8, 5.4, 6.4, and 6.9 percent per year at Aldrigole (Ireland) and Cape Meares (Oregon), Ragged Point (Barbados), Point Matatula (American Samoa), and Cape Grim (Tasmania), respectively, from July 1978 to June 1985. These measured trends, combined with knowledge of industrial emissions, were used in an optimal estimation inversion scheme to deduce a globally averaged methylchloroform atmospheric lifetime of 6.3 (+ 1.2, -0.9) years (1sigma uncertainty) and a globally averaged tropospheric hydroxyl radical concentration of (7.7 +/- 1.4) x 10(5) radicals per cubic centimeter (1sigma uncertainty). These 7 years of gas chromatographic measurements, which comprise about 60,000 individual calibrated real-time air analyses, provide the most accurate estimates yet of the trends and lifetime of methylchloroform and of the global average for tropospheric hydroxyl radical levels. Accurate determination of hydroxyl radical levels is crucial to understanding global atmospheric chemical cycles and trends in the levels of trace gases such as methane.     

Atmospheric trace gases in antarctica

Trace gases have been measured, by electron-capture gas chromatography and gas chromatography-mass spectrometry techniques, at the South Pole (SP) in Antarctica and in the U.S. Pacific Northwest (PNW) ( approximately 45 degrees N) during January of each year from 1975 to 1980. These measurements show that the concentrations of CCl(3)F, CCl(2)F(2), and CH(3)CCl(3) have increased exponentially at substantial rates. The concentration of CCl(3)F increased at 12 percent per year at the SP and at 8 percent per year in the PNW; CCl(2)F(2) increased at about 9 percent per year at both locations, and CH(3)CCl(3) increased at 17 percent per year at the SP and 11.6 percent per year at the PNW site. There is some evidence that CCl(4) ( approximately 3 percent per year) and N(2)O (0.1 to 0.5 percent per year) may also have increased. Concentrations of nine other trace gases of importance in atmospheric chemistry are also being measured at these two locations. Results of the measurements of CHClF(2)(F-22), C(2)Cl(3)F(3)(F-113), SF(6), C(2)-hydrocarbons, and CH(3)Cl are reported here.     

Atmospheric trace gases: trends and distributions over the last decade

Concentrations of the halocrbons CCl(3)F (F-11), CCl(2)F(2) (F-12), CCl(4), and CH(3)CCl(3), methane (CH(4)), and nitrous oxide (N(2)O) over the decade between 1975 and 1985 are reported, based on measurements taken every January at the South Pole and in the Pacific Northwest. The concentrations of F-11, F-12, and CH(3)CCl(3) in both hemispheres are now more than twice their concentrations 10 years ago. However, the annual rates of increase of F-11, F-12, and CH(3)CC1(3) are now considerably slower than earlier in the decade, reflecting in part the effects of a ban on their nonessential uses. Continued increases in these trace gas concentrations may warm the earth and deplete the stratospheric ozone layer, which may cause widespread climatic changes and affect global habitability.     

Continuing worldwide increase in tropospheric methane, 1978 to 1987

The average worldwide tropospheric mixing ratio of methane has increased by 11% from 1.52 parts per million by volume (ppmv) in January 1978 to 1.684 ppmv in September 1987, for an increment of 0.016 +/- 0.001 ppmv per year. Within the limits of our measurements, the global tropospheric mixing ratio for methane over the past decade is consistent either with a linear growth rate of 0.016 +/- 0.001 ppmv per year or with a slight lessening of the rate of growth over the past 5 years. No indications were found of an effect of the El Ni?o-Southern Oscillation-El Chichon events of 1982-83 on total global methane, although severe reductions were reported in the Pacific Northwest during that time period. The growth in tropospheric methane may have increased the water concentration in the stratosphere by as much as 28% since the 1940s and 45% over the past two centuries and thus could have increased the mass of precipitable water available for formation of polar stratospheric clouds.     

A Net Sink for Atmospheric CH3Br in the East Pacific Ocean

Surface waters along a cruise track in the East Pacific Ocean were undersaturated in methyl bromide (CH(3)Br) in most areas except for coastal and upwelling regions, with saturation anomalies ranging from + 100 percent in coastal waters to -50 percent in open ocean areas, representing a regionally weighted mean of -16 (-13 to -20) percent. The partial lifetime of atmospheric CH(3)Br with respect to calculated oceanic degradation along this cruise track is 3.0 (2.9 to 3.6) years. The global, mean dry mole fraction of CH3Br in the atmosphere was 9.8 +/- 0.6 parts per trillion, with an interhemispheric ratio of 1.31 +/- 0.08. These data indicate that approximately 8 percent (0.2 parts per trillion) of the observed interhemispheric difference in atmospheric CH3Br could be attributed to an uneven global distribution of oceanic sources and sinks.     

