Satellite cloud observations offer the advantage of much better spatial and temporal coverage compared to surface observations. However they require careful efforts to identify and correct for temporal discontinuities in the data sets associated with orbital drift, sensor degradation, and inter-satellite calibration differences. AR4 noted that there were substantial uncertainties in decadal trends of cloud cover in all satellite data sets available at the time and concluded that there was no clear consensus regarding the decadal changes in total cloud cover. Since AR4 there has been continued effort to assess the quality of and develop improvements to multi-decadal cloud products from operational satellite platforms (Evan et al., 2007; O’Dell et al., 2008; Heidinger and Pavolonis, 2009). Several satellite data sets offer multi-decadal records of cloud cover (Stubenrauch et al., 2013). AR4 noted that there were discrepancies in global cloud cover trends between ISCCP and other satellite data products, notably a large downward trend of global cloudiness in ISCCP since the late 1980s which is inconsistent with PATMOS-x and surface observations (Baringer et al., 2010). Recent work has confirmed the conclusion of AR4, that much of the downward trend in ISCCP is spurious and an artefact of changes in satellite viewing geometry (Evan et al., 2007). An assesment of long-term variations in global-mean cloud amount from nine different satellite data sets by Stubenrauch et al. (2013) found differences between data sets were comparable in magnitude to the interannual variability (2.5 to 3.5%). Such inconsistencies result from differnces in sampling as well as changes in instrument calibration and inhibit an accurate assessment of global-scale cloud cover trends.
Satellite observations of low-level marine clouds suggest no long-term trends in cloud liquid water path or optical properties (O’Dell et al., 2008; Rausch et al., 2010). On regional scales, trends in cloud properties over China have been linked to changes in aerosol concentrations (Qian et al., 2009; Bennartz et al., 2011) (Section 2.2.3). In summary, surface-based observations show region- and height-specific variations and trends in cloudiness but there remains substantial ambiguity regarding global-scale cloud variations and trends, especially from satellite observations. Although trends of cloud cover are consistent between independent data sets in certain regions, substantial ambiguity and therefore low confidence remains in the observations of global-scale cloud variability and trends.