REPORT OF THE
THIRD MEETING OF THE
SCIENTIFIC STEERING COMMITTEE
10-11 October 2001
Organized by the
World Meteorological Organisation
in co-operation with the
International Atomic Energy Agency
THIRD MEETING OF THE GNIP SCIENTIFIC STEERING COMMITTEE
10-11 October 2001
World Meteorological Organisation
The 1998 Memorandum of Understanding (MoU) between the IAEA and the WMO relating to the operation of the Global Network of Isotopes in Precipitation (GNIP) established a Scientific Steering Committee consisting of IAEA and WMO staff and members of the scientific community in order to provide scientific review and direction for the operation and use of the network. The 3rd meeting of the SSC took place at the WMO Headquarters in Geneva. A list of participants is attached in Annex I.
Mr. Arthur Askew of the WMO chaired the meeting. As per the MoU, the SSC Chair rotates between IAEA and WMO and this was the first year of WMO holding the Chair. The principal issues for this meeting were a review of the 2000-2001 activities, discussion of potential means of cooperation with climate related programmes of the WMO, and a review of the progress made in developing a sustainable global network.
Increasing Links with the International Community
The SSC was informed that the IAEA has initiated a dialogue with the GEWEX (Global Energy and Water Cycle Experiment) group for a collaborative research programme on the isotope content of major rivers. This cooperation will occur within the framework of a new CRP of the IAEA to be initiated in 2002 on the subject of a global network of isotopes in rivers. The SSC appreciated the development, noting that a river network will be a very useful contribution to isotope-based climate research.
At the July 2001 joint meeting of the Climate Variability Project (CLIVAR) of WMO's World Climate Research Programme (WCRP) and the International Geosphere Biosphere Programme's Past Global Changes project (PAGES), the GNIP activities related to climate studies were discussed. Keith Alverson, Executive Director of PAGES, provided a letter on behalf of the CLIVAR/PAGES Intersection working group expressing strong support for the GNIP (Annex II). CLIVAR/PAGES recognize the value of GNIP data for both the paleo and climate dynamics communities. WCRP Director, D. Carson, proposed that he would include GNIP on the agenda of the next CLIVAR meeting and will support and promote the idea of creating a joint PAGES/CLIVAR/IAEA activity group.
The SSC reviewed the working document on "Isotope Climatology and the Global Network of Isotopes in Precipitation: Directions for the Future", prepared by Tom Edwards, J. Birks of the University of Waterloo, Canada, and participants of the 2nd GNIP-SSC meeting (Annex III). The document was considered to be useful for discussion among other programmes and could form a basis for discussion at the next CLIVAR meeting.
A presentation was made by Mr. H. Teunissen, from WMO's Global Climate Observing System (GCOS). GCOS consists of several data networks related to atmospheric, oceanic, and terrestrial observations for climatology. GCOS was established to ensure that the observations necessary to address climate-related issues are defined, obtained and made available to all potential users.
It was suggested that perhaps GNIP could be included in the GCOS network to raise the profile of GNIP and facilitate an increased application of these data in climate and hydrology research. The requirements to have GNIP as a part of GCOS are to present a document on the application of the GNIP database for climate studies, to propose a sustainable long-term network of core stations and to describe the actions undertaken for quality control and quality assurance. Mr. H. Teunissen indicated that he would support IAEA/WMO efforts to incorporate GNIP in GCOS.
Mr. Soudine of WMO described the GAW (Global Atmosphere Watch) network. National Meteorological Services are running 22 stations of the GAW global network. In addition, there are about 300 local network stations for GAW. The sampling procedures for chemical analyses at GAW stations are similar to those for isotopes. At some stations, the observers are already performing rain sampling for both GAW and GNIP networks. GAW was identified as a potential partner for new GNIP stations that may be run on a long-term basis.
The World Hydrological Programme (WMO - Hydrology and Water Resources Programme) and the UNESCO-IHP programme may also be relevant partners for joint activities with GNIP in relation to hydrogeological and hydrological studies. The river network, proposed by the IAEA Isotope Hydrology Section would have to co-operate with the Global Runoff Data Centre in Koblenz as their objectives are similar. WHYCOS (World Hydrological Cycle Observing System) is also of potential interest for the river network.
The status of some of the national networks and GNIP stations was reported by the managers attending the meeting and the IAEA staff in charge of the GNIP. The Swiss network is maintained by the Federal Office for Water and Geology (FOWG) and has a legal base (the Federal law on the protection of waters) that ensures long-term sustainability. The network also covers rivers and aquifers. Up to 1991, five stations were included in the GNIP database. Since then, the FOWG is responsible for the network and did not provide any data to the GNIP database. A formal agreement between the IAEA and the FOWG would clarify some concerns about the acknowledgment to data providers and information on data requesters. These concerns are relevant for all national network managers.
The Canadian network, unlike that of the Swiss, is maintained by researchers on a voluntary basis. This situation makes the design of the network highly relevant for on-going research but the continuity of the monitoring is difficult to secure.
Building a Sustainable Global Network
A large part of the second day of the meeting was devoted to a discussion of the design of the network and selection of core stations (a map of GNIP network is given in Annex IV). The network design should reflect the needs of the climate and hydrology communities and be sustainable on a long-term basis.
There are multiple factors for selecting the stations of a core network. The selection process should take into account future development of the database and be based on the following criteria:
- availability of good quality meteorological data (air temperature, vapour pressure, precipitation)
- access to upper air elements (regional moisture, wind, air temperature…)
- reasonable length and high quality of historical record (isotope and rain gauge)
- strategic location within the general circulation
The SSC members representing the scientific community provided some advice on particular areas of interest and will provide, as soon as possible, the rationale for the selection of particular stations. The list of core stations will be finalised by WMO and IAEA after consultation with other scientists. Also the rationale and design of the GUAN (Global Upper Air Network) for meteorological observations would be consulted in defining the GNIP core stations. National networks will be asked to select the station(s) they would like to enter in the list of core stations and commit themselves to maintaining them.
Conclusions and recommendations:
This was the first time that the SSC meeting took place at WMO Headquarters and it was a good opportunity to identify links between the GNIP and other climate programmes of WMO. Stronger WMO involvement in the management of GNIP will give a push to the programme and higher recognition with regards to the climate community.
The preparation of a document providing a rationale and identifying a set of core network stations remains to be accomplished. In addition to facilitating participation of national authorities in network operation, such a document would also form a basis for having the GNIP accepted as part of the GCOS network.
The committee proposes to hold its next meeting either at WMO Headquarters in Geneva or at UNESCO Headquarters in Paris.
