S P A R C Stratospheric Processes And their Role in Climate A project of the World Climate Research Programme Home Initiatives Organisation Publications Meetings Acronyms and Abbreviations Useful Links

Summary

The SPARC project was set up by the World Climate Research Programme (WCRP) in 1992 to consolidate knowledge on the role of the stratosphere in climate and develop understanding of the processes involved. This Implementation Plan reviews the scientific objectives of the project and provides an overview of the activities needed to achieve the goals of the programme.

The implementation of the SPARC plan will be guided by the SPARC Scientific Steering Group (SSG) and carried out by ad hoc SPARC working groups. The SPARC Office will continue to coordinate the international activities of the programme, support its implementation, edit the SPARC newsletter and publish other scientific documents and reports produced by the programme.

Motivation

Until a few years ago, the community of scientists studying climate and climate change and the community studying the stratosphere were largely distinct and separate. The prevailing view was that the stratosphere, with just 10-20% of the atmosphere's mass, could not exert a strong influence on the climate of the troposphere. Recently, however, an increasing number of studies have examined the links between the two and have found strong connections.

It is now clear that, not only is the interannual variability in the troposphere (due to non-linear variability or sea surface temperature changes) a forcing mechanism for the stratosphere, but also that stratospheric variations can change significantly the radiative forcing on the troposphere. Decreases in stratospheric ozone, which are detectable over the past twenty years or so, have been shown to have caused changes in the radiative forcing of climate of comparable magnitude to those due to other atmospheric trace gases. Further, volcanic eruptions occasionally cause large enhancements to the stratospheric aerosol layer which lead to large, though relatively short-lived, perturbations to the radiative forcing of climate. More tentatively, it has been proposed that stratospheric circulation changes arising from natural stratospheric variability (as may be caused by the QBO and, more contentiously, the solar cycle) and anthropogenic effects (e.g., ozone depletion) can be communicated downwards and amplified to induce significant perturbations to the tropospheric circulation and climate.

Many general circulation models (GCMs) currently used for climate studies do not attempt to represent the stratosphere in much detail, and so studies are required with GCMs which resolve the stratosphere in order to verify the role of its downward influence on the troposphere. The GCM performance is particularly poor at higher altitudes, but even the winds and temperature in the lower stratosphere and upper troposphere, a region widely recognised as being of critical importance for climate and climate change, are not well simulated. With this level of performance, there is a likelihood that the feedbacks between temperature, dynamics, radiation and chemistry are being modelled correctly, thus introducing a further set of uncertainties into the GCM calculations of possible future climates.

Many research agencies around the world are increasing their research efforts in this area. This approach was supported by the conference on the World Climate Research Programme in Geneva, August 1997, which was attended by well over 300 members of the climate research and policy communities. They urged that nations reinforce their commitment to co-operative international research efforts through the WCRP, and they identified an improved understanding of the role of the stratosphere in the climate system and the impact of stratospheric change on climate as an outstanding challenge and research priority.

This recommendation specifically included the need to improve knowledge of the physical and chemical characteristics of the lower stratosphere and upper troposphere region, in particular the exchanges of energy, water vapour and chemicals. The effects of the major depletion of ozone in the lower stratosphere and of the observed increase of stratospheric water vapour on global climate need to be clarified and qualified. The potential effects of the increasing emissions from the growing fleet of civilian aircraft have also to be investigated. The conclusions from the WCRP Conference were passed to the third Conference of the Parties to the United Nations Framework Convention on Climate Change (Kyoto, December 1997) which strongly reiterated that arrangements for funding and support for climate research and essential supporting observations be put in place.

Response

The SPARC SSG has identified the key scientific issues underlying these larger concerns and has split them into three areas of activities which broadly answer the following questions. What changes have occurred in the atmosphere? What processes are / might be causing them? How well can models reproduce these changes?

(a) Stratospheric indicators of climate change

Atmospheric ozone, water vapour and temperatures have been routinely measured for many years. Unfortunately there are insufficient high quality data, particularly for upper tropospheric and stratospheric water vapour and for tropospheric ozone, to allow a complete global assessment of variability or trends. Much of the early work done within SPARC has been to catalogue and evaluate the measurements that do exist and to assess how to proceed. Parallel efforts have been made to develop climatologies - these are described in section (c) as development of the climatologies is closely connected to the models they are being used to evaluate.

