Global warming and desertification

The record from the polar ice core clearly shows that the increase in greenhouse gas concentrations in the atmosphere started with the Industrial Revolution in the eighteenth century, though the rate of increase accelerated after the middle of the twentieth century. It is the general consensus that global warming has occurred because of the increase in atmospheric concentrations of greenhouse gases due to human activities (Houghton et al. 1996). Human activities may influence local precipitation directly through changes in local vegetation cover, and indirectly through global warming.

The direct influence of deforestation of the Amazonian tropical rain forest on Amazonian climate has been discussed (e.g. Shukla et al. 1990). Almost all research results suggest a positive feedback effect of the decrease in local evapotranspiration by deforestation on local precipitation, i.e. the local precipitation will decrease following deforestation, although quantitative evaluation is still the subject of future research. If desertification caused by overgrazing or by other means were to occur in arid lands, the same kind of feedback effect as in the deforestation of the humid Amazonian forest might appear and the local precipitation would decrease. But it is also necessary to take into account the indirect influence of global warming on local precipitation, as is discussed below. The effect of this indirect influence is, in many regions, greater than that of the direct influence.

The global climate system redistributes energy from lower to higher latitudes. The energy surplus in the lower latitudes, bounded roughly by latitudes 35° North and South, results from the latitudinal gradient of the Earth's energy budget. The total energy distributed by ocean currents is greater than that distributed by the atmosphere.

The mean time during which a water molecule passes through a hydrological system (such as a lake, the Pacific Ocean, or the troposphere) is termed the mean hydrological residence time. The "memory" of a hydrological system increases with a longer residence time. The mean residence time is about 10 days for atmospheric vapour, but about 3,000 years for ocean. The atmosphere has a very short memory compared with the longer memory of the ocean. If we could stop the energy supply to the atmosphere, its motion would cease within a month, but the ocean would continue its circulation for a longer period following such an energy cut-off.

The total heat stored in hydrological systems should also be taken into account in assessing the future evolution of the global environment. A short memory is synonymous with a small heat capacity. Thus, the atmosphere contains insufficient heat to act as the source of future dynamic changes in the global climate. It can only respond to changes in forcing, such as changes in solar irradiation or heat supply from the ocean. Future atmospheric behaviour not only depends on increases in greenhouse gas concentrations and earth orbital changes but also depends heavily on changes in the sea surface temperature (SST), global ocean circulation, and the increased atmospheric turbidity (Kayane 1996).

Figure 1 shows the distribution of the SST trend calculated by the SST database for 1930-1989 released from the United Kingdom Meteorological Office for areas of 5 degrees latitude by 5 degrees longitude. The average rate of the SST increase for the whole ocean is about 0.9°C/100 years, which is greater than the rate of the global air temperature increase. The increase in ground surface temperature in the eastern part of North America since the middle of the nineteenth century also exceeds the increase in the surface air temperature (Deming 1995). It is worth noting that the SST increase is predominant in the tropical ocean, but the SST in the Atlantic Ocean north of 30°N was markedly decreased, probably owing to an increased cloud amount caused by water vapour transported from the low-latitude zone. The effect of global warming on the SST and the air temperature does not appear to follow the same trend: the temperature decreases in some regions and increases in others owing to global warming. This is also true for precipitation changes.

Figure 2 shows possible causal relations between global warming and hydrological processes, although the anthropogenic contribution to the SST remains the subject of ongoing research. Increases in ocean evaporation, ocean precipitation, and global precipitation, corresponding to the intensified global energy and water cycle, are processes directly deducible from the SST increase.

Figure 1 Global Trends of the Sea Surface Temperature (SST) for Each 5 by 5° Change in Latitude and Longitude Calculated Using the United Kingdom Meteorological Office (UKMO)/SST Database (Source: Kayane et al. 1995)

Figure 2 Causal Relationships between Global Warming and Regional Precipitation Changes (GHGs, Greenhouse Gases)

Figure 3 Long-Term Time Series (Circles) and Their Trends (Line) in the Boreal Summer Monsoon Rainfall (June-September) at Colombo (Open Circles and Dashed Line) and Nuwara Eliya (Solid Circles and Solid Line) for 1986 1992 (Source: Kayane et al. 1995)

The SST in the Indian Ocean increased by 0.5-1.0°C during 1930-1989. As a result of increased ocean evaporation due to the SST increase, the rainfall during the SW monsoon season from June to September at Colombo increased by about 30 per cent during 1869-1993. However, the rainfall at Nuwara Eliya, a station at an elevation of 1,895 m in the central high mountains in Sri Lanka, has decreased by about 40 per cent during the same period (fig. 3). An almost linear increase in rainfall during 1870-1970 was also observed at Calicut (Lengerke 1976), a coastal station in south-west India located to the west of the Western Ghats Mountains, where the same orographic effect on rainfall pattern is expected as on the Sri Lankan south-west coast. Such long-term changes in rainfall in Sri Lanka and southwest India can be interpreted as the result of intensified Indian monsoon circulation caused by global warming (Kayane et al. 1995). Changes in local rainfall may take opposite trends within a relatively small island like Sri Lanka, owing to global warming.

If the Hadley (north-south) circulation were intensified by global warming, the subtropical high would also be intensified, resulting in a decrease in precipitation in arid lands in the middle latitudes. One such example has recently been reported by Liu and Zhao (1996) for the Tibetan Plateau. During the last 40 years, the annual precipitation had an increasing trend in south-east Tibet and had no obvious change in north-east Tibet, while it had a decreasing trend by 5-10 per cent in the north-west and central Tibetan Plateau. Because of the combined effects of the increase in evaporation due to temperature increase and the decrease in precipitation, the river discharge in the middle reaches of the Yarlung Zangbo River decreased by 10 per cent, at the Lancang River by 5 per cent, and at the Lhasa River and the Nyang Chu River also by about 5 per cent.

Brenes Vargas and Saborio Trejos (1994) reported that, generally speaking, the increase in rainfall in the windward side and the decrease in the leeward side of the central mountain range in Costa Rica might be a result of the intensified North Atlantic high, which would strengthen the north-east trade winds to Costa Rica. However, a part of the coastal area in the leeward side shows an increasing trend, presumably caused by the intensified local atmospheric circulation from ocean to coast induced by the strengthened trade winds.

In Patagonia, located in the westerly zone of the southern hemisphere, definite increasing trends in annual precipitation during the past 100 years are observed at Rio Colorado, Neuquen, and Paso de los Indios, all located in northern Patagonia, but stations in southern Patagonia show no obvious change and certainly no decreasing trend (Quintela et al. 1995).

There have also been substantial annual precipitation changes in certain latitudes and regions, most notably a decrease in the African Sahel in the middle latitudes after the 1960s, and a fairly steady increase in the former USSR in the polar frontal zone during the past 100 years (Folland et al. 1990).

The observed precipitation trends described above can be interpreted as the result of an intensified global energy and water cycle, i.e. as indirect effects of human activities on local precipitation through global warming. It may be concluded that the precipitation variability has increased globally both in time and space. This may raise serious problems with respect to water resources and food supply in the future, especially in arid lands.