Increase In Tropospheric Ozone Research Articles

Influence of the Brewer‐Dobson circulation on stratosphere‐troposphere exchange

A three‐dimensional model of dynamics and photochemistry is used to investigate how transfers of mass and chemical constituents between the stratosphere and troposphere are influenced by changes of the residual mean circulation. Under climatological‐mean conditions, the integration recovers a transfer of ozone into the extratropical troposphere that is consistent with the observed increase of tropospheric ozone during winter. Large at middle and high latitude, it reflects the downward transfer of stratospheric air, which is compensated at lower latitude by an upward transfer of tropospheric air. Changes representative of those observed between years modulate these transfers by ∼10%. The structure at low latitude, however, is sensitive to the quasi‐biennial oscillation's residual circulation, which interferes with the upwelling that must return mass to the stratosphere at the same rate that it is rejected at middle and high latitude. These exchanges of mass couple the circulation in the troposphere to that in the stratosphere, implying the involvement of the Hadley circulation.

Long-term changes in the tropospheric column ozone from the ozone soundings over Europe

The tropospheric column of ozone is analyzed from the measurements of the vertical profile of ozone by balloon-born ozonesondes. The data base includes ∼16,000 ozone profiles collected above six European stations—three of them have begun the ozonesoundings since 1970. We present a trend analysis (with data up to 2005) focusing on detection of the long-term tropospheric ozone variability over the European network. The ozone time series have been examined separately for each station and season (DJF, MAM, JJA, SON) using a flexible trend model. A trend component of the model is taken as a smooth curve without a priori defined shape. A large increase in the European tropospheric ozone since the beginning of the 1970s (net change of ∼10% in summers and ∼30% in winters) and a kind of stabilization in the early 1990s have been corroborated by the study. This pattern comes from the most extensive data set of ozonesoundings over Hohenpeissenberg and Payern. The decadal differences in the trend pattern between these and other European stations are disclosed. The results of a stepwise regression model using various characteristics of the ozone and temperature profiles as explanatory variables for the tropospheric column ozone (TCO 3) variations show that the ozone changes may be reconstructed using the ozone mixing ratio at 500 hPa, the thermal tropopause (TT) height, and the difference between ozonepause and TT heights. It appears that the last two factors induce 20–30% of the net TCO 3 change over Hohenpeissenberg in the 1970–2004 period.

Role of tropospheric ozone increases in 20th‐century climate change

Human activities have increased tropospheric ozone, contributing to 20th‐century warming. Using the spatial and temporal distribution of precursor emissions, we simulated tropospheric ozone from 1890 to 1990 using the NASA Goddard Institute for Space Studies (GISS) chemistry model. Archived three‐dimensional ozone fields were then used in transient GISS climate model simulations. This enables more realistic evaluation of the impact of tropospheric ozone increases than prior simulations using an interpolation between preindustrial and present‐day ozone. We find that tropospheric ozone contributed to the greater 20th‐century warming in the Northern Hemisphere extratropics compared with the tropics and in the tropics compared with the Southern Hemisphere extratropics. Additionally, ozone increased more rapidly during the latter half of the century than the former, causing more rapid warming during that time. This is especially apparent in the tropics and is consistent with observations, which do not show similar behavior in the extratropics. Other climate forcings do not substantially accelerate warming rates in the tropics relative to other regions. This suggests that accelerated tropospheric ozone increases related to industrialization in the developing world have contributed to the accelerated tropical warming. During boreal summer, tropospheric ozone causes enhanced warming (>0.5°C) over polluted northern continental regions. Finally, the Arctic climate response to tropospheric ozone increases is large during fall, winter, and spring when ozone's lifetime is comparatively long and pollution transported from midlatitudes is abundant. The model indicates that tropospheric ozone could have contributed about 0.3°C annual average and about 0.4°C–0.5°C during winter and spring to the 20th‐century Arctic warming. Pollution controls could thus substantially reduce the rapid rate of Arctic warming.

