Relative radiative forcing consequences of global emissions of hydrocarbons, carbon monoxide and NOx from human activities estimated with a zonally-averaged two-dimensional model

A global two-dimensional (altitude-latitude) chemistry transport model is used to follow the changes in the tropospheric distribution of the two major radiatively active trace gases, methane and ozone, following step changes to the sustained emissions of the short-lived trace gases methane, carbon monoxide and non-methane hydrocarbons. The radiative impacts were dependent on the latitude chosen for the applied change in emissions. Step change global warming potentials (GWPs) were derived for a range of short-lived trace gases to describe their time-integrated radiative forcing impacts for unit emissions relative to that of carbon dioxide. The GWPs show that the tropospheric chemistry of the hydrocarbons can produce significant indirect radiative impacts through changing the tropospheric distributions of hydroxyl radicals, methane and ozone. For aircraft, the indirect radiative forcing impact of the NO _x emissions appears to be greater than that from their carbon dioxide emissions. Quantitative results from this two-dimensional model study must, however, be viewed against the known inadequacies of zonally-averaged models and their poor representation of many important tropospheric processes.     

Radiative forcing from surface NO x emissions: spatial and seasonal variations

The global three-dimensional Lagrangian chemistry-transport model STOCHEM has been used to follow changes in the tropospheric distributions of methane CH₄ and ozone O₃ following the emission of pulses of the oxides of nitrogen NO _x . Month-long emission pulses of NO _x produce deficits in CH₄ mixing ratios that bring about negative radiative forcing (climate cooling) and decay away with e-folding times of 10–15 years. They also produce short-term excesses in O₃ mixing ratios that bring about positive radiative forcing (climate warming) that decay over several months to produce deficits, with their attendant negative radiative forcing (climate cooling) that decays away in step with the CH₄ deficits. Total time-integrated net radiative forcing is markedly influenced by cancellation between the negative CH₄ and long-term O₃ contributions and the positive short-term O₃ contribution to leave a small negative residual. Consequently, total net radiative forcing from NO _x emission pulses and the global warming potentials derived from them, show a strong dependence on the magnitudes, locations and seasons of the emissions. These dependences are illustrated using the Asian continent as an example and demonstrate that there is no simple robust relationship between continental-scale NO _x emissions and globally-integrated radiative forcing. We find that the magnitude of the time-integrated radiative forcing from NO _x -driven CH₄ depletion tends to approach and outweigh that from ozone enhancement, leaving net time-integrated radiative forcings and global warming potentials negative (climate cooling) in contrast to the situation for aircraft NO _x (climate warming). Control of man-made surface NO _x emissions alone may lead to positive radiative forcing (climate warming).     

The Oxidation of Organic Compounds in the Troposphere and their Global Warming Potentials

Oxidation by hydroxyl radicals is the main removal process for organic compounds in the troposphere. This oxidation acts as a source of ozone and as a removal process for hydroxyl and peroxy radicals, thereby reducing the efficiency of methane oxidation and promoting the build-up of methane. Emissions of organic compounds may therefore lead to the build-up of two important radiatively-active trace gases: methane and ozone. Emission pulses of 10 organic compounds were followed in a global 3-D Lagrangian chemistry-transport model to quantify their indirect greenhouse gas impacts through changes induced in the tropospheric distributions of methane and ozone. The main factors influencing the global warming potentials of the 10 organic compounds were found to be their spatial emission patterns, chemical reactivity and transport, molecular complexity and oxidation products formed. The indirect radiative forcing impacts of organic compounds may be large enough that ozone precursors should be considered in the basket of trace gases through which policy-makers aim to combat global climate change.     

Abatement of Greenhouse Gases: Does Location Matter?

