Mysteries of the Caspian

Mysteries of the Caspian Sea (Mikhailov V.N., 2000), EARTH SCIENCES

The Caspian Sea has an unstable hydrological regime. In the 20th century the sea level experienced both a rapid fall and a sharp rise. The article answers the questions of how unusual these changes in sea level are and whether their consequences can be considered an ecological catastrophe. Possible causes of the unstable regime of the Caspian Sea are discussed.
MYSTERIES OF THE CASPIAN SEA

V. N. MIKHAILOV

Lomonosov Moscow State University

INTRODUCTION

The Caspian Sea is the largest endorheic lake on the planet. This body of water is called a sea because of its enormous size, brackish water and a regime similar to that of a sea. The level of the Caspian sea-lake lies far below the level of the World Ocean. At the beginning of 2000 it stood at about - 27 m abs. At this level the area of the Caspian is ~ 393 thousand km2 and the volume of water 78,600 km3. The mean and maximum depths are 208 and 1025 m respectively.

The Caspian Sea stretches from south to north (Fig. 1). The Caspian washes the shores of Russia, Kazakhstan, Turkmenistan, Azerbaijan and Iran. The sea is rich in fish, and its bed and shores in oil and gas. The Caspian Sea has been studied fairly well [1, 2, 5], yet many mysteries remain in its regime. The most characteristic feature of the sea is the instability of its level, with sharp falls and rises. The latest rise in the level of the Caspian took place before our eyes from 1978 to 1995. It gave rise to many rumours and speculations. Numerous publications appeared in the press speaking of catastrophic floods and an ecological catastrophe. It was often written that the rise in the level of the Caspian had led to the flooding of almost the entire Volga delta. What in these statements corresponds to reality? What is the reason for such behaviour of the Caspian?

WHAT HAPPENED TO THE CASPIAN IN THE 20TH CENTURY

Systematic observations of the level of the Caspian Sea began in 1837. In the second half of the 19th century the mean annual values of the Caspian level were in the range from - 26 to - 25.5 m abs. and showed a certain downward trend. This trend continued into the 20th century (Fig. 2). Between 1929 and 1941 the sea level dropped sharply (by almost 2 m - from - 25.88 to - 27.84 m abs.). In the following years the level continued to fall and, having dropped by approximately 1.2 m, reached in 1977 the lowest mark for the period of observations, - 29.01 m abs. Then the sea level began to rise rapidly and, having risen by 2.35 m by 1995, reached the mark of - 26.66 m abs. Over the next four years the mean sea level fell by almost 30 cm. Its mean marks were - 26.80 in 1996, - 26.95 in 1997, - 26.94 in 1998 and - 27.00 m abs. in 1999.

The fall in sea level in the 1930s-1970s led to the shallowing of coastal waters, the advance of the shoreline towards the sea and the formation of wide beaches. The latter was perhaps the only positive consequence of the fall in level. There were far more negative consequences. As the level fell, the area of feeding grounds for fish stocks in the northern Caspian shrank. The shallow Volga estuarine seashore quickly began to be overgrown with aquatic vegetation, which worsened the conditions for fish passing into the Volga to spawn. Fish catches fell sharply, especially of valuable species: sturgeon and sterlet. Shipping began to suffer because the depths in the approach channels decreased, especially near the Volga delta.

The rise in level from 1978 to 1995 was not only unexpected but also led to even greater negative consequences. After all, both the economy and the population of the coastal areas had already adapted to the low level.

Many sectors of the economy began to suffer losses. Considerable territories ended up in the zone of flooding and waterlogging, especially in the northern (lowland) part of Dagestan, in Kalmykia and in Astrakhan Oblast. The cities of Derbent, Kaspiysk, Makhachkala, Sulak, Kaspiysky (Lagan) and dozens of other smaller settlements suffered from the rise in level. Considerable areas of agricultural land were flooded and waterlogged. Roads and power lines, and engineering structures of industrial enterprises and municipal services are being destroyed. A threatening situation has developed with fish-breeding enterprises. Abrasion processes in the coastal zone and the impact of storm surges of seawater have intensified. In recent years, tangible damage has been done to the flora and fauna of the seashore and the coastal zone of the Volga delta.