Effects of elevated concentrations of atmospheric CO2 and tropospheric O3 on decomposition of fine roots

Rising atmospheric carbon dioxide (CO2) concentration ([CO2]) could alter terrestrial carbon (C) cycling by affecting plant growth, litter chemistry and decomposition. How the concurrent increase in tropospheric ozone (O3) concentration ([O3]) will interact with rising atmospheric [CO2] to affect C cycling is unknown. A major component of carbon cycling in forests is fine root production, mortality and decomposition. To better understand the effects of elevated [CO2] and [O3] on the dynamics of fine root C, we conducted a combined field and laboratory incubation experiment to monitor decomposition dynamics and changes in fine root litter chemistry. Free-air CO2 enrichment (FACE) technology at the FACTS-II Aspen FACE project in Rhinelander, Wisconsin, elevated [CO2] (535 microl 1-1) and [O3] (53 nl 1-1) in intact stands of pure trembling aspen (Populus tremuloides Michx.) and in mixed stands of trembling aspen plus paper birch (Betula papyrifera Marsh.) and trembling aspen plus sugar maple (Acer saccharum Marsh.). We hypothesized that the trees would react to increased C availability (elevated [CO2]) by increasing allocation to C-based secondary compounds (CBSCs), thereby decreasing rates of decomposition. Because of its lower growth potential, we reasoned this effect would be greatest in the aspen-maple community relative to the aspen and aspen-birch communities. As a result of decreased C availability, we expected elevated [O3] to counteract shifts in C allocation induced by elevated [CO2]. Concentrations of CBSCs were rarely significantly affected by the CO2 and O3 treatments in decomposing fine roots. Rates of microbial respiration and mass loss from fine roots were unaffected by the treatments, although the production of dissolved organic C differed among communities. We conclude that elevated [CO2] and [O3] induce only small changes in fine root chemistry that are insufficient to significantly influence fine root decomposition. If changes in soil C cycling occur in the future, they will most likely be brought about by changes in litter production.

U.S. nitrogen science plan focuses collaborative efforts

Nitrogen is a major nutrient in terrestrial ecosystems and an important catalyst in tropospheric photochemistry. Over the last century human activities have dramatically increased inputs of reactive nitrogen (Nr, the combination of oxidized, reduced, and organically bound nitrogen) to the Earth system (Figure 1). Nitrogen cycle perturbations have compromised air quality and human health, acidified ecosystems, and degraded and eutrophied lakes and coastal estuaries [Vitousek et al., 1997a, 1997b; Rabalais, 2002; Howarth et al., 2003; Townsend et al., 2003; Galloway et al., 2004].Increased Nr affects global climate. Use of agricultural fertilizers such as ammonium nitrate leads to increased soil production of nitrous oxide (N2O), which has 320 times the global warming potential of carbon dioxide (CO2). Emission of nitrogen oxides (NOx = nitric oxide, NO + nitrogen dioxide, NO2) from fossil fuel burning leads to increases in tropospheric ozone, another greenhouse gas. Ozone is phytotoxic, and may reduce terrestrial CO2 sequestration. To predict the effects of nitrogen cycling changes under changing climatic conditions, there needs to be a better understanding of the global nitrogen budget.

RNA interference-based (RNAi) suppression of AtMPK6, an Arabidopsis mitogen-activated protein kinase, results in hypersensitivity to ozone and misregulation of AtMPK3

The recent increase in tropospheric ozone (O 3) concentrations promotes additional oxidative stress through the production of reactive oxygen species (ROS) in plant tissues, resulting in the activation of genes whose products enable the stressed cells to retain their integrity and function. This response is made possible by an integration of highly regulated signaling networks that mediate the perception of, and response to, this oxidative assault. In Arabidopsis thaliana, ROS-induced signaling has been shown to flow through a protein phosphorylation cascade involving the mitogen-activated protein kinases (MAPKs) AtMPK3 (MPK3) and AtMPK6 (MPK6). We found that RNAi-mediated silencing of MPK6 renders the plant more sensitive to ozone, as determined by visible leaf damage. The MPK6-RNAi genotype also displayed a more intense and prolonged activation of MPK3 compared to that of WT plants. An MPK3 loss-of-function genotype is similarly very sensitive to ozone, and displays an abnormally prolonged MPK6 activation profile, suggesting reciprocity in regulation between these two MAPKs.