Today's climate policy is based on the assumption that the location of emissions reductions has no impact on the overall climate effect. However, this may not be the case since reductions of greenhouse gases generally will lead to changes in emissions of short-lived gases and aerosols. Abatement measures may be primarily targeted at reducing CO₂, but may also simultaneously reduce emissions of NO_x, CO, CH₄ and SO₂ and aerosols. Emissions of these species may cause significant additional radiative forcing. We have used a global 3-D chemical transport model and a radiative transfer model to study the impact on climate in terms of radiative forcing for a realistic change in location of the emissions from large-scale sources. Based on an assumed 10% reduction in CO₂ emissions, reductions in the emissions of other species have been estimated. Climate impact for the SRES A1B scenario is compared to two reduction cases, with the main focus on a case with emission reductions between 2010 and 2030, but also a case with sustained emission reductions. The emission reductions are applied to four different regions (Europe, China, South Asia, and South America). In terms of integrated radiative forcing (over 100 yr), the total effect (including only the direct effect of aerosols) is always smaller than for CO₂ alone. Large variations between the regions are found (53–86% of the CO₂ effect). Inclusion of the indirect effects of sulphate aerosols reduces the net effect of measures towards zero. The global temperature responses, calculated with a simple energy balance model, show an initial additional warming of different magnitude between the regions followed by a more uniform reduction in the warming later. A major part of the regional differences can be attributed to differences related to aerosols, while ozone and changes in methane lifetime make relatively small contributions. Emission reductions in a different sector (e.g. transportation instead of large-scale sources) might change this conclusion since the NO_x to SO₂ ratio in the emissions is significantly higher for transportation than for large-scale sources. The total climate effect of abatement measures thus depends on (i) which gases and aerosols are affected by the measure, (ii) the lifetime of the measure implemented, (iii) time horizon over which the effects are considered, and (iv) the chemical, physical and meteorological conditions in the region. There are important policy implications of the results. Equal effects of a measure cannot be assumed if the measure is implemented in a different region and if several gases are affected. Thus, the design of emission reduction measures should be considered thoroughly before implementation.     

Estimates of indirect global warming potentials for CH4, CO and NOX

Emissions may affect climate indirectly through chemical interactions in the atmosphere, but quantifications of such effects are difficult and uncertain due to incomplete knowledge and inadequate methods. A preliminary assessment of the climatic impact of changes in tropospheric O₃ and CH₄ in response to various emissions is given. For a 10% increase in the CH₄ emissions the relative increase in concentration has been estimated to be 37% larger. The radiative forcing from enhanced levels of tropospheric O₃ is estimated to 37% of the forcing from changes in CH₄. Inclusion of indirect effects approximately doubles the climatic impact of CH₄ emissions. Emissions of NO_x increase tropospheric O₃, while the levels of CH₄ are reduced. For emissions of NO_x from aircraft, the positive effects via O₃ changes are significantly larger than the negative through changes in CH₄. For NO_x emitted from surface sources, the effects through changes in O₃ and CH₄ are estimated to be of similar magnitude and large uncertainty is connected to the sign of the net effect. Emissions of CO have positive indirect effects on climate through enhanced levels of tropospheric o₃ and increased lifetime of CH₄. These results form the basis for estimates of global warming potentials for sustained step increases in emissions.     

Impact of precursor levels and global warming on peak ozone concentration in the Pearl River Delta Region of China

The relationship between the emission of ozone precursors and the chemical production of tropospheric ozone(O3) in the Pearl River Delta Region(PRD) was studied using numerical simulation.The aim of this study was to examine the volatile organic compound(VOC)-or nitrogen oxide(NOx =NO+NO2)limited conditions at present and when surface temperature is increasing due to global warming,thus to make recommendations for future ozone abatement policies for the PRD region.The model used for this application is the U.S.Environmental Protection Agency’s(EPA’s) third-generation air-quality modeling system;it consists of the mesoscale meteorological model MM5 and the chemical transport model named Community Multi-scale Air Quality(CMAQ).A series of sensitivity tests were conducted to assess the influence of VOC and NOx variations on ozone production.Tropical cyclone was shown to be one of the important synoptic weather patterns leading to ozone pollution.The simulations were based on a tropicalcyclone-related episode that occurred during 14-16 September 2004.The results show that,in the future,the control strategy for emissions should be tightened.To reduce the current level of ozone to meet the Hong Kong Environmental Protection Department(EPD) air-quality objective(hourly average of 120 ppb),emphasis should be put on restricting the increase of NOx emissions.Furthermore,for a wide range of possible changes in precursor emissions,temperature increase will increase the ozone peak in the PRD region;the areas affected by photochemical smog are growing wider,but the locations of the ozone plume are rather invariant.     