An assessment of trends in the vertical distribution of ozone has been completed in conjunction with the International Ozone Commission. This international effort involved over 100 scientists and determined where there is sufficient confidence in the measurements to allow the calculation of trends. The results of this assessment are being used in the preparation of the 1998 WMO-UNEP report. For the first time, best guess trends have been given, obtained by combining the trends from different measurement systems. To date this has only been deemed possible at northern mid-latitudes where there is good agreement between the different data sets at the altitudes where they overlap.

Work to extend the geographical coverage of these trends will continue. However there are just not enough good measurements to establish global tropospheric trends, especially near the tropopause where they contribute in a significant manner to the radiative forcing of climate change. To ensure that it is possible to do so in future, the SPARC ozone group will provide a climate perspective in the on-going international efforts to define and promote a global ozone measuring system based on a judicious balance of ground-based, ozonesonde and satellite measurements.

Hundreds of balloon soundings of temperature are made each day to be used in operational weather forecasting. A great deal of effort has been devoted to determining trends in the troposphere using their operational data, and this work has been comprehensively reviewed in the IPCC reports on climate change. SPARC has initiated an assessment of the data quality and of the trends in stratospheric temperatures which is based on raw observations and on analyses. This assessment will be available in 1998 and is being used for the WMO-UNEP report. The conclusion is that there has been a cooling in the lower stratosphere and that the observed ozone reduction has played a significant role in this cooling. In the future, the assessment will be extended to the upper stratosphere and more effort will be made to interpret the observed trends, using model studies to improve the quantification of how much of the stratospheric temperature trends are caused respectively by the decrease in ozone and the increase in greenhouse gases. These analyses will enable key inputs into the search for anthropogenic influences on the vertical profile of temperature trends, and thus aid in the detection/attribution of climate change due to human activity. Links with the mainstream climate modelling community will be developed to extend this work into an improved understanding of the observed tropospheric temperature records.

Fewer measurements of water vapour in the stratosphere exist than for either ozone or temperature, although rawinsondes routinely make good measurements in the mid- and lower troposphere. The quality has not been assessed to the same level as for ozone or temperature and much more needs to be done in this regard. A validated climatology of water vapour in the stratosphere (and upper troposphere) is valuable in itself. The viability of an assessment of trends in stratospheric and upper tropospheric water vapour is being investigated with the aim of subsequently organising such an exercise.

Many more water vapour measurements are planned in the coming years as several new satellite instruments with improved capabilities become operational. These measurements need to be validated; experience with both ozone and temperature suggests that independent, high resolution measurements will be invaluable. If such measurements can be made (and presumably they can only be done by in situ instruments), a great deal of additional insight will be gained into processes operating on different spatial and temporal scales to those which can be investigated by satellite instruments.

To date very few studies have been conducted to detect and interpret trends in dynamical activity. Dynamical trends in the stratosphere may be induced by changes in concentrations of radiatively active gases, (e.g., carbon dioxide and ozone) and any similar radiative or dynamical trends in the troposphere. A key challenge will be to determine whether any observed trends can be attributed to either or both of these. The SPARC SSG follows closely the development of research on this issue.

(b) Stratospheric processes and their relation to climate

The processes underlying these changes, as well as those which could be more important in a changed climate, occur in a very complex region of the atmosphere - the upper troposphere/lower stratosphere. This is also the region in which the potential impact of aircraft emissions is greatest. A lot is already known about this part of the atmosphere. However it is like an incomplete jigsaw puzzle with many individual pieces well defined (though many not), but the overall picture not clear. It is very important that a holistic picture of this region is developed which incorporates the chemical, radiative and dynamical components clearly and in balance.

Two SPARC initiatives have begun to address these complexities by organising a series of focused workshops, some of which have resulted in ‘tutorial’ review papers being published. For instance, a more global picture of stratosphere-troposphere exchange, which puts it in the context of the overall stratospheric circulation, has emerged as a result of a SPARC/NATO workshop on the subject in 1993. In this way SPARC activities complement and provide added value to those of existing national and international research programmes by providing a much needed and effective means of communication between the various specialised research communities studying this region of the atmosphere. In turn, the development of a more holistic picture will provide an improved underpinning on which to base future field campaigns, modelling studies and laboratory studies of the important processes.

