Total Ozone Trends Research Articles

Simulated lower stratospheric trends between 1970 and 2005: Identifying the role of climate and composition changes

We have analyzed a set of simulations aimed at understanding the mechanisms that drive observed trends in the lower stratosphere after 1970. The simulations were performed using a version of the Community Atmosphere Model version 3 (CAM3) updated with interactive tropospheric and stratospheric chemistry. Even with a relatively low model top (≈40 km), this model shows good ability at reproducing a variety of large‐scale changes in climate and chemical composition in the stratosphere when forced with the observed sea‐surface temperatures and surface concentrations of long‐lived trace gases and ozone‐depleting substances. We then used the same model framework to differentiate the role of chemically active composition (ozone, methane, and chlorofluorocarbons) and CO2 changes on observed trends in the stratosphere. Among the sensitivity factors analyzed, our simulations indicate that changes in CO2 over the simulated period do not lead to significantly different total ozone trend; however, changes in CO2 lead to important differences in ozone in the upper part of the model. On the other hand, changes in surface methane concentration are shown to play a significant role in driving changes in the globally averaged total ozone column, through a combination of changes in tropospheric and stratospheric ozone columns. We also show that the correlation between a change in tropical mean age of air and in vertical velocity breaks down above 20 hPa, in association with increased isentropic mixing above that level. Finally, we show that our model is capable of reproducing trends in the tropical age of air that were found in other studies; our simulations also indicate a significant impact of keeping methane and ozone‐depleting substances at their 1970 levels, indicating the potentially important role of controlling methane emissions.

Variability of the total ozone trend over Europe for the period 1950–2004 derived from reconstructed data

Abstract. The total ozone data over Europe are available for only few ground-based stations in the pre-satellite era disallowing examination of the spatial trend variability over the whole continent. A need of having gridded ozone data for a trend analysis and input to radiative transfer models stimulated a reconstruction of the daily ozone values since January 1950. Description of the reconstruction model and its validation were a subject of our previous paper. The data base used was built within the objectives of the COST action 726 "Long-term changes and climatology of UV radiation over Europe". Here we focus on trend analyses. The long-term variability of total ozone is discussed using results of a flexible trend model applied to the reconstructed total ozone data for the period 1950–2004. The trend pattern, which comprises both anthropogenic and "natural" component, is not a priori assumed but it comes from a smooth curve fit to the zonal monthly means and monthly grid values. The ozone long-term changes are calculated separately for cold (October–next year April) and warm (May–September) seasons. The confidence intervals for the estimated ozone changes are derived by the block bootstrapping. The statistically significant negative trends are found almost over the whole Europe only in the period 1985–1994. Negative trends up to −3% per decade appeared over small areas in earlier periods when the anthropogenic forcing on the ozone layer was weak . The statistically positive trends are found only during warm seasons 1995–2004 over Svalbard archipelago. The reduction of ozone level in 2004 relative to that before the satellite era is not dramatic, i.e., up to ~−5% and ~−3.5% in the cold and warm subperiod, respectively. Present ozone level is still depleted over many popular resorts in southern Europe and northern Africa. For high latitude regions the trend overturning could be inferred in last decade (1995–2004) as the ozone depleted areas are not found there in 2004 in spite of substantial ozone depletion in the period 1985–1994.

Recalculation of 11‐year total ozone of Brewer spectrophotometer 115

Ground‐based total ozone (O3) has been measured by Brewer 115 at Hong Kong (Cape D'Aguilar, 22°13′N, 114°15′E, 64 m above mean sea level). Long‐term observations between 1995 and 2005 show that the sensitivity of outdoor spectrophotometers exhibit long‐term drift. Despite the undesirable sensitivity shift, high‐quality total ozone observations can still be achieved so long as the spectrophotometer is well maintained and calibrated regularly, and appropriate corrections applied to the data. The recalculated ground‐based total ozone also shows good agreement with the satellite (Total Ozone Mapping Spectrometer (TOMS) version 8) data and Brewer 61 (Chengkung: 23°05′, 121°21′). The procedures and experience of a total ozone trend analysis using multiple linear regression that account for upper level temperatures, QBO, AOD, sun spot activities, and El Niño were reported for future reference. It can be concluded that no ozone depletion was observed over Hong Kong in this time period.