LIST OF PARTICIPANTS
Mr Arthur ASKEW
Director, Hydrology and Water Resources Department
Case Postale 2300
Tel: (41 22) 730 83 55
Fax: (41 22) 730 80 43
Mr Pradeep AGGARWAL
Head, Isotope Hydrology Section
Wagramer Strasse 5
P.O. Box 100
Tel: (43 1) 2600 21 735
Fax: (43 1) 26007
Mr David CARSON
Director, World Climate Research Programme
Case Postale 2300
Tel: (41 22) 730 82 46
Fax: (41 22) 730 80 36
Mr Thomas W.D. Edwards
Earth Sciences, University of Waterloo
200 University Avenue West
WATERLOO ON N2L 3G1
Tel: (1 519) 888 45 67 ext 3236
Fax: (1 519) 746 01 83
Ms Laurence GOURCY
Isotope Hydrology Section
Wagramer Strasse 5
P.O. Box 100
Tel: (43 1) 2600 21 734
Fax: (43 1) 26007
Mr Stefan HASTENRATH
Department of Atmospheric and Oceanic Sciences
University of Wisconsin
1225 West Dayton Street
MADISON, WI, 53706
Tel: (1 608) 262 36 59
Fax: (1 608) 262 01 66
Mr Ulrich SCHOTTERER
Physics Institute, University of Bern
Tel: (41 31) 631 44 84
Fax: (41 31) 631 44 05
Mr Keith Alverson
PAGES International Project Office
Tel: (41 31) 312 31 33
Fax: (41 31) 312 31 68
Mr Hartmut GRASSL
Max Planck-Institute for Meteorology
Tel: (49 40) 411 73 225
Fax: (49 40) 411 73 350
Mr Ronald KOZEL
Scientific collaborator in hydrogeology section
Federal Office for Water and Geology FOWG
Tel: (41 31) 324 77 64
Fax: (41 31) 324 76 81
Mr Willibald STICHLER
Head of Environmental Isotope Group
GSF Institute of Hydrology
Ingolstädter Landstrasse 1
Tel: (49 89) 3187 25 66
Fax: (49 89) 3187 33 61
Mr Alexandre V. Soudine
Atmospheric Research and Environment Programme Department
Case Postale 2300
Tel: (41 22) 730 84 20
Fax: (41 22) 730 80 49
Mr Hans W. Teunissen
Global Climate Observing System Department
Case Postale 2300
Tel: (41 22) 730 80 86
Fax: (41 22) 730 80 52
Isotope Climatology and the
Global Network of Isotopes in Precipitation:
Directions for the Future
T.W.D. Edwards and S.J. Birks
University of Waterloo, Canada
GNIP Scientific Steering Committee Meeting
7th September 2000, Max-Planck Institute of Meteorology
The IAEA/WMO Global Network of Isotopes in Precipitation (GNIP) is an international observational research network dedicated to documentation and understanding of the shifting distribution of water isotope tracers in global atmospheric moisture. The GNIP originated in the early 1960s out of the need to track the dispersal of radioactive 3H produced by atmospheric testing of nuclear weapons, but attention over the succeeding decades turned increasingly to monitoring and analysis of the systematic temporal and spatial variability of the naturally-occurring stable water isotopes 2H and 18O in precipitation. In addition to the original desire to develop annual hydrologic input functions for water resource studies, these investigations soon led to the recognition of water isotope tracers as highly sensitive climatological parameters and monitors of ongoing global climate change.
The GNIP database is an essential resource for calibrating isotopic indicators of paleoclimate from various natural archives. Perhaps even more importantly, it constitutes the only comprehensive source of data for evaluating simulations of the modern global isotope field generated by atmospheric general circulation models (GCMs) equipped with water isotope diagnostics.
Indeed, the growing ability of isotopic GCMs has been a major factor in the emerging science of isotope climatology, since the distribution of isotopes in precipitation provides fundamental information about the partitioning of the global atmospheric water budget that is not directly accessible or testable using other means. This has increased manyfold the value of the existing GNIP database and the ongoing GNIP program, as well as firmly establishing the importance of accurate mapping and modelling of global isotope climate. The use of water isotope tracers also provides unifying physical and conceptual links between the sometimes-disparate disciplines of climatology and hydrology.
Following in the tradition of conventional descriptive climatology, as well as simply to compensate for the patchiness of the GNIP database in time and space, considerable effort has been directed toward characterization of isotope climate "norms". However, growing awareness of the dynamic nature of climate and undeniable evidence for ongoing global climate change have recently fueled much more critical re-assessment of GNIP data, which has revealed systematic isotope climate variability at annual to decadal time-scales analogous to that discernible from other meteorological parameters. These observations present exciting new challenges and opportunities to more fully explore evolving isotope climate as captured in existing and future GNIP data.
This document provides a brief overview of current issues in isotope climatology and the central role played by the GNIP program in this emerging science. Recommendations are also made regarding the future of the GNIP and how it can continue to serve in the best interests of the international scientific community. Although it hardly needs pointing out, the seeds planted some 40 years ago in the early days of the IAEA/WMO GNIP initiative have borne remarkable fruit, and overwhelming justification exists for continuing and enhancing this joint activity as a unique contribution to international climate research.
2. Past and Present
Isotope climatology is founded on the meticulous descriptive analysis of the distribution of isotopes in contemporary global precipitation. This task has been spearheaded strongly by scientists affiliated with the IAEA/WMO GNIP program (e.g., Yurtsever 1975; Yurtsever and Gat 1981; Rozanski et al. 1992, 1993; Araguás-Araguás et al. 2000; Gonfiantini et al. 2001). Major benchmarks in the understanding and modelling of water isotope partitioning in the hydrologic cycle also include the seminal studies of Craig (1961), Dansgaard (1964), Friedman et al. (1964), Craig and Gordon (1965) and others, as well as numerous more recent discussions of isotopic labelling during formation of precipitation (e.g., Jouzel and Merlivat 1984; Ciais and Jouzel 1994), modern isotope-climate relations (e.g., Rozanski et al. 1982, 1992; Merlivat and Jouzel 1979; Rindsberger and Magaritz 1983), and analyses of global and regional atmospheric water balance (e.g., Gat 1980, 1996, 2000; Salati et al. 1979; Gat and Matsui 1981; Koster et al. 1993; Gat et al. 1994); Krishnamurthy and Bhattacharya 1991).
2.1 Global isotope climate modelling
Incorporation of water isotope diagnostics into atmospheric GCMs was pioneered in the 1980s using coarse-resolution configurations of the Laboratoire de Météorologie Dynamique (Joussaume et al. 1984) and the Goddard Institute for Space Sciences (Jouzel et al. 1987) models. These experiments represented a fundamentally new approach to the critical assessment of the ability of GCMs to simulate the global water cycle, introducing the need to conserve both mass and isotopes by accounting for the slight differences in the behaviour of the major water isotopomers during phase changes. Subsequent developments included sensitivity studies and simulations using the LMD and GISS models at finer spatial resolution, and also more recently the ECHAM (Max-Planck Institute, Hamburg) and GENESIS (National Center for Atmospheric Research) atmospheric GCMs, in various simulations of present global isotope climate (e.g., Jouzel et al. 1991, 1996, 1997a, 2000; Cole et al. 1993, 1999; Hoffmann and Heimann 1993; Charles et al. 1995; Hoffmann et al. 1998, 2000; Mathieu et al. in preparation), including examination of regional climatic features like the Southeast Asian monsoon cycle (Hoffmann and Heimann 1997). Differing global isotope climates have also been simulated, commonly focusing on the last glacial maximum (LGM; e.g., Joussaume and Jouzel 1993; Charles et al. 1994; Jouzel et al. 1994, 2000; Juillet-Leclerc et al. 1997), but also including consideration of more subtle differences represented by warm and cool ENSO phases (Cole et al. 1993), warmer-than-present global climate during both the mid-Holocene and possible future states forced by elevated atmospheric CO2 concentration (Jouzel et al. 1998, 2000; Hoffmann et al. 2000) and cold episodes induced by meltwater events in the North Atlantic (Werner et al. 2000b).
Prominent among emerging themes in the application of isotopic GCMs to address questions in global climate and paleoclimate research is the investigation of the isotopic expression of decadal-scale modes of climate variability like the El Nińo–Southern Oscillation (e.g. see Cole et al. 1993, 1999) and critical assessment of the isotopic composition of past precipitation as a paleothermometer (e.g., see Jouzel et al. 1997, 1998, 2000; Hendricks et al. 2000; Werner et al. 2000a). The latter has been driven strongly by growing confidence in non-isotopic methods for reconstructing or constraining paleotemperatures for comparison with simulated paleo-isotope data, notably including inverse modelling of ice borehole temperature profiles, which revealed striking deviations from the d-T relations previously used to estimate glacial-interglacial temperature changes from Greenland ice cores (e.g., Cuffey et al. 1994, 1995; Johnsen et al. 1995), as well as measurement of noble gas ratios in ground water (e.g., Stute et al. 1992, 1995a,b) and use of paleoecological data to estimate paleotemperature (e.g., Edwards et al. 1996; Hammarlund et al. 2001).