Tropospheric ozone climatology over Irene, South Africa, from 1990 to 1994 and 1998 to 2002

Ozonesonde measurements over Irene in South Africa are reported for the period 1990 to 1994 and a more recent period, 1998 to 2002, when the station became part of the Southern Hemisphere Additional Ozonesondes (SHADOZ) network. Irene displays the characteristic Southern Hemisphere springtime tropospheric ozone maximum, but its seasonal features are modulated by both tropical and midlatitude influences because of its location (25°54′S, 28°13′E) on the boundary of zonally defined meteorological regimes. The tropical savanna biomass burning signature, namely, the spring maximum, is less distinct in the lower troposphere than at stations closer to biomass burning source regions nearer the equator, although long‐range transport and recirculation in the subtropical anticyclonic gyre over southern Africa permit the buildup of relatively high springtime midtropospheric ozone. Midlatitude dynamical influences are evident, predominantly in winter when upper tropospheric ozone is enhanced as a result of stratospheric‐tropospheric injection of ozone. Mean tropospheric ozone values range between 40 and 60 ppbv throughout the year and increase by ∼20 ppbv in spring. The increase (∼10 ppbv) in surface and lower tropospheric ozone between the two time periods is attributed to an increase in urban‐industrial emissions. A classification of ozone profiles using a cluster analysis has enabled the delineation of a background and “most polluted” profile. Enhancements of at least 30% occur throughout the troposphere in spring, and in certain layers, increases close to 100% are observed.

Influence of stratospheric airmasses on tropospheric vertical O<sub>3</sub> columns based on GOME (Global Ozone Monitoring Experiment) measurements and backtrajectory calculation over the Pacific

Abstract. Satellite based GOME (Global Ozone Measuring experiment) data are used to characterize the amount of tropospheric ozone over the tropical Pacific. Tropospheric ozone was determined from GOME data using the Tropospheric Excess Method (TEM). In the tropical Pacific a significant seasonal variation is detected. Tropospheric excess ozone is enhanced during the biomass burning season from September to November due to outflow from the continents. In September 1999 GOME data reveal an episode of increased excess ozone columns over Tahiti (18.0° S; 149.0° W) (Eastern Pacific) compared to Am. Samoa (14.23° S; 170.56° W) and Fiji (18.13° S; 178.40° E), both situated in the Western Pacific. Backtrajectory calculations show that none of the airmasses arriving over the three locations experienced anthropogenic pollution (e. g. biomass burning). Consequently other sources of ozone have to be considered. One possible process leading to an increase of tropospheric ozone is stratosphere-troposphere-exchange. An analysis of the potential vorticity along trajectories arriving above each of the locations reveals that airmasses at Tahiti are subject to enhanced stratospheric influence, compared to Am. Samoa and Fiji. As a result this study shows clear incidents of transport of airmasses from the stratosphere into the troposphere.

Elevated CO 2 and O 3t concentrations differentially affect selected groups of the fauna in temperate forest soils

Rising atmospheric CO 2 concentrations may change soil fauna abundance. How increase of tropospheric ozone (O 3t) concentration will modify these responses is still unknown. We have assessed independent and interactive effects of elevated [CO 2] and [O 3t] on selected groups of soil fauna. The experimental design is a factorial arrangement of elevated [CO 2] and [O 3t] treatments, applied using Free-Air CO 2 Enrichment technology to 30 m diameter rings, with all treatments replicated three times. Within each ring, three communities were established consisting of: (1) trembling aspen ( Populus tremuloides) and paper birch ( Betula papyrifera) (2) trembling aspen and sugar maple ( Acer saccharum) and (3) trembling aspen. After 4 yr of stand development, soil fauna were extracted in each ring. Compared to the control, abundance of total soil fauna, Collembola and Acari decreased significantly under elevated [CO 2] (−69, −79 and −70%, respectively). Abundance of Acari decreased significantly under elevated [O 3t] (−47%). Soil fauna abundance was similar to the control under the combination of elevated [CO 2+O 3t]. The individual negative effects of elevated [CO 2] and elevated [O 3t] are negated upon exposure to both gases. We conclude that soil fauna communities will change under elevated [CO 2] and elevated [O 3t] in ways that cannot be predicted or explained from the exposure of ecosystems to each gas individually.

