Atmospheric Ozone and Methane in a Changing Climate (original) (raw)
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Climate Change - Geophysical Foundations and Ecological Effects, 2011
Climate Change-Geophysical Foundations and Ecological Effects 4 stratosphere was explained as a combination of special meteorological conditions and changed chemical composition induced by industrially manufactured (anthropogenic) chlorofluorocarbons (CFCs) and halons. 1.1 Ozone chemistry In the atmosphere, ozone (O 3) is produced exclusively by photochemical processes. Ozone formation in the stratosphere is initiated by the photolysis of molecular oxygen (O 2). This produces two oxygen atoms (O) which recombine with molecular oxygen to form ozone. Since ozone is created by photochemical means, it is mainly produced in the tropical and subtropical stratosphere, where sunshine is most intensive throughout the year. At the same time, the ozone molecules formed in this way are destroyed again by the photolysis of ozone and by reaction with an oxygen atom. These reactions form the basis of stratospheric ozone chemistry, the so-called Chapman mechanism (Chapman, 1930). But if stratospheric ozone amounts are determined via this simple reaction system and the known rate constants and photolysis rates, the results obtained are about twice as high as the measured values. Since the early 1950ies, it has been known that fast so-called catalytic cycles reduce the determined ozone amounts to the observed values. By the early 1970ies, the catalysts had been identified as the radical pairs OH/HO 2 and NO/NO 2 , which are formed from water vapour (H 2 O) and nitrous oxide (N 2 O) respectively (Bates and Nicolet, 1950; Crutzen, 1971; Johnston, 1971). In the mid-1970ies, the radical pairs Cl/ClO (from CFCs) and Br/BrO (from halons) were identified as further significant contributors (Molina and Rowland, 1974; Wofsy et al., 1975). The important point is that a catalyst can take part in the reaction cycle several thousand times and therefore is very effective in destroying ozone molecules. The increased occurrence of CFCs and halons due to anthropogenic emissions has significantly accelerated stratospheric ozone depletion cycle over recent decades, triggering a negative stratospheric ozone trend which is most obvious in the Southern polar stratosphere during spring time where the ozone hole is found. In the troposphere, CFCs and halons are mostly inert. Over time (several years), they are transported into the stratosphere. Only there they are photolysed and converted into active chlorine or bromine compounds. In particular, ozone is depleted via the catalytic Cl/ClO-cycle in polar spring. However, the kinetics of these processes are very slow, because the amount of UV radiation is limited due to the prevailing twilight conditions. In the polar stratosphere, it is mainly chemical reactions on the surface of stratospheric ice particles that are responsible for activating chlorine (and also bromine) and then driving ozone depletion immediately after the end of polar night (Solomon et al., 1986). In the very cold lower polar stratosphere, polar stratospheric clouds (PSCs) form during polar night (Figure 1). PSCs develop at temperatures below about 195 K (=-78 °C) where nitric acid trihydride crystals form (NAT, HNO 3 •3H 2 O). Under the given conditions in the lower stratosphere ice particles develop at temperatures below approx. 188 K (=-85 °C). Due to different land-sea distributions on the Northern and Southern Hemisphere, the lower stratosphere over the south pole cools significantly more in winter (June-August) than the north polar stratosphere (December-February) (see Section 1.2). The climatological mean of polar winter temperatures of the lower Arctic stratosphere is around 10 K higher than that of the lower Antarctic stratosphere. While the Antarctic stratosphere reaches temperatures below PSC-forming temperatures for several weeks every year, there is a pronounced year-on-year variability in the north polar stratosphere: relatively warm winters, where hardly any PSCs develop are www.intechopen.com
Atmospheric methane and global change
Earth-science Reviews, 2002
Methane (CH 4 ) is the most abundant organic trace gas in the atmosphere. In the distant past, variations in natural sources of methane were responsible for trends in atmospheric methane levels recorded in ice cores. Since the 1700s, rapidly growing human activities, particularly in the areas of agriculture, fossil fuel use, and waste disposal, have more than doubled methane emissions. Atmospheric methane concentrations have increased by a factor of 2 -3 in response to this increase, and continue to rise. These increasing concentrations have raised concern due to their potential effects on atmospheric chemistry and climate. Methane is important to both tropospheric and stratospheric chemistry, significantly affecting levels of ozone, water vapor, the hydroxyl radical, and numerous other compounds. In addition, methane is currently the second most important greenhouse gas emitted from human activities. On a per molecule basis, it is much more effective a greenhouse gas than additional CO 2 . In this review, we examine past trends in the concentration of methane in the atmosphere, the sources and sinks that determine its growth rate, and the factors that will affect its growth rate in the future. We also present current understanding of the effects of methane on atmospheric chemistry, and examine the direct and indirect impacts of atmospheric methane on climate. D
