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АРИДНЫЕ ЭКОСИСТЕМЫ, 2005, том 11, №26-27
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ГЕТЕРОГЕННОСТЬ АРИДИЗАЦИИ В ЦЕНТРАЛЬНОЙ АЗИИ В
КОНЦЕ 20-ГО ВЕКА
© 2005 г. А.Н. Золотокрылин Институт Географии РАН, 109017 Москва, Старомонетный переулок 29, Россия
Анализ междесятилетнего изменения климатических факторов и индикаторов аридности на равнинах Центральной Азии с 1982-2001 гг., свидетельствуют о том, что распределение индикатора на большое расстояние совпадает с территориальным перераспределением осадков холодного времени года. Отличительной чертой аридизации было одновременное уменьшение осадков в одном регионе и увеличение в другом, что наблюдалось в течение последних 20 лет. Длительность периода аридизации уменьшается в Прикаспийской низменности, так же как и в некоторых местах на территории между Каспийским и Аральским морями, включая западные территории Аральского побережья. Однако, увеличения в большей части происходит между морями к востоку 60°Е меридиана.
Регион, где происходит увеличение длительности периода аридизации, включает Восточные районы вблизи Аральского моря и среднюю часть бассейна реки Сыр-Дарья.
Из результатов этого исследования видно, что изменение регионального климата в Центральной Азии колеблется, согласно специфики субрегионов и поэтому нет оснований предполагать, что тенденция аридизации будет распространяться по всей территории в течение следующего десятилетия.
HETEROGENEITY OF ARIDIZATION OF CENTRAL ASIA AT THE END OF THE 20TH
CENTURY
© 2005. A.N. Zolotokrylin
Institute of Geography, Russian Academy of Sciences 109017, Staromonetny per. 29, Moscow, Russia
Introduction
Contemporary desertification of arid lands is increasingly affected by the interactions of natural and anthropogenic factors and their impacts. Of these factors, climate is of the greatest importance. Aridization of climate (climatic desertification) intensifies anthropogenic processes of desertification and makes the appeared ecological crises more profound. The current global warming trend, and aftereffects on the major arid regions of the world are not yet clearly understood,. but research increasingly points to regional climate change as a large influential factor.
The investigation is aimed at identification of decadal climate change in the Central Asia between 1982-2001, and its impact upon processes of aridization. Climatic desertification of the territory is estimated with the help of the normalised difference vegetation index (NDVI)-indicator (Zolotokrylin, 2003). This indicator reflects the zonality of heat exchange between arid land and atmosphere as the relation of radiation and evapotranspiration mechanisms in the regulation of thermal conditions of soil surface and the lower layer of the atmosphere (Zolotokrylin, 2002). The peculiar feature of this indicator is the fact that manifestation of climatic desertification is estimated in low-inertia components of ecosystems - vegetation cover and in soil (moisture reserves) on the scale of several decades (phrase removed for clarity). From this point of view, climatic desertification means: (1) reversible loss of part of vegetation cover and (2) degradation of part of vegetation cover with low restoration ability under the conditions of excessive anthropogenic load.
Materials and method
ГЕТЕРОГЕННОСТЬ АРИДИЗАЦИИ В ЦЕНТРАЛЬНОЙ АЗИИ В КОНЦЕ 20-Г0 ВЕКА 101
Territory of investigation (the latitude of 40-52o N and the longitude of 43-80o E) covers Central Asia and the adjoining arid lands. Climatic data are presented by daily average air temperature and daily precipitation totals over the period of 1982-2001. Decade statistical parameters of climate are calculated for different months and warm (cold) season, which are then compared. The period from April to October is considered to be warm, and the period from November to March, cold.
In this study, the important index of aridization, such as the number of days with precipitation, and the number of precipitation days of different intensity, was identified. Additionally, monthly precipitation totals and monthly sums of active temperature days (days with temperatures 10oC above the daily average temperature) were calculated. It is known that the sum of active temperature divided by 10 is numerically close to evaporability in millimeters. Then the proportion of sum of precipitation days to sum of active temperature days is recognized as index of humidification known as Selyaninov hydrothermal coefficient (HTC):
HTC= XP / 0.1 Xt>10,
Where XP denotes precipitation sum, Xt>i0 is the sum of active temperatures.