Climate response to the increase in tropospheric ozone since preindustrial times: A comparison between ozone and equivalent CO2 forcings

We examine the characteristics of the climate response to anthropogenic changes in tropospheric ozone. Using a general circulation model, we have carried out a pair of equilibrium climate simulations with realistic present‐day and preindustrial ozone distributions. We find that the instantaneous radiative forcing of 0.49 W m−2 due to the increase in tropospheric ozone since preindustrial times results in an increase in global mean surface temperature of 0.28°C. The increase is nearly 0.4°C in the Northern Hemisphere and about 0.2°C in the Southern Hemisphere. The largest increases (>0.8°C) are downwind of Europe and Asia and over the North American interior in summer. In the lower stratosphere, global mean temperatures decrease by about 0.2°C due to the diminished upward flux of radiation at 9.6 μm. The largest stratospheric cooling, up to 1.0°C, occurs over high northern latitudes in winter, with possibly important implications for the formation of polar stratospheric clouds. To identify the characteristics of climate forcing unique to tropospheric ozone, we have conducted two additional climate equilibrium simulations: one in which preindustrial tropospheric ozone concentrations were increased everywhere by 18 ppb, producing the same global radiative forcing as present‐day ozone but without the heterogeneity; and one in which CO2 was decreased by 25 ppm relative to present day, with ozone at present‐day values, to again produce the same global radiative forcing but with the spectral signature of CO2 rather than ozone. In the first simulation (uniform increase of ozone), the global mean surface temperature increases by 0.25°C, with an interhemispheric difference of only 0.03°C, as compared with nearly 0.2°C for the heterogeneous ozone increase. In the second simulation (equivalent CO2), the global mean surface temperature increases by 0.36°C, 30% higher than the increase from tropospheric ozone. The stronger surface warming from CO2 is in part because CO2 forcing (obscured by water vapor) is shifted relatively poleward where the positive ice‐albedo feedback amplifies the climate response and in part because the magnitude of the CO2 forcing in the midtroposphere is double that of ozone. However, we find that CO2 is far less effective than tropospheric ozone in driving lower stratospheric cooling at high northern latitudes in winter.

Reanalysis of total ozone measurements at Dombås and Oslo, Norway, from 1940 to 1949

Total ozone measurements from Dobson spectrometer number 8 at Dombås, Norway (62.1°N, 9.1°E), and Oslo, Norway (59.9°N, 10.7°E), from 1940 to 1949 have been examined and reanalyzed. New sets of Bass‐Paur absorption and Rayleigh scattering coefficients have been created, and total ozone values have been recalculated using the new coefficients. Approximately half of the ozone registrations at Dombås were based on direct Sun observations calculated from the CC′ wavelength pairs. The long C′ pair has provided valuable information about the effect of atmospheric aerosols on the ozone measurements. A method for determining aerosol corrections is presented which demonstrates that the monthly mean aerosol error can reach 4% total ozone, normally with a higher correction in the summer than in the winter. Also, the influence of SO2 on the Oslo ozone measurements is estimated. The D8 ozone series from 1940 to 1949 has been compared to ozone records from other European stations and to the D56 ozone series from Oslo 1978–1998. Studies of old and new Dobson data demonstrate that the annual variation in the ozone layer has changed during the last 50–60 years. The comparison indicates that the ozone decrease is relatively small for the summer months (2.9 ± 1.8%), whereas the average winter and spring values have deceased by 6.1 ± 3.9% from the 1940s to the 1990s. If the postwar increase in tropospheric ozone is taken into account, the depletion of the ozone layer is considerably higher, probably 8–9% for winter/spring. All the reanalyzed total ozone data from D8 for the period 1940 to 1949 are available at the World Ozone and Ultraviolet Radiation Data Center.

