Tracing Isotopic Composition of Past and Present Precipitation - Opportunities for Climate and Water Studies
The Workshop on "Tracing Isotopic Composition of Past and Present Precipitation Opportunities for Climate and Water Studies", was jointly organized by the World Meteorological Organization (WMO), the International Atomic Energy Agency (IAEA), Past Global Changes (PAGES) - a core project of the International Geosphere - Biosphere Programme (IGBP), and the International Association of Hydrological Sciences (IAHS). The meeting was attended by 43 experts and representatives of national and international organizations, and was held from 23—25 January 1995 in Rüttihubelbad near Bern, Switzerland. It represented a platform for scientists from isotope, water and climate communities to share their interest in this area of science and to discuss future directions of their joint activities.
The Global Network "Isotopes in Precipitation" (GNIP) was initiated by IAEA in 1958 and became operational in 1961. The main objective was to collect systematic data on isotopic content of precipitation on a global scale, and to establish temporal and spatial variations of environmental isotopes in precipitation. This, in turn, was intended to provide the basic isotopic data to underpin the use of environmental isotopes in hydrological investigations within the scope of water resources inventories, planning and development. Although these primary objectives are still important for the future, the network's database has also proved indispensable in palaeoclimatology, and provides important input data for verification and further improvement of atmospheric circulation models.
One of the major purposes of WMO is to facilitate world-wide cooperation in the establishment of networks of stations for the making of meteorological observations as well as for hydrological and other geophysical observations related to meteorology. Besides the classical meteorological and hydrological variables which are and have been routinely observed for a long time by national Meteorological and Hydrological Services, there are other variables which are also of interest and importance for WMO. This was the rationale for WMO to join the IAEA initiative to establish the GNIP.
IAHS represents the hydrological sciences community world-wide. There is still a great need for scientific and technical information to promote the comparison of hydrological investigation using isotope data and classical techniques respectively. For a better understanding of the various processes governing the dynamics of hydrological systems and to improve hydrological modelling, the complementary approaches of tracer hydrology and the application of isotope methods need to be integrated.
The basic objective of PAGES is to provide the palaeo-dimension to the global change programmes, such as IGBP, WCRP and HDP. Important information is derived from isotope studies of records of past precipitation. The isotopic ratios of water offer opportunities to study atmospheric temperature, origin of water vapour and atmospheric circulation patterns. This, in turn, provides estimates of past and present interactions within ecosystems, fluvial systems, and the cryosphere which are fundamental to assess the impact of changes in climate and the hydrological cycle.
The efforts of the IAEA in promoting isotope methods in studies related to the hydrological cycle, including continuous operation of GNIP over the past three decades, were acknowledged both by the representatives of the international organizations and by top-level scientists participating in the meeting. Specific steps towards strengthening the operation of the network, with broader participation and the support of national and international bodies dealing with water and climate, were discussed and agreed upon. In particular, support for national networks and their integration was expressed.
It became apparent that GNIP has gained international recognition as one of the key elements in global change studies related to evolution of the water cycle and climate. The network is now expected to serve additional purposes, namely as a benchmark for the interpretation of paleo-records, as a validation tool for Global Circulation Models, and for establishing large-scale regional (and continental-scale) water balances. Furthermore, the long-term temporal changes emerging from the available data sets need to be understood in terms of changing climate.
The recommendations of the working groups addressed, among others, the following issues:
1. The structure of GNIP should be strengthened. This includes the build-up of:
- stations located close to major natural climatic archives (e.g. Greenland, mountain areas),
- stations which represent climatically sensitive areas (indicated by GCM's and biome models).
There should be close liaison with WMO stations which measure other atmospheric parameters. Co-operation with such programmes as Global Atmospheric Watch (GAW) of WMO and Global Climate Observing System (GCOS) is recommended.
2. Specific, research-oriented pilot or research observing stations, to be operated in specific areas for a limited period of time, should be launched. More detailed sampling (e.g. event-based samples for air moisture and for precipitation) in selected climatic zones was deemed necessary to gain a better understanding of the physical mechanisms controlling spatial and temporal variability of the isotopic composition of precipitation (source regions, continental basins, air mass boundaries). Moisture collection and measurements will require substantial improvements in established methods. Efforts should be made to develop innovative techniques for in situ isotope measurements. Such programmes should be conducted on a national level by scientific institutions presently operating national isotope monitoring networks. Studies of this type already performed in several countries (e.g. UK, Brazil, Israel) have demonstrated the importance of this approach.
3. Isotope monitoring of river outflow from major continental basins should be initiated. This could be realized in co-operation with the UNEP/WHO Global Environmental Monitoring System-Water (GEMS-Water).
4. Hydrological/isotopic studies should be linked to the existing or planned programmes (e.g. GEWEX and its projects, such as BALTEX, GAME and LAMBADA) aimed at studying water and energy balance on a continental scale. Case studies to assess the sensitivity of isotopic tracers to extreme hydrological events (floods, droughts) were suggested. A programme was recommended to test linkages between selected palaeo-data sets, including isotope indicators, with the aim of measuring event-scale hydrology (flood stratigraphy).
5. There is an urgent need to improve the interpretation of isotope records available from various natural climatic archives. In particular, it was emphasized that the isotope data from ice cores need to be calibrated against instrumental records of meteorological parameters. To this end, more information is required on the isotopic composition of water vapour, sea surface temperature and prevailing winds in the vapour source regions. Shallow ice cores (polar and alpine) could then provide a link to the GNIP data sets and could extend the isotope calibration information to other natural archives.
6. The deuterium excess parameter (d) is of particular importance in climate modelling and in the understanding of hydro-meteorological pathways. The use of the deuterium excess imposes strict requirements on the accuracy of deuterium and oxygen-18 analysis. A GNIP-based world-wide documentation of quality control regarding sampling, shipping and measurements is needed.
7. The IAEA/WMO database and other isotope data sets should be included in the World Data Center A for palaeo-climatology.
8. National Science Foundations and funding agencies should be approached to provide funds for strengthening GNIP and national networks which could in turn contribute to the global data base.
9. A science and implementation plan for these recommendations should be developed by a WMO-IAEA-PAGES Steering Committee.
10. IAEA was requested to organize a follow-up meeting in autumn 1995 to finalize the revision of GNIP and take necessary actions. It is also intended, that at this meeting the WMO-IAEA-PAGES Steering Committee will be constituted and will hold its first session.