Impact of long‐range correlations on trend detection in total ozone

Total ozone trends are typically studied using linear regression models that assume a first‐order autoregression of the residuals [so‐called AR(1) models]. We consider total ozone time series over 60°S–60°N from 1979 to 2005 and show that most latitude bands exhibit long‐range correlated (LRC) behavior, meaning that ozone autocorrelation functions decay by a power law rather than exponentially as in AR(1). At such latitudes the uncertainties of total ozone trends are greater than those obtained from AR(1) models and the expected time required to detect ozone recovery correspondingly longer. We find no evidence of LRC behavior in southern middle‐and high‐subpolar latitudes (45°–60°S), where the long‐term ozone decline attributable to anthropogenic chlorine is the greatest. We thus confirm an earlier prediction based on an AR(1) analysis that this region (especially the highest latitudes, and especially the South Atlantic) is the optimal location for the detection of ozone recovery, with a statistically significant ozone increase attributable to chlorine likely to be detectable by the end of the next decade. In northern middle and high latitudes, on the other hand, there is clear evidence of LRC behavior. This increases the uncertainties on the long‐term trend attributable to anthropogenic chlorine by about a factor of 1.5 and lengthens the expected time to detect ozone recovery by a similar amount (from ∼2030 to ∼2045). If the long‐term changes in ozone are instead fit by a piecewise‐linear trend rather than by stratospheric chlorine loading, then the strong decrease of northern middle‐ and high‐latitude ozone during the first half of the 1990s and its subsequent increase in the second half of the 1990s projects more strongly on the trend and makes a smaller contribution to the noise. This both increases the trend and weakens the LRC behavior at these latitudes, to the extent that ozone recovery (according to this model, and in the sense of a statistically significant ozone increase) is already on the verge of being detected. The implications of this rather controversial interpretation are discussed.

The impact of interannual variability on multidecadal total ozone simulations

We have used a two‐dimensional chemistry and transport model to study the effects of interannual dynamical variability on global total ozone for 1979–2004. Long‐term meteorological data from the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis‐2 project and the European Center for Medium Range Weather Forecasts (ECMWF) updated reanalysis (ERA‐40) are used to construct yearly dynamical fields for use in the model. The simulations qualitatively resolve much of the seasonal and interannual variability observed in long‐term global total ozone data, including fluctuations related to the quasi‐biennial oscillation (QBO). We performed a series of model experiments to examine the relative roles of the interannual variability, changes in halogen and volcanic aerosol loading, and the 11‐year solar cycle in controlling the multidecadal ozone changes. Statistical regression is used to isolate these signals in the observed and simulated time series. At Northern midlatitudes, the simulated interannual dynamical variability acts to reinforce the chemical ozone depletion caused by the enhanced aerosol loading following the eruption of Mt. Pinatubo in 1991. However, at Southern midlatitudes, the interannual variability masks the aerosol‐induced chemical effect. The simulated solar cycle response in total ozone is generally consistent with observations and is primarily due to the direct photochemical effect. The halogen‐induced total ozone trend for 1979–1996 derived from the model is in good agreement with that derived from observations in the tropics and the Northern Hemisphere (NH). However, at Southern midlatitudes, the trend derived from the model is more sensitive to halogen loading than that derived from observations.

Assessment of temperature, trace species, and ozone in chemistry‐climate model simulations of the recent past

Simulations of the stratosphere from thirteen coupled chemistry‐climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long‐term trends are compared for an extended period (1960–2004). Comparisons of polar high‐latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long‐term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

The total ozone field separated into meteorological regimes – Part II: Northern Hemisphere mid-latitude total ozone trends

Abstract. Previous studies have presented clear evidence that the Northern Hemisphere total ozone field can be separated into distinct regimes (tropical, midlatitude, polar, and arctic) the boundaries of which are associated with the subtropical and polar upper troposphere fronts, and in the winter, the polar vortex. This paper presents a study of total ozone variability within these regimes, from 1979–2003, using data from the TOMS instruments. The change in ozone within each regime for the period January 1979–May 1991, a period of rapid total ozone change, was studied in detail. Previous studies had observed a zonal linear trend of −3.15% per decade for the latitude band 25°–60° N. When the ozone field is separated by regime, linear trends of −1.4%, 2.3%, and 3.0%, per decade for the tropical, midlatitude, and polar regimes, respectively, are observed. The changes in the relative areas of the regimes were also derived from the ozone data. The relative area of the polar regime decreased by about 20%; the tropical regime increased by about 10% over this period. No significant change was detected for the midlatitude regime. From the trends in the relative area and total ozone it is deduced that 35% of the trend between 25° and 60° N, from January 1979–May 1991 is due to movement of the upper troposphere fronts. The changes in the relative areas can be associated with a change in the mean latitude of the subtropical and polar fronts within the latitude interval 25° to 60° N. Over the period from January 1979 to May 1991, both fronts moved northward by 1.1±0.2 degrees per decade. Over the entire period of the study, 1979–2003, the subtropical front moved northward at a rate of 1.1±0.1 degrees per decade, while the polar front moved by 0.5±0.1 degrees per decade.