The NDVI-undicator of climatic desertification is defined as the multi-year time interval during vegetative season with NDVI < 0.07. Both indices are calculated for each decade during 1982-2001.
Results
Change of climate in the decade of 1992-2001 as compared with the decade of 1982-1991.
Climate and hydrological cycle of the northern parts of the territory (dry steppes, semi-deserts, partially northern deserts) are mainly determined by western transfer, and that of the southern part (northern deserts, southern deserts) - by subtropical circulation. Conventional boundary between these circulations lies approximately at 47°N latitude. Precipitation of the warm period exceed precipitation of the cold one northwards of the latitude of 47°N, but they become smaller than precipitation of the cold period southwards of this latitude.
Territory northwards of the 47° N latitude. Analysis of synoptic conditions proves growth of recurrence of cyclones in the western part of the territory in the cold season and to a lesser degree in the warm season in the last decade. As a result, winters become warmer, annual precipitation increases due to the precipitation of the cold season (Appendix 12, Fig.1). The greatest increase of precipitation occurred in Prikaspijskaya Lowland. Increase of the number of days with precipitation due to poor precipitation (less than 5 mm/day) is also registered there. It should be noted that increase of annual precipitation in the last decade manifests itself approximately up to the longitude of 60o E. Eastwards of this meridian, annual precipitation decreases.
Because of increase of precipitation of the cold season in Prikaspijskaya Lowland annual isohyets shifted southeastwards by 200-300 km. Analysis of drought index - vegetation conditions index (VCI) -proves decrease of droughts duration over the vegetation season (Zolotokrylin and Vinogradova, 2004). Winds of western rhumbs become more frequent in summer (especially in the western part of Prikaspijskaya Lowland) thus hindering atmosphere dust transport from Turanskaya Plain [label on fig. 1. map].
Territory southwards of the latitude of 47 ° N. Temperature rises in the second decade mainly in the cold season. But the season of active vegetation becomes cooler, which is confirmed by the decline in the total number of active temperature days and, therefore, decrease of evaporability on the greater part of the territory. The maximum decrease of sums of temperatures by 100°C and over is observed in the regions between the Caspian and the Aral seas and southwards, including regions adjoining the southern part of the Aral Sea. Decrease of aridity of this territory (increase of HTC from 0.2 to 0.4) in May-June is connected with decrease of the sum of temperatures.
Annual precipitation totals did not change significantly within the study area in the last decade with the exception of the regions eastwards of 60o E longitude including the mountainous part of the Syr-Darya River basin. This basin's annual precipitation decreased due to noticeable decrease of precipitation in the cold season. Before the beginning of drying out of the Aral Sea, precipitation of warm season prevailed north of the Aral Sea and surrounding it. In the last decade precipitation in the cold and warm seasons is almost equal above bare bottom and the remaining water surface.
102
ЗОЛОТОКРЫЛИН
Active wind activity in the Aral Sea region in 1970-80s caused sharp increase in number of dust storms and dust transfer to irrigated lands, which resulted in soils salinization (Zolotokrylin, 1996). At the end of the 20th century, the number of dust storms considerably decreased, due to decreases of recurrence of strong winds despite continuing drying out of the Aral Sea.
Estimation of impact of decade climate change upon aridization
Territory northwards of the latitude of 47° N. In the last decade, shortening of the period of aridization is mainly observed in Prikaspijskaya Lowland (Appendix 13, Fig. 2). It is determined by the fact that moisture reserves in soil in winter-spring period increased due to mild winters almost without frosts. Soil freezes very little, and excess of winter precipitation accumulates in soil. Contrast of soil moisture content between micro-lowerings and micro-risings decreased considerably. As a result, restoration of vegetation cover (especially considerable in the west of Prikaspijskaya Lowland) began, which is proved by the analysis of distribution of NDVI-data. Another cause of vegetation re-restoration was a sharp decrease of anthropogenic load at the end of the 20th century. It is worth mentioning that these climate changes took place mainly in dry steppe, semi-desert and partially northern desert regions adjoining the Caspian Sea.