Owing to the increase in depths in the shallows of the Northern Caspian and the reduction in the areas occupied by aquatic vegetation in these places, conditions for the reproduction of stocks of anadromous and semi-anadromous fish and the conditions for their migration into the delta to spawn improved somewhat. However, the predominance of negative consequences from the risen sea level led people to speak of an ecological catastrophe. Development of measures to protect economic facilities and settlements from the advancing sea began.

HOW UNUSUAL

IS THE PRESENT BEHAVIOUR OF THE CASPIAN?

Research into the life history of the Caspian Sea can help answer this question. Of course, there are no data from direct observations of the past regime of the Caspian, but there is archaeological, cartographic and other evidence from historical times and the results of palaeogeographic research covering a longer period [5].

It has been proven that during the Pleistocene (the last 700-500 thousand years) the level of the Caspian Sea underwent large-scale fluctuations within a range of about 200 m: from -140 to + 50 m abs. Within this period, four stages are distinguished in the history of the Caspian: the Baku, Khazar, Khvalyn and New Caspian stages [5] (Fig. 3). Each stage included several transgressions and regressions. The Baku transgression occurred 400-500 thousand years ago, when the sea level rose to 5 m abs. During the Khazar stage there were two transgressions: the Early Khazar (250-300 thousand years ago, maximum level 10 m abs.) and the Late Khazar (100-200 thousand years ago, highest level -15 m abs.). The Khvalyn stage in the history of the Caspian included two transgressions: the Early Khvalyn, the largest of the Pleistocene (40-70 thousand years ago, maximum level 47 m abs., which is 74 m higher than today), and the Late Khvalyn (10-20 thousand years ago, with the level rising to 0 m abs.). These transgressions were separated by the deep Yenotayevka regression (22-17 thousand years ago), when the sea level fell to - 64 m abs. and was 37 m lower than today.

Significant fluctuations of the Caspian level also took place during the New Caspian stage of its history, which coincided with the Holocene (the last 10 thousand years). After the Mangyshlak regression (10 thousand years ago, with the level dropping to - 50 m abs.), five stages of the New Caspian transgression were recorded, separated by minor regressions (Fig. 4). Following the fluctuations of the sea level - its transgressions and regressions - the outline of the water body also changed (Fig. 5).

Over historical time (2000 years), the range of change in the mean level of the Caspian Sea was 7 m - from - 32 to - 25 m abs. [5] (see Fig. 4). The minimum level over the last 2000 years was during the Derbent regression (6th-7th centuries AD), when it dropped to - 32 m abs. In the time since the Derbent regression, the mean sea level has varied within an even narrower range - from - 30 to - 25 m abs. This range of level change has been called the risk zone [5].

Thus, the level of the Caspian fluctuated before as well, and in the past these fluctuations were more significant than in the 20th century. Such periodic fluctuations are a normal manifestation of the unstable state of an enclosed water body with variable conditions at its external boundaries. Therefore, there is nothing unusual about the fall and rise of the Caspian Sea level.

CAN THE RECENT RISE IN THE CASPIAN LEVEL AND ITS CONSEQUENCES BE CONSIDERED AN ECOLOGICAL DISASTER

Fluctuations of the Caspian level in the past apparently did not lead to irreversible degradation of its biota. Of course, sharp drops in sea level created temporary unfavourable conditions, for example for fish stocks. However, as the level rose, the situation corrected itself. The natural conditions of the coastal zone (vegetation, bottom-dwelling animals, fish) undergo periodic changes along with the fluctuations of the sea level and apparently possess a certain reserve of stability and resistance to external influences. After all, the most valuable sturgeon stock has always existed in the Caspian basin, quickly overcoming temporary deterioration of living conditions regardless of sea level fluctuations.

Rumours that the sea level rise caused flooding throughout the Volga delta were not confirmed. Moreover, it turned out that the rise in water levels even in the lower part of the delta did not correspond to the magnitude of the sea level rise. The rise in water level in the lower part of the delta during low water did not exceed 0.2-0.3 m, and during floods it was almost not noticeable at all. At the maximum Caspian level in 1995, backwater from the sea spread along the deepest branch of the delta, the Bakhtemir, for no more than 90 km, and along other branches for no more than 30 km. Therefore, only the islands in the offshore zone and a narrow coastal strip of the delta were flooded. Flooding in the upper and middle parts of the delta was associated with high floods in 1991 and 1995 (which is a normal phenomenon for the Volga delta) and with the poor condition of the protective dams. The reason for the weak influence of the sea level rise on the regime of the Volga delta is the presence of a huge shallow offshore zone, which dampens the impact of the sea on the delta.