Comparison of recent modeled and observed trends in total column ozone

We present a comparison of trends in total column ozone from 10 two‐dimensional and 4 three‐dimensional models and solar backscatter ultraviolet–2 (SBUV/2) satellite observations from the period 1979–2003. Trends for the past (1979–2000), the recent 7 years (1996–2003), and the future (2000–2050) are compared. We have analyzed the data using both simple linear trends and linear trends derived with a hockey stick method including a turnaround point in 1996. If the last 7 years, 1996–2003, are analyzed in isolation, the SBUV/2 observations show no increase in ozone, and most of the models predict continued depletion, although at a lesser rate. In sharp contrast to this, the recent data show positive trends for the Northern and the Southern Hemispheres if the hockey stick method with a turnaround point in 1996 is employed for the models and observations. The analysis shows that the observed positive trends in both hemispheres in the recent 7‐year period are much larger than what is predicted by the models. The trends derived with the hockey stick method are very dependent on the values just before the turnaround point. The analysis of the recent data therefore depends greatly on these years being representative of the overall trend. Most models underestimate the past trends at middle and high latitudes. This is particularly pronounced in the Northern Hemisphere. Quantitatively, there is much disagreement among the models concerning future trends. However, the models agree that future trends are expected to be positive and less than half the magnitude of the past downward trends. Examination of the model projections shows that there is virtually no correlation between the past and future trends from the individual models.

Trends in laminae in ozone profiles in relation to trends in some other middle atmospheric parameters

Ozone profiles measured by ozonesondes often do not display a smooth shape, they exhibit relatively narrow layers of significantly enhanced or depressed ozone concentration, called positive and negative laminae, respectively. At northern middle latitudes, the overall ozone content in laminae per profile reveals a strong negative trend since the late 1960s till the early 1990s. The trend changes to a pronounced positive trend around the mid-1990s. The trend in total ozone at northern middle latitudes changes as that in laminae in the mid-1990s from negative to positive. This change of trend is more probably of dynamical than chemical origin. Zonal winds in the upper middle atmosphere near 52°N reveal a change of trend in the 1990s, as well. This indicates possibility that the overall Northern Hemisphere middle atmosphere dynamics could experience a change in long-term trends in the 1990s.

Trend analysis of total ozone data for turnaround and dynamical contributions

Statistical trend analyses have been performed for monthly zonal average total ozone data from both TOMS and SBUV satellite sources and ground‐based instruments over the period 1978–2002 for detection of a “turnaround” in the previous downward trend behavior and hence evidence for the beginning of an ozone recovery. Since other climatic and geophysical changes can impact ozone behavior and can influence the detection of turnaround and recovery, we also focus on accounting for ozone variations that may be ascribed to various physical and chemical influences. Thus we include in the statistical trend modeling and analysis the effects of various dynamical and circulation variations in the atmosphere, including those associated with the quasibiennial oscillation (QBO), Arctic Oscillation (AO) and Antarctic Oscillation (AAO), and Eliassen‐Palm (EP) flux influences, as well as influences of solar cycle. A notable result of the analysis is that for latitude zones of 40° and above in both hemispheres, large positive and significant estimates of a change in trend (since 1996) are obtained (on the order of 1.5 to 3 DU per year). The dynamic index series, AO/AAO and EP flux, are found to have a substantial influence on total ozone for these higher latitudes, and significant influences of lesser magnitude are also found for lower latitudes. The feature of positive significant change in trend in total ozone over recent years, however, is obtained both without and with the dynamical index terms included in the statistical models.

