Atoll islands are low, flat land on atolls with a maximum elevation of one to two meters. They are under crisis of submergence due to rising sea levels in this century. The formation and maintenance of atoll island landforms are regulated not only by physical but also by ecological processes. The source of sand constituting the islands is calcifying organisms, and coral reefs on the ocean side of the islands act as a natural breakwater and provide bioclastic materials. Bleaching induced by global warming degrades the ecological process of island formation. Moreover, the traditional vegetation management system acts to stabilize landforms, but urbanization of atoll islands disrupted this human process, thereby increasing the vulnerability of these islands to environmental stresses. For sustainable utilization and development of atoll islands, it is fundamental to evaluate the interaction between natural and human processes through interdisciplinary research among geomorphology, anthropology and coastal engineering. The ultimate goal of the research is to establish a diagnosis of the present status and future response of atoll islands to environmental changes, and to establish an ‘atoll island model’ to predict their landform changes in response to future sea levels. Another objective is to re-evaluate traditional human management systems of island landforms and vegetation in effective utilization of resources and land. Remote sensing and GIS would provide effective tools to extend the results obtained in model fields to other atoll islands in general.

　Reef islands are formed entirely of bioclasts produced in adjacent reefs that formed in response to Holocene sea-level changes. Thus reef islands are regarded as combined products of geological, physical, and ecological processes. They have diversity in size, constituents, sedimentation pattern, and dynamics, which would be a result of finer-scale variations in environmental factors. We provide an overview of the developmental history of reef islands and factors controlling their formation and maintenance. The factors are: 1) Geological: relative fall in sea level in the late Holocene, 2) Physical: waves, currents and cyclones, and 3) Ecological: distribution of reef-building organisms and land vegetation, and factors that influence coral-reef development. We show how the factors are changing or predicted to change in the future, and propose two integrated studies: a process-based modeling study and a mapping-based comparative study.

　Topographic changes in Majuro Atoll, Marshall Islands, are caused by various natural and artificial processes, for example, sediment transport induced by waves and currents, sediment supply from coral reefs, and the existence of constructed structures along the shoreline. For the present paper, numerical calculations of wave transformations of the lagoon of Majuro were carried out, and wave energy fluxes, which are the potentials of alongshore sediment transport, were also estimated. The waves in the lagoon were assumed to be generated in two ways: wind waves and diffracted waves from swells. The contribution of waves to topographic changes in Majuro were estimated by comparison with the process of past topographic change, which was obtained by satellite imagery.

　Coral atolls have been considered the most precarious landforms thus having an extremely severe environment for human settlements in the Oceanic realm. It appears that atoll islets of foraminiferal sand and coral shingle accumulated on reef flats are considerably weaker than high islands with regard to sea-level change and climatic fluctuation, but there also seems to be variety to their vulnerability. In fact our excavation on Laura, the largest and widest islet in Majuro atoll, Marshall Islands, has revealed that its earliest colonization surprisingly dated back to 2,000 BP, and agricultural pits for cultivation of aroid species – probably Cyrtosperma – were constructed some centuries later at the least, while a smaller islet shows a much younger age of 360 BP. The long-lasting landscape of pit agriculture comprising remarkable undulation and rich vegetation, which appears to be the richest and most diverse part of the present atoll environment, can be viewed as a symbol indicating the islets are in good condition in aspects of both land formation and human subsistence. Here, we will discuss this landscape as a historical product invented through the entanglement between two agencies, nature and human. Understanding this will be also helpful in framing policies regarding anticipated sea-level rising in the future.

　This paper gives a brief overview of the current status of reef island mapping, and reports on our efforts to generate maps for two low-lying atoll countries, the Republic of the Marshall Islands and Tuvalu, for which no recent detailed land-use maps or topographic maps are available. Three topics are presented: 1) Nationwide mapping by satellite remote sensing, 2) Shoreline change detection using maps, aerial photographs and satellite images, and 3) Topographic mapping by digital photogrammetry. We indicate improvements and future prospects for each topic, as well as possible analyses which could be performed by integrating these three topics.