As for the negative impact of the sea level rise on the economy and the life of the population in the coastal zone, the following should be recalled. At the end of the last century the sea level stood higher than it does now, and this was in no way perceived as an ecological disaster. And earlier the level was at even higher marks. Meanwhile, Astrakhan has been known since the mid-13th century, and it was here, from the 13th to the mid-16th century, that Sarai-Batu, the capital of the Golden Horde, was located. These and many other settlements on the Caspian coast did not suffer from high water levels, since they were situated on elevated ground, and during abnormal flood levels or storm surges people temporarily moved from low-lying places to higher ones.

Why, then, are the consequences of the sea level rising, even to lower marks, now perceived as a disaster? The cause of the enormous damage suffered by the national economy is not the rise in level, but the thoughtless and short-sighted development of the strip of land within the aforementioned risk zone, which was freed (temporarily, as it turned out!) from under the sea after 1929, that is, when the level dropped below - 26 m abs. Buildings erected in the risk zone were naturally flooded and partially destroyed. Now, when territory developed and polluted by humans is being flooded, a dangerous ecological situation is indeed being created, the source of which is not natural processes but unreasonable economic activity.

ON THE CAUSES OF CASPIAN LEVEL FLUCTUATIONS

When considering the question of the causes of Caspian level fluctuations, it is necessary to note the opposition of two concepts in this field: geological and climatic. Significant contradictions between these approaches became apparent, for example, at the international conference "Caspian-95".

According to the geological concept, the causes of changes in the level of the Caspian fall into two groups of processes. Processes of the first group, in the view of geologists, lead to changes in the volume of the Caspian basin and, as a consequence, to changes in sea level. These processes include vertical and horizontal tectonic movements of the Earth's crust, accumulation of bottom sediments and seismic phenomena. The second group includes processes that, as geologists believe, affect groundwater discharge into the sea, now increasing it, now decreasing it. Such processes are said to be the periodic squeezing out or absorption of waters that saturate bottom deposits under the influence of changing tectonic stresses (alternating periods of compression and extension), as well as technogenic destabilization of the subsurface caused by oil and gas production or underground nuclear explosions. The fundamental possibility that geological processes influence the morphology and morphometry of the Caspian basin and groundwater discharge cannot be denied. However, at present a quantitative link between geological factors and fluctuations in the level of the Caspian has not been proven.

Undoubtedly, tectonic movements played a decisive role in the initial stages of the formation of the Caspian basin. However, if one takes into account that the Caspian Sea depression lies within a geologically heterogeneous territory, which results in the periodic rather than linear nature of tectonic movements with repeated changes of sign [5], then a noticeable change in the capacity of the basin is hardly to be expected. Also arguing against the tectonic hypothesis is the fact that the shorelines of the New Caspian transgressions are at the same level on all sections of the Caspian coast (with the exception of certain areas within the Absheron Archipelago) [5].

There are no grounds for considering a change in the capacity of the basin due to sediment accumulation as the cause of fluctuations in the level of the Caspian. The rate at which the depression is being filled with bottom deposits, in which river-borne sediments play the main role, is estimated, according to current data, at about 1 mm/year or less, which is two orders of magnitude smaller than the changes in sea level observed now. Nor can seismic deformations, which are noted only near the epicenter and die out at short distances from it, have any significant effect on the volume of the Caspian depression.

As for periodic large-scale discharge of groundwater into the Caspian, its mechanism is still unclear. At the same time, according to E.G. Maev, this hypothesis is contradicted, first, by the undisturbed stratification of pore waters, which indicates the absence of noticeable water migration through the thickness of bottom deposits, and second, by the absence of proven strong hydrological, hydrochemical and sedimentation anomalies in the sea that would have to accompany a large-scale groundwater discharge capable of affecting changes in the level of the water body.

The main evidence of the insignificant role of geological factors at present, however, is the convincing quantitative confirmation of the plausibility of the second, climatic, or more precisely, water-balance concept of fluctuations in the level of the Caspian.