A model investigation of the impact of increases in anthropogenic NO x emissions between 1967 and 1980 on tropospheric ozone

Recent observations suggest that the abundance of ozone between 2 and 8 km in the Northern Hemisphere mid-latitudes has increased by about 12% during the period from 1970 to 1981. Earlier estimates were somewhat more conservative suggesting increases at the rate of 7% per decade since the start of regular observations in 1967. Previous photochemical model studies have indicated that tropospheric ozone concentrations would increase with increases in emissions of CO, CH₄ and NO _x . This paper presents an analysis of tropospheric ozone which suggests that a significant portion of its increase may be attributed to the increase in global anthropogenic NO _x emissions during this period while the contribution of CH₄ to the increase is quite small. Two statistical models are presented for estimating annual global anthropogenic emissions of NO _x and are used to derive the trend in the emissions for the years 1966–1980. These show steady increase in the emissions during this interval except for brief periods of leveling off after 1973 and 1978. The impact of this increase in emissions on ozone is estimated by calculations with a onedimensional (latitudinal) model which includes coupled tropospheric photochemistry and diffusive meridional transport. Steady-state photochemical calculations with prescribed NO _x emissions appropriate for 1966 and 1980 indicate an ozone increase of 8–11% in the Northern Hemisphere, a result which is compatible with the rise in ozone suggested by the observations.     

Satellite Measurements of Tropospheric Column O<Subscript>3</Subscript> and NO<Subscript>2</Subscript> in Eastern and Southeastern Asia: Comparison with a Global Model (MOZART-2)

Satellite measurements of tropospheric column O₃ and NO₂ in eastern and southeastern Asia are analyzed to study the spatial and seasonal characteristics of pollution in these regions. Tropospheric column O₃ is derived from differential measurements of total column ozone from Total Ozone Mapping Spectrometer (TOMS), and stratospheric column ozone from the Microwave Limb Sounder (MLS) instrument on the Upper Atmosphere Research Satellite (UARS). The tropospheric column NO₂ is measured by Global Ozone Monitoring Experiment (GOME). A global chemical and transport model (Model of Ozone and Related Chemical Tracers, version 2; MOZART-2) is applied to analyze and interpret the satellite measurements. The study, which is based on spring, summer, and fall months of 1997 shows generally good agreement between the model and satellite data with respect to seasonal and spatial characteristics of O₃ and NO₂ fields. The analysis of the model results show that the industrial emission of NO_x (NO + NO₂) contributes about 50%–80% to tropospheric column NO₂ in eastern Asia and about 20%–50% in southeastern Asia. The contribution of industrial emission of NO_x to tropospheric column O₃ ranges from 10% to 30% in eastern Asia. Biomass burning and lightning NO_x emissions have a small effect on tropospheric O₃ in central and eastern Asia, but they have a significant impact in southeastern Asia. The varying effects of NO_x on tropospheric column ozone are attributed to differences in relative abundance of volatile organic compounds (VOCs) with respect to total nitrogen in the two regions.     

Effects of tropical deforestation on global and regional atmospheric chemistry

A major portion of tropospheric photochemistry occurs in the tropics. Deforestation, colonization, and development of tropical rain forest areas could provoke significant changes in emissions of radiatively and photochemically active trace gases. A brief review of studies on trace-gas emissions in pristine and disturbed tropical habitats is followed by an effort to model regional tropospheric chemistry under undisturbed and polluted conditions. Model results suggest that changing emissions could stimulate photochemistry leading to enhanced ozone production and greater mineral acidity in rainfall in colonized agricultural regions. Model results agree with measurements made during the NASA ABLE missions. Under agricultural/pastoral development scenarios, tropical rain forest regions could export greater levels of N₂O, CH₄, CO, and photochemical precursors of NO _y and O₃ to the global atmosphere with implications for climatic warming.     