Eastwards of the Prikaspijskaya Lowland, there is a tendency of increase of duration of period with aridization processes is registered (Appendix 13 Fig. 2). It is especially evident on the territory eastwards of the meridian of 60° E.
Territory southwards of the latitude 47 ° N. On the territory between the Caspian and the Aral Seas shortening of aridization period alternates with some increase of it (Appendix 13, Fig. 2). Shortening of the period occurred in the regions where at the beginning of vegetation season dryness decreased. Increase of aridization period duration by 0.5 month is more characteristic of the territory eastwards of the Aral Sea where precipitation of the cold season decreased considerably, and days with low-intensity precipitation decreased in number. For example, in the region of lower reach of the Syr-Darya River the aridization period became longer by one month and more.
Discussion of results
Analysis of the results shows that regional climatic system on the whole remains stable in the last decades. Climate change in 1992-2001 decade as compared with 1982-1991 decade reflects cyclic increase of intensification of regional circulations, but does not point to shift of their trajectories, i.e. the beginning of stable change. Boundary between regional circulations remains stable. Climatic boundaries (meridional gradients of climate characteristics) correspond to the boundaries of natural zonality. We can say that observed change of climate humidity in the northern and southern parts of the territory is the recurrent fluctuation of regional circulations. There is not enough long-term evidence yet to state that this climate change has stabilized, although it can be considered to be a regional manifestation of global warming. Causes of variability of regional circulations can be different including increase of anthropogenic greenhouse gases in the atmosphere.
As is clear from the data analysis of the precipitation during the cold season and the number of days with precipitation, which form moisture reserves in soil at the beginning of vegetation season, change to the greatest extent. Regions of both increase and decrease of precipitation form simultaneously on the territory. Inter-decade change of precipitation and their frequency determine increase of duration of the period with aridization processes in the regions northwards, eastwards and southeastwards of the Aral Sea. It is important to note that this period does not practically change in the observed delta landscapes of Amu-Darya River Basin.
Observed decreases of precipitation during the cold season in the Syr-Darya basin, especially in its mountainous regions, increases the duration of aridization period in delta landscapes, such as in lower and middle course of the Syr-Darya and has caused the Aral Sea crisis to be more profound.
Strong non-linear responses of dynamic processes in delta landscapes to climate fluctuations (small accidental deviations of climate can cause considerable cyclic changes in dynamic components of landscape) is the cause of changes of water regime, vegetation, level of inland lakes, etc. for many years. Non-linear responses to extreme climatic phenomena are especially dangerous in populated regions where use of natural resources has become unsustainable. For example, the populated regions of
ГЕТЕРОГЕННОСТЬ АРИДИЗАЦИИ В ЦЕНТРАЛЬНОЙ АЗИИ В КОНЦЕ 20-ГО ВЕКА 103
the Amu-Darya and Syr-Darya River basins, where irrational economic activities since the 1950's have provoked the Aral Sea crisis.
Conclusion
Analysis of inter-decade change of climate characteristics and indicators of aridity on the plains of Central Asia from 1982-2001 testifies to the fact that indicator distribution to a great extent agrees with territorial redistribution of cold season precipitation. The peculiar feature of aridization in this region was that simultaneous decreases and increases in precipitation were observed. The duration of the aridization period decreases in the Prikaspijskaya Lowland, such as in some places of the territory between the Caspian and the Aral seas including the western areas adjoining the Aral Sea, but increases eastwards of the 60 ° E meridian. The regions of increase of duration of aridization period include the eastern regions near the Aral Sea and the middle course of the Syr-Darya River Basin.
From this study's observations, regional climate change in Central Asia is variable according to specific subregions, though in the regions where intense aridization has occurred, this trend will continue through the next decade.
REFERENCES
1. Zolotokrylin A.N. Sand Storms on Turan Lowland // Proc. Russian Acad. Sci. Ser. Geography, 1996, No. 6. PP. 48-54 .
2. Zolotokrylin A.N. Climatic desertification. In (Paper Name or Volume Name?) Ed. A.N.Krenke. M.: Nauka, 2003. 246 p.
3. Zolotokrylin A.N., Vinogradova V.V. Drought Climatology in Southeastern Russian Plain Obtained from Remote Sensing Data // Earth. Obs. Rem. Sens., 2004, No.1. PP.83-89.
