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1995-2010?

1980-on going 1997

Breakthrough to seasonal predictions through new observing system and coupled modeling

1. First quality-controlled global time series for precipitation, cloud parameters, water vapor column content, and surface radiation fluxes

2. Contribution of water storage in soils to medium-range weather and climate predictions

1. First observation of global ocean structure (except polar seas)

2. Sea level change time series every 10 days

1. Stratospheric temperature trend analysis

2. Realistic stratospheric circulation modules as part of general circulation models (GCMs)

1. Sea-ice modeling for AOGCMs

2. Three-dimensional surveys of the Arctic Ocean

1. First initiatives on climate variability in the Americas and Africa

2. Asian monsoon prediction focus

1. Extended weather forecasting

2. Realistic atmospheric circulation modules for coupled modeling

(Coupled model intercomparison operational predictions of the variations for use in climate services in support of sustainable development.

• Detect climate change, attribute causes, and project the magnitude and rate of human-induced climate change, its regional variations, and related sea level rise (as needed for input to the IPCC, UN FCCC, and other conventions).

How difficult it will be to rcach these objectives can best be indicated by the formulation of scientific questions that must be answered before we can speak of success:

• Can we predict the onset, strength, and breaks of monsoons weeks and months ahead? Only joint action by the projects GEWEX and CLIVAR will be able to give an answer.

• Are the strength and frequency of ENSOs influenced by humankind?

• Would an increase in strength be a major consequence, and would there be new weather extremes? Only a reconstruction of climate history beyond the instrumental record, as planned in CLIVAR, can give an answer.

• Will the North Atlantic deep water (NADW) formation rate be slowed or stop if the greenhouse gas concentration increase continues unabated? The prerequisite

286 martin h kim ann carbon cyclc on dccadal-to-ccntcnnial time scalcs to understand its interaction with the climate will be different from a strategy directed at the determination of national carbon balances in the context of the Kyoto Protocol, and it w ill also be different if the goal is the assessment of regional impacts induced by various forcing factors.

A further fundamental problem consists of possible nonlinear, rapid transitions ("surprises") in parts of the global system, for which, almost by definition, an adequate observing strategy is impossible or very difficult to conceive - as for example, w itnessed, by the detection history of the Antarctic ozone hole. Needed is an environment of good science that can detect surprises. Furthermore, observational systems should be designed to be flexible enough so that they can rapidly be adapted to new demands.

Monitoring large- and global-scale changes in biogeochemistry constitutes a formidable scientific challenge. For example, recent studies to assess continental-scale net carbon balances by inverse atmospheric transport modeling from global atmospheric CO2 concentration measurements have yielded very contradictory results. The reasons for these discrepancies arc related to largely unknown imperfections in the employed atmospheric models, to inaccuracies in the observational data because of measurement, calibration, and sampling errors, and also to the highly underdetermined nature of the underlying mathematical problem. The latter means that many different source/sink configurations will result in almost the same atmospheric concentration signatures at the relatively few observing sites, and that prevents an unambiguous discrimination among various source/sink scenarios. Significant progress might come from an integrative approach in which a meso-scale atmospheric model, combined with a process model of surface biogeochemical trace gas flux exchanges, is developed. This model would be run in an assimilation mode with high-density atmospheric concentration sampling including, for example, regular measurements from aircraft, surface-based flux measurements, and information from remote sensing platforms. Evidently, such an approach is based on the largely successful paradigm of the global meteorological data assimilation systems established under the auspices of WMO for operational weather forecasts. Several plans for feasibility studies to develop such an observing system for biogeochemical species (in particular for CO2) have recently been proposed -for example, the Carbon America Plan (Tans et al.. Global Change Biology, 2, 309-318, 1996) - or are being developed as part of new regional field projects - for example, the Large-Scale Biosphere Project in Amazonia (LBA, http://yabae.cptec.inpe.br/lba/, or http://www.PIK-Potsdam.DE/mirror/bahc/lba/) or the Eurosiberian Carbonflux project (http:/7www.bgc-jcna.mpg.de/projects/sibir/flux01.html). Similar initiatives are also being planned for Australia and South Africa. Whether these initiatives will be successful remains to be seen. In any case, there exists a need to assess and optimize these regional initiatives into a design for a global monitoring strategy. The Task Force on Global Analysis, Interpretation and Modeling of the IGBP has recently taken up the challenge to take the lead in this endeavour. However, it is clear that the implementation and coordination of such a global observing system for biogeochemical trace gases ultimately will have to be established within a UN body, such as WMO.

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