2.1.1 Steady-state isotope climate modelling
Isotopic GCMs are commonly run to simulate average "equilibrium" or "steady state" global isotope climate, characterized by essentially stationary annual cycles in each grid cell that can be compared directly with GNIP-type average monthly-composite data at individual stations (where appropriately located) or with interpolated grid values. The notion of generating equilibrium isotope climate is a logical extension of the traditional characterization of conventional climate parameters in terms of climatological norms, and provides a powerful first-order test of a GCM's basic ability to partition the atmospheric moisture budget. Further testing of the level of skill in simulating the major features of equilibrium global isotope climate can be undertaken by consideration of derived relations between isotope abundances and climate, such as the classic "effects" (temperature, continental, altitude, precipitation amount), as well as between the oxygen and hydrogen isotopes themselves (the d-excess parameter).
As with depiction of many other climatological processes, however, accurate simulation of isotopically-important mechanisms like rain-out and moisture recycling is highly dependent on the degree of detail with which the earth's surface can be represented, which is mainly constrained by computational limits on grid resolution. The simulated isotopic evolution (distillation) of a vapour parcel traversing a mountain range and its isotopic footprint in precipitation downwind, for example, will obviously be highly sensitive to the altitude and placement of that topographic barrier, as will other important features of regional air mass circulation. Similarly, differentiation between major land surface types, such as forests and lakes, can be expected to have a strong impact on the isotopic expression of moisture recycling in some regions via transpiration and evaporation.
Such limitations necessitate substantial expert judgement and reasoned latitude when comparing observational data with global equilibrium isotope climate simulations, since even the best simulations to date only approximate the observed spatial heterogeneity of the average distribution of water isotopes in global precipitation. An impression of the difficulty involved in evaluating isotopic GCM simulations can be gained from examination of Fig. 1, showing representations of the contemporary precipitation d18O field generated by two different models at differing grid resolutions. General patterns are certainly well-represented and similar in the two simulations, including the progressive depletion in heavy-isotope content inland and poleward (i.e., classical continental and latitudinal or temperature effects) and in the subtropics (amount effects), but the models are significantly restricted in their ability to reproduce the finer structure of the global isotope field. A somewhat fairer test of the ability of isotopic GCMs to capture the main features of global isotope climate, acknowledging the computational limits of current models, is provided by consideration of zonal means, which yields compelling evidence that poleward transport and distillation of atmospheric moisture (and hence energy) is being simulated realistically (Fig. 2).
In spite of their potential shortcomings, such simulations can serve as highly useful thinking tools, creating the need to critically address and explain deviations from observational data in control simulations, as a basic test of a model's ability and the utility of further experiments to hindcast or forecast isotope climate under different boundary conditions.
2.1.2 Transient-state isotope climate modelling
The skill of isotopic GCMs can also be probed using transient-state simulations, providing additional insight into how realistically a model's water cycle performs when forced by changing boundary conditions, such as the use of observed sea surface temperatures over a given period, rather than fixed average "climatological" SSTs. Transient simulations afford the opportunity to assess the magnitude and nature of the subsequent temporal variability in the simulated distribution of isotopes in atmospheric moisture and precipitation, as well as variability in the relations among isotopic and non-isotopic climate parameters. Direct comparison with observational data is limited by the restricted availability of continuous long time-series; nevertheless, promising representations of variability at time scales from interannual to daily offer further confirmation that isotopic GCMs can realistically mimic important aspects of the water cycle (e.g., Cole et al. 1993, 1999; Hoffmann et al. 1998). Potential to evaluate model-simulated annual to decadal time-scale variability in the past also exists, given the ability to obtain paleo-isotope data from ice cores, tree ring sequences and other finely resolved archives.
2.3 Continental isotope paleoclimatology
Starting well in advance of the first attempts to incorporate water isotope tracers in GCMs, and continuing in parallel with both descriptive and model-based isotope climatology, has been the wide application of isotope data as indicators of continental paleoclimate. These range from estimation of paleotemperature based on the use of empirical spatial isotope-temperature relations as transfer functions, to inferences of water balance or other environmental factors without explicitly attempting to deconvolute the isotopic composition of paleoprecipitation. This extensive literature details the generation of isotopic time-series from various continental archives, with the main ones being polar and non-polar ice cores (e.g., Aristarain et al. 1986; Johnsen et al. 1995; Fisher et al. 1995; Stuiver et al. 1995; Thompson et al. 1989, 1993, 1995, 2000), groundwaters and cave deposits (e.g., Rozanski 1985; Winograd et al. 1992; Stute et al. 1992, 1995a, 1995b; Stute and Talma 1998), lake sediments (e.g., Edwards and McAndrews 1989; von Grafenstein et al. 1992, 1999; Hammarlund et al. 2001), and terrestrial plant matter (e.g., Edwards and Fritz 1986; Becker et al. 1991; Buhay and Edwards 1995; Pendall et al. 1999; Feng et al. 1999; Anderson et al. 2001; see also review by Rozanski et al. 1997).
Highlights of these studies include testing of the validity of empirical d-T relations as temperature transfer functions from comparison of recent isotopic and instrumental temperature time-series, which indicates remarkable stability of this basic isotope-climate relation in some regions over particular time-scales, in spite of climate change (e.g., Johnsen 1977; von Grafenstein et al. 1996), as well as identification of significant variation in d-T relations at certain times in some locations as a prominent feature of climate change (e.g., Edwards et al. 1996; Stute and Talma 1998; Hammarlund et al. 2001). Success has also been achieved in efforts to tease out the isotopic expression of past climate episodes like the Little Ice Age (e.g., Hoffmann et al. 2001), characteristic modes of climate variability in isotope paleo-records, such as the El Nińo–Southern Oscillation (e.g., Cole and Fairbanks 1990; Thompson 1993) and the North Atlantic Oscillation (e.g., Barlow et al. 1993; Appenzeller et al. 1998), as well as in preliminary attempts to map variations in the spatial distribution of isotopes in precipitation at times in the past (e.g., Edwards et al. 1996; Wolfe et al. 2000).
Relatively little data-model comparison has been undertaken to evaluate isotopic GCM paleoscenarios, which have mainly targeted the LGM because of the strong contrast to present conditions and the availability of a paleo-SST (sea surface temperature) field to establish lower boundary conditions (CLIMAP Project Members 1981). Although considerable continental paleo-isotope data exist for the LGM, especially at polar latitudes, the best-constrained records at mid- and low latitudes primarily originate from glaciers at elevations that are too high to be adequately represented by GCMs. More definitive comparison should be possible with simulations of other climates, such as the mid-Holocene warm period (c. 6000 yr BP), for which considerable low-elevation paleo-isotope data exist (especially from lake sediments), but efforts to model this time-slice have been hampered by the lack of an appropriate SST field (Jouzel et al. 2000; Hoffmann et al. 2000).
