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Stratospheric Processes And their Role in Climate
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SPARC Brochure, June 2001 - The SPARC Initiatives

Stratospheric Indicators of Climate Change

The stratosphere-troposphere system is changing due to both natural and anthropogenic factors, and the challenge is to understand the system sufficiently well to predict future scenarios. The test of this understanding lies in explaining past changes, but these changes need to be adequately quantified. To this end, SPARC has conducted three major assessments on temperature, ozone, and water vapour. These results have been used in the Scientific Assessment of Ozone Depletion 1999, and in the IPCC-Third Assessment Report 2001. Trends in dynamical activity would also be crucial to determine the overall climate effects of stratospheric changes.

Stratospheric Temperature Trends

Chair: V. Ramaswamy (USA)

Objectives

The objectives are to:

  • Inter-compare and assess the stratospheric temperature variations and changes, available from observational datasets, and
  • Use model simulations and measurements to understand the causes of the observed changes, in particular the role of natural and anthropogenic causes.

Key findings

The first phase of the assessment has been completed (see Ramaswamy et al., Reviews of Geophysics). The results have also been reported in the recently released WMO (1999) and IPCC (2001) scientific assessments.

Inter-comparison of the observations over the 1979-1994 period reveals an annual-mean cooling of the global lower stratosphere (see Figure 1), with the trends being statistically significant mainly in the midlatitudes of the Northern Hemisphere. There is a remarkable coherence amongst the various observational datasets. The global-mean, annual-mean cooling is estimated from the various datasets to be about 0.6 K/decade. Over the longer period 1966-1994, the global-mean, annual-mean cooling is estimated to be about 0.35 K/decade. Substantial cooling (~3-4 K/ decade) is observed in the lower polar stratosphere during late winter/spring in both hemispheres. However, in the Arctic, the dynamical variability is large, and this introduces difficulties in establishing a statistical significance of the trends there.

The vertical profile of the annual-mean stratosphere change observed in the Northern Hemisphere midlatitudes (45°N) is quite robust among the different observations (see Figure 2). The mean trend consists of a ~0.75K/ decade cooling in the 20-35 km range, with the cooling trend increasing with height above. Model simulations indicate that changes in trace gas concentrations are major contributors to the observed cooling of the global-mean stratosphere. The trace gas changes identified are:

  • Increases in the well-mixed greenhouse gases (CO2, CH4, N2O and CFCs),
  • Depletion of stratospheric ozone (see Figure 3)
  • Increases in water vapour (see Figure 4).

The cooling trend of the global stratosphere has been punctuated by transient warmings (1-2 years) following the El Chichon (1982) and Mt Pinatubo (1991) volcanic eruptions, when temporary enhancements in stratospheric aerosol concentrations induced a radiative heating.

Future Plans

Further analyses of observations and model simulations will continue to advance the understanding of the variation and trends in the stratospheric thermal state.

Activities planned include:

Ensuring continuity, updating and consistency checks on the observed temperature time series to the end of the last decade.
Extension of the analyses to cover the observed seasonal trends.
Assessing the temperature variations on finer time and space scales e.g., interannual variations, zonal-mean trends.
Ascertaining causes of the observed features on the different space-time scales: trace gases, aerosols, SST variations, solar cycle, quasi-biennial oscillation and dynamical variations.

The plans call for providing inputs into the next scientific assessment of stratospheric ozone (WMO, 2002), which is already underway.
Annual-and-zonal-mean decadal temperature trends

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Figure 1: Annual-and-zonal-mean decadal temperature trends for the 1979-1994 period, as obtained from different datasets.
1974-1994 temperature trends

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Figure 2: Mean vertical profile of the temperature trend over the 1979-1994 period in the stratosphere at 45°N, as compiled using radiosonde, satellite and analysed datasets. The solid red line indicates the trend estimate while the dashed lines denote the uncertainty at the 2-s level.
Reference:
V. Ramaswamy, M-L. Chanin, et al., Stratospheric temperature trends: Observations and model simulations, Reviews of Geophysics, 39, 71-122, 2001.

Understanding Ozone Trends

Chair: N. Harris (UK)

Objectives

The objective is to improve the understanding of the past changes in ozone. This work involves assessing the quality of existing data records and understanding past ozone trends in the light of the suggested mechanisms.