The Potential effects of increased methane on atmospheric ozone
Geophysical Research Letters, 1982
Using a one dimensional atmospheric Miller et al. (1981) and the references found model, we investigate the possible influence of an therein. For these calculations, 30 chemical increase in atmospheric methane on ozone. The species were active, including all those currently couplings between methane and the catalytic identified as important to methane and ozone destruction of ozone by NO., HOx, and C1X are chemistry in the troposphere and stratosphere. discussed. Our model calculations suggest that Recent updates to the model include the addition doubling the ground-level flux of methane, with of clouds in the troposphere (50% cloud cover at 6 fixed atmospheric temperatures and currently km altitude) and the calculation of UV penetration recommended chemical reaction rates, would in the Schumann-Runge bands of molecular oxygen increase the total ozone column by 3.5%. based on the formulation of Frederick and Hudson Calculations showing the very significant (1980). The chemical reaction rates and incident moderating effects of a methane increase on ozone solar fluxes are those recommended by recent NASA perturbations due to N20 and chlorofluorocarbons panels (WMO, 1982). are discussed. The "initial" atmosphere used as a basis for comparison is one with no chlorofluorocarbons eg., Ehhalt and Schmidt, 1978, Logan et al.,
An emissions-based view of climate forcing by methane and tropospheric ozone
Geophysical Research Letters, 2005
1] We simulate atmospheric composition changes in response to increased methane and tropospheric ozone precursor emissions from the preindustrial to present-day in a coupled chemistry-aerosol-climate model. The global annual average composition response to all emission changes is within 10% of the sum of the responses to individual emissions types, a more policy-relevant quantity. This small non-linearity between emission types permits attribution of past global mean methane and ozone radiative forcings to specific emissions despite the well-known nonlinear response to emissions of a single type. The emissionsbased view indicates that methane emissions have contributed a forcing of 0.8−0.9WmAˋ2,nearlydoubletheabundance−basedvalue,whiletheforcingfromotherozoneprecursorshasbeenquitesmall(0.8-0.9 W m À2 , nearly double the abundance-based value, while the forcing from other ozone precursors has been quite small (0.8−0.9WmAˋ2,nearlydoubletheabundance−basedvalue,whiletheforcingfromotherozoneprecursorshasbeenquitesmall(À0.
The sensitivity of tropospheric methane to the interannual variability in stratospheric ozone
Chemosphere - Global Change Science, 2001
Importance of this paper: Many processes in the stratosphere have important impacts upon tropospheric behavior. The forcing of tropospheric methane by stratospheric ozone is one such interaction for which we have long and accurate observational records. We have empirically determined a quantitative measure of this interaction. This result provides the ®rst convincing demonstration in the data records of such a stratosphere±troposphere interaction. It can also be used to calibrate the modeling work being done in this area.
Journal of Geophysical Research, 2008
1] Reducing methane (CH 4 ) emissions is an attractive option for jointly addressing climate and ozone (O 3 ) air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that tropospheric O 3 responds approximately linearly to changes in CH 4 emissions over a range of anthropogenic emissions from 0-430 Tg CH 4 a À1 (0.11-0.16 Tg tropospheric O 3 or 11−15pptglobalmeansurfaceO3decreaseperTgaAˋ1CH4reduced).WefindthatneithertheairqualitynorclimatebenefitsdependstronglyonthelocationoftheCH4emissionreductions,implyingthatthelowestcostemissioncontrolscanbetargeted.Withaseriesoffuture(2005−2030)transientsimulations,wedemonstratethatcost−effectiveCH4controlswouldoffsetthepositiveclimateforcingfromCH4andO3thatwouldotherwiseoccur(fromincreasesinNOxandCH4emissionsinthebaselinescenario)andimproveO3airquality.WeestimatethatanthropogenicCH4contributes0.7WmAˋ2toclimateforcingand11-15 ppt global mean surface O 3 decrease per Tg a À1 CH 4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH 4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005-2030) transient simulations, we demonstrate that cost-effective CH 4 controls would offset the positive climate forcing from CH 4 and O 3 that would otherwise occur (from increases in NO x and CH 4 emissions in the baseline scenario) and improve O 3 air quality. We estimate that anthropogenic CH 4 contributes 0.7 Wm À2 to climate forcing and 11−15pptglobalmeansurfaceO3decreaseperTgaAˋ1CH4reduced).WefindthatneithertheairqualitynorclimatebenefitsdependstronglyonthelocationoftheCH4emissionreductions,implyingthatthelowestcostemissioncontrolscanbetargeted.Withaseriesoffuture(2005−2030)transientsimulations,wedemonstratethatcost−effectiveCH4controlswouldoffsetthepositiveclimateforcingfromCH4andO3thatwouldotherwiseoccur(fromincreasesinNOxandCH4emissionsinthebaselinescenario)andimproveO3airquality.WeestimatethatanthropogenicCH4contributes0.7WmAˋ2toclimateforcingand4 ppb to surface O 3 in 2030 under the baseline scenario. Although the response of surface O 3 to CH 4 is relatively uniform spatially compared to that from other O 3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O 3 formation regime is NO x -saturated. In the model, CH 4 oxidation within the boundary layer (below $2.5 km) contributes more to surface O 3 than CH 4 oxidation in the free troposphere. In NO x -saturated regions, the surface O 3 sensitivity to CH 4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH 4 . Accurately representing the NO x distribution is thus crucial for quantifying the O 3 sensitivity to CH 4 . Citation: Fiore, A. M., J. J. West, L. W. Horowitz, V. Naik, and M. D. Schwarzkopf (2008), Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality,
