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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.

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.

The question of whether anthropogenic emissions of aerosols or their precursors can contribute to droughts and heavy precipitation events is still an open one. While there is a microphysical link between an increase in aerosols and an increase in cloud albedo, a direct link to surface precipitation is less straight forward because it involves interactions between cloud microphysics and dynamics. Locally it has been suggested that in some instances increases in aerosols can increase heavy precipitation because less precipitation is formed in the lower parts of convective clouds so that more latent heat is released when the cloud glaciates. Globally the dominant effect of aerosols on precipitation is that aerosols cool the surface due to the increased aerosol and cloud optical depth, which then reduces evaporation and, hence, precipitation.

The severe drought during the second half of the 20th century has demonstrated that livelihood in Africa is highly sensitive to climatic conditions. From previous studies the complexity of the key factors in African climate variability can be inferred. This chapter discusses the various key factors and highlights the need of more complex scenarios to assess the human influence on African climate. In particular, the effect of ongoing land-use changes in the course of population growth, shifting cultivation and overgrazing has to be taken into account. New ensemble simulations with a regional climate model are presented, which are subject to increasing greenhouse gas concentrations and land cover changes. The model predicts dryer conditions, near-surface warming and an intensification of heat stress. There is some indication that reforestation and regional protection of natural vegetation may be more effective for mitigating climate change in Africa than reduced greenhouse gas emissions.

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.

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.

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.

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.

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.

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.