Knowledge of the distribution of isotopes in past global precipitation is urgently needed by paleoclimate researchers, in addition to climate modellers. This ranges from workers seeking to use paleoprecipitation isotope data as a temperature proxy (e.g., see Cuffey and Marshall 2000), to those wishing to separate important environmental signals like changing lake water evaporative enrichment or other factors from the signal of changes in the isotopic composition of local paleoprecipitation (e.g., see Wolfe et al. 2000). Highly detailed isotope paleodata are available in polar regions because of the concentration of international effort to obtain long glacier ice records, but the task of compiling and mapping the shifting distribution of isotopes in paleoprecipitation in other regions, based on diverse investigations in various archives, is less advanced. The latter need has been recognized and is now being addressed within the International Geosphere-Biosphere Programme - Past Global Changes Project through ISOMAP (Isotope Calibration and Mapping Study). The ISOMAP database will extend the range of observational records into the past and provide consistent time-slices and time-series of the changing distribution of isotopes in paleoprecipitation, ultimately serving as a paleo-counterpart to the GNIP (Edwards and Grafenstein, 1995; Edwards 1998).
2.4 Isotope hydroclimatology
Water isotope tracers also play a growing role in hydroclimatologic investigations, ranging from studies of catchment-scale water balance and runoff generation to basin-scale evaporation-transpiration partitioning. Investigations directly relevant to the spatial and temporal resolution of both GNIP data and isotopic GCMs include a pioneering use of water isotope data to obtain a "snapshot" of the water balance of the Mackenzie River Basin (Hitchon and Krouse 1972; see also Gibson et al. 1994; Gibson 2001), assessment of long time-series of stable and radioactive water isotope data from the Danube River (Rank et al. 1998), and ongoing efforts to use isotopic data to constrain predictions of the hydrologic and climatic impacts of Amazonian deforestation (Henderson-Sellers et al. 2001). Preliminary analysis of short-term records of stable water isotope tracers and other indicators has also been undertaken recently in the St. Lawrence (Yang et al. 1996) and Ottawa (Telmer and Veizer 2000) river systems. As emphasized by Hoffmann et al. (2000), such investigations can provide highly valuable data to aid in the evaluation of isotopic GCM scenarios because of the integration provided by runoff generation over large basins, which helps to compensate naturally for the mismatch between station-based observational data and gridded model output. As well, studies like that of Telmer and Veizer (2000) reflect increasing realization of the potential to use information inherent in the distribution of water isotopes in the hydrosphere to link water and energy cycles. Both viewpoints strongly underscore the value of incorporating water isotopes into regular monitoring of the discharge and chemistry of major rivers of the world and especially within targeted field programs like the continental-scale studies of the Global Water and Energy Cycle Experiment (e.g., see Edwards and Gibson 1995).
3. Directions for the Future
As the preceding discussions outline, the historical GNIP database and the ongoing GNIP program clearly have important roles to play in global climate and water research. Though noted above only briefly, it is important to keep in mind that the GNIP continues to be a source of valuable information for local hydrological studies, especially in regions where conventional meteorologic, hydrometric and hydrogeologic data are sparse, and where difficulties in managing water resources (and the human need) are often particularly acute. This would provide ample justification for maintaining the GNIP program in (at least) its current state, even in the absence of concerns about climate change and the need for better knowledge of the water cycle.
3.1 Mapping isotope climate
The increasing use and sophistication of isotopic GCMs has helped to fuel demand for more comprehensive global observational data, which are needed for improved documentation of the average distribution of isotopes in annual and monthly global precipitation, as well as for much better characterization of transient-state isotope climate at various time-scales. A significant challenge exists to provide global precipitation isotope data in a form that allows more effective comparison with isotopic GCM output, compensating for both the patchiness of GNIP data in time and space and the inherent mismatch between data from fixed stations on a complex surface and uniformly gridded GCM output on a highly simplified surface. Station data can be contoured and integrated over a grid cell for comparison with a corresponding model-generated grid value, but this can obviously engender large discrepancies in areas of high relief or highly variable surface characteristics. On the other hand, future-generation isotopic GCMs will certainly be able to simulate much finer spatial resolution and more realistic representation of Earth's surface, as can already be done with finer-resolution regional climate models nested within GCMs. As illustrated in Fig. 3, mapping of the average (steady-state) global isotope field appears to have progressed substantially from the early 1970s to 1990s, although a certain amount of this apparent progress simply reflects a substantially greater amount of interpolation, rather than a real increase in data coverage, driven by the desire for maps that could be visually comparable to isotopic GCM output. A more realistic impression of the current state of documentation of the annual and monthly average global isotope fields as contained in the GNIP data base (and the potential for better representation of such data) can be gained from examination of the series of maps in Figs. 4 and 5.
One possible strategy to bridge the gap between isotopic mapping and modelling could be to actively use isotopic GCMs as tools for interpolating and contouring GNIP station data, rather than simply comparing patterns and trends and isotope-climate effects. This could ultimately lead to the creation of a real-time global isotope re-analysis dataset (GIRAD) comprising best-approximation time-slice maps of the distribution of isotopes in global precipitation over actual months and years. Although founded on real and unchanging GNIP data, the GIRAD dataset would evolve in concert with the evolution of isotopic GCMs, providing the basis for much more detailed and effective data-model (and model-model) comparison for evaluation of equilibrium and/or transient isotopic GCM simulations. Ideally, GIRAD would incorporate all available GNIP data, weighted for reliability and with flexible, realistic error estimations reflecting the varying confidence to be placed in any given set of gridded values for d18O, d2H and d-excess. This database itself could also be directly updated, tested and corrected through ongoing monthly GNIP sampling, as well as through targeted sampling campaigns such as those of the ISOHYC (Isotopes in the Hydrological Cycle) initiative or GEWEX.
4. Key points and recommendations
§ GNIP data, both "historical" from the past four decades, and "contemporary" from ongoing sampling and analysis, are seeing widening use in global climate and water studies, far beyond that originally envisioned.
§ The GNIP database constitutes an extremely valuable resource, recording actual time-slices and time-series of the distribution of isotopes in precipitation in different regions and different times that can be linked directly with conventional meteorologic and hydrometric observations. This potential for development of "real-time" isotope climatology represents a fundamentally new way of analysing GNIP-style data, which has traditionally been heavily smoothed and averaged in both time and space in the interests of defining artificial norms and stationary cycles. Although this traditional approach initially met the demand for depictions of steady-state isotope fields for comparison with isotopic GCM simulations, this strategy has probably impeded the development of more sophisticated understanding of global isotope climate and, arguably, has significantly undermined the justification for continuing the GNIP program and its affiliated national networks, by seeming to suggest that the major benefits of the program have already been realized.
§ Maintenance of the GNIP station network world-wide at some minimum density remains essential, in order to ensure that real-time data continue to be collected. Determination of what that minimum density should be and which stations need to be re-activated or added poses crucial questions, yet the knowledge to properly inform such an evaluation does not currently exist. To a large extent, this is simply because sufficiently sophisticated analysis of existing and accumulating GNIP data remains to be undertaken. Efforts to motivate both producers (mappers) and users (modellers) of isotope climate data to undertake this challenge should be a high priority.
§ One approach could be to directly couple isotope climate mapping and modelling, with the aim of producing an evolving, global isotope re-analysis dataset ("GIRAD") comprising "soft" model-interpolated gridded data fields founded on "hard" GNIP station data. Such a dataset would provide a valuable common baseline description of real-time global isotope climate for more effective testing and comparison of isotopic GCMs, as well as a much more sophisticated basis for other uses of precipitation isotope data, ranging from better definition of hydrologic input functions and their variability to the calibration of isotopic paleo-indicators, and the opportunity to incorporate additional layers of data (e.g., isotopic characterization of atmospheric water vapour and surface waters).
