[1] We define the radiative forcings used in climate simulations with the SI2000 version of the Goddard Institute for Space Studies (GISS) global climate model. These include temporal variations of well-mixed greenhouse gases, stratospheric aerosols, solar irradiance, ozone, stratospheric water vapor, and tropospheric aerosols. Our illustrations focus on the period 1951-2050, but we make the full data sets available for those forcings for which we have earlier data. We illustrate the global response to these forcings for the SI2000 model with specified sea surface temperature and with a simple Q-flux ocean, thus helping to characterize the efficacy of each forcing. The model yields good agreement with observed global temperature change and heat storage in the ocean. This agreement does not yield an improved assessment of climate sensitivity or a confirmation of the net climate forcing because of possible compensations with opposite changes of these quantities. Nevertheless, the results imply that observed global temperature change during the past 50 years is primarily a response to radiative forcings. It is also inferred that the planet is now out of radiation balance by 0.5 to 1 W/m 2 and that additional global warming of about 0.5°C is already ''in the pipeline.''

Recent findings, based on both ground-based and satellite measurements, have established that there has been an apparent downward trend in the total column amount of ozone over mid-latitude areas of the Northern Hemisphere in all seasons. Measurements of the altitude profile of the change in the ozone concentration have established that decreases are taking place in the lower stratosphere in the region of highest ozone concentration. Analysis of updated ozone records, through March of 1991, including 29 stations in the former Soviet Union, and analysis of independently calibrated satellite data records from the Total Ozone Mapping Spectrometer and Stratospheric Aerosol and Gas Experiment instruments confirm many of the findings originally derived from the Dobson record concerning northern midlatitude changes in ozone. The data from many instruments now provide a fairly consistent picture of the change that has occurred in stratospheric ozone levels.

Abstract. International agreements for the limitation of ozone-depleting substances have already resulted in decreases in concentrations of some of these chemicals in the troposphere. Full compliance and understanding of all factors contributing to ozone depletion are still uncertain; however, reasonable expectations are for a gradual recovery of the ozone layer over the next 50 years. Because of the complexity of the processes involved in ozone depletion, it is crucial to detect not just a decrease in ozone-depleting substances but also a recovery in the ozone layer. The recovery is likely to be detected in some areas sooner than others because of natural variability in ozone concentrations. On the basis of both the magnitude and autocorrelation of the noise from Nimbus 7 Total Ozone Mapping Spectrometer ozone measurements, estimates of the time required to detect a fixed trend in ozone at various locations around the world are presented. Predictions from the Goddard Space Flight Center (GSFC) two-dimensional chemical model are used to estimate the time required to detect predicted trends in different areas of the world. The analysis is based on our current understanding of ozone chemistry, full compliance with the Montreal Protocol and its amendments, and no intervening factors, such as major volcanic eruptions or enhanced stratospheric cooling. The results indicate that recovery of total column ozone is likely to be detected earliest in the Southern Hemisphere near New Zealand, southern Africa, and southern South America and that the range of time expected to detect recovery for most regions of the world is between 15 and 45 years. Should the recovery be slower than predicted by the GSFC model, owing, for instance, to the effect of greenhouse gas emissions, or should measurement sites be perturbed, even longer times would be needed for detection.

A seasonal trend analysis of Dobson total ozone data that have been critically reevaluated and revised is performed for 29 northern hemisphere stations located between 19°N and 64°N latitude using data through 1986. The trend model considered for these data allows for a different linear trend for each month of the year, so that the seasonal as well as the latitudinal and regional nature of the total ozone trend behavior can be examined. The trend model also incorporates the 10.7‐cm solar flux series and the 50‐hPa equatorial zonal wind series as additional explanatory factors for solar and quasi‐biennial oscillation induced ozone variations. Regression random effects models are then used for the individual station seasonal trend estimates to obtain trend estimates as a function of latitude for different seasons of the year. The results of this seasonal trend analysis indicate significantly more negative trends during the winter months (December‐March) than during the summer months (May–August), notably at higher latitudes, with the trends in winter becoming more negative with increasing latitude. The trends in the winter are estimated to be of the order of −1.2%, −2.1%, and −3.0% per decade for latitudes 35°N, 45°N, and 55°N, respectively, while trends during the summer are of the order of −0.6% per decade with no distinct pattern as a function of latitude. The year‐round or annual trend over all latitudes is estimated to be about −0.84±0.82% per decade. The trends are found to display some regional variation, with trends in Japan being considerably less negative than those in North America and Europe. Sensitivity studies are also performed to investigate the effects on ozone trend estimates due to certain factors such as abnormal ozone behavior in 1983 and 1985, the use of ozone data prior to 1965, and nuclear weapons testing in the early 1960s. The seasonal trend analysis is also performed using published (unrevised) Dobson data. Trend results based on published data are on average less negative than trends from revised Dobson data for European stations, by about 1.0% per decade across all seasons, with only small average differences for stations in North America and Japan.

SUMMARYThe cause of the observed mid-latitude decline in ozone in a 20-year integration of a stratospheric chemical transport model forced by European Centre for Medium-Range Weather Forecasts analyses for the years 1979 to 1998 is investigated. A very simple chemical scheme for ozone is used which includes no interannual variation so that any modelled interannual variability, or trend, must arise from changes in the meteorology. The integration from 1979 to 1998 does show a downward trend in mid-latitude ozone in which many of the observed features are reproduced, especially between the middle 1980s and the early 1990s. A detailed statistical trend analysis shows that the quantitative comparison between modelled and observed trend is very sensitive to the choice of period considered. However, there is good qualitative agreement in terms of the latitudinal variation of the trend. For the different periods considered, the dynamically driven model trend accounts for at least half of the observed northern mid-latitude trend averaged over December to February. The vertical variation of the modelled and observed trends agree qualitatively. The modelled total ozone correlates well with the vertical winter EP-flux at 100 hPa and the North Atlantic Oscillation index, suggesting that long-term changes in stratospheric circulation are intimately connected to the observed mid-latitude ozone trend.