PUBLICATIONS. Journal of Geophysical Research: Atmospheres

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1 PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE Key Points: Few time trends in atmospheric transmission for clear or cloudy skies were found Parameters of linear relationships were unrelated to the fossil fuel combustion Cloud cover played the major role in determining solar dimming and brightening Correspondence to: G. Stanhill, Citation: Stanhill, G., O. Achiman, R. Rosa, and S. Cohen (2014), The cause of solar dimming and brightening at the Earth s surface during the last half century: Evidence from measurements of sunshine duration, J. Geophys. Res. Atmos., 119, 10,902 10,911, doi: / 2013JD Received 12 DEC 2013 Accepted 17 AUG 2014 Accepted article online 21 AUG 2014 Published online 23 SEP 2014 The cause of solar dimming and brightening at the Earth s surface during the last half century: Evidence from measurements of sunshine duration Gerald Stanhill 1, Ori Achiman 1, Rafael Rosa 1, and Shabtai Cohen 1 1 Department of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, ARO Volcani Center, Bet Dagan, Israel Abstract Analysis of the Angstrom-Prescott relationship between normalized values of global radiation and sunshine duration measured during the last 50 years made at five sites with a wide range of climate and aerosol emissions showed few significant differences in atmospheric transmissivity under clear or cloud-covered skies between years when global dimming occurred and years when global brightening was measured, nor in most cases were there any significant changes in the parameters or in their relationships to annual rates of fossil fuel combustion in the surrounding 1 cells. It is concluded that at the sites studied changes in cloud cover rather than anthropogenic aerosols emissions played the major role in determining solar dimming and brightening during the last half century and that there are reasons to suppose that these findings may have wider relevance. 1. Introduction Reviews of the evidence for changes in global radiation E g, the downwelling short wave radiation from the Sun and sky reaching the Earth s surface, during the second half of the last century [Cohen,2009;Wild, 2009] show that a significant and widespread reduction, termed global dimming occurred between the 1950s and 1980s after which recoveries, named global brightening took place. Many recent studies of the cause of the changes in global radiation E g conclude that anthropogenic aerosols play a major role [Alpert et al., 2005; Norris and Wild, 2009; Ohmura, 2009; Folini and Wild, 2011; Haywood et al., 2011; Sanchez-Lorenzo and Wild, 2012; Wang et al., 2012]. This conclusion, however, is not supported by studies of the changes in E g measured at a number of high precision measurement sites in the USA [Long et al., 2009; Augustine and Dutton, 2013] nor by reports of solar dimming and brightening occurring in areas with very low levels of anthropogenic aerosols [Stanhill and Moreshet, 1994;Stanhill and Cohen, 1997; Liley, 2009; Stanhill and Cohen, 2009]. The major difficulty in resolving this question arises from the interaction between the two most probable causes of changes in the amount of short wave radiation reaching the Earth s surface viz., changes in cloud cover and in aerosol load. The effect of aerosols on cloud formation, duration, and radiative properties is well established [Lohmann and Feichter, 2005] but difficult to quantify because of the limited accuracy of model estimates of aerosol effects on E g especially the indirect effect [Forster et al., 2007; Haertner et al., 2009; Lee et al., 2013]. Additional difficulties stem from the absence of aerosol load data prior to the Total Ozone Mapping Spectrometer estimates derived from the Nimbus satellite series starting in 1978, and the paucity of global radiation measurements before the International Geophysical Year 1957/1958. The limited accuracy of the standard, subjective observations of cloud cover [Galligan, 1953] poses a further problem. The lack of long-term measurements gave rise to the need for proxies for global radiation, cloud cover, and aerosol load. The most widely used proxy for global radiation is sunshine duration n, measurements of which began in Sunshine duration recorders operate as light switches or photonstats registering the time that direct radiation in the solar beam exceeds 120 W m 2 : a threshold corresponding to the irradiance from the unobscured Sun 3 above the horizon [World Meteorological Organization (WMO), 2008]. Despite its limitations measurements of n have long been shown to be highly correlated with measurements of E g [Angstrom, 1924; Kimball, 1919] and since 1940 have been very widely used as a proxy in the form of the linear Angstrom-Prescott equation with its empirically derived but physically meaningful parameters [Prescott, 1940]. Measurements of sunshine duration have also been widely used as a proxy for cloud cover [Jones and STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,902