Subsonic aircraft and ozone trends

Growth in subsonic air traffic over the past 20 years has been dramatic, with an annual increase of }6.1% over the decade between 1978 and 1988. Furthermore, aircraft activities in the year 2000 are predicted to be double those of 1990, with a shift towards more high-flying, longhaul subsonics. Aircraft exhaust gases increase the amount of nitrogen oxides (NO _x ) in the upper troposphere/lower stratosphere through injection at cruise altitudes. Given that NO _x is instrumental in tropospheric ozone production and stratospheric ozone destruction, it is important to determine the influence of subsonic aircraft NO _x emissions on levels of atmospheric ozone. This paper describes calculations designed to investigate the impact that subsonic aircraft may already have had on the atmosphere during the 1980s, run in a 2-D chemical-radiative-transport model. The results indicate a significant increase in upper tropospheric ozone over the decade arising from aircraft emissions. However, when comparing model results with observational data, certain discrepancies appear. Lower stratospheric ozone loss over the 1980s does not appear to be greatly altered by the inclusion of aircraft emissions in the model. However, given the trend in greater numbers of long-haul subsonic aircraft, this factor must be considered in any further calculations.     

Global and regional evolution of short-lived radiatively-active gases and aerosols in the Representative Concentration Pathways

In this paper, we discuss the results of 2000?C2100 simulations following the emissions associated with the Representative Concentration Pathways (RCPs) with a chemistry-climate model, focusing on the changes in 1) atmospheric composition (troposphere and stratosphere) and 2) associated environmental parameters (such as nitrogen deposition). In particular, we find that tropospheric ozone is projected to decrease (RCP2.6, RCP4.5 and RCP6) or increase (RCP8.5) between 2000 and 2100, with variations in methane a strong contributor to this spread. The associated tropospheric ozone global radiative forcing is shown to be in agreement with the estimate used in the RCPs, except for RCP8.5. Surface ozone in 2100 is projected to change little compared from its 2000 distribution, a much-reduced impact from previous projections based on the A2 high-emission scenario. In addition, globally-averaged stratospheric ozone is projected to recover at or beyond pre-1980 levels. Anthropogenic aerosols are projected to strongly decrease in the 21st century, a reflection of their projected decrease in emissions. Consequently, sulfate deposition is projected to strongly decrease. However, nitrogen deposition is projected to increase over certain regions because of the projected increase in NH₃ emissions.     

RCP2.6: exploring the possibility to keep global mean temperature increase below 2��C

The RCP2.6 emission and concentration pathway is representative of the literature on mitigation scenarios aiming to limit the increase of global mean temperature to 2°C. These scenarios form the low end of the scenario literature in terms of emissions and radiative forcing. They often show negative emissions from energy use in the second half of the 21st century. The RCP2.6 scenario is shown to be technically feasible in the IMAGE integrated assessment modeling framework from a medium emission baseline scenario, assuming full participation of all countries. Cumulative emissions of greenhouse gases from 2010 to 2100 need to be reduced by 70% compared to a baseline scenario, requiring substantial changes in energy use and emissions of non-CO₂ gases. These measures (specifically the use of bio-energy and reforestation measures) also have clear consequences for global land use. Based on the RCP2.6 scenario, recommendations for further research on low emission scenarios have been formulated. These include the response of the climate system to a radiative forcing peak, the ability of society to achieve the required emission reduction rates given political and social inertia and the possibilities to further reduce emissions of non-CO₂ gases.     