Atmospheric Chemistry in a Changing World

The world is changing,and the atmosphere's composition is changing with it. Human activity is responsible for much of this. Global population growth and migration to urban centers, extensive biomass burning, the spread of fertilizer‐intensive agribusiness, globalization of business and industry, rising standards of living in the developing world, and increased energy use fuels atmospheric change. If current practices continue, atmospheric increases are likely for the greenhouse gases carbon dioxide, methane, nitrous oxide; and for the chemically active gases nitric oxide, sulfur dioxide,and ammonia. Increases in global tropospheric ozone and aerosols are a distinct possibility.

Preindustrial-to-present-day radiative forcing by tropospheric ozone from improved simulations with the GISS chemistry-climate GCM

Abstract. Improved estimates of the radiative forcing from tropospheric ozone increases since the preindustrial have been calculated with the tropospheric chemistry model used at the Goddard Institute for Space Studies (GISS) within the GISS general circulation model (GCM). The chemistry in this model has been expanded to include simplified representations of peroxyacetylnitrates and non-methane hydrocarbons in addition to background NOx-HOx-Ox-CO-CH4 chemistry. The GCM has improved resolution and physics in the boundary layer, improved resolution near the tropopause, and now contains a full representation of stratospheric dynamics. Simulations of present-day conditions show that this coupled chemistry-climate model is better able to reproduce observed tropospheric ozone, especially in the tropopause region, which is critical to climate forcing. Comparison with preindustrial simulations gives a global annual average radiative forcing due to tropospheric ozone increases of 0.30 W/m2 with standard assumptions for preindustrial emissions. Locally, the forcing reaches more than 0.8 W/m2 in parts of the northern subtropics during spring and summer, and is more than 0.6 W/m2 through nearly all the Northern subtropics and mid-latitudes during summer. An alternative preindustrial simulation with soil NOx emissions reduced by two-thirds and emissions of isoprene, paraffins and alkenes from vegetation increased by 50% gives a forcing of 0.33 W/m2. Given the large uncertainties in preindustrial ozone amounts, the true value may lie well outside this range.

Analysis of 1970–1995 trends in tropospheric ozone at Northern Hemisphere midlatitudes with the GEOS‐CHEM model

The causes of trends in tropospheric ozone at Northern Hemisphere midlatitudes from 1970 to 1995 are investigated with the GEOS‐CHEM model, a global three‐dimensional model of the troposphere driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS). This model is used to investigate the sensitivity of tropospheric ozone with respect to (1) changes in the anthropogenic emission of nitrogen oxides and nonmethane hydrocarbons, (2) increases in methane concentrations, (3) variations in the stratospheric source of ozone, (4) changes in solar radiation resulting from stratospheric ozone depletion, and (5) increases in tropospheric temperatures. Model results indicate that local increases in NOx emissions have caused most of the increases seen in lower tropospheric ozone over Europe and Japan. Increases in methane are responsible for roughly one fifth of the anthropogenically induced increase in tropospheric ozone at northern midlatitudes. However, changes in ozone precursors do not adequately explain either the spatial differences in observed ozone trends across midlatitudes or the observed decreases in ozone over Canada throughout the troposphere. We argue that ozone depletion in the lowermost stratosphere is likely to have reduced the stratospheric source by as much as 30% from the early 1970s to the mid 1990s. Model simulations that account for such a reduction along with reported changes in anthropogenic emissions show steep declines of ozone in the upper troposphere and variable increases in the lower troposphere that are more consistent with observations. Differential temperature trends in summer between North America and Europe may account for at least some of the remaining spatial variation in tropospheric ozone trends. Increases in ultraviolet (UV) radiation due to stratospheric ozone depletion do not appear to significantly reduce tropospheric ozone, except at midlatitudes in the Southern Hemisphere following the breakup of the ozone hole.

