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The past 100 years are a key period for understanding climate variability and climate change as it marks the changeover from a climate system dominated by natural influences to one significantly dominated by anthropogenic activities. This volume is a compilation of contributions to a workshop dealing with different aspects of climate change, variability, and extremes during the past 100 years. The individual contributions cover a broad range of topics, from the re-evaluation of historical marine data to the effect of solar variability on the stratosphere. In this introductory chapter we provide an overview of the book in the context of recent research.

After the establishment of national weather services and the development of early weather forecasts towards the end of the 19th century, the Arctic region was seen as the home of cyclones, which very often resulted in violent storms in northern latitudes and influenced the weather of middle latitudes. Between 1882 and 1945 Arctic climate features were observed starting from ground-based measurements of a temporary network of stations in the framework of the First International Polar Year (1882–1883) and expanding to the third dimension with continuous observations of the upper atmosphere at Greenland and Svalbard (Spitsbergen), and ending with monitoring of the atmosphere around the Arctic in a dense network of stations during the Second International Polar Year (1932–1933). In addition, the airship “Graf Zeppelin” presented a platform for various kinds of instruments to measure meteorological parameters below, along, and above the flight track. During World War II, secret German weather stations were maintained in the Arctic, which sent their data to Norway to enter daily weather forecasts. Today these historical data series provide very important insight into climate features of the Arctic, which are mostly observed only during expeditions or through continuous observations during certain periods. They can be used to develop or test theories.

Available estimates of sea ice extent in the northern hemisphere cover the period from the early part of the 20th century to present day. We analyze changes in ice extent and thickness in the Arctic and its relation to surface air temperature over this period. Time series obtained from different data sets demonstrate better agreement after the 1950s and especially since 1979 with the onset of regular remote sensing observations from satellites. Statistics of time series show minima ice extent in August–September. Mean square deviations reach maxima in July–August. The distributions of trend coefficients show a more significant decrease of summer ice extent. Statistics of monthly ice extent in the Siberian Arctic seas show a similar distribution. September ice extent in the majority of the Siberian Arctic seas and in the Barents Sea reveal rapid shrinking during Arctic warmings in the 1920–1940s and 1990s. Significant correlation between surface air temperature and ice extent occurs in summer months with maximum in June under the influence of June maximum solar irradiation, and amplified by heat advection in the atmosphere and ice extent anomalies in the previous months. The relationship between variations of winter air temperature and ice extent is weaker because winter ice extent anomalies depend on air temperature anomalies as well as on the area occupied by a freshened upper layer. Good agreement between variations of the sum of summer air temperature in the marine Arctic and sea ice extent in September is found (correlation coefficient is 0.85). It confirms that summer melting plays the most important role in the sea ice volume decrease. The renewed observations in 2004–2005 at the Russian “North Pole” drifting stations revealed that the area-averaged perennial ice in the Arctic Basin decreased by 110cm relative to the 1990 value. But the land-fast ice thickness in the Kara and Laptev Seas show an insignificant positive linear trend for 1934–2005 in agreement with the sum of winter air temperature. The negative trend of land-fast ice thickness becomes apparent starting from the 1970s.

Sea surface temperature (SST) is a key oceanic variable – widely used for research, including global climate change assessments and atmospheric reanalyses. This paper reviews the evolution of the SST data and products available from the International Comprehensive Ocean-Atmosphere Data Set (ICOADS), since that project’s inception in 1981. Climate-scale SST products based on ICOADS (or related in situ data) are also reviewed. Measurements of SST have been made since around the early 1800s from ships, augmented in recent decades by in situ measurements from buoys and other automated Ocean Data Acquisition Systems (ODAS). SST, unlike some other ICOADS variables such as surface air temperature or humidity, is observed from space with reasonable accuracy. However, without reference to in situ measurements most satellite-based SST products will contain large-scale biases due to varying atmospheric composition and imperfect instrumental calibration. ICOADS is vital to the removal of such biases, which are especially large following volcanic eruptions. We describe products combining in situ and satellite SSTs that exploit the strengths of each type of measurement, to yield both high resolution and high accuracy. Finally, we discuss future developments anticipated for ICOADS and SST products, such as further blending of metadata and enhanced product uncertainty assessments.

The free atmosphere is an essential part of the climate system, and the processes in the free atmosphere are strongly interrelated with the processes in other climate system components. Temperature in the free atmosphere is better studied than other parameters defining the upper-air climate, but nevertheless there exist numerous problems in the upper-air temperature trends. This paper gives the overview of the current status of radiosonde based empirical studies of the upper-air temperature climate changes. Problems such as data availability, data quality and completeness, inhomogeneity detection in the upper-air temperature time series, temperature trend estimates, overall patterns of upper-air trends are discussed in the paper. Special attention is given to the problem of whether the reanalysis outputs can reproduce trends over long periods in the upper-air temperature series.

The Earth’s climate has traditionally been studied by statistical analysis of observations of particular weather elements such as temperature, wind and rainfall. Climatological information, usually expressed as long-term averages and variability, is then presented over a geographical area or at a single location and time series of these quantities or of the observations themselves are examined for evidence of warming, more-frequent severe storms, and so on.