A key area that has been identified for investigation is the role of gravity waves in the global circulation of the atmosphere. The parameterisation of the effects of unresolved gravity waves is one of the most significant uncertainties in numerical models of the global atmosphere. SPARC is now leading in developing plans for a field campaign to investigate the processes associated with gravity wave generation and propagation under well defined conditions. In parallel with this planning exercise, work has proceeded on establishing a global climatology of gravity wave activity. In particular, SPARC has encouraged meteorological agencies around the world to start archiving results from their operational balloon soundings at the highest possible vertical resolution, rather than just at standard levels. The response has been very encouraging with over a dozen countries now saving high resolution data.

(c) Modelling of stratospheric effects on climate

The role that the stratosphere plays in climate can be inferred from analysis of observations, but proof that the effects are real can only be obtained by the careful use of climate models which include a complete representation of the stratosphere. Such models allow the dynamical and physical processes which determine the climate to be investigated in the complex system of a modelled atmosphere. While simpler models can be applied to investigate mechanistic processes, the GCMs include all processes thought to be important for climate and allow them to interact. With such models, systematic numerical experimentation can clarify what role the middle atmosphere has on the climate, and particularly how the tropospheric climate responds to middle atmospheric change and is likely to react to future changes.

Current climate-middle atmosphere models capture the essence of the atmospheric structure, in that all dominant features of the real atmosphere can be recognised in the simulations. However, there are a host of persistent problems related to the representation of the stratosphere which require more attention. Accordingly a SPARC initiative has been set up to investigate the success with which climate GCMs simulate the stratosphere. Particular attention will be paid to issues such as the polar lower stratosphere (too cold in the models), the ability to accurately represent the thermal structure in the tropical tropopause region, and the variable success of the models in representing the transport barriers in the middle atmosphere (the polar vortex edge and the subtropical transport barrier). The ability of the models to represent these features (which all show in the climatological structure of the modelled fields) is being examined, as well as how the models react to imposed forcing mechanisms.

Studies to date have shown that a large amount of the interannual variability of the real atmosphere can be captured in the GCMs, but the structure may differ from reality in a manner which is possibly related to the deficiencies in the modelled climate. These deficiencies could also severely affect the ability of the models to accurately simulate the climate response to, say, ozone change. Quite apart from real stratospheric effects on climate, the numerical representation of the stratosphere in GCMs needs to avoid imposing an unrealistic upper boundary condition to the tropospheric circulation, for example by introducing spurious reflections at the model top which can contaminate the tropospheric stationary-wave pattern.

Understanding the performance of the GCMs under current climatic conditions, which is being supplemented by a range of activities where the components of the models are being tested, will increase our certainty in the robustness of the models. In order to be able to assess the model simulations a comprehensive climatology of the stratosphere is being compiled against which they can be compared. Future tests will examine the consensus of model reactions to imposed stratospheric change.

A documentation of stratospheric forcings on climate and their changes over the past century will also be compiled in order to provide the climate modelling community with the best estimates of these parameters.

Requirements of the programme

A number of scientific areas have been identified as being key components of SPARC. Great progress has been made on some facets, with major assessments of temperature and ozone trends already performed, a stratospheric climatology largely prepared, and the first phase of a major intercomparison of stratospheric climate models nearly completed. In other areas the progress has been less tangible, but equally valuable. Workshops and related activities have encouraged discussion of the scientific issues within SPARC, as reflected at the first SPARC General Assembly and by the contribution of leading scientists involved in SPARC in the 1998 WMO-UNEP Scientific Assessment of Ozone Depletion.

This Implementation Plan is focused on how understanding of the scientific issues can be promoted further. Here we list the main recommendations for future work in each section of this document:

Stratospheric indicators of climate change:

Stratospheric processes and their relation to climate:

Modelling of stratospheric effects on climate:

Finally, it cannot be stated strongly enough that the over-arching aim of SPARC is to promote the synthesis of the various strands of research described above. Combined measurements and modelling studies will be carried out which really test and advance our understanding of how processes in the stratosphere influence climate. Such studies will, in turn, improve our confidence in our understanding of past climate changes and in our ability to calculate possible changes in the future.

The SPARC SSG will oversee the development of this Implementation Plan. Implementation will take into account close co-ordination and co-operation with national programmes and other climate-related activities, including the other WCRP programmes, the IGBP core programme IGAC, the IPCC, and recommendations will be made to the GCOS AOPC and the CEOS IGOS on the data requirements of the programme. Joint projects will be developed as appropriate and maximum use will be made of existing infrastructure.