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[1] Newly available data from extended weather stations and time period reveal that much of China has experienced significant decreases in cloud cover over the last half of the Twentieth century. This conclusion is supported by the analysis of the more reliably observed frequency of cloudfree sky and overcast sky. The total cloud cover and low cloud cover have decreased 0.88% and 0.33% per decade, respectively, and cloud-free days have increased 0.60% and overcast days decreased 0.78% per decade in China from 1954 -2001. Meanwhile, both solar radiation and pan evaporation have decreased in China, with solar radiation decreasing 3.1 W/m 2 and pan evaporation decreasing 39 mm per decade. Combining these results with findings of previous studies, we speculated that increased air pollution may have produced a fog-like haze that reflected/absorbed radiation from the sun and resulted in less solar radiation reaching the surface, despite concurrent increasing trends in cloud-free sky over China.

Weather and climate extremes have been varying and changing on many different time scales. In recent decades, heat waves have generally become more frequent across the United States, while cold waves have been decreasing. While this is in keeping with expectations in a warming climate, it turns out that decadal variations in the number of U.S. heat and cold waves do not correlate well with the observed U.S. warming during the last century. Annual peak flow data reveal that river flooding trends on the century scale do not show uniform changes across the country. While flood magnitudes in the Southwest have been decreasing, flood magnitudes in the Northeast and north-central United States have been increasing. Confounding the analysis of trends in river flooding is multiyear and even multidecadal variability likely caused by both large-scale atmospheric circulation changes and basin-scale “memory” in the form of soil moisture. Droughts also have long-term trends as well as multiyear and decadal variability. Instrumental data indicate that the Dust Bowl of the 1930s and the drought in the 1950s were the most significant twentieth-century droughts in the United States, while tree ring data indicate that the megadroughts over the twelfth century exceeded anything in the twentieth century in both spatial extent and duration. The state of knowledge of the factors that cause heat waves, cold waves, floods, and drought to change is fairly good with heat waves being the best understood.

[1] Analysis of meteorological data made available by the China Meteorological Administration (CMA) reveals significant decreases in sunshine duration over the latter half of the 20th century over much of China, especially in the eastern half of the country. Most of the decrease has occurred since about 1980, and appears to be related to large increases in atmospheric anthropogenic aerosol loading that have increased the aerosol extinction coefficient (AEC), as calculated from observations of (decreasing) visual range. A finding of particular interest is a significant decrease in summertime maximum temperature over this time period for a large portion of southeastern China near the Sichuan Basin (while the rest of China-and indeed most of the northern hemisphere-has warmed). This same region of southeastern China shows the largest decreases in sunshine duration and largest increases in AEC. The consistent spatial and temporal patterns of change in these variables seem to support the theory that aerosol loading has decreased the duration and intensity of sunshine in this region and thus lowered summertime maximum temperatures.

Analysis of long‐term surface global and diffuse solar radiation, aerosol single scattering albedo (SSA), and relative humidity (RH) data reveals that much of China experienced significant decreases in global solar radiation (GSR) and increases in diffuse solar radiation under cloud‐free skies between the 1960s and 1980s. With RH and aerosol SSA being rather constant during that time period, we suggest that the increasing aerosol loading from emission of pollutants is responsible for the observed reduced GSR and increased diffuse radiation in cloud‐free skies. Although pollutant emissions continue to increase after the 1980s, the increment of aerosol SSA since 1980s can partly explain the transition of GSR from a decreasing trend to no apparent trend around that time. Preliminary analysis is also provided on the potential role of RH in affecting the global and diffuse solar radiation reaching the earth surface.

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Near-record warmth over much of the United States during March 2007 promoted early growth of crops and vegetation. A widespread arctic air outbreak followed in early April, resulting in extensive agricultural losses over much of the south-central and southeastern US. This 'false spring' event also resulted in widespread damage to newly grown tissues of native deciduous forest species, shown by previous researchers to have had measurable effects on the terrestrial carbon cycle. The current study reconstructed the historical occurrence of false springs over most of the southeastern quarter of the conterminous US (32-39 • N; 75-98 • W) from 1901 to 2007 using daily maximum and minimum temperature records from 176 stations in the Global Historical Climatology Network database, and enhanced vegetation index (EVI) data derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite observations. A false spring index was derived that examined the timing of the start of the growing season (SGS), or leaf emergence, relative to the timing of a potentially damaging last hard freeze (minimum temperature − 2.2 • C). SGS was modeled for the domain by combining EVI data with ground-based temperature 'degree day' calculations reflecting the rate of springtime warming. No significant area-wide, long-term SGS trend was found; however, over much of a contiguous region stretching from Mississippi eastward to the Carolinas, the timing of the last hard freeze was found to occur significantly later, this change occurring along with increased frequency of false springs. Earlier last hard freeze dates and decreased frequency of false springs were found over much of the northwestern part of the study region, including Arkansas and southern Missouri.

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