Desertification and other land depletion are serious environmental problems on a global scale, affecting developing countries and developed countries alike. Space-based observations are essential for studying these phenomena, which cover a wide area and are directly related to food shortages.
Desertification of farmland and rangeland
Brought about by the influence of human activity and natural disasters such as recurring droughts, desertification is thought to have a great impact on food production, rangeland for livestock, and systems for supplying water and energy. Such problems affect the social economies in most arid regions, and particularly in developing nations of Asia, Africa, and South America.
The advance of desertification adversely affects human society in many ways. However, its most direct effect is on the foundation of food production in the form of a depletion of rangeland and farmland.
According to a 1991 report from the United Nations Environment Program (UNEP), the effect of desertification on stock farming is the most widespread, with approximately 3.333 billion hectares of rangeland being affected. This corresponds to 73 percent of the total rangeland distributed in arid regions.
The next most affected region is rainfall-dependent farmland. Desertification affects 226 million hectares of farmland, corresponding to 47 percent of the total rainfall-dependent farmland in arid regions. Of the farmland in arid regions, approximately 43 million hectares are affected by desertification, mainly in the form of salinization.
Desertification has global repercussions
In 1992, the Earth Summit (the United Nations Conference on Environment and Development: UNCED) was held in Rio de Janeiro, Brazil to develop cooperative measures for environmental changes and achieve environmental preservation and sustainable development. This Summit was a large-scale conference attended by 100 leaders and heads of state from 180 participating countries and regions.
Agenda 21, adopted at the Earth Summit, defined desertification as the degradation of land in arid, semiarid, and dry sub-humid areas resulting from various factors including climatic changes and human activity.
According to this definition, desertification is not simply caused by natural factors (large-scale changes in atmospheric cycles and droughts), but also artificial factors.
The artificial factors include overgrazing, over-cultivation, poor irrigation, inadequate management of forests and deforestation practices, and destruction of vegetation. Each of these factors has its own causes. Over-cutting, for example, is probably caused by commercial logging, migrating cultivation, and increasing demand for fuel-wood (to secure firewood for everyday living in developing countries), and by having no adequate legal system regarding environmental preservation.
This problem of over-cutting leading to the destruction of forests is extremely grave in the Asia-Pacific region, and particularly in Indonesia, Malaysia, Thailand, and India.
Naturally, the decline in productivity of lands due to desertification can lead to a scarcity of food and have an adverse effect on the living conditions in the regions. In serious cases, this problem can threaten the foundation of human existence itself with famine and give rise to environmental refugees and other chaotic social situations. For example, a lot of human lives and livestock were lost during droughts in the Sahel region on the south edge of the Sahara Desert, which peaked in 1972, 1973, 1983, and 1984. During these periods, there were occurrences of environmental refugees over a wide area, causing a serious political and social problem.
Japan does not view desertification as a problem that will have much impact on it. However, desertification in such large regions as Asia, Africa, and South America lowers food production, increases poverty in developing nations, influences climatic changes, and impacts diversity of life forms. These problems are gradually transcending the borders of countries and regions to affect the entire world.
Monitoring desertification from space
The advance of desertification is indeed having grave adverse effects on the environment, but we must also be particularly concerned with the wide-ranging impact of desertification on climatic changes. Much information has been gathered on this issue to date. However, we must work on this problem more actively in the future with the aid of satellite observation data. Base data for developing and implementing technology and policies to prevent desertification in the regions will be gathered by regularly monitoring vegetation, soil, soil moisture, and the like over large expanses in arid and semiarid regions and by measuring quantities of changes in this data.
However, surveys of vegetation and soil in regions experiencing desertification are very expensive and difficult for humans to conduct due to the large areas that the surveys must cover and the severity of the natural environment. Therefore, surveys using such advanced technology as remote sensing are indispensable, as they enable us to gather data over large areas simultaneously.
Since satellites orbit the Earth in regular cycles, they are capable of performing regular observations of large regions that people would have great difficulty reaching. It is possible to access data acquired by LANDSAT Multispectral Scanner(MSS) from as far back as 1972. Accordingly, we can compare current data with data from nearly 30 years ago, enabling us to identify the progression of such environmental changes as desertification of land and changes in land use.
Figure 2 shows a false color composite image of the southwest portion of Chad Lake in Central Africa, observed on December 8, 1972, by LANDSAT MSS. In this image, vegetation appears red, water blue, bare land off-white, and clouds and the like white.
Figure 3 is a false color composite image of the same region observed by OPS aboard JERS-1 on September 28, 1994.
By comparing Figure 2 and Figure 3, the environmental changes occurring over the 22-year span can be seen. We can see how an area of the lake 22 years earlier has transformed into a marsh that is grown over with vegetation. This kind of environmental change is thought to be due not only to weather changes, but also to artificial factors such as the influence of human activity upstream on the river that flows into this lake.
The region of vegetation seen in the more recent image is distributed over a portion that used to be a water region, illustrating the magnitude of environmental change around the lake. However, this is only a simple comparison using images from two time periods. We can identify changes in the vegetation region by calculating special physical quantities such as the Normalized Difference Vegetation Index (NDVI). Visible and near-infrared satellite data is most commonly used to achieve this.
 By observing visible, near-infrared satellite data and all-weather Synthetic Aperture Radar (SAR) data, which is not affected by weather or time of day, it is possible to regularly observe regions with a high probability of cloud cover. We can also detect changes in the structure and function of ecosystems and regularly update land use maps.
After launching JERS-1 in 1992, NASDA has been observing nearly all land area on the Earth, collecting observation data at multiple time periods for some regions. NASDA has accomplished much from JERS-1 SAR data, including creation of high resolution mosaic images showing the forest distribution in regions of Africa, Southeast Asia, and the Amazon River area.
NDVI was calculated with global cloud-free composite images created from ADEOS OCTS data. There are plans to construct a high-performance SAR such as PALSAR aboard ALOS, and a high-resolution optical sensor such as GLI and AVNIR-II aboard ADEOS-II. Acquisition of this satellite data is expected to gradually increase mosaic images on a global scale. After data of multiple time periods becomes available, we should be able to accumulate many useful data sets for identifying and predicting such environmental changes as desertification.
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