Climate Research, 1991
The response of the atmosphere to increasing emissions of radatively active trace gases (COz, CH,, N 2 0 , o3 and CFCs) is calculated by means of a l-dimensional coupled chemical-radiativetransport model. We identify the sign and magnitude of the feedback between the chemistry and thermal structure of the atmosphere by examining steady state changes in stratospheric ozone and surface temperature in response to perturbations in trace gases of anthropogenic origin. Next, we assess the possible decline in stratospheric ozone and its effect on troposphere-stratosphere temperature trends for the period covering the pre-industrial era to the present. Future trends are also considered using projected 'business-as-usual' trace gas scenario (scenario BAU) and that expected as a result of global phase-out of production of CFCs by the year 2000 (scenario MP). The numerical experiments take account of the effect of stratospheric aerosol loading due to volcanic eruptions and the influence of the thermal inertia of the ocean. Results indicate that the trace gas increase from the period 1850 to 1986 could already have contributed to a 3 to 10 % decline in stratospheric ozone and that this decline is expected to become more pronounced by the year 2050, amounting to a 43 % ozone loss at 40 km for the scenario BAU. The total column ozone Increase of 1.5 % obtained in our model calculations for the present-day atmosphere is likely to change sign with time leading to a net decrease of 12.7 % by the middle of next century. The equilibrium surface warming for the period 1850 to 1986 is found to be 0.7 K and our calculations indicate that thls warming will reach 2.6 K by the year 2050. As a result of radiativechemical interactions, a large stratospheric cooling (16.4 K) is likely by the middle of the next century. In case the control measures on production of CFCs as laid down in the London Amendment of the Montreal Protocol are implemented by all countries and a global phase-out of CFCs takes place by the year 2000, the ozone loss at 40 km could be restricted to 17 % by the year 2050 (net column decrease of only 2.9%). This would, however, only marginally reduce the stratospheric cooling to 12.6 K. The greenhouse warming at the surface expected by the middle of the next century with enforcement of the Montreal Protocol restrictions is likely to be 2 K.
Atmospheric composition change: Climate–Chemistry interactions
Atmospheric Environment, 2009
future changes. Reported results include new estimates of radiative forcing based on extensive model studies of chemically active climate compounds like O 3 , and of particles inducing both direct and indirect effects. Through EU projects like ACCENT, QUANTIFY, and the AeroCom project, extensive studies on regional and sector-wise differences in the impact on atmospheric distribution are performed. Studies have shown that land-based emissions have a different effect on climate than ship and aircraft emissions, and different measures are needed to reduce the climate impact. Several areas where climate change can affect the tropospheric oxidation process and the chemical composition are identified. This can take place through enhanced stratospheric-tropospheric exchange of ozone, more frequent periods with stable conditions favoring pollution build up over industrial areas, enhanced temperature induced biogenic emissions, methane releases from permafrost thawing, and enhanced concentration through reduced biospheric uptake. During the last 5-10 years, new observational data have been made available and used for model validation and the study of atmospheric processes. Although there are significant uncertainties in the modeling of composition changes, access to new observational data has improved modeling capability. Emission scenarios for the coming decades have a large uncertainty range, in particular with respect to regional trends, leading to a significant uncertainty range in estimated regional composition changes and climate impact.
Permafrost degradation and methane: low risk of biogeochemical climate-warming feedback
Environmental Research Letters, 2013
Climate change and permafrost thaw have been suggested to increase high latitude methane emissions that could potentially represent a strong feedback to the climate system. Using an integrated earth-system model framework, we examine the degradation of near-surface permafrost, temporal dynamics of inundation (lakes and wetlands) induced by hydro-climatic change, subsequent methane emission, and potential climate feedback. We find that increases in atmospheric CH 4 and its radiative forcing, which result from the thawed, inundated emission sources, are small, particularly when weighed against human emissions. The additional warming, across the range of climate policy and uncertainties in the climate-system response, would be no greater than 0.1 • C by 2100. Further, for this temperature feedback to be doubled (to approximately 0.2 • C) by 2100, at least a 25-fold increase in the methane emission that results from the estimated permafrost degradation would be required. Overall, this biogeochemical global climate-warming feedback is relatively small whether or not humans choose to constrain global emissions.