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This study documents multidecadal variations in low-level, upper-level, and total cloud cover over land and ocean independently obtained from surface synoptic observations and from satellite data produced by the International Satellite Cloud Climatology Project. Substantial agreement exists between global mean time series of surface- and satellite-observed upper-level cloud cover, indicating that the reported variations in this cloud type are likely to be real. Upper-level cloud cover has decreased over almost all land regions since 1971 and has decreased over most ocean regions since 1952. Global mean time series of surface- and satellite-observed low-level and total cloud cover exhibit very large discrepancies, however, implying that artifacts exist in one or both data sets. The global mean satellite total cloud cover time series appears spurious because the spatial pattern of correlations between grid box time series and the global mean time series closely resembles the fields of view of geostationary satellites rather than geophysical phenomena. The surface-observed low-level cloud cover time series averaged over the global ocean appears suspicious because it reports a very large 5%-sky-cover increase between 1952 and 1997. Unless low-level cloud albedo substantially decreased during this time period, the reduced solar absorption caused by the reported enhancement of cloud cover would have resulted in cooling of the climate system that is inconsistent with the observed temperature record. [ABSTRACT FROM AUTHOR]

We provide here a brief review on the role of land-atmosphere interactions for climate variability, with a special focus on the European continent. First, an overview of the land energy and water balances and of the underlying physical, biophysical, and biogeochemical soil-vegetation-atmosphere processes is presented. Further, we highlight how land-atmosphere feedbacks can impact seasonal to interannual climate variability in transitional climate zones and midlatitude regions along three main paths: Soil moisture-temperature interactions, soil moisture-precipitation interactions, and vegetation-climate interactions. In this context, we discuss recent results based on findings from terrestrial observational networks, satellite observations, and numerical climate models across a number of spatial and temporal scales. These results illustrate the extent to which land-surface processes, land-atmosphere interactions, and associated memory effects can modulate the dynamics of the climate system. Finally, the concluding section addresses current areas of uncertainty and open questions for research in this field. [ABSTRACT FROM AUTHOR]

The 11-year sunspot cycle (SSC) strongly affects the lower stratosphere. However, in order to detect the solar signal it is necessary to group the data according to the phase of the Quasi-Biennial Oscillation (QBO). Although this is valid throughout the year the effect of the SSC and the QBO on the stratosphere was largest during the northern winters (January/February). As the stratosphere can affect weather at the ground, the SSC effect on the lower stratosphere might provide a mechanism for solar-climate links. Here we analyse an extended, 65-year long data set of solar variability, QBO, and lower stratospheric dynamics. The results fully confirm earlier findings and suggest a significant effect of the SSC on the strength of the stratospheric polar vortex and on the mean meridional circulation. [ABSTRACT FROM AUTHOR]

Some solar eruptions lead to solar proton events (SPEs) at the Earth, which typically last a few days. High energy solar protons associated with SPEs precipitate on the Earth's atmosphere and cause increases in odd hydrogen (HOx) and odd nitrogen (NOy) in the polar cap regions (>60° geomagnetic). The enhanced HOx leads to short-lived ozone depletion (~days) due to the short lifetime of HOx constituents. The enhanced NOy leads to long-lived ozone changes because of the long lifetime of the NOy family in the stratosphere and lower mesosphere. Very large SPEs occurred in 1972, 1989, 2000, 2001, and 2003 and were predicted to cause maximum total ozone depletions of 1-3%, which lasted for several months to years past the events. A long-term data set of solar proton fluxes used in these computations has been compiled for the time period 1963-2005. Several satellites, including the NASA Interplanetary Monitoring Platforms (1963-1993) and the NOAA Geostationary Operational Environmental Satellites (1994-2005), have been used to compile this data set. [ABSTRACT FROM AUTHOR]

Recent satellite observations show that the solar ultraviolet irradiance is much more variable than the total solar irradiance. Atmospheric effects of the solar irradiance variations during 11-year solar activity cycle are investigated using different numerical models and observation data sets. It is shown that the direct and indirect (via ozone production) heating in the upper and middle stratosphere due to enhancement of the solar spectral irradiance leads to an acceleration of the polar night jets and suppression of the Brewer-Dobson circulation resulting in the ozone increase and warming of the lower tropical stratosphere. These stratospheric changes alter the tropospheric circulation leading to a statistically significant warming of the surface air over Russia, Europe and North America. The importance of the solar spectral irradiance variability for the attribution of the temperature changes in the upper stratosphere is also shown by the comparison of the simulated and observed temperature evolution during the last 25 years of the 20th century. [ABSTRACT FROM AUTHOR]

Through three examples taken from the history of ozone research, this paper illustrates that knowledge has been created by using simultaneously different approaches and methodologies, and by confronting the information resulting from laboratory studies, observational programs and modeling activities. New knowledge on chemical and dynamical processes in the atmosphere has been produced from detailed studies of the vertical and meridional ozone distributions in the stratosphere, and from investigations on the cause of the formation of the Antarctic ozone hole. [ABSTRACT FROM AUTHOR]

Short- and long-term changes in the intensity and persistence of the Arctic and Antarctic stratospheric polar vortices during spring have been analyzed, using NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalyses. For the Arctic the results confirm the existence of low frequency variability in the winter stratosphere. During the 1980s and early to mid-1990s the northern hemisphere (NH) polar vortex was intensified in spring and broke up late. Since the late 1990s however, major stratospheric warmings occurred more frequently, so that the polar vortex in spring still intensified in March but with a smaller magnitude. As some of the major warmings occurred early in winter, the polar vortex was able to recover leading to late breakup dates in spite of the dynamical disturbances. In the long-term, there is no statistically significant change in Arctic vortex intensity or lifetime. In the Antarctic, the significant intensification of the polar vortex found in the 1980s and 1990s has been considerably reduced due to an unexpected enhancement of dynamical activity in southern hemisphere (SH) winter since 2000, masking the significant increase in polar vortex persistence found for the period 1979-1999. Still on the long-term, the Antarctic vortex shows a significant deepening and shift towards later spring transitions. [ABSTRACT FROM AUTHOR]