2 Figure 1. The relationships between monthly values of normalized sunshine duration n and global radiation E g during years of global dimming (triangle) and global brightening (circle). Regression parameters for the pooled data appear in the figure and are detailed in Table 2. (a) Valentia, (b) Aldergrove all data , (c) Potsdam, (d) Hong Kong, (e) Bet Dagan, and (f) all data pooled. Henderson-Sellers, 1992; Steurer and Karl, 1991; Palle and Butler, 2002] and also have been related to aerosol load under special circumstances when values of aerosol optical depth (AOD) exceed 2 [Horseman et al., 2008]. In this study annual values of fossil fuel combustion F, served as a proxy for anthropogenic aerosol emissions, using the century long database with global coverage compiled by Boden et al. [2009]. The aim of this study is to analyze long-term series of E g and n measured under varying rates of aerosol emission F, in order to explore the causes of solar dimming and brightening in the last half century. STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,903

3 Table 1. Data Sources and General Details Site/Country Valentia Ireland Aldergrove United Kingdom Potsdam Germany Hong Kong Bet Dagan Israel Climate type Temperate marine Temperate Continental Tropical Mediterranean Coordinates Latitude 51 56ʹN 54 39ʹN 52 23ʹN 22 18ʹN 32 00ʹN Longitude 10 15ʹW 6 13ʹW 13 04ʹE ʹE 34 49ʹE Elevation (m) Period measured Number of months analyzed Number of missing months Mean irradiance E g (W m 2 ) Interannual range Mean sunshine duration n(h d 1 ) Interannual range Mean fossil fuel combustion F(g C m 2 yr 1 ) Interannual range Methods and Data Sources Changes in the transmissivity and reflectivity of the atmosphere will lead to changes in the parameters of the Angstrom-Prescott equation. In this equation E g /E 0 = an/n + b, global radiation E g, normalized to E 0 solar irradiance at the top of the atmosphere, is linearly related to sunshine duration n, normalized by day-length N. As discussed later the fraction n/n serves as the inverse proxy for cloud cover. The intercept parameter b of the equation represents normalized global radiation E g /E 0, under overcast sky conditions; the slope parameter a represents the effect of cloud cover on E g /E 0, and the sum of the two parameters a + b yields the value of E g /E 0 for a cloudless sky. If changes in aerosol load had led to significant changes in the radiative properties of the atmosphere during the last 50 years, this would have altered the parameters of the A-P equation. Specifically under cloudless sky conditions, an increase in the direct aerosol effect would have Table 2. Parameters of the Relationships Between Monthly Values of Normalized Values of Sunshine Duration and Global Radiation During Conditions of Global Dimming and Brightening as Shown in Figure 1 a Site Years Slope ±CI Intercept ±CI R 2 Aldergove All data Potsdam Dimming Brightening and All data Bet Dagan Dimming and Brightening and All data Hong Kong Dimming , , and Brightening , , , , , and All data Valentia Dimming Brightening All data Data pooled Dimming Brightening All data a CI indicates the 95% confidence interval for the parameter to the left. STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,904

4 Figure 2. (a) Parameters of the Angstrom-Prescott equations for each 6 year period of measurement with their standard errors for Valentia. (b) Mean rates of fossil fuel combustion for corresponding 6 year periods, g C m 2 yr 1. Figure 3. (a) Parameters of the Angstrom-Prescott equations for each 6 year period of measurement with their standard errors for Aldergrove. (b) Mean rates of fossil fuel combustion for corresponding 6 year periods, g C m 2 yr 1. STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,905