Responses of the tropospheric ozone and odd hydrogen radicals to column ozone change

The response of tropospheric ozone to a change in solar UV penetration due to perturbation on column ozone depends critically on the tropospheric NO _x (NO+NO₂) concentration. At high NO _x or a polluted area where there is net ozone production, a decrease in column ozone will increase the solar UV penetration to the troposphere and thus increase the tropospheric ozone concentration. However, the opposite will occur, for example, at a remote oceanic area where NO _x is so low that there is net ozone destruction. This finding may have important implication on the interpretation of the long term trend of tropospheric ozone. A change in column ozone will also induce change in tropospheric OH, HO₂, and H₂O₂ concentrations which are major oxidants in the troposphere. Thus, the oxidation capacity and, in turn, the abundances of many reduced gases will be perturbed. Our model calculations show that the change in OH, HO₂, and H₂O₂ concentrations are essentially independent of the NO _x concentration.     

A global model study of natural bromine sources and the effects on tropospheric chemistry using MOZART4

Halogens in the atmosphere chemically destroy ozone. In the troposphere, bromine has higher ozone destruction efficiency than chlorine and is the halogen species with the widest geographical spread of natural sources. We investigate the relative strength of various sources of reactive tropospheric bromine and the influence of bromine on tropospheric chemistry using a 6-year simulation with the global chemistry transport model MOZART4. We consider the following sources: short-lived bromocarbons (CHBr₃, CH₂BrCl, CHBr₂Cl, CHBrCl₂, and CH₂Br₂) and CH₃Br, bromine from airborne sea salt particles, and frost flowers and sea salt on or in the snowpack in polar regions. The total bromine emissions in our simulations add up to 31.7 Gmol(Br)/yr: 63 % from polar sources, 24.6 % from short-lived bromocarbons and 12.4 % from airborne sea salt particles. We conclude from our analysis that our global bromine emission is likely to be on the lower end of the range, because of too low emissions from airborne sea salt. Bromine chemistry has an effect on the oxidation capacity of the troposphere, not only due to its direct influence on ozone concentrations, but also by reactions with other key chemical species like HO _x and NO _x . Globally, the impact of bromine chemistry on tropospheric O₃ is comparable to the impact of gas-phase sulfur chemistry, since the inclusion of bromine chemistry in MOZART4 leads to a decrease of the O₃ burden in the troposphere by 6 Tg, while we get an increase by 5 Tg if gas-phase sulfur chemistry is switched off in the standard model. With decreased ozone burden, the simulated oxidizing capacity of the atmosphere decreases thus affecting species associated with the oxidation capacity of the atmosphere (CH₃OOH, H₂O₂).     

Estimates of the Climate Response to Aircraft CO2 and NO x Emissions Scenarios

A combination of linear response models is used to estimate the transient changes in the global means of carbon dioxide (CO₂) concentration, surface temperature, and sea level due to aviation. Apart from CO₂, the forcing caused by ozone (O₃) changes due to nitrogen oxide (NO_x) emissions from aircraft is also considered. The model is applied to aviation using several CO₂ emissions scenarios, based on reported fuel consumption in the past and scenarios for the future, and corresponding NO_x emissions. Aviation CO₂ emissions from the past until 1995 enlarged the atmospheric CO₂ concentration by 1.4 ppmv (1.7% of the anthropogenic CO₂ increase since 1800). By 1995, the global mean surface temperature had increased by about 0.004 K, and the sea level had risen by 0.045 cm. In one scenario (Fa1), which assumes a threefold increase in aviation fuel consumption until 2050 and an annual increase rate of 1% thereafter until 2100, the model predicts a CO₂ concentration change of 13 ppmv by 2100, causing temperature increases of 0.01, 0.025, 0.05 K and sea level increases of 0.1, 0.3, and 0.5 cm in the years 2015, 2050, and 2100, respectively. For other recently published scenarios, the results range from 5 to 17 ppmv for CO₂ concentration increase in the year 2050, and 0.02 to 0.05 K for temperature increase. Under the assumption that present-day aircraft-induced O₃ changes cause an equilibrium surface warming of 0.05 K, the transient responses amount to 0.03 K in surface temperature for scenario Fa1 in 1995. The radiative forcing due to an aircraft-induced O₃ increase causes a larger temperature change than aircraft CO₂ forcing. Also, climate reacts more promptly to changes in O₃ than to changes in CO₂ emissions from aviation. Finally, even under the assumption of a rather small equilibrium temperature change from aircraft-induced O₃ (0.01 K for the 1992 NO_x emissions), a proposed new combustor technology which reduces specific NO_x emissions will cause a smaller temperature change during the next century than the standard technology does, despite a slightly enhanced fuel consumption. Regional effects are not considered here, but may be larger than the global mean responses.     