§ Although GNIP data availability has been greatly improved with the advent of electronic, web-based publication, the lack of readily interpretable and usable global and regional "isomaps" has almost certainly contributed to under-utilization of the GNIP database, simply because of the difficulty in visualizing dynamic variability in seemingly static data. Presentation of existing data in various mapped forms, beginning with traditional "steady-state" views and leading ultimately to real-time "GIRAD" maps, should also be a priority of the GNIP program.
Anderson, W.T., Bernasconi, S.M., McKenzie, J.A., Saurer, M., and Schweingruber, F. 2001. Model evaluation for reconstructing the oxygen isotopic composition in precipitation from tree ring cellulose over the last century. Chemical Geology (in press).
Appenzeller C., Stocker T.F., and Anklin M. 1998. North Atlantic Oscillation dynamics recorded in Greenland ice cores. Science 282, 446-449.
Araguás-Araguás, L., Froehlich, K., and Rozanski, K. 2000. Deuterium and oxygen-18 composition of precipitation and atmospheric moisture. Hydrological Processes 14, 1341-1355.
Aristarain, A.J., Jouzel, J., and Pourchet, M. 1986. Past Antarctic Peninsula climate (1850-1980) from an ice core isotope record. Climate Change 8, 69-89.
Barlow, L.K., White, J.W.C., Barry, R.G., and Grootes, P. 1993. The North Atlantic Oscillation signature in deuterium and deuterium excess in the Greenland ice sheet project 2 ice core, 1840-1970. Geophysical Research Letters 20, 2901-2904.
Becker, B., Kromer, B., and Trimborn, P. 1991. A stable-isotope tree-ring timescale of the Late Glacial/Holocene boundary. Nature 353, 647-649.
Buhay, W.M., and Edwards, T.W.D. 1995. Climate in southwestern Ontario, Canada, between AD 1610 and 1885 inferred from oxygen and hydrogen isotopic measurements of wood cellulose from trees in different hydrologic settings. Quaternary Research 44, 438-446.
Buhay, W.M., Edwards, T.W.D., and Aravena, R. 1996. Evaluating kinetic fractionation factors used for ecologic and paleoclimatic reconstructions from oxygen and hydrogen isotope ratios in plant water and cellulose. Geochimica et Cosmochimica Acta 60, 2209-2218.
Charles, C., Rind, D., Jouzel, J., Koster, R., and Fairbanks, R. 1994. Glacial-interglacial changes in moisture sources for Greenland: influences on the ice core record of climate. Science 261, 508-511.
Charles, C., Rind, D., Jouzel, J., Koster, R., and Fairbanks, R. 1995. Seasonal precipitation timing and ice core records. Science 269, 247-248.
Ciais, P., and Jouzel, J. 1994. Deuterium and oxygen-18 in precipitation: an isotopic model including mixed cloud processes. Journal of Geophysical Research 99, 16,793-16,803.
CLIMAP Project Members. 1981. Climate: Long-range investigation, mapping and prediction (CLIMAP): Seasonal reconstruction of the Earth's surface at the last glacial maximum. Geological Society of America, MC 36.
Cole, J.E., and Fairbanks, R.G. 1990. The southern oscillation recorded in the oxygen isotopes of corals from Taraw atoll. Paleoceanography 5, 669-683.
Cole, J.E., Rind, D., and Fairbanks, R.G. 1993. Isotopic responses to interannual variability simulated by the GISS GCM. Quaternary Science Reviews 12, 387-406.
Cole, J.E., Rind, D., Webb, R.S., Jouzel, J., and Healy, R. 1999. Climatic control on interannual variability of precipitation d18O: The simulated influence of temperature, precipitation amount and vapour source region. Journal of Geophysical Research 104, 14,223-14,235.
Craig, H. 1961. Isotopic variations in meteoric waters. Science 133, 1833-1834.
Craig, H., and Gordon, L.I. 1965. Deuterium and oxygen-18 variations in the ocean and marine atmosphere. In Stable Isotopes in Oceanographic Studies and Paleotemperatures. Edited by E. Tongiorgi, Pisa, 9-130.
Cuffey, K.M., Alley, R.B., Grootes, P., Bolzan, J.M., and Anandakrishnan, S. 1994. Calibration of the d18O isotopic paleothermometer for central Greenland, using borehole temperatures. Journal of Glaciology 40, 341-349.
Cuffey, K.M., Clow, G.D., Alley, R.B., Stuiver, M., Waddington, E.D., and Saltus, R.W. 1995. Large Arctic temperature change at the Wisconsin-Holocene glacial transition. Science 270, 455-458.
Cuffey, K.M., and Marshall, S.J. 2000. Substantial contribution to sea-level rise during the last interglaciation from the Greenland ice sheet. Nature 404, 591-594.
Dansgaard, W. 1964. Stable isotopes in precipitation. Tellus 16, 438-468.
Edwards, T.W.D. 1998. Workshop Report. ISOMAP - Reconstructing the Isotopic Composition of Past Precipitation from Continental Archives. PAGES Newsletter 6, 10.
Edwards, T.W.D., and Fritz, P. 1986. Assessing meteoric water composition and relative humidity from 18O and 2H in wood cellulose: Paleoclimatic implications for southern Ontario, Canada. Applied Geochemistry 1, 715-723.
Edwards, T.W.D., and Gibson, J.J. 1995. Comments on the use of isotopic tracers in GCIP-LSA-NC and MAGS. Proceedings, International GEWEX Workshop on Cold-Season/Region Hydrometeorology, Banff, May 1995, IGPO Publication 15, 120-123.
Edwards, T.W.D., and McAndrews, J. H. 1989. Paleohydrology of a Canadian Shield lake inferred from 18O in sediment cellulose. Canadian Journal of Earth Sciences 26, 1850-1859.
Edwards, T.W.D., and von Grafenstein, U. 1995. Discussion document for an international project to map and model the isotopic composition of past and present global precipitation - "ISOMAP". In Final Report, Coordinated Research Programme on Use of Isotope Techniques in Palaeoclimatology - Continental Isotope Indicators of Palaeoclimate. Edited by K. Rozanski. IAEA, München, November 1995, Annex 4, 1-5.
Edwards, T.W.D., Wolfe, B.B., and MacDonald, G.M. 1996. Influence of changing atmospheric circulation on precipitation d18O-temperature relations in Canada during the Holocene. Quaternary Research 46: 211-218.
Feng, X., Cui, H., Tang, K., and Conkey, L.E. 1999. Tree-ring dD as an indicator of Asian monsoon intensity. Quaternary Research 51, 262-266.
Fisher, D.A., Koerner, R.M., and Reeh, N. 1995. Holocene records from Agassiz Ice Cap, Ellesmere Island, NWT, Canada. The Holocene 5, 19-24.
Friedman, I., Redfield, A.C., Schoen, B., and Harris, J. 1964. The variation of the deuterium content of natural waters in the hydrologic cycle. Review of Geophysics 2, 177-224.
Gat, J.R. 2000. Atmospheric water balance - The isotopic perspective. Hydrological Processes 14, 1357-1369.
Gat, J.R. 1996. Oxygen and hydrogen isotopes in the hydrologic cycle. Annual Review of Earth and Planetary Sciences 24, 225-262.
Gat, J.R. 1980. The isotopes of hydrogen and oxygen in precipitation. In Handbook of Environmental Isotope Geochemistry, Vol. 1. Edited by P. Fritz and J.-Ch. Fontes. Elsevier, Amsterdam, 21-47.
Gat, J.R., and Matsui, E. 1991. Atmospheric water balance in the Amazon Basin: an isotopic evapotranspiration model. Journal of Geophysical Research 96, 179-188.