Increasing river discharge to the Arctic Ocean

Synthesis of river-monitoring data reveals that the average annual discharge of fresh water from the six largest Eurasian rivers to the Arctic Ocean increased by 7% from 1936 to 1999. The average annual rate of increase was 2.0 +/- 0.7 cubic kilometers per year. Consequently, average annual discharge from the six rivers is now about 128 cubic kilometers per year greater than it was when routine measurements of discharge began. Discharge was correlated with changes in both the North Atlantic Oscillation and global mean surface air temperature. The observed large-scale change in freshwater flux has potentially important implications for ocean circulation and climate.     

Kinetics of the OH Reaction with Methyl Chloroform and Its Atmospheric Implications

The rate coefficients for the reaction of hydroxyl (OH) radicals with methyl chloroform (CH(3)CCI(3)) were measured between 243 and 379 kelvin with the pulsed photolysis-laserinduced fluorescence method. The measured rate coefficients at 298 and 277 kelvin were approximately 20 and approximately 15%, respectively, lower than earlier values. These results will increase the tropospheric OH concentrations derived from the CH(3)CCI(3) budget analysis by approximately 15%. The predicted atmospheric lifetimes of species whose main loss process is the reaction with OH in the troposphere will be lowered by 15% with consequent changes in their budgets, global warming potentials, and ozone depletion potentials.     

Rapid wastage of Alaska glaciers and their contribution to rising sea level

We have used airborne laser altimetry to estimate volume changes of 67 glaciers in Alaska from the mid-1950s to the mid-1990s. The average rate of thickness change of these glaciers was -0.52 m/year. Extrapolation to all glaciers in Alaska yields an estimated total annual volume change of -52 +/- 15 km3/year (water equivalent), equivalent to a rise in sea level (SLE) of 0.14 +/- 0.04 mm/year. Repeat measurements of 28 glaciers from the mid-1990s to 2000-2001 suggest an increased average rate of thinning, -1.8 m/year. This leads to an extrapolated annual volume loss from Alaska glaciers equal to -96 +/- 35 km3/year, or 0.27 +/- 0.10 mm/year SLE, during the past decade. These recent losses are nearly double the estimated annual loss from the entire Greenland Ice Sheet during the same time period and are much higher than previously published loss estimates for Alaska glaciers. They form the largest glaciological contribution to rising sea level yet measured.     

New observational constraints for atmospheric hydroxyl on global and hemispheric scales

Dramatic declines in emissions of methyl chloroform (1,1, 1-trichloroethane) resulting from the Montreal Protocol provide an unprecedented opportunity to improve our understanding of the oxidizing power of Earth's atmosphere. Atmospheric observations of this industrial gas during the late 1990s yield new insights into the global burden and distribution of the hydroxyl radical. Our results set firm upper limits on the global and Southern Hemispheric lifetimes of methyl chloroform and confirm the predominance of hydroxyl in the tropics. Our analysis suggests a global lifetime for methyl chloroform of 5.2 (+0.2, -0.3) years, a Southern Hemispheric lifetime of 4.9 (+0.2, -0.3) years, and mean annual concentrations of OH that are 15 +/- 10% higher south of the intertropical convergence zone than those north of this natural mixing boundary between the hemispheres.     

Deuterated methane observed on saturn

Absorptions for the V(2) band of deuterated methane (CH(3)D) have been observed in the 5-micron spectrum of Saturn, obtained with a Fourier transform spectrometer. Analysis of the band yields a CH(3)D abundance of 2.6 +/- 0.8 centimeter-amagat and a temperature of 175 +/- 30 K for the mean level of spectroscopic line formation. This temperature indicates that a substantial portion of Saturn's flux at 5 microns is due to thermal radiation, and that we are therefore looking fairly deep into its atmosphere, as is the case for the Jupiter 5-micron window. This CH(3)D abundance leads to a deuteriumlhydrogen ratio of about 2 x 10(-5) in Saturn's atmosphere. This ratio is much lower than the terrestrial value but comparable to that determined for Jupiter and may be taken as representative of the deuteriumlhydrogen ratio in the solar system at the time of its formation.     

