S. M. Hollandsworth

Climate forcings in Goddard Institute for Space Studies SI2000 simulations

[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.’’ INDEX TERMS: 1620 Global Change: Climate dynamics (3309); 1635 Global Change: Oceans (4203); 1650 Global Change: Solar variability;

Forcings and chaos in interannual to decadal climate change

We investigate the roles of climate forcings and chaos (unforced variability) in climate change via ensembles of climate simulations in which we add forcings one by one. The experiments suggest that most interannual climate variability in the period 1979–1996 at middle and high latitudes is chaotic. But observed SST anomalies, which themselves are partly forced and partly chaotic, account for much of the climate variability at low latitudes and a small portion of the variability at high latitudes. Both a natural radiative forcing (volcanic aerosols) and an anthropogenic forcing (ozone depletion) leave clear signatures in the simulated climate change that are identified in observations. Pinatubo aerosols warm the stratosphere and cool the surface globally, causing a tendency for regional surface cooling. Ozone depletion cools the lower stratosphere, troposphere and surface, steepening the temperature lapse rate in the troposphere. Solar irradiance effects are small, but our model is inadequate to fully explore this forcing. Well-mixed anthropogenic greenhouse gases cause a large surface wanning that, over the 17 years, approximately offsets cooling by the other three mechanisms. Thus the net calculated effect of all measured radiative forcings is approximately zero surface temperature trend and zero heat storage in the ocean for the period 1979–1996. Finally, in addition to the four measured radiative forcings, we add an initial (1979) disequilibrium forcing of +0.65 W/m2. This forcing yields a global surface warming of about 0.2°C over 1979–1996, close to observations, and measurable heat storage in the ocean. We argue that the results represent evidence of a planetary radiative imbalance of at least 0.5° W/m2; this disequilibrium presumably represents unrealized wanning due to changes of atmospheric composition prior to 1979. One implication of the disequilibrium forcing is an expectation of new record global temperatures in the next few years. The best opportunity for observational confirmation of the disequilibrium is measurement of ocean temperatures adequate to define heat storage.

Long-term ozone trends derived from the 16-year combined Nimbus 7/Meteor 3 TOMS Version 7 record

Ozone measurements from the Nimbus 7 TOMS instrument, which operated from November 1978 through early May 1993, have been extended through December 1994 using data from the TOMS instrument on-board the Russian Meteor 3 satellite. Both TOMS data records have recently been recalibrated, and then reprocessed using the Version 7 retrieval algorithm. Long-term trend estimates obtained from a multiple regression analysis show ozone losses in the extended data record similar to those reported in previous studies using Version 6 TOMS and SBUV data, and ground-based Dobson data. Ozone continues to decline through the end of 1994, with the most significant ozone losses occurring in the high southern latitudes during October (−20% per decade) and in the northern mid- to high-latitudes during March/April (−6 to −8% per decade). There is no significant ozone trend in the tropics. Annual-average trends derived from the Nimbus 7 Version 7 data are 0–2.5% per decade less negative than those derived over the same time period using Version 6 data.

Ozone trends deduced from combined Nimbus 7 SBUV and NOAA 11 SBUV/2 data

The long-term time series of global ozone from the Nimbus-7 SBUV (Nov. 1978–June 1990) are extended through June 1994 by using measurements from the NOAA-11 SBUV/2. The data sets are merged based upon comparisons during the 18-month overlap period in which both instruments were operational. During this period, the average offset between the two time series is less than 2% in total ozone, and less than 6% in Umkehr layers 1–10. A linear-regression trend model is applied to the extended time series to calculate updated trends as a function of latitude and altitude. Trends through June 1994 are 1.5-2% per decade less negative than through June 1990 in the tropical middle stratosphere (35–40 km) and in the upper stratosphere (45–50 km) at mid-latitudes. In the lower stratosphere, the trends are nearly 1.5% per decade more negative in the southern hemisphere tropical regions to 25°S, but remain relatively unchanged elsewhere. The seasonal structure of the total ozone trends is similar to past trend study results, but the magnitude of the seasonal trend can vary by 2% per decade depending on the length of the time series. Both TOMS (through April 1993) and SBUV total ozone time series show small negative trends in the equatorial region, though they are not statistically significant at the 2-σ level.

Altitude dependence of stratospheric ozone trends based on Nimbus 7 SBUV data

A multiple regression statistical model is applied to estimate the altitude, latitude, and seasonal dependences of stratospheric ozone trends using 11.5 years of Nimbus 7 SBUV data for the period November 1978 to June 1990. In the upper stratosphere, the derived trends agree in both latitude dependence and approximate amplitude with published predictions from stratospheric models that consider gas-phase chemical processes together with the observed [approximately]0.1 ppbV per year increase in tropospheric chlorine. The dominant contribution to column ozone trends occurs in the lower stratosphere where significant negative trends are present at latitudes >20[degrees] in both hemispheres. The observed latitude dependence is qualitatively consistent with model predictions that include the effects of heterogeneous chemical ozone losses on lower stratospheric aerosols.