Gat, J.R., Bowse, C.J., and Kendall, C. 1994. The contribution of evaporation from the Great Lakes to the continental atmosphere: estimate based on stable isotope data. Geophysical Research Letters 21, 557-560.
Gibson, J.J. 2001. Forest-tundra water balance signals traced by isotopic enrichment in lakes. Journal of Hydrology 251, 1-13.
Gibson, J.J., Edwards, T.W.D., and Prowse, T.D. 1994. Evaporation in the North: overview of quantitative studies using stable isotopes. In Mackenzie Basin Impact Study (MBIS) Interim Report #2. Edited by S.J. Cohen. Environment Canada, 138-150.
Gonfiantini, R., Roche, M.-A., Olivry, J.-C., Fontes, J.-C., and Zuppi, G.M. 2001. The altitude effect on the isotopic composition of tropical rains. Chemical Geology 181, 147-167.
Hammarlund, D., Buchardt, B., Edwards, T.W.D., Barnekow,L., and Birks, H.J.B. 2001. Holocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Scandinavia. The Holocene (in press).
Henderson-Sellers, A., McGuffie, K., and Chambers, S. 2001. Isotopes as validation tools for predictions of the impact of Amazonian deforestation on climate and regional hydrology. International Conference on the Study of Environmental Change Using Isotope Techniques, IAEA, Vienna, 23-27 April 2001, IAEA-CN-80.
Hendricks, M.B., DePaolo, D.J., and Cohen, R.C. 2000. Space and time variation of d18O and dD in precipitation: Can paleotemperature be estimated from ice cores? Global Biogeochemical Cycles 14, 851-861.
Hitchon, B., and Krouse, H.R. 1972. Hydrogeochemistry of surface waters of the Mackenzie River drainage basin, Canada - III. Stable isotopes of oxygen, carbon, and sulphur. Geochimica et Cosmochimica Acta 36, 1337-1357.
Hoffmann, G., and Heimann, M. 1993. Water tracers in the ECHAM general circulation model. Proceedings, Symposium on Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere, IAEA, Vienna, 19-23 April 1993, IAEA-SM-329/7, 3-14.
Hoffmann, G., and Heimann, M. 1997. Water isotope modeling in the Asian monsoon region. Quaternary International 37, 115-128.
Hoffmann, G., Jouzel, J., and Johnsen, S. 2001. Deuterium excess record from central Greenland over the last millennium: Hints of a North Atlantic signal during the Little Ice Age. Journal of Geophysical Research 106, 14265-14274.
Hoffmann, G., Jouzel, J., and Masson, V. 2000. Stable water isotopes in Atmospheric General Circulation Models. Hydrological Processes 14, 1385-1406.
Hoffmann, G., Werner, M., and Heimann, M. 1998. Water isotope module of the ECHAM atmospheric general circulation model: A study on timescales from days to several years. Journal of Geophysical Research 103, 16,871-16,896.
Johnsen, S.J. 1977. Stable isotope profiles compared with temperature profiles in firn and with historical temperature records. In Isotopes and Impurities in Snow and Ice. Proceedings, Grenoble Symposium, August-September 1975. IAHS Publication No. 118, 388-392.
Johnsen, S.J., Dahl-Jensen, D., Dansgaard, W., and Gundestrup, N. 1995. Greenland palaeotemperatures derived from GRIP borehole temperature and ice core isotope profiles. Tellus 47B, 624-629.
Joussaume, S., and Jouzel, J. 1993. Paleoclimatic tracers: An investigation using an atmospheric General Circulation Model under ice age conditions, 2: Water isotopes. Journal of Geophysical Research 98, 2807-2830.
Joussaume, S., Sadourny, R., and Jouzel, J. 1984. A general circulation model of water isotope cycles in the atmosphere. Nature 311, 24-29.
Jouzel, J., Alley, R.B., Cuffey, K.M., Dansgaard, W., Grootes, P., Hoffmann, G., Johnsen, S.J., Koster, R.D., Peel, D., Schuman, C.A., Stievenard, M., Stuiver, M., and White, J.W.C. 1997b. Validity of the temperature reconstruction from ice cores. Journal of Geophysical Research 102, 26471-26487.
Jouzel, J., Fröhlich, K., and Schotterer, U. 1997a. Deuterium and oxygen-18 in present-day precipitation: data and modelling. Hydrological Sciences Journal 42, 747-764.
Jouzel, J., Hoffmann, G., Koster, R.D., and Masson, V. 2000. Water isotopes in precipitation: Data/model comparison for present-day and past climates. Quaternary Science Reviews 19, 363-379.
Jouzel, J., Koster, R., and Joussaume, S. 1996. Climate reconstruction from water isotopes: What do we learn from climate models? In Climate Variations and Forcing Mechanisms of the Last 2000 Years. Edited by R.S. Bradley, J. Jouzel, and P.D. Jones. Springer-Verlag, Berlin, 213-241.
Jouzel, J., Koster, R.D, Suozzo, R.J., Russell, G.L., White, J.W.C., Broecker, W.S. 1991. Simulations of the HDO and H218O atmospheric cycles using the NASA GISS General Circulation Model: Sensitivity experiments for present-day conditions. Journal of Geophysical Research 96, 7495-7507.
Jouzel, J., Koster, R.D., Hoffmann, G., and Armengaud, A. 1998. Model evaluations of the water isotope-climate relationships used in reconstructing palaeotemperatures. Proceedings, Symposium on Isotope Techniques in the Study of Environmental Change, International Atomic Energy Agency, Vienna, 14-18 April 1997, 485-502.
Jouzel, J., Koster, R.D., Suozzo, R.J., and Russell, G.L. 1994. Stable water isotope behaviour during the LGM: A GCM analysis. Journal of Geophysical Research 99, 25791-25801.
Jouzel, J., Russell, G.L., Suozzo, R.J., Koster, R.D., White, J.W.C., Broecker, W.S. 1987. Simulations of the HDO and H218O atmospheric cycles using the NASA GISS General Circulation Model: The seasonal cycle for present-day conditions. Journal of Geophysical Research 92, 14,739-14,760.
Jouzel, J., and Merlivat, L. 1984. Deuterium and oxygen-18 in precipitation: modelling of the isotope effects during snow formation. Journal of Geophysical Research 89, 11,749-11,757.
Juillet-Leclerc, A., Jouzel, J., Labeyrie, L., and Joussaume, S. 1997. Modern and last glacial maximum sea surface d18O estimated from an isotopic general circulation model of the atmosphere: Some paleoceanographic implications. Earth and Planetary Science Letters 146, 591-605.
Koster, R.D., de Valpine, D.P., and Jouzel, J. 1993. Continental water recycling and H218O concentrations. Geophysical Research Letters 20, 2215-2218.
Krishnamurthy, R.V., and Bhattacharya, S.K. 1991. Stable oxygen and hydrogen isotope ratios in shallow ground waters from India and a study of the role of evapotranspiration in the Indian monsoon. In Stable Isotope Geochemistry: A Tribute to Samuel Epstein. Edited by H.P. Taylor, Jr., J.R. O'Neil, and I.R. Kaplan. The Geochemical Society, Special publication No. 3, 187-193.
Mathieu, R., Pollard, D., Cole, J., Webb, R.S., and White, J.W.C. in prep. Simulation of stable water isotopes variations by the GENESIS GCM for present-day conditions. Journal of Geophysical Research.
Merlivat, L., and Jouzel, J. 1979. Global climatic interpretation of the deuterium-oxygen-18 relationship for precipitation. Journal of Geophysical Research 84, 5029-5033.
Pendall, E., Betancourt, J.L., and Leavitt, S.W. 1999. Paleoclimatic significance of dD and d13C values in pińon pine needles from packrat middens spanning the last 40,000 years. Palaeogeography Palaeoclimatology Palaeoecology 147, 53-72.