5 Figure 4. (a) Parameters of the Angstrom-Prescott equations for each 6 year period of measurement with their standard errors for Potsdam. (b) Mean rates of fossil fuel combustion for corresponding 6 year periods, g C m 2 yr 1. resulted in a reduction in the sum of the slope and intercept of the equation. Under both complete and partial cloud cover, an increase in the indirect aerosol effect would have led to reductions in both the slope and the intercept parameters unless the changes in the aerosol load altered the extent of cloud cover without changing their radiative properties. The limited effect of changes in aerosol load on sunshine duration is discussed in section 4. It should be noted that changes in cloud cover unrelated to aerosol load could also affect the parameters of the A-P equation as different types of clouds are known to differ in their radiative properties [Neumann, 1954; Matuszko, 2012]. The five sites studied were selected to provide a wide range of climates and rates of fossil fuel combustion. All were part of national meteorological service climate networks and subject to routine inspection, calibration, and data quality control: with the exception of Aldergrove, the measurements were made at observatories with facilities for radiation calibration. The mean monthly values of E g and n used in this study to calculate the parameters of the A-P equations were those supplied by the national meteorological services and published in the bulletins of the World Radiation Data Center established by the World Meteorological Organization (WMO) in The bulletins list the models of calibrated thermopile pyranometers used to measure E g : at Hong Kong, Potsdam, and Valentia, the three sites which provided data before 1964, details of the pyranometer models that were then used are specified in the supporting information. At all five sites the Campbell- Stokes sunshine duration recorder was used to measure n. The mean monthly values of cloud cover c, at Potsdam, used for comparison with measurements of sunshine duration, are available at de/. They are based on the means of three daily observations of the sky dome made by trained observers at 07, 14, and 21 h Central European Time, times which remained unchanged during the 120 years of measurements. The data processing and quality control methods applied to all of the measurements analyzed were those recommended by the WMO [Aguilar et al., 2003]. The parameters of the Angstrom- Prescott equations were calculated separately for periods when global dimming and brightening occurred at each site; the years of change were identified by inspection of time series of annual totals of E g. The changes in annual values of E g measured at Aldergrove were smaller and more variable than at the other sites; the mean value before 1985/1986, was ± 4.7 W m 2, slightly but not significantly, greater than after this period, ± 4.1 W m 2. In the absence of any evidence of global dimming or brightening at this site the parameters of the A-P equation at Aldergrove presented in Figure 1 are for the complete data set. A-P parameters were also calculated for successive 6 year periods: these provided sufficient data to combine high values of correlation coefficients with low values of standard errors while allowing an adequate number of periods to identify trends. Details of the sites with mean values of E g, n, and F are presented in Table 1 together with the numbers of monthly means, both those analyzed and those with missing data not included in the analyses. The values of F for the 1 cell surrounding the measurement sites were converted from an angular coordinate to an area basis and expressed in units of grams of Carbon per square meter land surface per year. STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,906

6 Figure 5. (a) Parameters of the Angstrom-Prescott equations for each 6 year period of measurement with their standard errors for Hong Kong. (b) Mean rates of fossil fuel combustion for corresponding 6 year periods, g C m 2 yr Results The relationships between the normalized monthly values of global radiation and sunshine duration calculated separately for periods of global dimming and brightening at each site are presented in Figure 1 and their parameters and confidence limits listed in Table 2. The A-P parameters calculated for successive 6 year periods are presented as time series in Figures 2a 6a; in all 42 equations, the linear relationships between normalized global radiation and sunshine duration were statistically highly significant (P < 0.01), the mean coefficient of determination was R 2 = 0.80, ranging between 0.95 and The annual rates of fossil fuel combustion, averaged over the same 6 year periods, are shown in Figures 2b 6b. At three of the four sites (i.e., excluding Valentia) where the periods of dimming and brightening could be clearly distinguished the slope and intercept parameters of the A-P equations were slightly but not significantly larger during years when solar brightening occurred than during years of dimming. The linear relationships between the slope and intercept parameters and their sum, and the midyear of measurement and rates of fossil fuel combustion are presented in Tables 3 and 4, respectively, together with their statistical significance. The relationships between the pooled parameters from all 42 Angstrom-Prescott equations from the five sites and the midyear of measurement and rates of fossil fuel consumption are given in the last lines of Tables 3 and 4; neither of these pooled relationships were significant, P > Discussion and Conclusions The uncertainties (95% confidence level) for monthly mean values of E g measurements set out in the WMO Guide are currently 2%, and we assume a higher value of 4% to be appropriate for measurements before 1964 when a variety of less accurate pyranometers were in use, and the methods of data processing and quality control may have been less stringent than those adopted by the World Radiation Data Center [Robinson, 1964]. The uncertainty in sunshine duration measurements made with the Campbell-Stokes recorder has been specified at 0.1 h [WMO, 2008]. These WMO estimates of uncertainties agree with the analysis of errors in a long series of E g and n measurements made at Izana Atmospheric Observatory between 1933 and The uncertainties in daily values of E g during the first 14 years of measurement with an uncalibrated pyranometer averaged 7%, doubling to mean value of 15% during a one and a half year period in this early part of the series. The uncertainty in daily values of n, assessed by comparison with an electronic sunshine recorder was 5.8% [Garcia et al., 2014]. Assuming that the errors are randomly distributed, the standard error of the mean decreases with the inverse square root of the length of the measurements being averaged, which would reduce these daily uncertainties to 2 and 4%, respectively, for mean monthly values of E g and 1% for n. The major question to be resolved before discussing the results presented is the validity of using sunshine duration as a proxy for cloud cover. Analysis of the very extensive data sets of the national climate networks of the USA [Steurer and Karl, 1991] and China [Xia, 2010] yielded highly significant, but seasonally varying, STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,907