Observations of stratosphere‐to‐troposphere transport events over the eastern Mediterranean using a ground‐based lidar system

In order to cover a substantial amount of stratospheric intrusions during the project Influence of Stratosphere‐Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), coordinated measurements were carried out at several locations in central and southern Europe, based on quasi‐operational stratosphere‐troposphere exchange forecasts. In this context, lidar measurements of tropospheric ozone were performed at Thessaloniki (23°E, 40.5°N), Greece, for the investigation of stratosphere‐to‐troposphere transport (STT) events over the southeastern Mediterranean region, during 2000–2002. The study of STT in this area is of particular interest not only because of its geographic location, which is more southern than the typical position of the polar front jet, but also because of the sparseness of detailed studies in this area. A summary of the main characteristics of the STT events that were detected by lidar during the investigation period reveals a tropospheric ozone increase of the order of 10% between 4.5 and 6.5 km above sea level, a coincident decrease in relative humidity, and elevated values of potential vorticity, thus providing a direct indication of the occurrence of stratospheric air in the middle troposphere. No direct effect on surface ozone was recorded. Furthermore, the majority of the STT cases detected reveal a common pathway of the stratospheric air masses that reached Thessaloniki originating from the North Sea. In addition, the event of 9 January 2001, during which the clearest and strongest descent of stratospheric air occurred, is further analyzed. An ozone‐rich layer of the order of 60–90 ppbv between 5 and 6.5 km above sea level is clearly depicted, which was the result of the intermixture of originally stratospheric and boundary layer air originating from North America.

Design and Analysis of Climate Model Experiments for the Efficient Estimation of Anthropogenic Signals

Abstract Presented herein is an experimental design that allows the effects of several radiative forcing factors on climate to be estimated as precisely as possible from a limited suite of atmosphere-only general circulation model (GCM) integrations. The forcings include the combined effect of observed changes in sea surface temperatures, sea ice extent, stratospheric (volcanic) aerosols, and solar output, plus the individual effects of several anthropogenic forcings. A single linear statistical model is used to estimate the forcing effects, each of which is represented by its global mean radiative forcing. The strong colinearity in time between the various anthropogenic forcings provides a technical problem that is overcome through the design of the experiment. This design uses every combination of anthropogenic forcing rather than having a few highly replicated ensembles, which is more commonly used in climate studies. Not only is this design highly efficient for a given number of integrations, but it also allows the estimation of (nonadditive) interactions between pairs of anthropogenic forcings. The simulated land surface air temperature changes since 1871 have been analyzed. The changes in natural and oceanic forcing, which itself contains some forcing from anthropogenic and natural influences, have the most influence. For the global mean, increasing greenhouse gases and the indirect aerosol effect had the largest anthropogenic effects. It was also found that an interaction between these two anthropogenic effects in the atmosphere-only GCM exists. This interaction is similar in magnitude to the individual effects of changing tropospheric and stratospheric ozone concentrations or to the direct (sulfate) aerosol effect. Various diagnostics are used to evaluate the fit of the statistical model. For the global mean, this shows that the land temperature response is proportional to the global mean radiative forcing, reinforcing the use of radiative forcing as a measure of climate change. The diagnostic tests also show that the linear model was suitable for analyses of land surface air temperature at each GCM grid point. Therefore, the linear model provides precise estimates of the space–time signals for all forcing factors under consideration. For simulated 50-hPa temperatures, results show that tropospheric ozone increases have contributed to stratospheric cooling over the twentieth century almost as much as changes in well-mixed greenhouse gases.

