Download Uncertainties of Climate Change in Arid Environments of Central Asia

Reviews in Fisheries Science, 14:29–49, 2006
Copyright © Taylor & Francis Inc.
ISSN: 1064-1262 print
DOI: 10.1080/10641260500340603
Uncertainties of Climate Change in Arid
Environments of Central Asia
ELENA LIOUBIMTSEVA AND ROY COLE
Department of Geography and Planning, Grand Valley
State University, Allendale, Michigan, USA
This article examines the key uncertainties of climate change in the Central Asian republics of the former USSR—a vast arid region and a classic example of complex and
poorly understood interactions between the regional responses to global climate change
and the local human-induced desertification. Based on paleoanalogous scenarios, Central Asian deserts are often predicted to become wetter as a result of global warming
because they are located north of 30◦ latitude. However, despite some similarities between the paleoclimate changes and greenhouse warming, such predictions have very
serious limitations. Climate models predict that the temperature in arid Central Asia
will increase by 1–2◦ C by 2030–2050, with the greatest increases in wintertime. Some
models project greater aridity in the future though others project less aridity, and it
is becoming increasingly apparent that climate change modeling in arid zones is extremely uncertain because of the extreme natural variability (both temporal and spatial)
of the desert climate. The physical differences of climate change forcings imply that one
might expect quite different regional responses to future human-induced climate change
compared to the Holocene climate in terms of their rapidity and amplitude. Local and
regional human impacts, such as massive irrigation, may have a stronger impact on the
climatic system at the regional level than global climate change.
Keywords paleoclimatology, greenhouse warming, climate models, paleogeographic
data, human-induced desertification
Introduction
Sound use of aquatic and land resources in arid zones cannot be developed without a
profound understanding of the processes of climate change. Much paleogeographic research
in many arid regions of the world indicates that deserts and semi-deserts are prone to rapid
geomorphological, hydrological, biological, and biogeochemical changes in response to
global climatic change (Goudie, 1994; Varushchenko et al., 1987; Lioubimtseva et al.,
1998) and have often been predicted to be among the most responsive ecosystems to higher
carbon dioxide concentrations and global warming (Melillo et al., 1993; Bazzaz et al.,
1996; Smith et al., 2000). Recent modeling studies (Wang and Eltahir, 2000; Zolotokrylin,
2002; Claussen et al., 2003) have shown the strong sensitivity of arid climates to changes
in the surface parameters that can be triggered by human activities. Despite the growing
understanding of the international scientific community of the regional impacts of global
climate change, relatively little attention has been given to the climate and environmental
Address correspondence to Elena Lioubimtseva, Grand Valley State University, Department of
Geography and Planning, 1 Campus Drive, Allendale, MI 49401-9403, USA. E-mail: lioubime@
gvsu.edu
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change of Central Asia. The recent reports of the Intergovernmental Panel on Climate
Change (IPCC, 2001) and the international scientific literature in general, contain very little
discussion of possible implications of global change for this enormous arid region. However,
the question of trends and cycles of climate change in the arid lands of Central Asia have
been attracting the attention of Russian geographers since the 19th century (Berg, 1904).
Based on the evidence of paleoenvironmental changes and historical and archaeological
data many Russian scientists attributed the decline of the classical and medieval civilization
of Turkistan to an increase of aridity (Shnitnikov, 1969; Doluhanov, 1985; Varushchenko
et al., 1987). Other researchers rejected the hypothesis of natural climate trends in this region
suggesting instead that the climate and environmental changes of the last millennium were
caused exclusively by human-induced processes (Markov, 1951; Kharin et al., 1998). Yet
the question formulated by Markov more than half a century ago “is Central Asia getting
drier?” still remains open.
It is extremely difficult to draw a clear boundary between local and regional responses
to global climate change and the trends caused by local land use changes, such as massive
irrigation, overgrazing, and the consequent desertification processes. The Central Asian
deserts are not pristine environments; grazers have exploited them for thousands of years,
and in the 20th century, large irrigation schemes have been added to these landscapes.
Although the drama of the Aral Sea crisis has been attracting the attention of the Russian and
international scientific communities during the last decades, still remarkably little thought
has been given to connections between the local and global changes in the region and
the effects of massive irrigation on the climate of the Central Asian republics. To fully
understand the impact of human activities, it is necessary to consider the extent to which
anthropogenic effects have modified the background level of carbon storage, and whether
change in the intensity of either process has any evident potential to take up or release
carbon from the desert-zone carbon reservoir. Great uncertainties still exist about carbon
pools in the desert soils. Figures on soil organic carbon given for this region in some widely
used global datasets (Zinke et al., 1984) seem to be seriously overestimated (Adams and
Lioubimtseva, 2002) and, although much of attention has been given to estimations of
vegetation biomass and production in this region (Nechaeva, 1984; Bazilevich, 1995), the
question of organic carbon in desert and semi-desert soils is poorly documented (Lal and
Kimble, 2000). Confusion between organic and inorganic carbon in desert and semi-desert
soils, or inadequate translation from Russian into English, are often the main reasons why
Western authors tend to overestimate the levels of organic carbon in these soils. Realistic
assessment of the region’s carbon budget is important for governmental decisions on land use
and understanding of the implications for climate change (Lioubimtseva and Adams, 2002).
The purpose of this article is to discuss the state of our current understanding of climate
change in the republics of Central Asia and to outline the major uncertainties of climate and
environmental change in this region. Particular attention is given to interactions between
the effects of global change and the regional human-induced land degradation in the region.