CHANGES IN THE COMPONENTS OF THE CASPIAN WATER BALANCE AS THE MAIN CAUSE OF ITS LEVEL FLUCTUATIONS

Fluctuations in the level of the Caspian were first explained by changes in climatic conditions (more specifically, river runoff, evaporation and precipitation on the sea surface) as early as E.Kh. Lenz (1836) and A.I. Voeikov (1884). Later, the leading role of changes in the water balance components in sea level fluctuations was proven again and again by hydrologists, oceanologists, physical geographers and geomorphologists.

Key to most of the studies mentioned are the compilation of the water balance equation and the analysis of its components. The meaning of this equation is as follows: the change in the volume of water in the sea is the difference between the incoming (river and groundwater inflow, precipitation on the sea surface) and outgoing (evaporation from the sea surface and outflow of water into the Kara-Bogaz-Gol Bay) components of the water balance. The change in the level of the Caspian is the quotient of the change in its water volume divided by the area of the sea. The analysis showed that the leading role in the water balance of the sea belongs to the ratio between the runoff of the Volga, Ural, Terek, Sulak, Samur and Kura rivers and the apparent or effective evaporation, the difference between evaporation and precipitation on the sea surface. The analysis of the water balance components revealed that the largest contribution (up to 72% of the variance) to level variability comes from river inflow, and more specifically, from the runoff formation zone in the Volga basin. As for the causes of changes in the Volga runoff itself, many researchers believe they are linked to the variability of precipitation (mainly winter precipitation) in the river basin. The precipitation regime, in turn, is determined by atmospheric circulation. It was proven long ago that the zonal type of atmospheric circulation favors increased precipitation in the Volga basin, and the meridional type favors decreased precipitation.

V.N. Malinin found that the root cause of moisture reaching the Volga basin should be sought in the North Atlantic, specifically in the Norwegian Sea. It is there that increased evaporation from the sea surface leads to an increase in the amount of moisture carried to the continent and, accordingly, to increased precipitation in the Volga basin. The latest data on the water balance of the Caspian Sea, obtained by R.E. Nikonova and V.N. Bortnik of the State Oceanographic Institute, are given with the author's refinements in Table 1. These data convincingly prove that the main causes of both the rapid fall in sea level in the 1930s and the sharp rise in 1978-1995 were changes in river runoff, as well as in apparent evaporation.

Bearing in mind that river runoff is one of the main factors affecting the water balance and, as a consequence, the level of the Caspian Sea (and the Volga's runoff accounts for no less than 80% of the total river inflow into the sea and about 70% of the incoming part of the Caspian water balance), it would be interesting to find a relationship between the sea level and the runoff of the Volga alone, which is measured most accurately. A direct correlation of these quantities does not give satisfactory results.

However, the relationship between the sea level and the Volga runoff can be clearly traced if the river runoff is taken not for each individual year, but as the ordinates of the residual mass curve of runoff, that is, the cumulative sum of normalized deviations of annual runoff values from the long-term mean (the norm) [4]. Even a visual comparison of the course of the mean annual levels of the Caspian and the residual mass curve of the Volga runoff (see Fig. 2) reveals their similarity.

Over the entire 98-year period of observations of the Volga runoff (the village of Verkhneye Lebyazhye at the head of the delta) and of the sea level (Makhachkala), the correlation coefficient between the sea level and the ordinates of the residual mass curve of runoff was 0.73. If the years with small changes in level (1900-1928) are discarded, the correlation coefficient rises to 0.85. If the analysis is limited to the periods of rapid fall (1929-1941) and rise of the level (1978-1995), the overall correlation coefficient is 0.987, and 0.990 and 0.979 for the two periods separately [4].

These calculation results fully confirm the conclusion that during periods of sharp fall or rise of the sea level, the levels themselves are closely related to runoff (more precisely, to the sum of its annual deviations from the norm).

A special task is to assess the role of anthropogenic factors in the fluctuations of the Caspian level, above all the reduction of river runoff due to its irretrievable losses to filling reservoirs, evaporation from the surface of artificial water bodies, and water withdrawal for irrigation. It is believed that from the 1940s onward, irretrievable water consumption grew steadily, which led to a reduction in river inflow to the Caspian and an additional lowering of its level compared with the natural one. According to V.N. Malinin, by the end of the 1980s the difference between the actual sea level and the reconstructed (natural) level reached almost 1.5 m. At that time, total irretrievable water consumption in the Caspian basin was estimated at 36-45 km3/year (of which about 26 km3/year was accounted for by the Volga). Had it not been for the withdrawal of river runoff, the rise of the sea level would have begun not in the late 1970s but in the late 1950s.

