The Arctic Seas ice cover has been regularly monitored since active development of the Northern Sea Route began in the 1930s. The monitoring is accomplished by a network of polar stations, using airplanes, ships, and, since the late 1960s, satellites.

Studies of long-term changes in the ice cover aim to address both economic and purely scientific objectives. Ice cover affects construction of ports and other onshore structures and operation of offshore platforms for production of hydrocarbons as well as their transportation to the mainland. In the science realm, it is impossible to understand the main mechanisms of Earth's climate changes—and hence predict future changes—without investigating long-term changes in the state of the Arctic Seas ice cover.

For the last few years, in connection with current climate warming, studies by Russian and other scientists have predicted a significant decrease in sea-ice extent in the Arctic and even its complete disappearance in the summertime by the end of the twenty-first century. This monograph presents results of studies of climatic system changes in the Arctic focused on the ice cover that do not justify such extreme conclusions.

Alternating periods of warming and cooling were typical in the Arctic during the twentieth century. The authors show that the duration of the main climate-shaping cycle of these fluctuations was about 60 years, but there were also 20- and 10-year cycles. The authors analyze the spatial-temporal peculiarities of these cycles and their influence on sea-ice extent variability.

They show relationships between long-term changes in area of the ice cover and climatic fluctuations in air temperature, atmospheric circulation indices, characteristics of water masses, and river runoff volume. There is also an analysis of possible natural causes of intra-secular climate fluctuations that influence the state of the Arctic ice cover (its area, thickness, concentration and multiyear ice edge). Based on the data presented for the twentieth century, the authors project Arctic Seas ice cover conditions for the twenty-first century: they expect that an oscillatory (rather

than a unidirectional) background of ice area changes in the Arctic Seas will be preserved during the current century, with a gradual increase by the 2030s and a subsequent decrease by the 2060s.

Many studies and international projects, such as the Arctic Climate Impact Assessment (ACIA), attribute the air temperature increase during the last quarter of the twentieth century exclusively to accumulation of greenhouse gases in the atmosphere. However, these studies typically do not account for natural hydro-meteorological fluctuations whose effects on multiyear variability, as this monograph shows, can far exceed the anthropogenic impact on climate.

Academician V. M. Kotlyakov Director of the Institute of Geography Russian Academy of Science

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