The following section provides a brief overview of the study region, its climate and
physiography. Section three addresses the Holocene changes in Central Asia based on the
regional biostratigraphic, geomorphological, and archeological data. Section four discusses
current trends of climate change based on meteorological and remote sensing data series
and also highlights some major sources of uncertainties and contradictions affecting climate change predictions and modeling in arid zones of Central Asia. Section five analyzes
possible implications of climate change for desert and semi-desert landscapes and their
carbon budget based on field and laboratory experimental data on direct physiological responses to CO2 in the study area and other arid regions of the world. It also discusses
Climate Change in Central Asia
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some contradictions between the existing biogeography model scenarios and the recent
experimental data. Finally, section six addresses several major factors of human-induced
degradation of arid landscapes.
Brief Overview of the Study Region, Climate, and Physiography
The five central Asian states of the Commonwealth of Independent States (CIS), formed
in December 1991 after the break-up of the former USSR, Kazakhstan, Kyrgyzstan,
Tadjikistan, Turkmenistan, and Uzbekistan, occupy an area of about 3.5 million km2 with
a combined population of more than 57 million people (PRB, 2002). Deserts cover more
than 95% of the total territory of Turkmenistan and more than a half of Uzbekistan and
Kazakhstan.
The Central Asian arid region comprises the entire Turan Lowland and the southern
margin of the Kazakh Hills (Figure 1) and is bounded by the Middle Asian mountains (up
to 7450 m) on its southern and southeastern edges. In the southwest, the somewhat lower
mountains of the Kopet Dagh (2000 m) allow monsoon precipitation to reach the western
slopes of the Tian Shan and Pamiro-Alaı̈ ranges. In the north, the Turanian plain descends
Figure 1. The study region and its climate.
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progressively northward and westward and opens out towards the Caspian lowland. The
northern boundary of this vast arid zone is rather poorly defined but it lies at approximately
48◦ N.
The deserts and semi-deserts of Central Asia have a typical continental climate and
can be divided into two climate subregions—northern (mainly Kazakhstan) and southern
or Irano-Turanian (Petrov, 1976; Lioubimtseva, 2002). In the north of the semi-desert zone,
the winters are very cold with a mean winter minimum of −26◦ C and absolute minimum
around −40◦ C or colder and the annual precipitation from 155 to 270 mm. Precipitation in
the northern deserts is associated mainly with the prevailing westerlies and has a distinct
maximum in spring-summer as the influence of the Siberian high diminishes and convective activity becomes stronger. In the southern Iranio-Turanian part of the region winters
are milder with mean January temperature ranging between −10 and 0◦ C. The average
July temperatures are about 32◦ C with a maximum of 52◦ C in the eastern Kara Kum. Precipitation in this subregion has a spring maximum that is associated with the northward
migration of the Iranian branch of the Polar front. Most frequently, rain is brought by the
depressions, which develop over the Eastern Mediterranean, migrate northeastwards, and
regenerate over the Caspian Sea (Lioubimtseva, 2002). Westerly cyclones of the temperate
zone change their trajectories in summer over the Aral Sea from a west-east to a northsouth direction and approach the zone affected by the Indian monsoon over the Zagros
Mountains.
Evidence of the Holocene Climate Change
Numerous biostratigraphic, geomorphological, and archaeological proxy data indicate that
the climate of the region has experienced many changes over time. Climatic variations
through the late Pleistocene resulted in multiple shifts from hyper-arid deserts to subhumid shrub lands (Kes et al., 1993; Varushchenko et al., 1987; Velichko et al., 1987;
Tarasov, 1992; Tarasov et al., 1998). However, in contrast with tropical deserts (i.e., the
Sahara Desert), these environmental changes had relatively small amplitudes in the Late
Pleistocene and Holocene, when the plains of Central Asia remained arid during long periods
of time interrupted by relatively short humid intervals. Pollen evidence suggests that these
fluctuations of precipitation did not exceed 150–200 mm. Among the relict features, the
activity of ancient water flows during the periods of glacial melting in the mountains of
Pamir, and Tien Shan left its distinctive traces both on the piedmonts, dissected by deep
dry valleys, and on the aeolian plains, where alluvial fans formed, altering their location at
each stage of alluvial deposition.
Available biostratigraphic, geomorphologic, and archaeological data suggest that a
severe arid phase of the Last Glacial Maximum (about 21,000 BP) was followed by the
increase of humidity at the beginning of the Holocene. According to available pollen data
in northern and central Kazakhstan, annual rainfall during the LGM was lower than now
by 100–150 mm (Aubekerov et al., 1989; Tarasov, 1992). The dry cold steppes of the
Younger Dryas were replaced in the Holocene by mesophytic forest-steppe vegetation with
a maximum increase of arborescent species.
The Holocene global optimum caused an increase of precipitation in many desert
regions of the world. In the Kyzyl Kum desert, the Holocene climatic optimum is known
as the Lavliakan humid phase and has been dated by radiocarbon from 8 to 4000 BP,
with a maximum around 6000 BP (Doluhanov, 1985; Kes, 1993). Such climatic conditions
favored the development of subhumid steppes (Artemisia and Graminea) on the currently
desert Ustyurt plateau (Varushchenko et al., 1987; Tarasov, 1992).
Climate Change in Central Asia
33
Marine fossils, relict shore terraces, archeological sites, and historical records point
to repeated major recessions and advances of the Aral during the past 10,000 years. Until
the present century, fluctuations in its surface level were at least 20 m and possibly more
than 40 m. Significant cyclical variations of sea level during this period resulted from major
changes in river discharge into it, caused by climatic alteration and natural diversions of
the Amudarya away from the Aral sea (Micklin, 1988; Kes et al., 1993). Multiple smaller
fluctuations of aridity in this region occurred also at a finer temporal scale. Historical
documents and archaeological data in western Turkmenistan and Kazakhstan indicate that
precipitation was slightly higher than today between the Caspian and Aral Sea between
the 9th and 14th century AD, followed by the present day conditions (Varushchenko et al.,
1987; Doluhanov, 1985).