Anthropogenic CO Emissions: Implications for the Atmospheric CO-OH-CH4 Cycle

Present anthropogenic emissions of CO are apparently large enough to perturb the natural CO-OH-CH(4) cycle, which plays a crucial role in the self-cleansing processes in the troposphere. A significant increase in global concentrations of CO, CH(4) CH(3)Cl, and other trace gases may result from a decrease in the OH concentration caused by continued CO emissions. Even if the CO emissions were maintained at the present rate, increases in CO and CH(4) by the year 2025 might be as large as 50 and 25 percent, respectively. The time constants associated with the perturbations of the CO-OH-CH(4) cycle are of the order of a few decades. Perturbation of this cycle may also indirectly affect stratospheric chemistry.     

Quantum Yield for Production of CH3NC in the Photolysis of CH3NCS

The ultraviolet spectrum of methyl isothiocyanate (CH(3)NCS) and the quantum yield for its dissociation into methyl isocyanide (CH(3)NC) and atomic sulfur at 308 nanometers, Phi = 0.98 +/- 0.24, were measured. Methyl isothiocyanate is widely used as an agricultural fumigant and readily enters the atmosphere during and after application. The results indicate that photodissociation by sunlight is an effective pathway for its removal from the atmosphere.     

Measurements of time-variable gravity show mass loss in Antarctica

Using measurements of time-variable gravity from the Gravity Recovery and Climate Experiment satellites, we determined mass variations of the Antarctic ice sheet during 2002-2005. We found that the mass of the ice sheet decreased significantly, at a rate of 152 +/- 80 cubic kilometers of ice per year, which is equivalent to 0.4 +/- 0.2 millimeters of global sea-level rise per year. Most of this mass loss came from the West Antarctic Ice Sheet.     

Continental-scale partitioning of fire emissions during the 1997 to 2001 El Niño/La Niña period

During the 1997 to 1998 El Ni?o, drought conditions triggered widespread increases in fire activity, releasing CH4 and CO2 to the atmosphere. We evaluated the contribution of fires from different continents to variability in these greenhouse gases from 1997 to 2001, using satellite-based estimates of fire activity, biogeochemical modeling, and an inverse analysis of atmospheric CO anomalies. During the 1997 to 1998 El Ni?o, the fire emissions anomaly was 2.1 +/- 0.8 petagrams of carbon, or 66 +/- 24% of the CO2 growth rate anomaly. The main contributors were Southeast Asia (60%), Central and South America (30%), and boreal regions of Eurasia and North America (10%).     

A large terrestrial carbon sink in north america implied by atmospheric and oceanic carbon dioxide data and models

Atmospheric carbon dioxide increased at a rate of 2.8 petagrams of carbon per year (Pg C year-1) during 1988 to 1992 (1 Pg = 10(15) grams). Given estimates of fossil carbon dioxide emissions, and net oceanic uptake, this implies a global terrestrial uptake of 1.0 to 2. 2 Pg C year-1. The spatial distribution of the terrestrial carbon dioxide uptake is estimated by means of the observed spatial patterns of the greatly increased atmospheric carbon dioxide data set available from 1988 onward, together with two atmospheric transport models, two estimates of the sea-air flux, and an estimate of the spatial distribution of fossil carbon dioxide emissions. North America is the best constrained continent, with a mean uptake of 1.7 +/- 0.5 Pg C year-1, mostly south of 51 degrees north. Eurasia-North Africa is relatively weakly constrained, with a mean uptake of 0.1 +/- 0.6 Pg C year-1. The rest of the world's land surface is poorly constrained, with a mean source of 0.2 +/- 0.9 Pg C year-1.     

The oceanic sink for anthropogenic CO2

Using inorganic carbon measurements from an international survey effort in the 1990s and a tracer-based separation technique, we estimate a global oceanic anthropogenic carbon dioxide (CO2) sink for the period from 1800 to 1994 of 118 +/- 19 petagrams of carbon. The oceanic sink accounts for approximately 48% of the total fossil-fuel and cement-manufacturing emissions, implying that the terrestrial biosphere was a net source of CO2 to the atmosphere of about 39 +/- 28 petagrams of carbon for this period. The current fraction of total anthropogenic CO2 emissions stored in the ocean appears to be about one-third of the long-term potential.     

Freshwater methane emissions offset the continental carbon sink

Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH(4)) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets. Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least 103 teragrams of CH(4) year(-1), corresponding to 0.65 petagrams of C as carbon dioxide (CO(2)) equivalents year(-1), offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and freshwaters need to be recognized as important in the global carbon cycle.