Introduction

Arctic sea ice is an important climate parameter that regulates processes of heat, salt, and momentum exchange between the Arctic Ocean and the Arctic atmosphere. As part of the global climate system, Arctic sea ice directly influences the climate of the Northern Hemisphere and is also influenced by global climate changes. The major goals of this book are to describe the state and variability of the Arctic sea ice cover, to demonstrate methods for sea ice studies, and to describe and test hypotheses that will allow us to understand and predict future Arctic sea ice conditions. In order to reach these goals, we synthesize the data collected and experience gained by Arctic and Antarctic Research Institute (AARI) scientists during their more than 85 years of Arctic exploration.

Climate is usually defined as the "average weather'' or as a statistical description that includes the mean and the variability of atmospheric, oceanic, and sea ice parameters over a period of time ranging from months to thousands or millions of years. The classical period for averaging is 30 years, as defined by the World Meteorological Organization (WMO). In meteorology, the most relevant parameters for characterizing climate are atmospheric variables such as temperature, pressure, precipitation, and wind. For sea ice, these parameters are ice concentration, thickness, drift, sea ice area, and sea ice extent.

Observational evidence indicates that significant climate changes have taken place throughout Earth's history. Geological data, isotopic evidence, dendrological and pollen analyses, vegetation and its fluctuations, glacier dynamics, lake-level variability, and instrumental observations over thousands or millions of years all confirm this.

Gribbin and Lamb (1978) describe four phases of climate change since Earth's last glaciation (approximately 10kyr bp). The first postglacial climate warming (7-5 kyr bp) was characterized by a major decrease in glaciers and sea ice and a significant increase in mean air temperature, which was 2-3°C higher in summer compared to present conditions. The second phase was cooling that occurred in the

Iron Age (a period between 900 and 300 bc), distinguished by a decrease in air temperature, southward retreat of the northern forestry line, and changes in the precipitation regime. In the eleventh and twelfth centuries (or in the eighth to fourteenth centuries according to Borisenkov, 1982), there was an epoch of "small" climatic warming, which was characterized by favorable navigation conditions in the North Atlantic (and Viking colonization of the Greenland coast and part of North America). The average air temperature in this epoch was approximately 1.5°C higher than in the Little Ice Age (eighteenth to early nineteenth centuries) and slightly above current temperatures. This phase, often referred to as the Medieval Warm Period, was replaced by the Little Ice Age, when the coasts of Greenland and Iceland were bounded by sea ice, and glaciers expanded in the Alps and other regions. The surface water temperature in the North Atlantic at that time was 2-3 °C lower than in the 1920s-1930s, the period of the first Arctic warming in the twentieth century (Lamb and Johnson, 1964). Zakharov in Formation and Dynamics of a Modern Climate of Arctic Regions (2004) reconstructed sea ice conditions in the eastern Barents Sea during the Little Ice Age by analyzing observations of Russian navigators during their cruises to Novaya Zemlya in the eighteenth and nineteenth centuries. He concluded that typical August ice conditions during that epoch corresponded approximately to sea ice conditions observed today in late June.

It should be noted that the Arctic climate described above was unstable. During both the cold and warm phases, there were interspersed shorter colder and warmer periods. The intensity of changes in various climate characteristics and their effects varied strongly from region to region. Under these conditions, it is very difficult to determine the major frequencies and magnitudes of climatic variations because these parameters depend on a variety of characteristics as well as the quality of the data. In this study, we express climate variability in the form of quasi-fluctuations at different frequencies (i.e., climate variability has a polycyclic character).

Monin (1969) and Monin and Sonechkin (2005) provide a detailed system of temporal-scale classification for weather and climate, with emphasis on important and robust climate changes at:

— Pleistocene glacial periods (hundreds of thousands of years)

— Inter-secular periods (from hundreds to thousands of years)

— Intra-secular periods (decades)

In this classification interannual and shorter signals are not related to climate change, which are often used in contemporary climate analysis publications. Orvig's (1973) analysis of Arctic climate variability concludes that the WMO climate definition (30-year mean) is not applicable to the Arctic because polar climate fluctuations are very large. Dobrovolsky (2000, 2002) supports the idea of stochastic climate variability and also supports Hasselmann's (1967) temporal climate classification, which distinguishes only two main ranges of atmosphere-ice-ocean system change, namely, synoptic and climatic, where variability with a period longer than one month is climatic. In this study, we assume that climate variability is a variability with a period 10-year or greater.

Twentieth century climate change research repeatedly led to projections ranging from either the complete disappearance of Arctic sea ice or, on the contrary, increases in ice area and thickness. Most of these projections were based on linear extrapolations of prolonged climatic tendencies accepted by investigators as permanent. In spite of well known failures of linear extrapolation of climatic data, extrapolation was repeatedly applied during the second half of the twentieth century, up to the present. Thus, after the "Arctic warming epoch'' in the 1920s-1940s, some concern about the consequences of continued global warming was expressed (Budyko, 1969). However, the average temperature in the northern hemisphere began to decrease beginning in the middle of the twentieth century. This gave rise to concern about the possible extended continuation of this process (Gribbin and Lamb, 1978). Based on analysis of changes in ice conditions and air temperatures in the Arctic from the end of the 1960s to the mid-1970s, Volkov and Zakharov (1977) predicted further cooling and increased ice cover area in the Arctic Seas up to the 1990s. It was supposed that climatic and ice conditions by that time would approximate those that were observed in the Arctic at the beginning of the twentieth century.

But, again, nature prepared a surprise: a new warming event began in the middle of the 1970s, and by the middle of the 1990s Arctic ice conditions were the mildest of the twentieth century. The scientific community again emphasized the implications of "global warming'' and predicted its catastrophic consequences. In many studies and international projects (e.g., Arctic Climate Impact Assessment, 2005), increased air temperature recorded during the last quarter of the twentieth century is attributed exclusively to accumulation of greenhouse gases in the atmosphere. In the opinion of supporters of the catastrophic consequences of the "global warming'' scenario, escalating air temperatures are expected throughout the twenty-first century. Based on mathematical models that incorporate this continuous air temperature increase, a decrease in ice area is predicted up to the middle of the twenty-first century (e.g., Vinnikov et al., 1999; Johannessen et al., 2004). Published predictions range from the complete disappearance of Arctic Ocean ice to the onset of a new glacial epoch within a restricted time frame. All of these studies ignore natural hydrometeorological fluctuations, which, as this monograph shows, contribute to multiyear variability and can exceed by many times the anthropogenic impact on climate. This monograph is devoted to investigating the manifestations of natural fluctuations of sea ice extent and of other characteristics of climate on varying scales.

Joint scientific programs undertaken by scientists of different countries will contribute to further study of these problems. The atmosphere, the ocean, and sea ice were among the major topics of study included in programs undertaken for the 2007-2008 International Polar Year and its legacy for the period after March 2009, headed by the International Council for Science and the World Meteorological Organization. In addition to thematic work, Russian plans include undertaking annual large scientific expeditions onboard R/V Akademik Fedorov to deploy and support North Pole drifting research stations (NP-35 during September 2007-July 2008, NP-36 since September 2008) and to establish other new research bases in the Arctic.

These studies will make it possible to obtain more detailed knowledge of the Arctic and the Antarctic and to develop observation systems, thus taking steps forward in investigating the changes occurring in the climatic system as well as in understanding their major causes.

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