Explanation of the precipitation changes in the deserts of Central Asia center on the
shifting westerly cyclonic circulation, dependent on the position of the Siberian high. The
level of the Caspian and Aral Seas and their impact on local precipitation through the contribution of moisture and heat to the lower atmosphere are also very important in controlling
precipitation. The impact of the Caspian Sea on precipitation is evidence of a very strong
connection between the climate of Central Asia and the climate of European Russia because
the level of water in the Caspian Sea depends entirely on run-off from the Volga and Ural
basins. The level of the Aral Sea depends on the run-off from the Syrdarya and Amudarya
rivers and ultimately on the rhythm of the glaciations in the Pamir and Tian Shan mountains.
The Holocene optimum and earlier interglacials (for example, isotopic stage 5e, about
125,000 BP) are often suggested as possible analogous scenarios for a predicted global
warming caused by increasing concentrations of greenhouse gases in the atmosphere. Although palaeodata indeed provide valuable insights on possible geomorphological, hydrological, and biological responses to climate change, caution should be exercised in predicting
future trends with palaeoanalogues. Strong agreement exists between climatologists that
the climate transition from the LGM conditions to the Holocene was triggered by changes
in seasonal sunlight distribution caused by oscillations in the Earth’s orbit and axial tilt
(Berger, 1978). The physical differences of climate change forcing imply that one might
expect quite different regional responses to future human-induced climate change compared
to the Holocene climate in terms of their rapidity and amplitude.
Current Trends and Model Scenarios
Based on palaeoanalogous scenarios, Central Asian deserts are often predicted to become
moister as a result of global warming because they are located north of 30◦ latitude and are
expected to benefit from the southward shift and probable intensification of the westerly
cyclones similar to the early Holocene conditions. While palaeodata may improve our
understanding of the global and regional mechanisms of climate change during relatively
long time intervals (centuries and millennia), they usually do not capture the pattern of finer
fluctuations of temperature and precipitation.
The Intergovernmental Panel on Climate Change (IPCC) report Regional Impacts of
Climate Change, 2001 provides very limited information about climate change in arid Central Asia (IPCC, 2001). “There were no discernible trends in annual precipitation during
1900–1995 for the region as a whole, nor in most parts of this region” (IPCC, 2001, Chapter
7). The report does point out a likely 1–2◦ C/century temperature increase for Central Asia.
The aridity index shows no consistent trends for Central Asia as a whole (IPCC, 2001). A
most commonly used measure of aridity is Thornthwaite’s index of aridity (Thornthwaite,
1948) defined as a ratio of precipitation and evapotranspiration (P:PET). Most of Central
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Figure 2. Temperature increase from 1900 to 2000. (a) Toshkent station (adapted from Chub, 2000).
(b) temperature anomaly for 1880 to 2000 for arid zones of Central Asia (based on GHCN dataset,
Peterson and Vose (1997)).
Asia lies in the zone in which the aridity index varies between 0.05 and 0.5 and consequently is defined as “arid” and “semi-arid” by the United Nations Environmental Program
(UNEP) classification (1992). It is clear that one of the most important consequences of the
temperature increase in arid lands is the increase of evapotranspiration, and therefore aridity.
Meteorological data series show a steady increase of annual and winter temperatures in
this region since the beginning of the past century (Figures 2a and 2b). However, few stations
have data for longer than a century; most have observations for just half that time. In addition,
many stations have stopped functioning since the collapse of the USSR: the number of snowgauging stations in the river basins of Kazakhstan, Kyrgyzstan, Tajikistan, and Uzbekistan,
for example, has decreased from 257 in 1985 to 30 in 1995 (Borovikova, 1997).
Available historical precipitation data from the Hulme (1998) global dataset do not
show any significant changes in precipitation in Central Asia since 1900 (Figure 3). This
Figure 3. Precipitation in Central Asia, 1900–2000 (based on the CRU dataset, Hulme (1998)).
Climate Change in Central Asia
35
database, most commonly used in the modeling studies, is based on a very small number
of stations located in the region, and is prone to errors caused by the extrapolation of data
from only few stations over a very large area. Furthermore, there is uncertainty regarding
these data because most of the sites are located near irrigated areas.
Despite the absence of an overall precipitation change noticeable for the entire region,
great spatial variability has been observed at the landscape scale. For example, a study by
Neronov (1997) in eastern Turkmenistan revealed significant differences between the trends
of annual precipitation and aridity index in the sandy desert of the eastern Kara Kum and the
neighboring irrigated lands. He analyzed annual and monthly temperature and precipitation
data from the Repetek meteorological station (located in the quasi-pristine landscape of
sandy desert within the Repetek biosphere reserve) and Bayramaly stations (located in the
Murghab oasis with extensive irrigated lands, 110 km southeast from Repetek), covering
the periods of 1919–1997 and 1890–1997, respectively. Both sites have experienced very
similar temperature increases during the period of instrumental observations and especially
since the middle of the past century. Steady temperature increases in these two stations and
in the Turanian deserts in general (Figures 4a and 4b) might be an indication of some general
factor affecting atmospheric circulation in Central Asia. For example, the steady increase
in both mean annual and mean winter and summer temperature trends could be the result
of the decreasing effect of the southwestern periphery of the Siberian high in winter and
the intensification of summer thermal depressions over Central Asia. Precipitation trends
show a dramatic difference between the two sites as well, and are apparently responsible for
the difference between the trends of the mean annual aridity index. Although the index of
aridity increased almost twice (from 0.42 in 1891 to 0.71 in 1994) in Bayramaly, there was
only a slight change from 1910 to 1954 (0.41 to 0.46). A steady increase of precipitation,
especially since the middle of the 20th century was also reported for several oases in Central
Asia (e.g., Kirsta and Rihlov, 1988) and can be attributed to the micro-regional, humaninduced climate change caused by the expansion of irrigated lands. Such trends usually
show remarkable spatial variability and are much more intensive in the big oases where the
area of irrigated lands has dramatically grown during the past century (such as Urganch,
Bokhora, and Toshkent oasis in western Uzbekistan or Murgab, Tedjen, and Ashgabat oases
in Turkmenistan).