Summertime total ozone variations over middle and polar latitudes

The statistical relationship between springtime and summertime ozone over middle and polar latitudes is analyzed using zonally averaged total ozone data. Short‐term variations in springtime midlatitude ozone demonstrate only a modest correlation with springtime polar ozone variations. However by early summer, ozone variations throughout the extratropics are highly correlated. Analysis of correlation functions indicates that springtime midlatitude ozone, not polar ozone, is the best predictor for summertime polar ozone. Long‐term total ozone trends at middle and high latitudes are also different for spring and nearly identical for summer. About 39% of the observed southern midlatitude ozone decline in December can be attributed to the polar ozone depletion up to November. In the Northern Hemisphere, the corresponding contribution is about 15%, but the error bars are too large to make an accurate estimate.

Statistical analysis of total ozone measurements in Oslo, Norway, 1978–1998

The total ozone series from Dobson spectrophotometer 56 (D56) operating at the University of Oslo (59.9°N, 10.7°E), Norway, from March 1978 to May 1998 has recently been reevaluated. Ozone trends for the time series are calculated using multiple regression models with explanatory variables for the 11 year solar cycle (lagged 22 months), the 2.5 year Quasi‐Biennial Oscillation (QBO) cycle, stratospheric aerosol loading from volcanoes, detrended temperatures at 100 hPa and 500 hPa levels, El Niño–Southern Oscillation (ENSO) events, and various teleconnection patterns. Six of the teleconnection explanatory variables examined are significant to a 95% confidence level in describing the observed total ozone variations. The year‐round total ozone trend in Oslo (±SD) is estimated to −4.2 ± 0.4 % per decade, whereas the winter and summer trends amount to −6.2 ± 0.9 % per decade and −2.4 ± 0.5 % per decade, respectively. The analyses imply that climatic and dynamical changes account for up to a 20% increase in winter ozone trend and a 40% decrease in summer ozone trend. The East Atlantic Jet is the single most dominating teleconnection pattern in Oslo and contributes to about 0.7 percentage points per decade change in summer total ozone trend. The regression analyses demonstrate that the start and termination dates of the ozone record are highly important for the trend results: The summer and winter ozone trend can change by 0.9 percentage points per decade if 1 year is included or omitted from the analyses.

Long-term variations of the UV-B radiation over Central Europe as derived from the reconstructed UV time series

Abstract. The daily doses of the erythemally weighted UV radiation are reconstructed for three sites in Central Europe: Belsk-Poland (1966–2001), Hradec Kralove-Czech Republic (1964–2001), and Tõravere-Estonia (1967–2001) to discuss the UV climatology and the long-term changes of the UV-B radiation since the mid 1960s. Various reconstruction models are examined: a purely statistical model based on the Multivariate Adaptive Regression Splines (MARS) methodology, and a hybrid model combining radiative transfer model calculations with empirical estimates of the cloud effects on the UV radiation. Modeled long-term variations of the surface UV doses appear to be in a reasonable agreement with the observed ones. A simple quality control procedure is proposed to check the homogeneity of the biometer and pyranometer data. The models are verified using the results of UV observations carried out at Belsk since 1976. MARS provides the best estimates of the UV doses, giving a mean difference between the modeled and observed monthly means equal to 0.6±2.5%. The basic findings are: similar climatological forcing by clouds for all considered stations (~30% reduction in the surface UV), long-term variations in UV monthly doses having the same temporal pattern for all stations with extreme low monthly values (~5% below overall mean level) at the end of the 1970s and extreme high monthly values (~5% above overall mean level) in the mid 1990s, regional peculiarities in the cloud long-term forcing sometimes leading to extended periods with elevated UV doses, recent stabilization of the ozone induced UV long-term changes being a response to a trendless tendency of total ozone since the mid 1990s. In the case of the slowdown of the total ozone trend over Northern Hemisphere mid-latitudes it seems that clouds will appear as the most important modulator of the UV radiation both in long- and short-time scales over next decades. Key words. Atmospheric composition and structure (biospheric-atmosphere interaction) – Meteorology and atmospheric dynamics (climatology; radiative processes)

Reconstruction of erythemal UV irradiance and dose at Hohenpeissenberg (1968?2001) considering trends of total ozone, cloudiness and turbidity

