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Chapter 14 Soils on a Warmer Earth
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Chapter 14
SOILS ON A WARMER EARTH Projecting the effect of increased CO, and gaseous emissions on soils in Mediterranean and subtropical regions
Dan H. Yaalon Institute of Earth Sciences, Hebrew University. Givat Ram Campus Jerusalem 91904, Israel
ABSTRACT The subtropical region is defined here as roughly the zone of 10' to 35" N and S of the equator, with a pronounced seasonal rainfall, with about 2 to 4.5 months in which precipitation exceeds evapotranspiration by 75%. Rain falls mainly in summer in contrast to the Mediterranean Region with a winter rainfall season. The area covered by the subtropics is around 18 million km2, mainly in Africa South of Sahara and smaller regions in Asia (India), Australia and South and North America. The area of the Mediterranean region cover close to 9 million km2 mostly in Southern Europe, North Africa and the Near East. General circulation models for the condition of a double C02 concentration are not consistent in predicting the moisture conditions. Some models predict a decrease in precipitation and soil moisture, especially in the summer months, which would result in a considerably change in the moisture regime both on cotenary slopes and in individual pedons. In general these are very fragile environments, sensitive to climatic change, partly because of the high variability and seasonality of the climatic parameters. The major soil groups in the two regions are Luvisols, Acrisols, Vertisols, Cambisols, Arenosols and Fluvisols. Six major soil responses can become evident within a short time of less than 50 years: 1) organic matter reduction; 2) carbonate and salt regime changes; 3) erosion; 4) crusting; 5 ) waterlogging; and 6) fire frequency. Among these the reduction of organic matter due to increased temperature should be most easily modelled and is of special importance because of a positive feed back effect on additional temperature increase. The soil carbonate regime and salt balance may be affected by both decreasing and increasing precipitation. Salinization on footslopes may be caused by increased rainfall upslope. Changes in intensity and seasonality of rainfall will result in increased runoff and erosion, in other cases surface crusting. Reduced infiltration in clayey soils may cause waterlogging downslope. Prolonged drought periods may cause an increased fire
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frequency and result in subsequent severe erosion. The response action will depend not only on more precise prediction of the climatic parameters but also on the nature of the soils, their texture, structure, depth and spatial variability. Much detailed research on soil distribution and sensitivity to changes is needed. The basic soil processes are well known and understood, but their actual rates and possible feedbacks are known to a much lesser extent. Note from the editors: Unfortunately Dr. Yaalon could not deliver the final paper in time due to unexpected hospitalization. The reader may wish to contact the author directly for more detailed information.
Chapter 14
SOILS ON A WARMER EARTH Projecting the effect of increased CO, and gaseous emissions on soils in Mediterranean and subtropical regions
Dan H. Yaalon Institute of Earth Sciences, Hebrew University. Givat Ram Campus Jerusalem 91904, Israel
ABSTRACT The subtropical region is defined here as roughly the zone of 10' to 35" N and S of the equator, with a pronounced seasonal rainfall, with about 2 to 4.5 months in which precipitation exceeds evapotranspiration by 75%. Rain falls mainly in summer in contrast to the Mediterranean Region with a winter rainfall season. The area covered by the subtropics is around 18 million km2, mainly in Africa South of Sahara and smaller regions in Asia (India), Australia and South and North America. The area of the Mediterranean region cover close to 9 million km2 mostly in Southern Europe, North Africa and the Near East. General circulation models for the condition of a double C02 concentration are not consistent in predicting the moisture conditions. Some models predict a decrease in precipitation and soil moisture, especially in the summer months, which would result in a considerably change in the moisture regime both on cotenary slopes and in individual pedons. In general these are very fragile environments, sensitive to climatic change, partly because of the high variability and seasonality of the climatic parameters. The major soil groups in the two regions are Luvisols, Acrisols, Vertisols, Cambisols, Arenosols and Fluvisols. Six major soil responses can become evident within a short time of less than 50 years: 1) organic matter reduction; 2) carbonate and salt regime changes; 3) erosion; 4) crusting; 5 ) waterlogging; and 6) fire frequency. Among these the reduction of organic matter due to increased temperature should be most easily modelled and is of special importance because of a positive feed back effect on additional temperature increase. The soil carbonate regime and salt balance may be affected by both decreasing and increasing precipitation. Salinization on footslopes may be caused by increased rainfall upslope. Changes in intensity and seasonality of rainfall will result in increased runoff and erosion, in other cases surface crusting. Reduced infiltration in clayey soils may cause waterlogging downslope. Prolonged drought periods may cause an increased fire
176
D.H . Yaalon
frequency and result in subsequent severe erosion. The response action will depend not only on more precise prediction of the climatic parameters but also on the nature of the soils, their texture, structure, depth and spatial variability. Much detailed research on soil distribution and sensitivity to changes is needed. The basic soil processes are well known and understood, but their actual rates and possible feedbacks are known to a much lesser extent. Note from the editors: Unfortunately Dr. Yaalon could not deliver the final paper in time due to unexpected hospitalization. The reader may wish to contact the author directly for more detailed information.