Another area where the regional weather records show a significant climate change
since the 1960s is in the vicinity of the Aral Sea, where a general trend towards a more
continental climate has been suggested by data from stations located near the shore. Such
data indicate an increase in summer and decrease in winter air temperatures of 1.5–2.5◦ C,
a decline in the mean annual relative humidity of 2–3%, and an increase in the occurrence
of drought days by 300% (Middleton, 2002). Precipitation records also show a shift in
seasonality. While in the 1950s the maximum precipitation in the Aral Sea area occurred
during February–March and the minimum in September, in the 1970s the maximum was
observed in April and the minimum in July. Spring frosts have been recorded later and
autumn ones earlier (Glazovsky, 1995; Middleton, 2002). The reduction of the sea surface
area also caused a significant decrease of precipitation since the 1960s and saline dust
from the exposed lakebed has been implicated in rapid climate and vegetation change.
Paradoxically, despite the increasing water shortage in the Amudarya delta, there has been
a rather stable trend of increasing precipitation during the last decade indicated by data from
several stations situated in irrigated areas of the Amudarya delta, such as Nukus, Takiatish,
Chimbay, and Kungrad (Victor Statov, personal communication, 2003).
It is important to note, however, that there is still significant uncertainty in the estimations of the extent of arid lands in the temperate zone of Eurasia as a result of ambiguity
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Figure 4. Precipitation, temperature, and aridity index (P/PET). (a) Repetek Station, Turkmenistan,
1910 to 1995. (b) Bayramaly Station, Turkmenistan, 1910 to 1995. Adapted from Neronov (1997).
in the indices of aridity used in different studies. The boundary between arid and semi-arid
climates as shown on the commonly used UNEP map (1992), which was based on the
Thornthwaite index, is located 150–200 km south of the arid/semi-arid boundary of the
USSR climate classification. Long temporal series of remotely sensed data offer another
useful approach to climate change-monitoring based on the estimation of the statistical
relationship between climate aridity and vegetation phytomass estimations from satellite
imagery (Kogan, 1995; Lambin, 1997; Nicholson et al., 1998). Most of this research focused
on climate change in the Sudan-Sahelian zone of Africa and the western U.S. Zolotokrylin
(2002) developed a new empirical aridity index for Central Asian deserts and semi-deserts,
Climate Change in Central Asia
37
which can be defined as a duration of the period with a normalized difference vegetation index (NDVI) less than 0.07. The NDVI is defined as the normalized difference of reflectance
in red and infrared bands of the electromagnetic spectrum. 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 atmosphere (Zolotokrylin, 2002). Analyses of the National Atmospheric
and Oceanic Administration AVHRR temporal series since the 1980s showed a decrease in
aridity from 1991–2000 compared to 1982–1990 in the northern part of the region (mainly
Kazakhstan) and a southward shift of the northern boundary of the desert zone in Central
Asia (Zolotokrylin, 2003).
Kharin et al. (1998), using the NOAA advanced very high resolution radiometer
(AVHRR) series for the assessment of precipitation and land degradation since 1982, also
points to a possible decrease of aridity in this region during the past decades. Their studies in
Uzbekistan and Turkmenistan indicate an increase in vegetation phytomass associated with
local precipitation increase from 10 to 30% from 1982 to 1992. Nonclimatic factors, such as
land-use changes, may affect the growth and vigor of green vegetation, and NDVI changes
may not always be perfectly correlated with precipitation change. Although remote sensing
data provide useful insights into the relationship between the NDVI and precipitation, the
duration of such observations is still too short to describe climate change variations and
trends realistically. Data have been collected in several parts of Central Asia that seem to
indicate that precipitation has increased at the end of the 20th century. However, because
this trend has been identified only for a very short period of time it is difficult to say it is
evidence of a manifestation of some cyclic pattern of precipitation at a decadal scale or a
part of some longer-term trend responding to either regional or global factors. The very existence of arid ecosystems is related to their temporal climatic variability, and together with
vegetation, cover characteristics that represent the major challenge for climate monitoring
and modeling.
Climate models predict that the temperature in arid Central Asia will increase by 1–
2◦ C by 2030–2050, with the greatest increase in winter. Precipitation projections vary from
one model to another and projected changes in the aridity index for different model runs
show no consistent trend for this regions (Figures 5a and 5b). Some models project greater
aridity in the future and some lower, and it is becoming increasingly apparent that climatechange modeling in arid zones is extremely uncertain, partly because of the extreme natural
variability (both temporal and spatial) of the desert climate and partly because of inherent
uncertainties in global and regional climate modeling. Atmospheric dynamics are known to
be very sensitive to natural climate variability at relatively short time-scales and the effect of
short-time variability on longer (decadal-to-millennial) time-scales are not fully understood.