Observational study of the quasi‐biennial oscillation in ozone

The structures of the quasi-biennial oscillations (QBOs) in zonal wind, temperature, and layer ozone amounts are investigated using 11.5 years (January 1979 to June 1990) of National Meteorological Center (NMC) global geopotential height data and global ozone data from the solar backscatter ultraviolet spectrometer (SBUV) on Nimbus 7. The QBO signals are isolated by computing lagged correlations between the deseasonalized, detrended variable fields and a reference signal representative of the equatorial QBO. Lagged correlations are calculated for the full time series and for each season separately to determine seasonal effects. The results depict an equatorial zonal wind QBO in good agreement with the observed QBO in ground-based equatorial zonal wind measurements, although the amplitude of the derived QBO in the NMC data is ∼30% too weak. The vertical extent of the oscillation is significantly higher (2 mbar) than that previously reported. The temperature QBO is consistent with ground-based observations in the lower stratosphere but weakens with height above ∼50 mbar. The ozone QBO is strong at all levels from 5 mbar into the lower stratosphere. Though the annual average total ozone QBO is quite symmetric about the equator, the oscillation is highly variable from layer to layer. The phase of the ozone QBO near the equator is consistent with that of the zonal wind and temperature in the middle and upper stratosphere, but the vertical resolution of the SBUV data in the lower stratosphere is too low to accurately represent the vertical phase of the ozone QBO in this region. Subtropical temperature and total ozone anomalies are found to be dependent on season.

Comparisons of observed ozone trends and solar effects in the stratosphere through examination of ground-based Umkehr and combined solar backscattered ultraviolet (SBUV) and SBUV 2 satellite data

Within the past year, two papers have been published which present updated profile ozone trends from the recently revised ground-based Umkehr record [Miller et al., 1995] and the combined Nimbus 7 solar backscattered ultraviolet (SBUV) and NOAA 11 SBUV 2 satellite data record [Hollandsworth et al., 1995]. In this paper we compare the ozone trends and responses to the 11-year solar cycle (represented by the F10.7 cm radio flux) derived from these two data sets for the period June 1977 to June 1991 (November 1978 to June 1991 for the satellite data). We consider data at northern midlatitudes (30°–50°N) at altitudes between 25 and 45 km derived from these two data sets. In particular, we investigate the effects of spatial sampling differences between the data sets on the derived signals. The trends derived from the two independent data sets are nearly identical at all levels except 35 km, where the Umkehr data indicate a somewhat more negative trend. The trend is approximately zero near 25 km but becomes more negative in the upper stratosphere, reaching nearly −7% per decade in the 40–45 km region. The upper stratospheric decreases are consistent with model results and are associated with the gas-phase chemical effect of chlorofluorocarbons CFCs and other ozone-destroying chemicals [World Meteorological Organization, 1995]. The ozone correlations in the two data sets with the F10.7 cm solar flux are similar, with near-zero solar-induced ozone variations in the 25–30 km region and statistically significant in-phase variations at higher altitudes. Estimates of the solar cycle in the ozone time series at 40–45 km from a regression model indicate variations of about 4.5% from solar cycle maximum to minimum. Analysis of the satellite overpass data at the Umkehr station locations shows that the average of the data from the 11 Umkehr stations is a good approximation for the 30°–50°N zonal mean.

Information content of Umkehr and solar backscattered ultraviolet (SBUV) 2 satellite data for ozone trends and solar responses in the stratosphere

Within the past few years, several papers have been published which present updated profile ozone trends from the recently revised ground-based Umkehr record (Miller et al., 1995) and the combined Nimbus 7 solar backscattered ultraviolet (SBUV) and NOAA 11 SBUV 2 satellite data record (Hollandsworth et al., 1995; Miller et al., 1996). Within these papers, however, there has remained an overriding question as to the actual information content of the measurement systems and their ability to detect atmospheric responses. In this paper, we compare the ozone trends and responses to the l 1-year solar cycle (derived from model and/or data specifications of these effects) to results of forward model/retrieval algorithm computations through the algorithms. We consider data at northern midlatitudes (30o-50oN) so that we may compare the satellite results with those of the ground-based systems. Our results indicate that the Umkehr data contain only four independent pieces of information in the vertical and that the SBUV system contains five. In particular, we find that consideration should be restricted to the following regions; Umkehr: the sum of Umkehr layers 1-5, and layers 6, 7, and 8+ (the sum of layers 8 and above), SBUV: the sum of layers 1-5, and layers 6, 7, 8, and 9+ (the sum of layers 9 and above). Additionally, we compare the actual trends and solar coefficients derived in these layers for the periods 1968-1991 and 1979-1991 for the Umkehr and SBUV data. Finally, we include within the latter comparisons the stratospheric aerosol and gas experiment (SAGE) I and II results from Wang et al. (1996) and the computations from the ozonesondes.

Trends in global ozone as of 1995

The possibility of ozone destruction caused by the anthropogenic release of chemicals into the atmosphere has been recognized since the 1970s. The development of the antarctic ozone hole has provided a clear example where enhanced levels of chlorine can be directly linked to enhanced ozone destruction. Ozone levels in October in the antarctic have declined from historical levels near 300 DU (1958–1975) to record low levels below 100 DU by 1993 and 1994. Both ground‐based and satellite‐based measurements of ozone at mid‐latitudes clearly show long‐term ozone loss in both hemispheres relative to the mid‐1970s. At 45°N the ozone decline from 1979 to 1994 has been at the rate of 6% per decade in February/March, while the summer trend is much less –2–3% per decade. These declines are larger than predicted by current models. Annual average ozone trends in the equatorial region are small and not statistically significant.