Achievements

On-going assessment of the quality of ozone measurements by ground-based and satellite instruments is coordinated by the World Meteorological Organisation’s Global Atmospheric Watch (WMO-GAW) programme. The main activity of the SPARC Ozone Trend working group has been to enhance the routine quality assessment through the organisation of the international "Assessment of Trends in the Vertical Distribution of Ozone" in conjunction with the International Ozone commission and WMO-GAW. This was published as SPARC Report N°1. The main conclusion is that the trends derived from the measurements made by a number of techniques above 20 km altitude are consistent (see Figure 3).

Future Plans

There have been an increasing number of studies investigating how decadal changes in dynamics affect the observed long-term ozone trends. Downward trends in mid-latitude ozone have been partly caused by changes in atmospheric dynamics. Changes in regional dynamic phenomena over the last 30 years such as the Arctic Oscillation are linked with reduced ozone amounts over much of Europe. Accurate quantification of the role of dynamics, vis a vis, that of chemical processes, remains unresolved.

A new joint SPARC/IOC initiative to assess their relative importance started with a workshop in March 2001 organised in cooperation with the WMO-UNEP assessment co-chairs. In conjunction with this, continuing discussions are being held within SPARC to see how progress can be made toward a comprehensive description of past stratospheric changes.
Stratospheric ozone trend 1980-1996

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Figure 3: Mean trend in the vertical distribution of ozone over northern mid-latitudes for 1980-1996 (heavy solid line) calculated using the trends derived from SAGE I-II, ozonesondes, SBUV and Umkehr measurements. Combined uncertainties are shown as 1s (light solid lines) and 2s (dashed lines). The combined trends and uncertainties are extended down to 10 km as shown by the light dotted lines but the results below 15 km should be viewed with caution.

Reference:
SPARC Report N°1, Assessment of Trends in Vertical Distribution of Ozone, May 1998.

Objectives

The overall goal was to provide an assessment of the state of knowledge of the water vapour distribution, variability and long-term changes. Accurate knowledge of the water vapour concentration in the atmosphere and its changes is critical for understanding and predicting long-term temperature changes both at the surface and throughout the atmosphere.

Activity

A large number of tropospheric and stratospheric water vapour measurements have been reported over the past 50 years. Instrumentation has evolved from in situ to satellite instrumentation but only a small number of measurements have records longer than 10 years. The objective was to gather the largest possible number of independent water vapour observations in the lower stratosphere and upper troposphere, to validate accuracy and uncertainty of the data sets, to look for consistencies between the data sets and to reach conclusions about observed changes.

Key finding

Taken together, ground-based, balloon, aircraft, and satellite measurements reveal a global stratospheric water vapour increase of ~2 parts per million by volume (ppmv) over the past 45 years or ~0.05 ppmv per year. This increase of over 75% has significant climate implications. Modeling studies by scientists from the University of Reading (UK) show that since 1980 the stratospheric water vapour increase has produced a surface temperature rise of about one half of that due to the increase of carbon dioxide alone. The reasons for this water vapour increase are unknown. One possible contributing factor is methane that has increased in the atmosphere since the 1950's, but this can only account for at most one half of the increase in stratospheric water vapour mixing ratio over this time period.

Upper tropospheric humidity (UTH), i.e. ~9 to 16 km above the earth's surface, has been measured by several consecutive generations of instruments on operational satellites. The satellite record shows a 2% increase over the last 20 years in the equatorial region. However, the uncertainty in this determination is too large to allow a clear conclusion as to whether this is a result of climate change. The combined uncertainty of the tropospheric relative humidity and temperature data sets is too large to allow definite conclusions to be drawn about long-term changes of upper tropospheric water vapour mixing ratio.

Future plans

Since recent publications show that the long-term change of upper tropospheric temperature and tropospheric cold-point temperature are negative, the increase of the stratospheric water vapour mixing ratio is incompatible with the simple Brewer freeze drying mechanism. The trend minimum near 100 hPa also awaits analysis and explanation. Future research is needed both in chemistry and dynamics to attempt to explain these observations. The water vapour data used in the SPARC study are available upon request at the SPARC Data Center.
Vertical profiles of water vapour trend

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Figure 4: Vertical profiles of the estimated linear trends for data sets from in situ , satellite and ground-based observations between 30º-50ºN. Colors and valid years are annotated on the inset. [Courtesy of K. Rosenlof.]
Reference:
SPARC Report N°2, SPARC Assessment of Upper Tropospheric and Stratospheric Water Vapour, December 2000.

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Last update: July 04, 2001