The existence of the Turanian deserts of Central Asia results from their inland location,
orographic effects on the flow of air masses, and is largely governed by topography. The
mechanisms of mid-latitude aridity, however, are not limited to “rain shadow effects” of the
mountains but are largely the effects of mountains on the polar jet stream. The interaction of
mountain ranges with the polar jet determines regions of frequent passage of intertropical
disturbances. The spatial scale of existing models and downscaling techniques are still
insufficient in understanding climatic data in their regional and local context, especially in
the zones with complex topography.
Both theoretical considerations and numerical models have shown a significant sensitivity of the climate of arid regions to vegetation distribution. Ground-cover parameters can
significantly alter the modeled climate (Claussen et al., 2003; Wang and Eltahir, 2000). Climate change scenarios, unfortunately, do not incorporate regional controls on climate. The
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Figure 5. GCM scenarios for arid zones of Central Asia, 2020 to 2080 (derived from IPCC-DCC
data). (a) Temperature range (degrees C). (b) Precipitation range (mm per day).
Climate Change in Central Asia
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impacts of the extensive redirection of montane and lacustrine water resources to irrigated
agriculture in Central Asia and the degradation of the Aral Sea remain unmeasured in current
climate models, yet may be of significant importance in regional climate change. Although
it is clear that both observed and predicted climate changes might be partly caused by global
climate change and partly by local anthropogenic processes, it is extremely difficult, if not
impossible, to delineate the boundary between these two factors.
Possible Implications of “Greenhouse” Warming for Central Asian Deserts
While the predictions based on the paleoanalogues, recent climate trends, and General Circulation Models (GCM) scenarios still rather strongly disagree about the direction, frequency,
rapidity, and amplitude of the possible regional climate changes that might be caused by
the global greenhouse warming, it is likely that any changes in the regional circulation and
climate regime will affect various landscape components, such as vegetation, soils, hydrological regimes, biogeochemical cycles, and landforms. It is not our goal here to evaluate the
probability of one or another regional climate change scenarios but rather to discuss possible
implications of the probable climate changes for the arid environments of Central Asia.
Possible Hydrological and Geomorphological Responses
Arid environments are prone to rapid and often unpredictable hydrological and geomorphological changes in response to apparently modest climatic stimuli, switching from one state
to another when a particular threshold is reached (Goudie, 1994). Such responses may include changes in surface and underground runoff, sediment transport, groundwater and lake
levels, erosion and deposition rates, and many other related processes. In order to discern
the effect of climate change on hydrological processes, changes in precipitation, evapotranspiration, infiltration, groundwater storage, and various components of total runoff must be
considered. This is possible only through the implementation of a continuous hydrological
simulation models that couple land-atmosphere interactions and subsurface hydrological
surfaces together (Loaiciga, 2003). Runoff scenarios for arid regions are still very imprecise,
but the evidence suggests that even insignificant changes in precipitation and evapotranspiration are likely to result in proportionally larger changes in runoff. Shiklomanov (1999)
showed that an increase in mean annual air temperature by 1–2◦ C and a 10% decrease in
precipitation could reduce annual river runoff by up to 40–70% for river basins in areas
of water deficit. Up to the present time, most assessments of changes in the hydrological regime of Central Asia have been based on GCM scenarios coupled with hydrological
models and have involved tremendous simplification caused by the coarse resolution of
the numerical grids. Nested hydrological models within regional climate models (RCMs)
linked with GCM could provide much more detailed analysis but such simulations are still
very expensive. Global warming could have a dramatic effect on the Caspian Sea, where
a 2–3◦ C increase in air temperature would lead to increased precipitation and greater discharge from the rivers such as the Volga and Ural and a further increase of the sea level
(Shiklomanov, 1999). Climate change might also affect snow and ice storage and melting
starting in the Pamir and Tian Shan Mountains and hydrological regimes of two major Central Asian rivers—the Syrdarya and Amudarya that drain into the Aral Sea. It is, however,
very unlikely that increased river runoff would be able to balance the shrinking of the Aral
Sea caused by irrigation.
Another probable effect of climate change in this region has to do with recent changes in
wind erosion and dust storm activity. Changing dust storm frequencies in this region during
the past century have been largely attributed to human-induced desertification (Middleton,
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2002; Glazovsky, 1995). It is still unclear how such processes might be modified in the
future, given their recursive nature and the complexity of their relations with climatic,
vegetation, soil, and land-use variables.
Possible Implications of CO2 Increase for Desert Vegetation and Soils
One of the most intriguing questions about possible implications of global climate change
for desert ecosystems is how arid vegetation and other photosynthesizing organisms would
adapt to the increasing concentrations of CO2 and how this would affect other processes
in arid ecosystems (e.g., hydrological cycle, biogeochemical cycles, etc.). In addition to
its effect on climate, an increased atmospheric CO2 concentration has direct and relatively
immediate effects on two important physiological processes in plants: it increases the photosynthetic rate, but decreases stomatal opening and, therefore, the rate at which plants
lose water. The combination of these two factors—increased photosynthesis and decreased
water loss—implies a significant increase of water efficiency (the ratio of carbon gain per
unit water loss) and productivity and a reduction in the sensitivity to drought stress in desert
vegetation as a result of elevated atmospheric CO2 (Smith et al., 2000). No regional modeling studies have been conducted in this region but global biogeography models (Melillo
et al., 1993; Woodward et al., 1998) predict relatively strong responses of arid ecosystems
to global climatic change.