7 Figure 6. (a) Parameters of the Angstrom-Prescott equations for each 6 year period of measurement with their standard errors for Bet Dagan. (b) Mean rates of fossil fuel combustion for corresponding 6 year periods, g C m 2 yr 1. inverse linear relationships between measured values of n/n and c, the observed fraction of sky covered with cloud. A similar result is presented here on the basis of an analysis of the 120 year long data set from Potsdam, Germany. The linear relationships for each month and for the entire data set are presented in Table 5 based on mean monthly values from this station, taken from the station website The linear and curvilinear relationships for all monthly values are shown in Figure 7. The reason for the seasonal variation in the linear cloud to sunshine relationship could be due to seasonal changes in cloud type with their different radiative characteristics [Matuszko, 2012]. It also could be explained by the important limitations in standard synoptic cloud cover data which include the large human observer error and the very limited sampling of diurnal variations in cloud cover, at best confined to a single morning, noon, and evening observation. Between-observer variation was shown by Galligan [1953] to reach its maximum, a standard deviation 20% of the mean, for a cloud cover of six tenths, the annual average for four of the sites examined in this study. A change of observers was shown by Palle and Butler [2002] to introduce nonhomogeneity into long-term series of cloud observations, and from their study of four long-term n/n and c series in Ireland, the authors concluded that sunshine measurements are likely to provide the most reliable indicators of long-term trends in cloudiness. A similar conclusion, which is sunshine duration may therefore be a more accurate indication of daytime cloudiness than cloud cover observations themselves, was reached by Weber [1994] on the basis of a study of measurements at Berlin between 1955 and It should be noted that recently developed objective methods of measuring cloud cover with all-sky camera systems or by estimation from direct and diffuse radiation measurements [Long et al., 2006] have eliminated many of the above limitations in the historic data sets. The effect of aerosol load on sunshine duration is limited to very low solar elevations < 3 when the direct solar beam is transmitted through a very long atmospheric pathway [Horseman et al., 2008], and the rare occasions when the aerosol optical depth is sufficiently high, AOD > 2, to reduce the direct solar beam to below 120 W m 2, the threshold value of the sunshine recorders used in this study. Limitations in the use of Table 3. Trends for the Parameters of the Angstrom-Prescott Equation as a Linear Function of the Midyear of the 6 Year Period of Measurement a a + b Intercept b Slope a Station Slope Intercept R 2 Slope Intercept R 2 Slope Intercept R 2 Valentia Aldergrove Potsdam Hong Kong Bet Dagan Pooled data e e-05 a R 2 values in boldface indicate statistically significant relationships at P < STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,908

8 Table 4. Relationships Between Angstrom-Prescott Equation Parameters and Mean Rate of Fossil Fuel Combustion a Station a + b Intercept b Slope a Slope Intercept R 2 Slope Intercept R 2 Slope Intercept R 2 Valentia Aldergrove Potsdam Hong Kong Bet Dagan Pooled data 1.1e e e a Parameters for each 6 year period of measurement were regressed on mean F for that period. R 2 values in boldface indicate statistically significant relationships at P < fossil fuel combustion as a proxy for aerosol load should be mentioned. These include the changing efficiency of fossil fuel combustion, the neglect of the footprint of the measurement site in relation to the sites of combustion within the 1 cell, especially important in coastal areas, and the arbitrary size of the cell which varies with latitude. The absence of a significant differences in the parameters of the Angstrom-Prescott relationships measured during periods of global brightening and dimming (except for Valentia, where the slope decreased during brightening), seen in Figure 1 and Table 2, support the conclusion that changes in the extent of cloud cover rather than in the radiative characteristics of the atmosphere were the major cause of changes in global radiation. At Potsdam, the site with the longest period of measurement, there was no difference in the parameters calculated for periods of global dimming and of global brightening. At Bet Dagan and Hong Kong, the parameters were, as might be expected, slightly higher during brightening, indicating a small but not statistically significant increase in atmospheric transmissivity. At Valentia, surprisingly, the slope decreased significantly during brightening. Although statistically significant trends were found in some of the parameters of the A-P equation at three of the sites: Hong Kong, Bet Dagan, and Valentia (Table 3), the magnitude of the trend was not large nor was there a clear indication in Figures 2a 6a of a change around 1980 when dimming changed to brightening [Wild, 2009]. The most significant trend (R 2 = 0.86)was observed at Hong Kong, where E g /E 0 under cloudless sky conditions decreased by 0.03 per decade, equivalent to a reduction of 10 W m 2 or 7.6% per decade. The only statistically significant relationships between the A-P parameters and rates of fossil fuel combustion were also found at Hong Kong (Table 4) where an increase of 100 g C m 2 yr 1 in the combustion rate was associated with reductions of 0.06 in the transmission of a cloudless sky, 0.04 for cloud covered skies, and a 0.02 reduction in the slope parameter, i.e., in the transmission of cloud cover; this was not statistically significant. Thus, at Hong Kong, the direct aerosol effect was 3 times greater than the indirect effect. When the data from all sites were pooled, the relationships between the parameters of the A-P equations and both the midyear of the period analyzed and the mean annual rate of fossil fuel combustion did not differ significantly. The equations of these pooled relationships, presented in the last lines of Tables 3 and 4, indicate the relationships with time and fossil fuel combustion were weak and not statistically significant. It is concluded that it is probable that changes in cloud cover rather than aerosol load were the major cause of solar dimming and brightening during the last half century at the sites studied. The conclusion that aerosol effects are minor applies to both the direct, clear sky and indirect, cloud cover aerosol effects, as shown by the absence of significant changes in values of both the offset parameter b and the slope parameter a. The Table 5. Regression Parameters for the Relationship Between Monthly Values of Normalized Sunshine Duration n/n and Cloud Cover C at Potsdam, a Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec All Data Slope a Intercept b R a n/n = ac + b. STANHILL ET AL American Geophysical Union. All Rights Reserved. 10,909