Impacts of Global Emissions of CO, NOx, and CH4 on China Tropospheric Hydroxyl Free Radicals

Using the global chemistry and transport model MOZART,the simulated distributions of tropospheric hydroxyl free radicals(OH) over China and its sensitivities to global emissions of carbon monoxide(CO),nitrogen oxide(NO x),and methane(CH 4) were investigated in this study.Due to various distributions of OH sources and sinks,the concentrations of tropospheric OH in east China are much greater than in west China.The contribution of NO + perhydroxyl radical(HO 2) reaction to OH production in east China is more pronounced than that in west China,and because of the higher reaction activity of non-methane volatile organic compounds(NMVOCs),the contributions to OH loss by NMVOCs exceed those of CO and take the dominant position in summer.The results of the sensitivity runs show a significant increase of tropospheric OH in east China from 1990 to 2000,and the trend continues.The positive effect of double emissions of NO x on OH is partly offset by the contrary effect of increased CO and CH 4 emissions:the double emissions of NO x will cause an increase of OH of 18.1%-30.1%,while the increases of CO and CH 4 will cause a decrease of OH of 12.2%-20.8% and 0.3%-3.0%,respectively.In turn,the lifetimes of CH 4,CO,and NO x will increase by 0.3%-3.1% with regard to double emissions of CH 4,13.9%-26.3% to double emissions of CO and decrease by 15.3%-23.2% to double emissions of NO x.     

Changing trends in tropospheric methane and carbon monoxide: A sensitivity analysis of the OH-radical

A sensitivity analysis is performed in order to study recently observed changes in atmospheric methane and carbon monoxide trends. For the analysis we have adapted a one-dimensional transport/chemistry model in order to comply with changes in vertical transport, stratosphere-troposphere flux of ozone, the water vapour cycle and the short-wave radiative transfer. In addition we have formulated an improved relationship which expresses the steady state OH concentration in terms of longer lived compounds which has a fair agreement with the one-dimensional model results. An analysis of the observed changes and trends in methane and carbon monoxide shows that both emissions and changes in global OH concentrations can be main causes for the observed changes. Average methane emissions have slowed down, particularly in the NH, in the last five years, though perhaps not very significantly. Carbon monoxide emissions are decreasing faster in the last couple of years than in the period 1983–1990. The study suggests that climate fluctuations (tropospheric water vapour, temperature and convective activity) and the stratospheric ozone depletion (tropospheric UV radiation) have a significant influence on tropospheric composition and thus on trends in methane and carbon monoxide concentrations.The IMAU is partner in the Netherlands Centre for Climate Research (CCR).     

Simultaneous measurements of ozone and its precursors on a diurnal scale at a semi urban site in India