The Kara Kum and Kyzyl Kum Deserts of Central Asia are strongly dominated by vegetation with C3 photosynthetic pathway with only few C4 and Crassulacae Acid Metabolism
species (mainly succulents). It is often expected that plants using the C3 photosynthetic pathway will respond more strongly to raised CO2 than species with the more water-efficient and
CO2 -efficient C4 photosynthetic system (Table 1). The significance of different photosynthetic pathways in the adaptation of perennial plants to life in extreme desert environments
is still hotly debated (Whitford, 2002). Most publications on this subject are based on chamber experiments and the recent free-air CO2 -Enrichment experiments studying responses of
desert vegetation to increased CO2 levels conducted in the southwestern U.S. where desert
vegetation cover is dominated by C4 species.
Much research on vegetation responses to elevated CO2 concentrations was conducted
in the former Soviet Union in the 1970s and 1980s. CO2 -enrichment experiments (both
chamber and free-air) conducted in the Kara Kum (Voznesensky, 1977) and Kyzyl Kum
(Voznesensky, 1977; Zelensky, 1977) Deserts showed a 2 to 4 times increase in the photosynthetic rate under the saturating CO2 concentrations during several years. Three Kara
Kum species (Eminium lehmanii, Rhemum turkestanuikum, and Ephedra stobilacea) responded with a six-fold increase in photosynthetic rate (Nechaeva, 1984). This probably
can be explained by their very ancient origin dating back to past intervals with higher CO2
concentrations. Although theoretically, C4 plants have photosynthetic rates more than double those of C3 plants, among the Kara Kum species both C3 species (Ferula litwinowiana
and Smirnowia turestana) and C4 plants (Stpagroistis karlinii) showed significant responses
to CO2 fertilization. It is important, however, to realize that these results are primarily based
on chamber experiments and should be treated with caution. The results of CO2 fertilization experiments in other arid regions of the world, such as the Negev Desert in Israel
(Grüenzweig and Köerner, 2000) and the Mojave Desert in the U.S. (Smith et al., 2000)
are rather mixed—which only contributes to the uncertainty about the implications of the
doubled CO2 concentrations for desert ecosystems.
The CO2 fertilization affects not only higher vegetation but also microphytic communities including mosses, lichens, fungi, algae, and cyanobacteria (blue-green algae).
41
Smirnowia turkestana
Sunecio subdentatus
Rheum turkestanicum
Tamarix ramosissim
Populus pruinosa
Ephedra strobilacea
Atriplex dimorphostegia
Stipagrostis karelinii
Haloxilon persicum
Salsola richteri
Calligonum caput-medusae
Species
Compiled from Whitford (2002) and Nechaeva (1984).
CAM
C4
C3
Photosynthetic
pathway
Fixes C at night
Higher than sunlight
1/4 full sunlight
Light saturation
Water loss rate
(g H2 O/gC fixed)
∼600
∼250
∼50
Optimum
temperature
∼25◦ C
∼45◦ C
∼35◦ C
Table 1
Comparison of the general characteristics of C3, C4, and CAM
3–4
55–65
20–30
Photosynthetic
rate (mgCO2 /
dm2leaf h)
42
E. Lioubimtseva and R. Cole
These microphytic communities form biogenic crusts on the soil surface varying from a
few millimeters to several centimeters in thickness and play a significant role in the desert
ecosystems controlling such processes as water retention and carbon and nitrogen fixation
in soils. In the Kara Kum Desert of Turkmenistan, the accelerated growth of such biogenic
crusts, known among the local herders as “karaharsang” (turk.-black moss), has been observed during the past 40–50 years and usually has been attributed to the undergrazing
caused by the decrease of the wild fauna and insufficient pressure on the desert rangelands.
It is possible, however, that the main reason for the accelerated growth of “black mosses”
in the Kara Kum is a response of microphyts to increasing concentrations of CO2 in the
atmosphere.
If the increased CO2 concentrations result in the significant increase of vegetation
productivity, one might also expect significant changes in runoff, precipitation regime, and
circulation. It was demonstrated in several modeling studies in other arid regions (Wang
and Eltahir, 2000; Claussen et al., 2003) that vegetation plays a prominent role in the
energy, moisture, and carbon-exchange between the land surface in arid and semi-arid
zones and the atmosphere. However, vegetation-climate feedbacks and their role in global
and regional climate change are still relatively poorly understood and it is difficult to predict
feedbacks caused by these possible changes in the desert vegetation cover. If indeed, the
desert vegetation can respond to CO2 fertilization, it is still unclear how it would affect the
surface runoff. So far these relations are not taken into account by climate models.
Uncertainty about the changes in water-use efficiency and photosynthetic activity of
desert vegetation due to increased CO2 in the atmosphere raises a very important question:
have the vegetation changes in Central Asia, which were recently reported by several authors
(Zolotokrylin, 2002; Kharin et al., 1998; Lioubimtseva, 2004) on the basis of field- and
remote-sensing observations, been the result of precipitation changes (as these authors
assume) or by physiological changes in arid plants?
Uncertainties About Ecosystem Carbon Budget
There is much uncertainty about the possible positive or negative impacts of global climate
change on carbon sequestration in the vegetation and soils of the arid zones of Central Asia.
The seasonal dynamics of CO2 fluxes on deserts and semi-desert vegetation of the region
have not been measured until very recently and no data exist on their possible and longterm changes. Based on research on organic matter reserves and net primary productivity,
Central Asian arid lands have the potential to sequester considerable amounts of carbon,
if pasture productivity could be improved through more effective management techniques
(Lioubimtseva and Adams, 2002; Guilmanov, 2002).
Even more uncertain is the soil carbon budget of the region. It seems that commonly used
global databases (e.g., Zinke et al., 1984) have tended to overestimate the carbon content
in the deserts of the former USSR and Mongolia (i.e., 10.0 tC/ha in Zinke et al., 1984).