In recent years numerical models describing dynamical, physical and chemical processes of the Earth's atmosphere have been significantly improved. Simulations with so-called Climate-Chemistry Models (CCMs) do not only allow investigations of single processes but also enable analyses of feedback mechanisms. Detailed inspections of model results with observations can help to identify gaps in our understanding and to improve our knowledge of atmospheric processes. This is the basis for model systems which are needed for estimates of the future evolution of atmospheric behaviour. In the following some examples are discussed which clearly indicate the need for such models. The role of natural as well as manmade forcings of the atmospheric system is examined. Moreover, the interaction of climate change effects and atmospheric chemistry, especially ozone, is demonstrated with results derived from multi-year CCM simulations. [ABSTRACT FROM AUTHOR]

In this review we attempt to critically evaluate the availability of reliable tropospheric ozone measurements suitable for long-term trend analysis. The focus is on large-scale changes deduced from measurements, which are used for comparison with numerical simulations of the tropospheric ozone cycle. These are required to quantify the influence of anthropogenic ozone precursor emission changes on climate. Long-term tropospheric ozone measurements show that ozone over Europe has increased by more than a factor of two between World War II and the early 1990s which is consistent with the large increase in anthropogenic ozone precursor emissions in the industrialized world. However, the further increase in background ozone over Europe and North America since the early 1990s cannot be solely explained by regional ozone precursor changes because anthropogenic ozone precursor emissions decreased in the industrialized countries as consequence of air pollution legislation. Measurements also indicate large increases in ozone in the planetary boundary layer over the tropical Atlantic since the late 1970s, which have been attributed to large increases in fossil fuel related emissions. Measurements at southern midlatitudes, which are limited in number, show a moderate increase in tropospheric ozone since the middle of the 1990s. [ABSTRACT FROM AUTHOR]

Distribution changes in seasonal mean 2 m temperature are investigated in Central Europe and the extratropical northern hemisphere for the most recent period 1961-2005 and the 21st century. Data from both observations and climate model runs are used. The latter are taken from scenarios prepared for the fourth IPCC assessment report (AR4). All data sets and model runs are scaled by their corresponding interannual variability to facilitate the comparison between the data sets. Time series are detrended to investigate changes in internal variability. For the last 45 years (1961-2005) the strongest changes in mean are found for in the summer mean temperature, both in observations and AR4 runs. With the exception of autumn, changes in the mean are captured reasonably well by the models on the regional as well as hemispheric scale. For Central Europe, estimates for variability changes show a weak increase (decrease) in summer (winter) observations. Both are not statistically significant at the 10% level. For the 21st century all climate scenario runs suggest large relative increases in mean temperature for all seasons. The AR4 model-to-model differences in the mean changes are largest in summer and not substantially smaller than those from the third assessment report (TAR). Model uncertainties are in the same order or even larger than the uncertainty introduced by the different scenarios. Compared to TAR, the AR4 runs show a much more consistent tendency for increases in Central Europe summer temperature variability especially towards the end of the 21st century (for 2070-2099: ~22-47% increase). No clear changes in seasonal mean temperature variability are found for the other seasons and the averaged northern hemisphere land time series. [ABSTRACT FROM AUTHOR]

Summer heat waves and extremely hot temperatures are a serious threat to society, the environment and the economy of Europe. In this chapter we present an overview of selected recent literature which looks specifically at European heat waves and extreme temperature events, their past change and expected future change from 1880 to 2100. Della-Marta et al. (2007b) show that over the period 1880-2005 the length of summer heat waves over western Europe has doubled and the frequency of hot days has almost tripled. These changes are seen in the probability density function (PDF) of western European daily summer maximum temperature (DSMT) as a significant change in the mean (+1.6 ± 0.4°C) and variance (+6 ± 2%). The relatively small change in variance over the last 126 years can explain approximately 40% of the change in hot days. We see a continuation of the observed trends in the future regional projections. Beniston et al. (2007) show that regional surface warming causes the frequency, intensity and duration of heat waves to increase over Europe. By the end of the 21st century, countries in central Europe will experience the same number of hot days as are currently experienced in southern Europe. The intensity of extreme temperatures increases more rapidly than the intensity of more moderate temperatures over the continental interior due to increases in temperature variability. [ABSTRACT FROM AUTHOR]

We provide here a brief overview of the impacts of climate variability and recent climate change on the European plant phenology across the 20th century. Facing recent climate changes, phenology has two major functions. Firstly, it reveals measurable impacts of climate change on nature, which at the same time clearly demonstrate global climate change in people's backyards. Secondly, long-term phenological data allow the reconstruction of temperature and its variability in the last centuries. The most prominent temperature driven changes in plant phenology are an earlier start of spring in the last three to five decades of, on average, 2.5 days/decade, mainly observed in midlatitudes and higher latitudes of the northern hemisphere. More heterogeneous changes in autumn are not as pronounced as in spring and cannot be linked to climate factors. A marked spatial and temporal variability of spring and summer onset dates and their changes can be mainly attributed to regional and local temperature. In this context, we discuss the temperature responses of the growing season and other phenological phases and their relation to the North Atlantic Oscillation. These results illustrate main feedbacks in biogeochemical cycles and land-surface interactions of the climate system. [ABSTRACT FROM AUTHOR]