Rank, D., Adler, A., Araguás-Araguás, L., Froehlich, K., Rozanski, K., and Stichler, W. 1998. Hydrological parameters and climatic signals derived from long term tritium and stable isotope time series of the River Danube. Proceedings, Isotope Techniques in the Study of Environmental Change, Vienna, IAEA-SM-349/16, 191-205.
Rindsberger, M., and Magaritz, M. 1983. The relation between air-mass trajectories and the water isotope composition of rain in the Mediterranean Sea area. Geophysical Research Letters 10, 43-46.
Rozanski, K., Sonntag, Ch., Munnich, K.O. 1982. Factors controlling stable isotope composition of modern European precipitation. Tellus 34, 142-150.
Rozanski, K., Araguás-Araguás, L., and Gonfiantini, R. 1992. Relation between long-term trends of oxygen-18 isotope composition of precipitation and climate. Science 258, 981-985.
Rozanski, K., Araguás-Araguás, L., and Gonfiantini, R. 1993. Isotopic patterns in modern global precipitation. In Climate Change in Continental Isotopic Records. Edited by P.K. Swart, K.L. Lohmann, J. McKenzie, and S. Savin. Geophysical Monograph 78, American Geophysical Union, Washington, DC, 1-37.
Rozanski, K. 1985. Deuterium and oxygen-18 in European groundwaters - Links to atmospheric circulation in the past. Chemical Geology (Isotope Geoscience Section) 52, 349-363.
Rozanski, K., Johnsen, S.J., Schotterer, U., and Thompson, L.G. 1997. Reconstruction of past climates from stable isotope records of palaeo-precipitation preserved in continental archives. Hydrological Sciences Journal 42, 725-745.
Salati, E., Dall'ollio, A., Matsui, E., and Gat, J.R. 1979. Recycling of water in the Amazon Basin, an isotopic study. Water Resources Research 15, 1250-1258.
Stuiver, M., Grootes, P.M., and Braziunas, T.F. 1995. The GISP2 d18O climate record of the past 16,500 years and the role of the sun, ocean and volcanoes. Quaternary Research 44, 341-354.
Stute, M, and Talma, A.S. 1998. Glacial temperatures and moisture transport regimes reconstructed from noble gases and d18O, Stampriet aquifer, Namibia. Isotope Techniques in the Study of Environmental Change, Vienna, IAEA-SM-349/53, 307-318.
Stute, M., Clark, J.F., Schlosser, P., Broecker, W.S., and Bonani, G. 1995a. A 30,000 yr continental paleotemperature record derived from noble gases dissolved in groundwater from the san Juan Basin, new Mexico. Quaternary Research 43, 209-220.
Stute, M., Forster, M., Frischkorn, H., Serejo, A., Clark, J.F., Schlosser, P., Broecker, W.S., and Bonani, G. 1995b. Cooling of tropical Brazil (5°C) during the last glacial maximum. Science 269, 379-383.
Stute, M., Schlosser, P., Clark, J.F., and Broecker, W.S. 1992. Paleotemperatures in the southwestern United States derived from noble gases in groundwater. Science 256, 1000-1002.
Telmer, K., and Veizer, J. 2000. Isotopic constraints on the transpiration, evaporation, energy, and gross primary production budgets of a large boreal watershed: Ottawa River basin, Canada. Global Biogeochemical Cycles 14, 149-165.
Thompson, L.G. 1993. Reconstructing the paleo ENSO records from tropical and subtropical ice cores. Bulletin de l'Institut Francais d'Etudes Andines 22, 65-83.
Thompson, L.G., Mosley-Thompson, E., and Henderson, K.A. 2000 Ice-core palaeoclimate records in tropical South America since the last glacial maximum. Journal of Quaternary Science 15, 377-394.
Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.N., Dai, J., Yao, T. Gundestrup, N., Wu, X., Klein, L., and Xie, Z. 1989. Holocene-Late Pleistocene climatic ice core records from Qinghai-Tibetan Plateau. Science 246, 474-477.
Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.N., Henderson, K.A., Cole-Dai, J., Bolzan, J.F., and Liu, K.B. 1995. Late glacial stage and Holocene tropical ice core records from Huascaran, Peru. Science 269, 46-50.
Thompson, L.G., Mosley-Thompson, E., Davis, M.E., Lin, P.N., Yao, T., Dyurgerov, M., and Dai, J. 1993. "Recent warming": Ice core evidence from tropical ice cores with emphasis on Central Asia. Global and Planetary Change 7, 145-156.
von Grafenstein, U., Erlenkeuser, Brauer, A., Jouzel, J., and Johnsen, S.J. 1999. A mid-European isotope-climate record from 15,500 to 5000 years BP. Science 284, 1654-1657.
von Grafenstein, U., Erlenkeuser, H., Müller, J., and Kleinmann-Eisenmann, A. 1992. Oxygen isotope records of benthic ostracods in Bavarian lake sediments: Reconstruction of late and post glacial air temperatures. Naturwissenschaften 79, 145-152.
von Grafenstein, U., Erlenkeuser, H., Müller, J., Trimborn, P., and Alefs, J. 1996. A 200-year mid-European air temperature record preserved in lake sediments: An extension of the air temperature-d18Op relation into the past. Geochimica et Cosmochimica Acta 60, 4025-4036.
Werner, M., Mikolajewicz, U., Heimann, M., and Hoffmann, G. 2000a. Borehole versus isotope temperatures on Greenland: Seasonality does matter. Geophysical Research Letters 27, 723-726.
Werner, M., Mikolajewicz, U., Hoffmann, G., and Heimann, M. 2000b. Possible changes of d18O in precipitation caused by a meltwater event in the North Atlantic. Journal of Geophysical Research 105, 10161-10167.
Winograd, I.J., Coplen, T.B., Landwehr, J.M., Riggs, A.C., Ludwig, K.R., Szabo, B.J., Kolesar, P.T., and Revesz, K. 1992. Continuous 500,000-year climate record from vein calcite in Devils Hole, Nevada. Science 258, 255-260.
Wolfe, B.B., Edwards, T.W.D., Aravena, R., Forman, S.L., Warner, B.G., Velichko, A.A., and MacDonald, G.M. 2000. Holocene paleohydrology and paleoclimate at treeline, north-central Russia, inferred from oxygen isotope records in lake sediment cellulose. Quaternary Research 53, 319-329.
Yang, C., Telmer, K, and Veizer, J. 1996. Chemical dynamics of the "St. Lawrence" riverine system: dDH2O, d18OH2O, d13CDIC, d34Ssulfate, and dissolved 87Sr/86Sr. Geochimica et Cosmochimica Acta 60, 851-866.
Yurtsever, Y., and Gat, J.R. 1981. Atmospheric waters. In Stable Isotope Hydrology: Deuterium and Oxygen-18 in the Water Cycle. Edited by J.R. Gat and R. Gonfiantini, International Atomic Energy Agency, Vienna.
Yurtsever, Y. 1975. Worldwide survey of stable isotopes in precipitation. Report of the Hydrology Section, International Atomic Energy Agency, Vienna, 40 pp.
MAP OF THE GNIP NETWORK
status on 01-11-2001
THOUGHTS ON CHOICE OF STATIONS FOR GNIP
prepared by Mr. Hastenrath
The choice of stations may be guided by these considerations/criteria:
(A) strategic location within the general circulation
(B) existence of long-term rain gauge station
(C) existence of an established institution to be in charge
(D) existence of previous isotope measurement series
The input below is organized into themes of circulation and climate dynamics.