Erythemal ultraviolet (UV) doses reaching the earth’s surface depend in a complex manner on the amount of total ozone, cloud cover, cloud type and the structure of the cloud field. A statistical model was developed allowing the reconstruction of UV from measured total ozone and a cloud modification factor (CMF) for the GAW site Hohenpeissenberg, Germany (48°N, 11°E). CMF is derived from solar global radiation G, normalized against a Rayleigh scattering atmosphere. By this way the complex influence of the cloud field is accounted for by introduction of a measured parameter, exposed also to this complex field. The statistical relations are derived from the period 1990–1998 where UV measurements and relevant meteorological parameters are available. With these relations daily UV doses could be reconstructed back to 1968. Tests show that the model works remarkably well even for time scales of a minute except for situations with high albedo. The comparison of measured and calculated UV irradiances shows that the model explains 97% of the variance for solar elevations above 18° on average over the period 1968–2001. The reconstruction back to 1968 indicates that maximum UV irradiances (clear days) have increased due to long-term ozone decline. Clouds show seasonally depending long-term changes, especially an increase of cirrus. Consequently the UV doses have increased less or even decreased in some months in comparison to the changes expected from the ozone decline alone. In May to August total cloud frequency and cloud cover have decreased. Therefore, the average UV doses have increased much more than can be explained by the ozone decline alone. It is also shown that the optical thickness of cirrus clouds has increased since 1953. The higher frequency of cirrus is caused in part by more frequent contrails. Besides that an observed long-term rise and cooling of the tropopause favors an easier cirrus formation. However, whether climate change and an intensification of the water cycle is responsible for the cirrus trends has not been investigated in detail.

Modeling the effect of ozone loss on photobleaching of chromophoric dissolved organic matter in the St. Lawrence estuary

Temporal trends in total ozone for the St. Lawrence estuary were estimated from ground‐based measurements at the NOAA/CMDL station in Caribou, Maine. Linear regression analysis showed that from 1979 to 1999 total ozone has decreased by about 3.3% per decade on an annual basis and ≤6.2% per decade on a monthly basis relative to unperturbed (pre‐CFC) levels. The influence of increased ultraviolet‐B (280–320 nm) radiation associated with ozone depletion on water column photochemical processes was evaluated by modeling the photobleaching of chromophoric dissolved organic material (CDOM). Linear regression analysis showed small (<0.5% per decade), but statistically significant upward trends in maximum noontime photobleaching rates. Most notably, positive trends in relative rates for May, June, and July, when maximum absolute rates are expected, were predicted. A global model based on TOMS ozone data revealed increases in photobleaching of ≤3% per decade at high latitudes in the Southern Hemisphere. Radiation amplification factors for increases in photochemically weighted UV (280–400 nm) in response to ozone depletion were estimated at 0.1 and 0.08 for photobleaching of CDOM absorbance at 300 and 350 nm, respectively. Application of the laboratory‐based model to conditions that more closely resembled those in situ were variable with both overestimation and underestimation of measured rates. The differences between modeled rates and observed rates under quasi‐natural conditions were as large or larger than the predicted increases due to ozone depletion. These comparisons suggest that biological activity and mixing play an important, but as yet ill‐defined, role in modifying photochemical processes.

On the longitude dependence of total ozone trends over middle-latitudes

It has recently been observed that the total ozone trends derived from certain geographical regions such as the Mediterranean and Athens (Greece) show similar values to those derived from the 40°N zonal averaged column ozone data. In this Letter, the total ozone concentration, collected by the Total Ozone Mapping Spectrometer (TOMS) flown on Nimbus-7 and Meteor-3 during the time-period January 1979-December 1993, as well as by Earth Probe during the time-period January 1997-May 2001, for the Mediterranean, Athens (Greece) and Srinagar (India), is analysed. Further, the harmonic analysis performed on total ozone time-series provides a proper tool to interpret the observed similarity in total ozone seasonal trends, which may probably be attributed to the effect of planetary waves on the ozone distribution.