9 Sunshine duration normalized Cloud cover fraction Figure 7. Relationship between monthly values of normalized sunshine duration n/n and cloud cover C at Potsdam, Parameters of the monthly equations for n/n = ac + b are given in Table 5. The polynomial regression fit for the data in this figure is the following: n/n = 0.80 C C (R 2 = 0.78) and for the linear fit n/n = C +1.12(R 2 = 0.77). role of anthropogenic aerosol emissions in bringing about changes in the extent of cloud cover without altering their radiative characteristics remains an open question [Lohmann and Feichter, 2005]. This conclusion may be of wider significance than the very small number of sites examined in this study would suggest as the sites sampled Temperate-Maritime, Mediterranean, Continental, and Tropical climates, see Table 1 for details, and covered a wide range of rates of anthropogenic aerosol emission. Similar conclusion concerning the minor role of aerosols in determining changes in solar radiation during the last half century have emerged from recent analyses of measurements at a number of research sites in the USA [Long et al., 2009; Augustine and Dutton, 2013] as well as from the widespread extent of the global dimming and brightening phenomena that has been documented in areas with low concentrations of anthropogenic aerosol [Stanhill and Moreshet, 1994; Cohen, 2009; Wild, 2009; Liley, 2009; Stanhill and Cohen, 2009]. Further evidence for a widespread role of changes in cloud cover as the cause of global dimming and brightening is provided by measurements of global cloud cover, both by satellite ( products/onlinedata.html) and from the more limited coverage available from subjective, land-based observations [Eastman and Warren, 2013], which show that a global reduction in cloud cover occurred during the recent period of recovery in levels of global radiation [Wild, 2009]. Acknowledgments The authors thank the anonymous reviewers for their constructive comments and helpful suggestions. We are grateful to the following colleagues for providing information on the measurements and their data processing procedures: at Aldergrove to Joan Self of the UK National Meteorological Library and Archives, at Bet Dagan to Vera Lubiansky and Avner Furshpan of the Israel Meteorological Service, at Hong Kong to K. M. Leung of the Hong Kong Observatory, at Potsdam to P. Werner of the Potsdam Institute for Climate Impact Research and F. Kaspar of the German Weather Service, at Valentia to Noelle Gillespie of the Valentia Observatory, Met Eireann. We also wish to acknowledge the work of the many weather observers responsible for the 250 years of measurements analyzed in this study. From 1964 the data used in this paper are available in the Bulletins of the World Radiation Data Center ( Before this year data for Potsdam are available at the portal of the Potsdam Institute for Climate Impact Research at ( the early data from Hong Kong and Valentia are available from the appropriate national meteorological services, payment may be required. References Aguilar, E., I. Auer, M. Brunet, T. C. Peterson, and J. Wieringa (2003), Guidelines on climate metadata and homogenization, WCDMP 53, WMD/TD 1186, World Meteorol. Organ., Geneva, Switzerland. Alpert, P., P. Kishcha, Y. J. Kaufman, and R. Schwarzbard (2005), Global dimming or local dimming? Effect of urbanization on sunlight availability, Geophys. Res. Lett., 32, L17802, doi: /2005gl Angstrom, A. (1924), Solar and terrestrial radiation. Report to the international committee for solar research on actinometric measurements of solar and terrestrial radiation, Q. J. R. Meteorol. Soc., 50, Augustine, J. A., and E. G. 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