I. SOUTH AMERICA
1. Amazon basin
This is the domain of one of the large equatorial centers of deep convection, of the world's largest freshwater stream, and major remnant of tropical rainforest. The fate of the atmospheric water vapor along its trajectory from the tropical Atlantic over the vast expanses of jungle to the eastern slope of the Andes, and the recycling processes involved, have long aroused the interest of meteorologists (Heinz Lettau) and geochemists (Eneas Salati) alike. Because of this strategic situation and the changing surface conditions, the region merits foremost attention under the auspices of GNIP. There is a number of longterm raingauge stations. Coooperation may plausibly be sought with Instituto Nacional de Meteorologia, INMET, Brasilia. Salati pioneered with the establishment of a network monitoring isotopes in precipitation. Resumption of the program at MANAUS and BELEM could be a plausible start, but it would be most desirable to resurrect the entire previous network.
Northeast Brazil, the Nordeste, has since the l9th century been recognized as one of the "problem climates" of the tropics. It is a semi-arid region, the short rainy season is narrowly concentrated around March-April, there are large year to year variations, and the Secas, or droughts, have a severe socio-economic impact. Empirical diagnostic research since the l970's has progressively elucidated the atmospheric-oceanic mechanisms of annual cycle, interannual and longer-term climatic variability, and laid the foundations for seasonal forecasting. Accordingly, the time is ripe for the exploration of isotopes in precipitation. As a bare minimum, FORTALEZA and QUIXERAMOBIM offer themselves as obvious locations for such monitoring. They have long series of raingauge measurements. Cooperation is to be sought with an established government institution, the Fundacao Cearense de Meteorologia e Recursos Hidricos, FUNCEME, with headquarters in Fortaleza.
3. Southern Tropical Andes
The region is located near the subtropical anticyclonic axis or boundary between the tropical easterlies and the midlatitude westerlies. The alternations between these circulation regimes are reflected in the water level of the large inland Lake Titicaca, as well as in the mass balance and vertical profiles of high mountain glaciers. Ice cores have been retrieved from the Quelccaya Icecap of Peru in the l980's, from Sajama and Ilimani in Bolivia in the l990's, and from Cerro El Tapado in northern Chile later on. In this context, measurements of isotopes in precipitation are highly desirable. There is a record of water level of Lake Titicaca since the
early part of the 20th century, with a comparably continuous series of raingauge measurements at the Jesuit Colegio San Calixto in La Paz. As a bare minimum one, or better two stations are desired for the monitoring of isotopes in precipitation. The obvious choices are in the capital LA PAZ, Bolivia, and in PUNO, Peru. Collaboration should be sought with the repective national meteorological services of these countries, and possible also with IRD-ORSTOM.
4. West coast meridional profile
Chile encompasses the range of zonal circulation regimes from the subtropics across the midlatitudes to the subpolar region. Long-term raingauge stations are operated at essential locations, and there is a functioning national meteorological service. For the measurement of isotopes in precipitation, the following stations offer themselves, proceeding from North to South: LA SERENA, SANTIAGO, PUERTO MONTT, and USHUAIA in Argentina, where again the national meteorological service may be an obvious partner.
A station in this location at the edge of the near-equatorial oceanic cold tongue and the equatorial dry zone would be very desirable. For GALAPAGOS collaboration should be sought with the Instituto Nacional de Meteorologia e Hidrologia of Ecuador, or possible by some other national institution.
6. Northern Argentina
There have been suggestions for a GNIP station in SALTA. As an extension of a complete "Salati-size" network in the Amazon basin, as urged in "I.1." above, this would be welcome, albeit with lower priority.
This short label is meant to encompass Central America, Mexico and the Caribbean. One can think of a meridional profile along the Mesoamerican land bridge from Panama over Central Americ into Mexico, and some islands in the Caribbean, once higher priorities elsewhere are met.
1. West African monsoon
Extensive research over the past several decades has elucidated the climate dynamics of the boreal summer monsoon, in particular the seasonal latitude displacement of the Intertropical Convergence Zone (ITCZ) and the remarkable zonal arrangement of climatic zones, ranging from the tropical rainforest in the South over the semi-arid Sahel to the Sahara desert and beyond. On this basis, a meridional profile of GNIP stations appears highly desirable. This would capture not only the succession of climatic conditions to the South of the Intertropuical Discontinuity (ITD or FIT) but also sample the rare hydrometeors in the central Sahara, where
influences from the tropics and midlatitudes may prevail according to the season of the year. There are long-term raingauge stations at key locations, as well as functioning national meteorological services. A plausible transect of stations would be as follows: LAGOS and KANO in Nigeria, NIAMEY (AGRYMET) in Niger, BAMAKO in Mali, ASSEKREM-TAMANRASSET and ALGER or ORAN in Algeria, on the Mediterranean coast. Undubitably, the high esteem enjoyed by the Secretary-General of WMO will be an important
asset for arrangements in his home country Nigeria as well as with services thoughout the region.
2. Eastern Africa
With regard to river, lake, and groundwater hydrology, the lakes of the great rift, Lake Victoria, and the Nile system, are obvious targets for isotope hydrology. As plans for such objectives mature, thought may be appropriately given to the atmospheric component, with some stations for the sampling of isotopes in precipitation.
III. MONSOON ASIA
Given the long meteorological tradition, thorough sustained research, long-standing raingauge network, and institutional stability, it seems plausible to aspire to a strong GNIP network here. Such objectives merit further consideration.
From the large-scale setting within the general circulation, established institutions and long raingauge records one should think that GNIP may undertake a major initiative here. This needs to be given more thought.
IV. SOUTHERN OSCILLATION
1. Australasian region
Because of its role in the near-global context, this hub of the SO requires attention. Long-term rasingauge records exist for DJAKARTA and other stations in Indonesia, and for DARWIN in northern Australia. For Indonesia partnership may be sought with the national meteorological service or a university. For Australia prospects seem optimal with the Bureau of Meteorology.
2. Pacific Islands
In addition to GALAPAGOS mentioned in "I.5." above, some other islands merit consideration from their strategic location near the subsidence/high-pressure hub of the SO, long-term raingauge record, and institutional stability, namely ISLA DE PASCUAS (EASTER ISLAND), Chile, and TAHITI, France. Contact with the respective meteorological services seems a plausible start.
IV. GLACIER MONITORING
Given the implications for the climatic interpretation of icecore profiles, isotope monitoring is desired at the very few sites were mass balance and kinematics of glaciers are still monitored. These are regrettably few and have decreased in number.
1. Central Asia
In all of China, one glacier is being monitored, URUMQI-No.1 in the Tienshan, under the auspices of the Institute for Glaciology and Geocryology of the Chinese Academy of Sciences, in Lanzhou. In the Tienshan of Kazakhstan, monitoring continues for the TUYUKSU Glacier, under the auspices of the Institute of Geography of the Kazakh Academy of Sciences. Monitoring of the ABRAMOV Glacier in the Pamir has been disrupted.
Turning from Central Asia to South America, the French IRD-ORSTOM has started monitoring programs on the ZONGO and CHACALTAYA glaciers in Bolivia, and on a glacier of ANTIZANA (15) in Ecuador. Cintinuation of these programs into the coming decades seems uncertain. Contacts with IRD should be considered.
Maintenance of networks over decades is a challenge, so that an appraisal of priorities is essential. In this spirit, the following of the components presented above appear most essential: I.1. Amazon basin, I.2. Nordeste, I.3.West coast profile, III.1. West African monsoon, IV.1. Australasian region, V.1. Central Asia. Before initiating the measurement programs, a careful appraisal is needed of the prospect for maintaining them without interruption indefinitely.