Global and zonal total ozone variations estimated from ground‐based and satellite measurements: 1964–2000

Six data sets of monthly average zonal total ozone were intercompared and then used to estimate latitudinal and global total ozone temporal variations and trends. The data sets were prepared by different groups and are based on TOMS, SBUV‐SBUV/2, GOME, and ground‐based measurements. Different approaches have been used to homogenize the records over the period 1979–2000. Systematic differences of up to 3% were found between different data sets for zonal and global total ozone area weighted average values. However, when these systematic differences were removed by deseasonalizing the data, the residuals agreed to within ±0.5% of the long‐term mean ozone values. All data sets show changes in the rate of the total ozone decline in recent years. While global ozone was fairly constant during the 1990s, the average values of the 1990s are about 2–3% lower than those of the late 1970s. About 38% of the global ozone is located between 25°S and 25°N where the data show no decline. The strongest decline and the largest variability occur over the 35°N–60°N zone during the winter‐spring season with the largest negative deviations occurring in 1993 and 1995. The decline in autumn is much smaller at these latitudes. Over the 35°S–60°S zone the ozone decline shows less seasonal dependence, and the largest deviations there were observed in 1985 and 1997. Sliding 11‐year trends were calculated to estimate ozone changes over different time intervals. The first interval was from 1964 to 1974, and the last interval was from 1990 to 2000. The steepest year‐round trends, of up to −5% per decade, occurred in the 11‐year periods ending between 1992 and 1997 over the 35°–60°N zone and between 1985 and 1993 over the 35°–55°S zone. More recent 11‐year trends have smaller declines.

Very strong negative trends in laminae in ozone profiles

A very strong negative trend in the overall ozone content in the positive laminae in ozone profiles, a decrease by 50% during about 20–25 years, was found in the 1970s, 1980s and early 1990s in data of eight Northern Hemisphere ozone sounding stations. They were located at high and middle latitudes in Europe, Canada and Japan. Only the station Wallops Island in US ( 38 ° N ) revealed much smaller reduction of ozone in laminae. This means among others that the negative trend is basically the same for Resolute Bay at 75 ° N and Tateno at 36 ° N in spite of twice as high ozone content in laminae at Resolute Bay. The trends have been broadly studied for large laminae with the peak ozone concentration higher by 40 nbar and more than the background ozone concentration. However, analyses of laminae with the peak ozone concentration higher than 30 or 20 nbar against background for a few stations revealed similar strong negative trends. The trends are almost height independent, but the contribution of laminae to overall ozone content clearly peaks in the lowermost stratosphere. The overall ozone content in laminae has a strong seasonal variation (factor 5 or even more) with a maximum in late winter/early spring, when the negative trend in laminae may be responsible for as much as one third of the observed ozone depletion in European middle latitudes. Laminae seem to contribute substantially to seasonal variation of trends in total ozone.

Long-term ozone and UV variations at Thessaloniki, Greece

Total columnar ozone has been decreasing over middle latitudes (20°–60°) between 1979 and December 2000 at rates of about 4% in winter/spring and about 2% per decade in the summer. Total ozone trends through 1997 and through 2000 differ only over the middle and high latitudes of the northern hemisphere. This long-term trend was interrupted by periods of enhanced volcanic activity, which resulted to significant ozone losses (more than 5% per year) following large volcanic eruptions (El Chichon in 1982 and particularly Mt Pinatubo in 1991). Linked to the ozone decrease are well-documented global increases of the harmful UV-B solar radiation levels reaching ground level. Through 1997 the erythemal dose over middle latitudes in clear skies was increasing at rates exceeding 5% per decade while the solar UV irradiance at the lower wavelengths was increasing by more than 10% per decade, partly enhanced by the decrease of air pollutants such as sulfur dioxide in the case of Thessaloniki. However, evidence is presented in this study that during the last decade (1990–2001) over Thessaloniki, the ozone trend was smaller the order of −1.5% per decade. Also the associated increasing trends in the erythemal dose and at 305 nm are smaller not exceeding +2% and +4% per decade respectively under clear skies. The long-term UV variability has a QBO component, which cannot be overlooked, even in comparison with the amplitude of the annual cycle.

A new method for describing long‐term changes in total ozone

A new method for describing long‐term changes in total ozone was developed so that variations on time scales greater than that of the QBO may be examined. The technique fits a flexible tendency curve to total ozone data after explained variations have been removed. The derivative of the tendency curve is the growth rate curve. The average along this curve is comparable to total ozone trends reported in the past. Statistical uncertainty of the growth rate is determined using bootstrap techniques. Dobson column ozone measurements with long‐term (30+ years) records from the NOAA/CMDL Cooperative Dobson Network were analyzed. Total ozone decreases ranged from 1‐2%/decade since the 1960s and from 2‐4%/decade since 1979 at all sites except Mauna Loa where results were not significant. The tendency curves indicate that the total ozone decline began in the 1970s and that there are no clear signs of recovery as of the end of 2000.