Thiruvananthapuram, Jan. 20
THE overall area experiencing either very dry or very wet conditions is showing a tendency to occupy a greater fraction of the Earth's land area as the globe becomes increasingly warmer.
Even as drought has expanded across land areas, the amount of water vapour in the air also has increased over the past few decades, according to scientists with the US-based National Centre for Atmospheric Research (NCAR).
It may be recalled that members of the World Meteorological Organisation (WMO) had earlier observed that the global mean surface temperature in 2004 is expected to be +0.44°C above the 1961-1990 annual average (14°C).
This value of 0.44°C placed 2004 as the fourth warmest year in the temperature record since 1861, just behind 2003 (+0.49°C).
Droughts and floods are extreme climate events that are likely to change more rapidly than the average climate. Because they are among the world's costliest natural disasters and affect a very large number of people each year, it is important to monitor them and perhaps predict their variability.
Average global precipitation has risen slightly, the scientists said. However, surface air temperatures over land areas have increased sharply since the 1970s.
The large warming increases the tendency for moisture to evaporate from land areas. The study also said that the percentage of Earth's land area stricken by serious drought has more than doubled from the 1970s to the early 2000s.
Almost half of that change is due to rising temperatures rather than decreases in rainfall or snowfall. Widespread drying has occurred over much of Europe and Asia, Canada, western and southern Africa, and eastern Australia.
The fraction of global land experiencing very dry conditions rose from about 10-15 per cent in the early 1970s to about 30 per cent by 2002.
The warming-induced drying has occurred over most land areas since the 1970s. The largest effects have been felt in northern mid- and high latitudes.
In contrast, rainfall deficits alone were the main factor behind expansion of dry soils in Africa and East Asia. These are regions where El Nino, a more frequent visitor since the 1970s, tends to inhibit precipitation.