Russian sources indicate much lower values for the same areas (between 1 and 3.5 tC/ha in
dark sierozems and sandy soils and relatively high only in the light sierozems—up to 4.5
tC/ha in the first top 50 cm of soils) (Gladishev and Rodin, 1977; Lioubimtseva and Adams,
2002). The soil carbon values that were obtained for the Central Asian deserts seem to
be overestimated due to the miss-assignment of sampling sites (Adams and Lioubimtseva,
2002).
Preliminary estimations by Lioubimtseva and Adams (2002) suggest that the arid environments of Central Asia currently store between 7.0 and 9.0 Gt of organic carbon in the
top 50 cm of soils. This is 3 or 4 times less than during the Liavliakan Pluvial (Holocene
Climate Change in Central Asia
43
climatic optimum), when a warmer global climate and lack of human impact favored richer
vegetation in this area (Lioubimtseva et al., 1998). However, these estimations are very
rough and further research is needed to assess the carbon budget of this region.
Human Impact and Land Degradation
The practice of farming is ancient in Central Asia. Numerous artifacts of Mesolithic and
Neolithic settlements are found all over this vast region and primitive forms of artificial
irrigation existed in Turkmenistan and Uzbekistan more than 2000 years ago. The second
major form of land use of this arid area that goes back many thousands of years ago is
pastoral nomadism—an extensive, mobile form of land use that can have negative local
environmental impacts in its traditional forms. Other human impacts include technological
developments such as the excessive use of woody vegetation for domestic use and road
building and drilling for water and oil. For this reason, any attempt to trace a clear boundary
between the desert responses to global warming and the effects of local human activities
would be extremely difficult. It is very likely that the effects of global change would be
rather insignificant compared to the local human-induced processes, such as irrigation,
overgrazing, ground water depletion, and many others effects of land use. Arid areas all
over the world have shown themselves to be highly susceptible to the effects of human
intervention (Goudie, 1994; Dregne and Chou, 1992; Glantz, 1999).
Grazing Impact on Vegetation and Soils
Grazing can exert both positive and negative effects on total vegetation cover, and by
implication on soil organic carbon (which is derived from plant material). Moderate and
rationally managed grazing generally favors sequestration of organic carbon in soils both
directly (organic matter from excrement) and indirectly (preserving perennial grasses such
as Carex physodes with well-developed root systems and high underground biomass). An
overgrazed soil is easily eroded and blown away by the wind to form crescent dune (barchan)
fields. In arid and semi-arid rangelands, overgrazing is the main reason for losses of both
vegetation and soil carbon and nitrogen. The impact of grazing on vegetation, however, has
been decreasing in the region during the past decades and is currently insignificant with the
exception of Priaralye (the Aral Sea area) (Neronov, 1997; Kharin et al., 1998). In many parts
of the Kara Kum and Kyzyl Kum Deserts, undergrazing rather than overgrazing has been the
main reason for environmental changes. Cattle, camel, horse, and sheep populations have
decreased dramatically in the Central Asian countries by 27%, 18%, 28%, 53%, respectively,
over the last 10 years (FAO, 2003). Goats, which accounted for only 5% of the small stock
(sheep and goats) in 1992, increased by 31% to account for 14% of the total small stock
population in 2002. But overall, the numbers of small stock have decreased over the past
10 years by over 30 million head. With undergrazing, the loosening of the soil is too little
and annual grasses or moss gradually replace the sparser natural perennial vegetation, and
the surface becomes more compacted. Unprecedented growth of karaharsangs (microphytic
communities) during the last decades in response to increasing CO2 fertilization usually
occurs only in the undergrazed areas. Similarly, the pressing of seeds underfoot to the
optimal depth occurs only with moderate numbers of animals per hectare of pasture.
Perturbation of Climate by Irrigation
Although artificial irrigation in oases of Central Asia has been known for almost three
millennia, the most important environmental changes in this region have occurred during
44
E. Lioubimtseva and R. Cole
the past 30–40 years as a result of massive irrigation schemes started in the 1960s (Glazovsky,
1995; Glantz, 1999). Irrigated arable land, principally for cotton monoculture, has increased
by 60% in the region from 1962 to 2002 (FAO, 2003). The total irrigated area in the
study area increased by over half a million hectares (5.2%) just from 1992 to 2002 (FAO,
2003). Turkmenistan alone accounted for 59% of the change, increasing its irrigated area
by 300,00 hectares (20%). Total water withdrawals (all uses) were 125% of the average
annual water resources in 1988 (Glazovsky, 1995). Such dramatic human-induced changes
in the hydrological cycle led not only to a significant decrease of river runoff, changes in
the number and area of lakes and rise of groundwater levels, but also to significant changes
in evapotranspiration and precipitation.
Both theoretical considerations and modeling experiments (Wang and Eltahir, 2000;
Zolotokrylin, 2002) show that changes in albedo and other biophysical parameters of vegetation cover caused by massive irrigation might establish totally new equilibria in the
climate-vegetation relations and reverse previously existing feedback mechanisms. While
a desert environment is featured by strong negative biosphere-atmosphere feedbacks, perturbation of a large area by irrigation induces a positive feedback that brings the system to
more humid climatic conditions with a new climate-vegetation equilibrium.