The growth of water consumption in the Caspian basin by 2000 was forecast first at up to 65 km3/year and then at up to 55 km3/year (36 of which were to be accounted for by the Volga). Such an increase in irretrievable losses of river runoff was supposed to lower the Caspian level by another 0.5 m or more by 2000. With regard to assessing the impact of irretrievable water consumption on the Caspian level, we note the following. First, the estimates found in the literature of the volumes of water withdrawal and evaporation losses from the surface of reservoirs in the Volga basin are apparently significantly overstated. Second, the forecasts of growth in water consumption turned out to be wrong. The forecasts assumed rates of development of water-consuming sectors of the economy (especially irrigation) that not only proved unrealistic but were replaced by a decline in production in recent years. In reality, as A.E. Asarin points out (1997), by 1990 water consumption in the Caspian basin amounted to about 40 km3/year, and at present it has decreased to 30-35 km3/year (in the Volga basin, to 24 km3/year). Therefore, the "anthropogenic" difference between the natural and actual sea level is currently not as large as was forecast.

ON POSSIBLE FUTURE FLUCTUATIONS

OF THE CASPIAN LEVEL

The author does not set out to analyze in detail the numerous forecasts of Caspian Sea level fluctuations (that is a separate and difficult task). The main conclusion from evaluating the results of forecasting Caspian level fluctuations can be stated as follows. Although the forecasts were based on completely different approaches (both deterministic and probabilistic), there was not a single reliable forecast. The main difficulty in using deterministic forecasts built on the sea's water balance equation is the undeveloped state of the theory and practice of ultra-long-range forecasts of climate change over large territories.

When the sea level was falling in the 1930s-1970s, most researchers predicted its further decline. In the last two decades, when the sea level began to rise, most forecasts predicted an almost linear and even accelerating rise of the level to -25 and even -20 abs. m and higher at the beginning of the 21st century. Three circumstances were not taken into account. First, the periodic nature of level fluctuations in all endorheic water bodies. The instability of the Caspian level and its periodic nature are confirmed by analysis of its present and past fluctuations. Second, at a sea level close to -26 abs. m, flooding will begin of the large sor bays on the northeastern coast of the Caspian that dried out during the low stand of the level, Mertvy Kultuk and Kaydak, as well as of low-lying areas elsewhere on the coast. This would lead to an increase in the area of shallow waters and, as a consequence, to increased evaporation (up to 10 km3/year). At a higher sea level, the outflow of water into Kara-Bogaz-Gol will increase. All this should stabilize, or at least slow down, the rise of the level. Third, level fluctuations under the conditions of the present climatic epoch (the last 2000 years), as shown above, are confined to the risk zone (from -30 to -25 abs. m). Taking into account the anthropogenic reduction of runoff, the level is unlikely to exceed the mark of -26 to -26.5 abs. m.

The decline in mean annual levels over the last four years by a total of 0.34 m may indicate that in 1995 the level reached its maximum (-26.66 m abs.) and that the trend in the Caspian level has changed. In any case, the prediction that the sea level is unlikely to exceed the -26 m abs. mark [5] is apparently being borne out.

CONCLUSION

In the 20th century the level of the Caspian Sea varied within a range of 3.5 m, first falling and then rising sharply. Such behaviour of the Caspian is the normal state of an enclosed water body as an open dynamic system with variable input conditions.

Each combination of the incoming (river runoff, precipitation on the sea surface) and outgoing (evaporation from the water surface, outflow into the Kara-Bogaz-Gol Bay) components of the Caspian water balance corresponds to its own equilibrium level. Since the components of the sea's water balance also change under the influence of climatic conditions, the level of the water body fluctuates, striving to reach a state of equilibrium but never reaching it. Ultimately, the trend in the Caspian level at any given time depends on the ratio of precipitation minus evaporation over the catchment (in the basins of the rivers that feed it) and evaporation minus precipitation over the water body itself. There is in fact nothing unusual about the recent 2.3 m rise in the Caspian level. Such changes in level occurred many times in the past and did not cause irreparable damage to the natural resources of the Caspian. The current rise in sea level became a disaster for the economy of the coastal zone only because of unwise human development of this risk zone.