Simultaneous observations of surface ozone (O₃) with its precursors namely, carbon monoxide (CO) and oxides of nitrogen (NO_x) have been taken on diurnal scale from a tropical semi-urban site, Pune (18.54°N, 73.81°E) in India. We present the data for one year (2003–2004) period to study the salient features of these trace gases. The peak in amplitude of ozone is found during the noontime whereas in CO and NO_X it is observed in the morning hours between 0800 and 0900 H. The concentration of these pollutants drop down considerably during southwest monsoon months and the diurnal pattern also become very weak. The diurnal trends of these gases are found to be different for different seasons, which are specific to the receptor site. Model simulations using 3-D chemical-transport model with regional emission inventories and observed winds have also been carried out. The comparison of model results with observations, on seasonal basis yielded a reasonable qualitative agreement. The relative role of local emissions and long range transport in the diurnal pattern for different seasons has been outlined, which reveals that the ozone is highly influenced by regional/long range transport in this region. The effect of precursor amounts in the morning on afternoon ozone peak levels has been investigated using the lag correlation study, which reveals that a time lag of 5–7 h is required for most of these precursor gases to photo-chemically produce ozone to its maximum potential. Results are discussed in the light of available topographic and meteorological conditions.     

Aerosol and ozone changes as forcing for climate evolution between 1850 and 2100

Global aerosol and ozone distributions and their associated radiative forcings were simulated between 1850 and 2100 following a recent historical emission dataset and under the representative concentration pathways (RCP) for the future. These simulations were used in an Earth System Model to account for the changes in both radiatively and chemically active compounds, when simulating the climate evolution. The past negative stratospheric ozone trends result in a negative climate forcing culminating at ?0.15 W m^?2 in the 1990s. In the meantime, the tropospheric ozone burden increase generates a positive climate forcing peaking at 0.41 W m^?2. The future evolution of ozone strongly depends on the RCP scenario considered. In RCP4.5 and RCP6.0, the evolution of both stratospheric and tropospheric ozone generate relatively weak radiative forcing changes until 2060–2070 followed by a relative 30 % decrease in radiative forcing by 2100. In contrast, RCP8.5 and RCP2.6 model projections exhibit strongly different ozone radiative forcing trajectories. In the RCP2.6 scenario, both effects (stratospheric ozone, a negative forcing, and tropospheric ozone, a positive forcing) decline towards 1950s values while they both get stronger in the RCP8.5 scenario. Over the twentieth century, the evolution of the total aerosol burden is characterized by a strong increase after World War II until the middle of the 1980s followed by a stabilization during the last decade due to the strong decrease in sulfates in OECD countries since the 1970s. The cooling effects reach their maximal values in 1980, with ?0.34 and ?0.28 W m^?2 respectively for direct and indirect total radiative forcings. According to the RCP scenarios, the aerosol content, after peaking around 2010, is projected to decline strongly and monotonically during the twenty-first century for the RCP8.5, 4.5 and 2.6 scenarios. While for RCP6.0 the decline occurs later, after peaking around 2050. As a consequence the relative importance of the total cooling effect of aerosols becomes weaker throughout the twenty-first century compared with the positive forcing of greenhouse gases. Nevertheless, both surface ozone and aerosol content show very different regional features depending on the future scenario considered. Hence, in 2050, surface ozone changes vary between ?12 and +12 ppbv over Asia depending on the RCP projection, whereas the regional direct aerosol radiative forcing can locally exceed ?3 W m^?2.     

Impact of the Montreal protocol and its amendments on the rate of change of global radiative forcing

Increases in chlorinated and brominated halocarbons are believed to be responsible for the depletion of stratospheric ozone observed over much of the globe in the past decade or so. Ozone depletion is in turn believed to lead to a negative radiative forcing, tending to cool the stratosphere and the surface. We show that the increasing atmospheric concentrations of ozone-depleting halocarbons and onset of related ozone depletion likely led to a negative forcing of the climate system in the 1980s that slowed significantly the rate of change of total anthropogenic radiative forcing due to the combined effect of all greenhouse gases over that decade. Within the next decade, emissions of these halocarbons are expected to rapidly decrease, with corresponding impacts on ozone and radiative forcing. As the emissions of ozone-depleting gases are reduced and eventually phased out, the rate of ozone depletion is expected to decrease and eventually reverse. All other things being equal, we show that the change from deepening ozone depletion in the 1980s to ozone increases in the future should lead to a pronounced increase in the decadal rate of change of anthropogenic greenhouse forcing of the next few decades, perhaps to levels unprecedented in this century.