Radiative Forcing Due to Reactive Gas Emissions

Reactive gas emissions (CO, NOx, VOC) have indirect radiative forcing effects through their influences on tropospheric ozone and on the lifetimes of methane and hydrogenated halocarbons. These effects are quantified here for the full set of emissions scenarios developed in the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. In most of these no-climate-policy scenarios, anthropogenic reactive gas emissions increase substantially over the twenty-first century. For the implied increases in tropospheric ozone, the maximum forcing exceeds 1 W m−2 by 2100 (range −0.14 to +1.03 W m−2). The changes are moderated somewhat through compensating influences from NOx versus CO and VOC. Reactive gas forcing influences through methane and halocarbons are much smaller; 2100 ranges are −0.20 to +0.23 W m−2 for methane and −0.04 to +0.07 W m−2 for the halocarbons. Future climate change might be reduced through policies limiting reactive gas emissions, but the potential for explicitly climate-motivated reductions depends critically on the extent of reductions that are likely to arise through air quality considerations and on the assumed baseline scenario.

Evolution of tropospheric ozone under anthropogenic activities and associated radiative forcing of climate

The budget of ozone and its evolution associated with anthropogenic activities are simulated with the Model for Ozone and Related Chemical Tracers (MOZART) (version 1). We present the changes in tropospheric ozone and its precursors (CH4, NMHCs, CO, NOx) since the preindustrial period. The ozone change at the surface exhibits a maximum increase at midlatitudes in the northern hemisphere reaching more than a factor of 3 over Europe, North America, and Southeast Asia during summer. The calculated preindustrial ozone levels are particularly sensitive to assumptions about natural and biomass burning emissions of precursors. The possible future evolution of ozone to the year 2050 is also simulated, using the Intergovernmental Panel on Climate Change IS92a scenario to estimate the global and geographical changes in surface emissions. The future evolution of ozone stresses the important role played by the tropics and the subtropics. In this case a maximum ozone increase is calculated in the northern subtropical region and is associated with increased emissions in Southeast Asia and Central America. The ozone future evolution also affects the more remote regions of the troposphere, and an increase of 10–20% in background ozone levels is calculated over marine regions in the southern hemisphere. Our best estimate of the global and annual mean radiative forcing associated with tropospheric ozone increase since the preindustrial era is 0.43 W m−2. This value represents about 20% of the forcing associated with well‐mixed greenhouse gases. The normalized tropospheric ozone radiative forcing is 0.048 W m−2 DU−1. An upper estimate on our forcing of 0.77 W m−2 is calculated when a stratospheric tracer is used to approximate background ozone levels. In 2050 an additional ozone forcing of 0.26 W m−2 is calculated, providing a forcing from preindustrial to 2050 of 0.69 W m−2.

Is the climate sensitivity to ozone perturbations enhanced by stratospheric water vapor feedback?

The climate response to a set of idealized ozone perturbations is investigated by integrations with a coupled atmosphere‐ocean model. Although all perturbations, including a homogeneous CO2 increase, induce the same stratosphere adjusted, tropopause radiative forcing, the climate response is quite variable within the set of experiments. Except for an upper tropospheric ozone increase, our model is more sensitive to ozone perturbations than to an equivalent CO2 perturbation. This applies in particular to a lower stratospheric ozone increase. The accompanying changes in the stratospheric water vapor (SWV) distribution are found to impose additional forcings on climate that may well exceed the forcings due to the original perturbations. Without SWV feedback on radiation the climate sensitivity to a lower stratospheric ozone increase draws remarkably near the respective value for equivalent CO2. This emphasizes the crucial role SWV may have in the forcing‐response relationship.

Atmospheric Ozone and Climate Change

This paper reviews the evolution of our understanding of the impact of changes in atmospheric ozone on the Earth's climate. Ozone has an impact on climate because it absorbs radiation in the ultra-violet, visible and infrared regions of the spectrum. There has been uncertainty whether changes in stratospheric ozone act to cool or warm the climate, because of the competing impact of changes at different wavelengths. It is now believed that the observed changes in stratospheric ozone over the past two decades have acted to cool die climate, offsetting around 20% of the warming effect due to increases in carbon dioxide and other greenhouse gases over the same period. Increases in tropospheric ozone are believed to have caused a warming effect which is 15% of that due to increases in carbon dioxide and other greenhouse gases over the same period.