Climate Change Caused by the Aral Sea Crisis
Most research on climate change in the Central Asian republics during the past decades
has been focused on the Aral Sea areas and an enormous number of publications exist on this subject, essentially in Russian (e.g., Molosnova et al., 1987; Semenov, 1990;
Glazovsky, 1995; Zolotokrylin and Hmelevskaya, 1999; Chub, 2000). The Aral Sea desiccation caused prominent climate change not only in the coastal area but affected the entire
atmospheric circulation system in its basin. Summer and winter air temperatures at the
stations near the seashore increased by 1.5–2.5◦ C and diurnal temperatures increased by
0.5–3.3◦ C (Glazovsky, 1995; Chub, 2000). Near the coast, the mean annual relative humidity decreased by 23% and recurrence of drought days increased by 300% (Glazovsky,
1995). The annual cycle of temperature and precipitation has also changed. A seven-fold
rise in the albedo of the area previously occupied by the Aral Sea caused a three-fold increase in reflected solar radiation and overall continentality of the climate (Chichasov, 1990;
Glazovsky, 1995).
In addition, the exposure of the former lakebed areas, especially on the eastern side of
the Aral Sea, represents an enormous source of highly saline wind-blown material (up to
1.5% of salt in the total mass of hard particles transported by the wind). Deposition of the
Aral Sea dust has been reported at a distance of several thousand miles from the source, and
many data suggest that the aeolian deposition of salts has adversely affected the vegetation
of vast adjacent areas. According to Semenov (1990) the amount of aeolian redeposition
from the former Aral seabed is exceeding 7.3∗ 106 tons per year, comprised of between
5 and 7∗ 104 tons of salt per year, equivalent to 3000 commercial trains of 40 cars each.
Today the drying bottom of the Aral Sea has became one of the biggest sources of dust
aerosols in the world and must be considered in model simulations of both global and
regional climates. Dust tends to cool the earth by reflecting sunlight back into space, and
it decreases rainfall by suppressing atmospheric convection. GCM simulations of future
climate, which include changes in dust loading, tend to yield different results from those
that do not, and the incorporation of iterative feedbacks between simulated future climate,
vegetation cover, and dust flux from changes in vegetation cover, tends to further amplify
the changes in arid-land climates. Following an initial increase in rainfall over arid areas,
Climate Change in Central Asia
45
dust flux decreases due to increased stabilizing vegetation cover and moister soils. This
decrease in dust flux tends to cause rainfall to increase further. Although the world is
generally forecast to become moister as greenhouse gas levels increase, this effect is patchy
and according to certain models some desert areas may become drier. In this case increased
dust flux may increase aridity and also suppress rainfall outside the desert areas themselves.
However, at present the incorporation of dust feedbacks in models is in its early stages
(Lioubimtseva and Adams, 2004).
Conclusion
Arid environments are projected to undergo significant changes under greenhouse warming,
but there is considerable variability and uncertainty between different climatic scenarios.
The complexities of precipitation changes, vegetation-climate feedbacks, and direct physiological effects of CO2 on vegetation present particular challenges for climate-change modeling of arid regions. Great uncertainties exist in prediction of arid ecosystem responses to
elevated CO2 , as well as to the regional precipitation and evaporation changes caused by
global climate change.
There has been a general warming trend in arid Central Asia on the order of 1–2◦ C since
the beginning of the 20th century with some fluctuations along the way. Such fluctuations
of the past century seem to be comparable in their amplitude and frequency with the earlier
climate variability during the second half of the Holocene. These short-term trends also
show significant spatial variability caused by regional topography and other factors.
Palaeodata provide important information about spatial patterns of change in arid environments in the past that can significantly improve our knowledge about the global and
regional controls on climate change in arid regions. However, because the mechanisms
of palaeoclimatic variability were different from those caused by the current greenhouse
warming, any paleoanalogies should be treated with great caution.
Precipitation projections in this region vary from one model to another but are unlikely
to be significant. Despite the great progress in global climate modeling, the GCMs give
very variable results, with large spatial differences in the areas forecast to receive higher
or lower precipitation. The lack of integration of such factors as dust aerosols, biophysical
and biochemical feedbacks caused by land-cover changes, as well as the regional factors
of human-induced climate change such as irrigation, are the major sources of uncertainties.
For example, the dust aerosols from the drying Aral Sea bottom may have a very significant
impact on regional climate but are not taken into account by the models.
Projections based on biogeographic models suggest considerable changes in desert
and semi-desert vegetation due to a combination of greenhouse-related climate change
and direct physiological CO2 effects on vegetation, such as changes in photosynthesis and
water-use efficiency over the coming century. This could have implications for crop growth
in desert-marginal areas, favoring greater productivity, and perhaps increase productivity
and biomass of natural desert vegetation and soil organic matter. However, the very limited
number of CO2 enrichment experiments in the Kara Kum and Kyzyl Kum does not always
confirm this thesis. The accelerated growth of biogenic crusts (observed during the past 40–
50 years) in this arid region might be a response of microphyts to increasing concentrations
of CO2 in the atmosphere. However, because such responses occur only on the undergrazed
desert rangelands, it is still unclear if the CO2 increase is really the major cause or such
growth or rather the land-use change. Local and regional human impact in arid zones can
significantly modify surface albedo, as well as water exchange and nutrient cycles that
might have potential impact on the climatic system both at the regional and global scales.
46
E. Lioubimtseva and R. Cole
On the other hand, improved management techniques can increase the carbon sequestration
capacity of semi-desert rangelands and arable lands.
Acknowledgments
The GCM scenarios were obtained from the IPCC Data Distribution Centre (http://ipccddc.cru.uea.ac.uk). Historical monthly precipitation data were obtained from Tyndall Centre
for Climate Change Research, University of East Anglia. We are grateful to Philip Micklin
and two anonymous reviewers for helpful comments, and Kin Ma who helped with editing
the final version of this manuscript.
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