A powerful new approach to climate analysis has emerged in recent years. It applies the tools and techniques of modern everyday weather forecasting in a process called reanalysis. The products, reanalyses, have applicability far beyond that of traditional climate information. Reanalyses have become established as an important and widely utilized resource for the study of atmospheric and oceanic processes and predictability also over the data sparse polar regions. They are used in a range of applications that require a comprehensive record of the state of either the atmosphere or its underlying land and ocean surfaces. The reanalysis products, unlike their operational counterparts, do not suffer from inhomogeneities introduced by changes in the data assimilation system. Thus they are in principle better suited for use in studies of low frequency variability and climate trends that complement studies of climate change based on individual instrumental records and climate-model simulations.

Climate quality requirements can be met by reanalyses for the decades with good upper-air data coverage by satellites or at least radiosonde data. The possibility of extending reanalyses to cover earlier periods when only surface observations are available in reasonable numbers (e.g., from the 1850s to the 1930s) is nevertheless of interest, and has been explored in pilot studies comparing analyses with good coverage of satellite and other upper-air data with analyses using only surface-pressure observations.

Satellite-borne microwave sounding instruments have been making measurements of the temperature of the Earth’s atmosphere for several decades. In order to construct a single atmospheric temperature data set from these measurements, data from a number of satellites must be combined, since each satellite operated only during a small part of the longer time period. If the combined data set is to be of sufficient quality to evaluate changes on the decadal or longer time scales, a number of calibration issues and time-varying biases must be addressed, and their effects removed from the data to the extent possible. Other sources of atmospheric temperature data, such as in situ measurements made by radiosondes and the output of the various reanalysis efforts, have not been demonstrated to be of high-enough quality to validate the satellite data. Because of this, satellite data is typically intercalibrated using a detailed analysis of data from periods of simultaneous operation by two or more satellites. When this type of calibration is used, long periods of simultaneous observation are needed to reduce uncertainties in the calibration procedure.

Ozone plays a key role in the physics and chemistry of the atmosphere. Total ozone, that is, the amount of ozone in an air column, is therefore a variable of vital climatic and environmental importance. The operational measurement of total ozone reaches back to the pioneering work of G. M. B. Dobson in the 1920s. Here, we give a brief overview of total ozone observations during the past 80 years, including the development of ground-based monitoring networks as well as the more recent satellite sensors. We summarize the measurement techniques, the available data as well as issues related to quality and comparability.

Observational data and modeling results are analyzed to describe changes in the Arctic sea ice cover during the last half century. Accelerated melt of sea ice cover is reported during the late 1990s and 2000s both based on satellite observations of sea ice extent and model simulations of sea ice thickness. The observed and modeled changes are in qualitative agreement but model results imply higher rate of ice thickness decline compared to sea ice extent. Possible causes of variability in sea ice cover include increased surface air temperatures, changes in atmospheric circulation and changes in the absorption of incoming radiative flux. However, atmospheric forcings, such as the Arctic Oscillation (AO), explain less than half of the total variance in Arctic sea ice cover. Model results analyzed in the Greenland Sea as well as observations in the western Arctic Ocean indicate that oceanic forcing might be an important overlooked factor in driving recent sea ice melt. The main oceanic processes relevant to variability of sea ice cover include advection of heat and melting of sea ice in marginal ice zones and at the ice-ocean interface downstream of the warm water paths. Such changes have potential significant ramifications to the entire pan-Arctic region, including the physical environment, regional ecosystems, native communities, and use of the region for commercial exploration and transportation. Continued studies including in situ and remote sensing observations and modeling are critical to advancing the knowledge of Arctic climate change and predicting scenarios of future change.

The major anthropogenic impact on climate over the 20th century occurred through a modification of the earth radiation balance by changing the amount of greenhouse gases and aerosol in the atmosphere. Radiative energy reaching the ground is particularly important for mankind as it is a key determinant of the climate of our environments and strongly influences the thermal and hydrological conditions at the Earth surface. Recent evidence suggests that significant anthropogenic-induced variations occurred in both surface solar and thermal radiation over the past decades, related to anthropogenic air pollution and greenhouse gas emissions, respectively. Observed solar radiation incident at the surface showed a continuous decrease (“global dimming” or “surface solar dimming”) since the beginning of worldwide measurements in the mid-20th century up to the 1980s, when a widespread trend reversal towards an increase (“global brightening” or “surface solar brightening”) occurred. This trend reversal was favoured by an increasing transparency of the cloud-free atmosphere, due to air pollution regulations and the breakdown of the economy in former communist countries. In the thermal spectrum of radiation, which is directly modified by changes in atmospheric greenhouse gas concentrations, a gradual increase in surface downwelling thermal radiation over recent years can be seen, in line with our expectations from an increasing greenhouse effect. This increasing greenhouse effect has become only fully apparent after the decline of solar dimming, which effectively masked greenhouse warming prior to the 1980s. The present article discusses the variations in surface radiation and their impact on various aspects of the climate system over the past decades.