　Reef-island countries constitute the smallest and most isolated low-lying habitable land on Earth, thus they are vulnerable to the environmental consequences of climatic change. To effectively provide measures for adapting to climate change situations, a better understanding of the diversity of the natural and socioeconomic conditions of reef islands is needed. We have established a global geographic database that includes data on: (1) natural factors affecting the geological, physical or ecological processes that control the formation and maintenance of reef islands; (2) factors characterizing the socioeconomic state of reef-island countries; and (3) geographic features that can be referred to for location on a map. The geographic database could serve as a basis for breaking the natural and socioeconomic conditions down into several patterns, and help in identification of reef islands where similar adaptation measures could be applicable. This report outlines the data we have collected and the processes we have employed to develop our geographic database.

　Numerous studies were undertaken for scientific support of the APN-funded project of the network system for monitoring and predicting ENSO events and the sea temperature structure of the warm pool in the western Pacific. This paper is a brief summary of scientific highlights derived from this project which include: (1) Development of some new indices for monitoring ENSO events and sea temperatures and associated convective activity over the warm pool in the western Pacific. (2) Development of a number of dynamic and statistical models for prediction of ENSO events on the basis of which multi-model ensemble predictions for both dynamical and statistical models are made. Verification of predictive skills shows good model performance. (3) A proposal for an air-sea interactive conceptual model applicable to predicting sea temperatures over the warm pool by using the subsurface currents and temperature dataset from the TOGA-TAO array. Observational evidence is presented to reveal the formation mechanism and eastward propagation features of subsurface current anomalies driven by the strong westerly wind anomalies over the warm pool in the western Pacific Ocean.

　The Himalayas with their unique physical, biological and human systems provide environmental goods and services to almost one-tenth of the global population and affect in the long run the global environment. The Himalayan Mountains, however, because of their young and fragile nature coupled with sharp gradients, are particularly vulnerable to impacts of a rapidly changing climate often coupled with anthropogenic alteration of mountain landscapes due to population changes and economic activities. People in the regions have been long marginalized and have the fewest resources and capacities to adapt to these changes and hence are the most vulnerable.

　The crucial aspects of global change impacts in the Himalayan mountain regions are ultimately about their potential effects on ecosystems and resources as well as on the societies that depend on them. As many of these effects are interactive with some specific thresholds, an understanding of the response of the system to global change and the associated vulnerabilities and potential adaptations, is crucial especially for preserving the natural resources and encouraging environmental management and sustainable development of mountain regions.

　Some of the results of the research studies on global change impact assessment for the Himalayan mountain regions are presented and discussed in this paper. The observed climatic changes are briefly presented as are their impacts on water resources, food security and people’s livelihoods. Likewise the effects of the process of globalization and economic liberalization on the Himalayan mountain communities are also briefly discussed.

　Increases in atmospheric CO₂ concentration exert several positive and negative effects on litter dynamics, but the net impact of climate change on litter decomposition and the carbon balance of the terrestrial ecosystem is yet unclear. The objective of this review is to summarize the current state of knowledge on litter dynamics under elevated CO₂ levels with a view to using it as a basis for mitigating the negative effects of rising anthropogenic CO₂ emissions. To understand the biogeochemical processes in response to elevated CO₂ levels, we propose a conceptual model illustrating the interactive effects of rising CO₂ levels on litter dynamics, which assumes that the expected positive effects on photosynthesis will be translated into higher detritus production. The general dilution of nitrogen within plant tissues grown under elevated CO₂ and the consequent N constraints on microbes, however, may have a strong feedback effect on photosynthetic efficiency through depressed nutrient mineralization. Again, a concomitant rise in the atmospheric temperature may accelerate the process of litter decomposition. Significant regional variations in litter production and decomposition rates are also probable in view of the changes in precipitation intensity/patterns and/or biodiversity changes associated with global warming. Gains in litter accumulation on the forest floor under elevated CO₂ levels may lead to increased carbon sequestration and could partially offset the negative effects of anthropogenic CO₂ emissions into the atmosphere.