World Meteorological Organization and Agricultural Droughts - Monitoring and Predicting Agricultural Drought: A Global Study - page 404

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changes in the normal patterns of hydrometeorological parameters. Based
on evidence from climate models, together with observations from instru-
mental and other available records, the IPCC (2001) Third Assessment Re-
port concluded that “there is new and stronger evidence that most of the
warming observed over the last 50 years is attributable to human activi-
ties.” Human activities will continue to change atmospheric composition
throughout the 21st century. By 2100, the concentration of CO 2 is expected
to increase from the present figure of 370 ppm to 540-970 ppm. Over the
period from 1990 to 2100, it is projected that the average global air tem-
perature will increase by 1.4 to 5.8°C, and global mean sea-level will rise
by 9 to 88 cm. It is reported in the IPCC (2001) assessments that recent
regional climate changes, particularly temperature increases, have already
affected many physical and biological systems.
As the earth warms, models project that arid and semiarid land areas
in southern Africa, the Middle East, southern Europe, and Australia will
become even more water stressed than they are today, causing a decrease in
agricultural production in many tropical and subtropical areas. Indeed, the
year 2000 was the 22nd consecutive year with a global mean surface tem-
perature above the 1961-91 normal temperature and was the 7th warmest
year in the past 140 years, despite the persistent cooling influence of the
1997-98 La Niña event.
The predicted increase in temperature would most probably have the
effect of increasing potential evapotranspiration rates in the drylands, and
in the absence of any large increases in precipitation, many drylands are
predicted to become even more arid in the next century. Desertification is
more likely to become irreversible if the environment becomes drier and
the soil becomes further degraded through erosion and compaction.
Based on experimental research, crop yield responses to climate change
vary widely, depending on species and cultivars, soil properties, pests and
pathogens, the direct effects of carbon dioxide on plants, and the interac-
tions between carbon dioxide, air temperature, water stress, mineral nu-
trition, air quality, and adaptive responses. Those countries with the least
resources have the least capacity to adapt to climate change and are the
most vulnerable. For example, the adaptive capacity of societies in Africa is
low due to lack of economic resources and technology, and vulnerability is
high as a result of heavy reliance on rainfed agriculture, frequent droughts
and floods, and poverty. Grain yields are projected to decrease, diminishing
food security, particularly in small food—importing countries. Desertifica-
tion would be exacerbated by reductions in average annual rainfall, runoff,
and soil moisture. Significant extinctions of plants and animal species are
projected and would impact rural livelihood.

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Ea rly Warning Systems for Drought

At a global level, the World Weather Watch (www.wmo.ch/web/www/
www.html) and Hydrology and Water Resources programs, which are